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EFFECT OF DIFFERENT TEMPERING METHODS ON SORGHUM MILLING

A Thesis

Submitted to the Faculty

of

Purdue University

by

Yumeng Zhao

In Partial Fulfillment of the

Requirements for the Degree

of

Master of Science in Agricultural and Biological Engineering

i

August 2016

Purdue University

West Lafayette, Indiana

ii

To my parents, Yuqing Wu and Mingqian Zhao,

for their love and encouragements.

ii

iii

ACKNOWLEDGEMENTS

I would like to thank everyone who has helped me over the course of this thesis study, especially my major professor, Dr. Kingsly Ambrose. His guidance and patience helped me improve step by step. I also greatly appreciate the guidance and suggestions I have received throughout my research from Dr. P. V. Venkat Reddy, milling consultant, and

Dr. Hulya Dogan, Kansas State University.

I would like to thank Dr.Thomas J. Herald and Dr. Prini Gadgil, from USDA-ARS, Prof.

Francis Churchill (Kansas State University), and Dr. Osvaldo H Campanella, Dr.

Bhavesh Patel for their help and for the opportunity to use their facilities for a portion of my research.

Sincere appreciation is also due to Dr. Bruce R. Hamaker and Dr. Richard Stroshine for serving in my committee.

Finally, I would like to express special thanks to all my research group members for their support and help and also to my colleagues and friends at K-State and Purdue for sharing their experiences and encouragement. iii

iv

TABLE OF CONTENTS

Page ACKNOWLEDGEMENTS ...... iii TABLE OF CONTENTS ...... iv LIST OF TABLES ...... vii LIST OF FIGURES ...... viii ABSTRACT ...... ix CHAPTER 1. INTRODUCTION ...... 1 1.1 Problem Statement ...... 1 1.2 Research Hypothesis and Goals ...... 4 1.2.1 Research Objectives ...... 4 1.3 Thesis Outline ...... 4 CHAPTER 2. LITERATURE REVIEW ...... 6 2.1 Sorghum ...... 6 2.1.1 Kernel Structure ...... 7

iv

2.1.2 Composition ...... 10 2.1.3 Nutritional Value of Sorghum ...... 11 2.2 Food and Non-Food Use of Sorghum ...... 12 2.3 Sorghum for Industrial use ...... 12 2.3.1 Sorghum Food Production ...... 13 2.4 Sorghum Milling ...... 13 2.4.1 Dry Milling ...... 14 2.4.2 Wet Milling ...... 16 2.4.3 Tempering Methods ...... 16 2.4.3.1 Tempering with Cold Water ...... 17

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Page 2.4.3.2 Hot Tempering ...... 18 2.4.3.3 Steam Tempering ...... 19 2.4.3.4 Sorghum Tempering Methods ...... 19 2.5 Sorghum Properties ...... 20 2.6 Conclusion ...... 21 CHAPTER 3. EFFECTS OF PRETREATMENT ON SORGHUM KERNEL CHARACTERISTICS ...... 23 3.1 Introduction ...... 23 3.2 Materials and Methods ...... 27 3.2.1 Samples ...... 27 3.2.2 Tempering Methods ...... 27 3.2.3 Physical Property Measurements ...... 28 3.3 Results and Discussion ...... 30 3.3.1 Kernel Hardness ...... 31 3.3.2 Abrasive Hardness Index ...... 32 3.3.3 Structural Changes in Sorghum Kernels ...... 33 3.3.4 Texture Analysis ...... 36 3.3.5 Correlation Analysis ...... 38 3.4 Conclusions ...... 39

v

CHAPTER 4. EFFECTS OF TEMPERING METHODS ON SORGHUM MILLING PROPERTIES……………………………………………………………………………41 4.1 Introduction ...... 41 4.2 Materials and Methods ...... 44 4.2.1 Milling Experiment ...... 44 4.2.2 Flour Properties ...... 46 4.2.3 Flour Composition ...... 46 4.3 Results and Discussion ...... 47 4.3.1 Milling Properties of Tempered Sorghum ...... 47 4.3.2 Yield of Sorghum Milling Stock ...... 52

vi

Page 4.3.3 Sorghum Flour Composition ...... 54 4.3.4 Particle Size and Shape Distribution of Sorghum ...... 56 4.3.5 Sorghum Flour Pasting Properties ...... 60 4.4 Conclusions ...... 62 CHAPTER 5. SUMMARY OF CONCLUSION ...... 63 5.1 Restatement of Research Objectives and Goals ...... 63 5.2 Project Overview ...... 64 5.3 Discussion of Major Findings ...... 65 5.4 Future Work ...... 66 REFERENCES ...... 68 APPENDICES Appendix A Preliminary Milling Studies on Sorghum at Different Moisture Contents ...... 85 Appendix B Example Calculation on Moisture Addition During Tempering ...... 86

vi

vii

LIST OF TABLES

Table ...... Page Table 2.1 Hardness of measured by Perten single kernel characterization system

...... 14

Table 3.1Kernal physical characteristics ...... 33

Table 3.2 Kernel pericarp thickness ...... 36

Table 3.3 Texture of pretreated sorghum kernels ...... 38

Table 3.4 Correlation between sorghum kernel characteristics ...... 39

Table 4.1 Sorghum milling stock yields ...... 53

Table 4.2 Chemical composition of sorghum flour ...... 56

Table 4.3 Sorghum flour pasting properties ...... 61

Appendices Table

Table A.1 Stocks yields over 8W of cold tempered sorghum through First break roll .... 85

vii

viii

LIST OF FIGURES

Figure ...... Page Figure 2.1 Sorghum kernels ...... 8

Figure 2.2 Sorghum kernel structure ...... 8

Figure 3.1. Cross sectional SEM images of differently tempered sorghum kernels...... 35

Figure 3.2. Typical compression pattern of sorghum kernels (steam tempered for 2.0 min)...... 37

Figure 4.1 Laboratory scale sorghum milling process ...... 45

Figure 4.2 Effects of different tempering methods on the mill stream stock distribution at different stages of milling...... 51

Figure 4.3 Sorghum milling stocks (a. bran; b. shorts; c. fine grits; d. coarse grits) ...... 54

Figure 4.4 Sorghum flour particle size and shape distribution ...... 59

viii

Figure 4.5 Selected sorghum flour particle images (steam-2.0 min) ...... 59

Figure 4.6 Sorghum flour pasting properties ...... 61

ix

ABSTRACT

Zhao, Yumeng. M.S.E., Purdue University, August 2015. Effect of Different Tempering Methods on Sorghum Milling. Major. Major Professor: Kingsly Ambrose.

Current sorghum milling processes result in high milling losses and inconsistent flour quality. Appropriate tempering methods could improve the quality and quantity of flour extracted from sorghum.

In this study, the effects of cold water, hot water, and steam tempering on sorghum kernel physical and mechanical properties were studied. Single kernel characteristics (SKCS), abrasive hardness, structural changes, and texture of kernels were evaluated. SKCS hardness indices decreased from 75.41 to a range of 53.50–64.20 after tempering. In terms of reducing the hardness value, hot water tempering was more efficient than the cold water tempering process. The abrasive hardness index, which represents the pericarp properties, did not show any correlation with moisture content. Abrasive hardness of steam tempered kernels increased from 14.61 to 22.11 when the time of treatment was extended. Smaller deformations and larger rupture forces were found for cold water ix

tempered kernels, indicating the brittleness of cold tempered sorghum kernels. The energy required to rupture sorghum kernels increased (63.41–70.04 N.mm) when the duration of steam tempering was extended. Untreated sorghum kernels were associated

x with the lowest energy (35.41 N.mm). Pericarp thickness decreased from 57.98 mm

(untreated) and 58.49 mm (cold tempered) to 24.49–37.60 mm after hot water and steam tempering. The changes in pericarp structure and endosperm texture induced by steam tempering influence the kernel mechanical properties.

The effects of cold water, hot water, and steam tempering on the roller milling performance and flour quality of sorghum were also evaluated. Steamed sorghum had high bran yield with a larger proportion of bran particles after the milling process that might be due to the gradual scrapping of endosperm from the bran. Bran yield increased with increasing steam duration (from 1 to 2 min) but decreased at 2.5 min steam treatment. Flour yield was not significantly different among the three treatments. Crude fiber content of the 1-min steam tempered sorghum was 1.28 ± 0.09%, which was significantly higher than that of flour obtained from cold water tempered sorghum (0.87 ±

0.19%). The damaged content was the highest in the cold water treated (5.96 ±

0.24%) sorghum and steam tempering resulted in a damaged starch content of 3.63–

4.18%. The circularity equivalent diameters of sorghum flour from different treatments

x mainly below 50 μm. Steam treatments for 1 and 2 min resulted in flour with more convex and circular particles compared to other treatments, indicating a better separation of starch granules from endosperm cells. Viscosity profiles of sorghum flour did not show significant differences among tempering treatments. Steam tempering of sorghum kernels at high temperature and pressure led to a better separation of bran and endosperm, without negatively impacting flour quality.

1

CHAPTER 1. INTRODUCTION

This thesis presents a study on the effect of different tempering methods on sorghum milling characteristics. Cold water tempering, hot water tempering and steam tempering methods were assessed for their effect on sorghum flour extraction. Changes in sorghum kernel physical characteristics, milling properties, and flour qualities after tempering and milling were studied.

1.1 Problem Statement

Sorghum is the 5th most important in the world. In 2015, about 597 million bushels of sorghum were produced in the United States (USDA-NASS, 2016). In the

U.S., sorghum is primarily used as an animal feed, while about 12% of sorghum is also used for ethanol production and for human consumption (US Grain Council, 2010).

1

However, with the expansion of the -free food market, sorghum, a potential source of gluten-free food, has been gaining more importance as human food. Sorghum is a major cereal crop in and South , due to its severe drought tolerant characteristics.

Sorghum kernels have a unique pericarp, which is higher in starch content than the pericarp of other cereal grains. This makes the pericarp brittle (Taylor, 2003). A specific component, , found in pigmented sorghum pericarp and testa (between the pericarp

2 and aleurone), affects the taste and nutritional value of sorghum based food products.

Sorghum kernels are primarily milled into flour, or flaked for further processing. With large germs, a friable pericarp and its very variable proportion of horny to floury endosperm, sorghum milling is difficult and the flour can be easily contaminated with bran. Most methods used in the commercial scale industry are based on the use of a dehuller for the decortication of grain, and a hammer mill for the production of meal

(Rohrbach, 2000). However, the properties of the flour produced using dehullling method is not constant, and the production the rate is low. In addition, hammer milling results in high starch damage. The roller milling process used for has been adapted to sorghum, but a high contamination of bran fraction has been observed due to its brittle pericarp (Taylor and Dewar, 2001).

Tempering, or conditioning, is the process of adding water to grains before milling to toughen the bran and mellow the endosperm of the kernel. Tempering improves the efficiency of flour extraction, resulting in fewer pericarp particles in the flour. Bradbury et al., (1960) described conditioning methods such as cold, warm and hot tempering

2 which are designed according to the temperature of the water used for conditioning wheat. Cold tempering is the adjustment of moisture without addition of heat, while warm conditioning is with the use of heated below 46°C. Hot conditioning is adjustment of moisture using water heated above 46°C. Several special treatments, such as the use of steam, vacuum and infrared heating have also been used in warm and hot conditioning methods (Kathuria and Sidhu, 1984; Dienst, 1951; Linder, 1954). During tempering, the moisture uptake rate is not only influenced by kernel size, but also by the initial kernel moisture content, endosperm structure, temperature, target moisture content, and

3 tempering time (Posner and Hibbs 1997; Stenvert and Kingswood, 1977). Tempering conditions will not only influence the physical properties of bran and endosperm, but will also affect the milling temperature, grindding energy and final flour properties. Kweon et al., (2009) found that with an increase in kernel moisture, flour extraction rate decreased, but the flour quality improved with decreased flour ash and reduced polyphenoloxydase

(PPO) activity. Therefore, both flour extraction and bran contamination level should be considered while selecting the appropriate tempering procedure (Hook et al., 1982).

Gomez (1993) conditioned sorghum to 16% moisture at 4°C before roller milling, and produced better sorghum meal with higher flour extraction, and slightly lower ash and content compared to dehulling and hammer milling. However, tempering at low temperatures will be a challenge at commercial facilities. Cecil (1992) optimized the moisture content of the grain to 26% at 60°C that resulted in maximum production of bran, with a higher rate of extraction of fat, fiber, and tannin. But the moisture content of flour is high, which resulted in poor keeping quality.

Sorghum grain kernels have very similar structure as that of corn kernel, except the size,

3 so a similar processing technology could be adapted (Taylor, 2003).

Therefore, the tempering methods of hot tempering at 60°C, steam tempering, as used in corn processing, and cold tempering under room temperature were studied. A laboratory scale sorghum milling flow was designed for this research to study the milling properties of the differently conditioned sorghum. The kernel physical properties after conditioning and properties of the flour obtained from processing were also studied.

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1.2 Research Hypothesis and Goals

It is important to understand the kernel physical property changes and the kernel structural changes under different conditioning methods. The hypothesis of this study is that the temperature and pressure affect the moisture penetration and kernel structure, and further influence the milling behavior of sorghum kernels.

1.2.1 Research Objectives

To obtain consistent and high quality flour from pigmented sorghum, it is important to develop an optimum conditioning method for the sorghum roller milling process. The use of a roller mill to process the sorghum into flour would increase the capacity for producing flour as compared to the use of a decorticator. Therefore, the conditioning methods were used on samples processed with a laboratory roller mill. In this study, cold, hot and steam conditioning were used as tempering methods to develop a suitable sorghum milling process using roller mills. The overall objective of this study was to study the effect of different tempering methods on sorghum milling. The specific

4 objectives of the study were:

1. To determine the effect of different tempering methods on sorghum kernel characteristics.

2. To study the effect of tempering on percent flour extraction and flour properties.

1.3 Thesis Outline

This thesis is divided into 5 chapters including this chapter. Chapter 2 covers the review of literature on the sorghum kernel structure and composition, current sorghum milling

5 practices and conditioning methods, and food and nonfood uses of sorghum.

Conditioning methods used for other cereal grains are also discussed in chapter 2. In chapter 3, the effect of different tempering methods on sorghum kernel properties, such as kernel resistance to abrasion, resistance to compression, and structural changes are discussed. Chapter 4 describes the milling behavior of differently conditioned sorghum and flour characteristics. A lab scale milling process was developed to determine the milling properties, including bran and flour yield. Flour particle size and shape distribution, flour composition and flour pasting properties were also studied to evaluate the quality of flour.

5

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CHAPTER 2. LITERATURE REVIEW

2.1 Sorghum

Africa is the largest producer of sorghum, harvesting about 22.1 million metric tons in

2015/2016 (USDA-FAS), while the United States is the second largest producer and biggest exporter of sorghum in the world. Compared with 2014, the U.S., sorghum production increased by 38% in 2015. The average planted area in US in 2015 was estimated to be 8.46 million acres, 19 percent higher than the previous year. The two major sorghum production states are Kansas and Texas, harvesting about 431 million bushels in 2015 (USDA-NASS). Researchers in Japan, the largest U.S. sorghum importer, and North America have been working on creating whiter sorghum flour for food (US Grains Council, 2010).

Food grade (without condensed ) sorghum has been widely planted in the U.S. for its food and feed processing qualities (Rooney, 1996). Condensed tannin is found in pigmented pericarp and testa, so the sorghum grain is always a brown or purple color

(Waniska, 2000). Food grade sorghum is denser, lighter in color and its caryopsis is 6

harder than grains produced from white purple-plant or red purple-plant hybrids. Food grade sorghum contains a higher percentage of floury endosperm, and yields more flour

(Gomez, 1993). Even though white food are bland in flavor, and have excellent processing properties (Rooney and Awika, 2005), they are more vulnerable to grain mold

7 than those with brown and red kernel pericarp (Thakur et al., 2006). Sorghum with a high level of condensed tannins has high bird and mold resistance (Dykes and Rooney, 2006).

Especially in Southern Africa, where bird-predation is a major problem, small-scale farmers intercrop tannin and tannin-free sorghums in areas of high bird predation in order to reduce field losses (Awika and Rooney, 2004).

2.1.1 Kernel Structure

Sorghum grain is an oval-shaped naked caryopsis (Fig. 2.1). Commercial sorghum kernels grown in the US are generally 4 mm long, 2 mm wide, and 2.5 mm thick, the kernels weigh from 3-80 mg, and have a narrow density range (1.28-1.36 g/cm3) (Serna-

Saldivar and Rooney, 1995). The test weight of sorghum ranges from 55-61 lb/bu (Serna-

Saldivar and Rooney, 1995). A sorghum kernel is composed of three main parts (Fig.

2.2): the pericarp, endosperm and the germ, and the relative proportions of the pericarp, germ, and endosperm in kernels depend on varieties and growth conditions. Sorghum grains have both horny and floury endosperm, and a large fat-rich germ. Kernel size,

7 shape, and germ inside the kernel affect milling properties and water uptake (Bradbury,

1960).

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Figure 2.1 Red Sorghum kernels (Source: D. M. Hikeezi, 2010. The importance of sorghum grain color and hardness, and their causes and measurement)

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Figure 2.2 Red Sorghum kernel structure (Potassium iodide stained)

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Pericarp: The pericarp layers include the epicarp, mesocarp and endocarp (Earp and

Rooney, 1982). Sorghum has a unique mesocarp because starch granules are presented.

The friable nature of the sorghum pericarp is probably related to its unique starch (Earp et al., 2004). Pericarp thickness ranges from 8 to 160 μm (Blakely et al., 1979; Earp and

Rooney, 1982) and varies within an individual kernel. Thickness of the pericarp, which is mainly determined by the mesocarp, affects milling properties. Since the pericarp breaks at the mesocarp layer, a thick pericarp hold together better and is removed in larger pieces during dehulling, thus requiring less decorticating time (Earp and Rooney, 1982).

Testa: the testa is develops from inner integument cells. The testa is a single and often continuous layer between the pericarp and the aleurone layer. The thickness of the testa ranges from 8 to 40 μm and varies within the individual caryopses (Blakely et al., 1979;

Earp and Rooney, 1982).

Endosperm: The sorghum endosperm consists of the aleurone layer, and the peripheral corneous and floury portions. The aleurone cell layer is a single layer of rectangular cells.

The peripheral area consists of small, blocky cells containing matrix of small starch

9 granules and bodies. Sorghum has both corneous and floury endosperm, and the proportion of the endosperm parts affects the texture of a kernel. The pericarp is easy to separate from corneous endosperm; thus a higher percentage of corneous endosperm sorghum gives a higher flour yields during dry milling (Rooney et al., 1981). Floury endosperm gives a higher starch yield by wet milling (Norris, 1971).

Germ: The germ consists of two major parts: the embryonic axis and the scutellum. The embryonic axis will develop into the new plant. The scutellum is the germ reserve tissue that contains oil globules, protein bodies, and a few starch granules (Rooney et al., 1981).

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Glueck and Rooney (1980) showed that the embryo is the key point for water uptake and mold susceptibility in sorghum kernels.

2.1.2 Composition

Sorghum proximate composition varies due to genetic and growth conditions. The pericarp is rich in fiber, the germ is high in protein, fat, and ash, and the endosperm contains mostly starch, some protein, and a small amount of fat and fiber.

Carbohydrates: The majority of the in sorghum is starch. Small amounts of soluble , pentosans, cellulose, and hemicellulose are also found in sorghum.

Generally, Sorghum contains about 20 to 30% amylose, but waxy varieties contain less than 5% amylose (Rooney and Pflugfelder, 1986). Sorghum is a good source of fiber

(mainly found in the pericarp), of which 86.2% of the fiber is insoluble (Leder, 2004).

Proteins: Sorghum protein content and overall composition differ between varieties and are affected by growth conditions. The protein content of sorghum is usually 11-13%

(Serna-Saldivar and Rooney, 1995). Prolamins (kafirins) and glutelins are the two major

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protein fractions found in sorghum. Sorghum proteins, unlike wheat protein matrix, do not have the ability to form a gas-holding and visco-elastic dough, and wild-type protein bodies remain distinct and separate (Hamaker and Bugusu, 2003).

Lipids: Sorghum crude fat content is about 3%. The fatty acid of sorghum is composed of linoleic (49%), oleic (31%), and palmitic acids (14%). Sorghum is the only cereal containing a significant amount of β-carotene and provitamin of , which are important in human physiology (Leder, 2004).

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Anti-nutrients in Sorghum Grain: The anti-nutrient compounds include protease inhibitors, galacto-oligosaccharides, lectins, ureases, phytates, tannins and phenolics.

These compounds have biological functions during plant development, but have at least a partially negative impact on human and animal organisms. Some of the components could reduce the digestibility of nutrients and the absorption of minerals by combining with nutrients (Rooney and Pflugfelder, 1986; Duodu et al., 2003; Mamary et al., 2001).

They may also inhibit the growth of animals and cause pathological alterations in the liver (Ortiz et al., 1994; Mamary et al., 2001). These compounds can affect the color, flavor and nutritional value of the sorghum grain and its products.

Condensed tannin is the major anti-nutrient compound found in sorghum. It can bind with both the grain proteins (Emmambux and Taylor, 2003) and digestive tract enzymes (Price and Butler, 1980) to reduce the nutritional value of the grain. The tannins also have a negative effect on the qualities by reducing the enzyme activity (Daiber, 1975).

2.1.3 Nutritional Value of Sorghum

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Sorghum is a substitute cereal for celiac and gluten sensitive people. Celiac disease is a serious chronic disease that affects approximately 1% of the population of the United

States and (Wieser and Koehler, 2008). Celiac disease is inheritable, so the only treatment for celiac disease is lifelong avoidance of gluten ingestion (Hill et al., 2005).

Therefore, sorghum, as a non-gluten cereal, is often recommended as a safe food and a gluten protein substitute for celiac patients (Kasarda, 2001).

Polyphenols, such as condensed tannins, are a potential source of antioxidants. The grain and bran fractions of tannin sorghums with added ingredients make excellent quality

12 that contains high levels of antioxidants, , and a natural dark brown color (Gordon, 2001).

Sorghum also contains phytochemicals, mainly phytosterols and policosanols. There are long chain fatty alcohols, which can potentially reduce serum cholesterol levels and, thus promote human health (Varady et al, 2003; Hwang et al, 2004; Carr et al., 2005).

However, these components are in small amounts, and often exist in cell layers between the pericarp and endosperm, thus the extraction and purification procedures are costly.

2.2 Food and Non-Food Use of Sorghum

Sorghum has been used for human food and as an animal feed in the United States,

Mexico, South America, and . Sorghum grain could be used for alcohol products; and the and foliage are used for green chop, hay, silage, and pasture.

In some areas, the stem is used as a building material, and plant remains may be used for fuel (House, 1985). In the United States, about 12% of sorghum is used to produce ethanol and other fuel sources (US Grains Council, 2010).

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2.3 Sorghum for Industrial use

Sorghum is a substrate cereal for industrial production of alcohol, distilled spirits, starch, dextrose, syrups, and edible oil (Hahn, 1966). Sorghum has a larger proportion of starch and higher oil content than wheat, which makes it an excellent source for starch and oil.

The starch could be further converted into syrups, dextrose, modified starch and other value-added products. Sorghum could be a adjunct in conventional lager production, which involves the use of unmalted sorghum as an adjunct, sorghum as

13 a substitute at various levels for malt, or using only sorghum malt (Hahn, 1966;

Olatunji et al., 1993).

2.3.1 Sorghum Food Production

About 40% of the world sorghum production is used for human food in Africa and , while the remainder has been used primarily as animal feed in Western countries (Murty and Kumar, 1995; Rooney and Waniska, 2000). Many studies have been conducted on improving the use of sorghum as food, including use in bread manufacturing

(Cauvain,1998; Schober et al., 2007; Schober et al., 2005; and Trappey et al., 2015), in tortillas (Rooney and Waniska, 2000; Winger et al., 2014), in noodles (Suhendro et al.,

2000; Liu, 2012), as parboiled sorghum (Young et al., 1990), snack foods (Serna-Saldivar et al., 1988), and in cookies (Morad et al., 1984; Rai et al., 2014). Sorghum is already commercially available in gluten-free bread, pasta, cookies, cereal, beer, and bakery mixes for brownies, cakes, and pancakes.

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2.4 Sorghum Milling

Grain products for food or industrial use come largely from dry milling, wet milling, malting, and . The main purpose of dry milling is to achieve a clean physical separation of bran, endosperm, and germ (Hahn, 1969), while wet milling is used to separate pure starch, lipid and protein compounds from the endosperm and germ. Grain shape, pericarp friability and the variable proportion of horny to floury endosperm in grain are factors that affect the sorghum milling process. Challenges of sorghum milling is also assigned to its extremely hard endosperm compared to other cereals (Table 2.1),

14 the sorghum kernels are harder than hard wheat, barley and soft wheat. Appropriate tempering and milling will yield a larger proportion of low-fat grits and low proportion of bran. For dry milling of sorghum, knowledge of grain properties is important. Wet milling of sorghum grain could use the same process as is used for corn, because the sorghum kernel is very similar in structure to the corn kernel, except for its size (Watson,

1984).

Table 2.1 Hardness of cereals measured by Perten single kernel characterization system

Sorghum1 Hard wheat2 Soft wheat2 Barley3

Hardness index 77.5 72.56 31.20 56.98 1Bean et al., 2006; 2Liu, 2008; 3Iwami et al., 2003.

2.4.1 Dry Milling

The most commonly used processing equipment consists of a traditional mortar, dehuller, hammer mill, and roller mill. These mills differ in the forces they apply to the kernel and the way the kernel size reduction occurs. The most widely used commercial milling

plants in Africa are hammer and stone mills, with capacities of 100-1800 kg/h and 25- 14

1800 kg/h, respectively (Beshata et al., 2006).

The traditional grinding process: This process is by compressing the sorghum with addition of water in a mortar, and winnowing to remove the bran. Pounded grain is then roasted in a pan, ground using a quern, and sifted through a sieve (Carr, 1961). The traditional grinding method causes relatively high losses of important nutrients and the use of stone quern leads to high iron content. John and Muller (1973) compared the traditional pestle and mortar method with roller milling, and found that roller mill with

15 three break and three reduction rolls removed more germ and bran than the traditional process.

Decortication or dehulling method: This is the method most used process in small to medium scale commercial mills. Dehulling removes the outer layer of the grain, i.e. pericarp, before the dry milling process of hammer milling or roller milling (Anderson et al., 1969). The equipment used for decortication is usually an abrasive mechanism, such as a decortication or debranning machine, or pearlers containing stones or resinoid disks (Rooney and Waniska, 2000). The purpose of this process is to produce a low fiber intermediate product for further particle size reduction and separation. Rooney et al.

(1972) studied the correlation of flour protein content with bran removal level, and suggested that the removal of 18% of the kernel during debranning might increase flour yield 8-10% and give flour with a higher protein content. , with a similar gain structure as sorghum, is widely processed with dehullling methods (Munck, 1995).

Roller milling: Roller milling is the most common size reduction process for cereal grains. It consists of several consecutive breaking steps to open the kernel and reduce the

15

particle size of the grain into grits and flour (Hoseney et al, 1994). Abdelrahman et al

(1981) dry milled sorghum using a three-break roll and three-sifter system. The pre- break roll cracked open the kernel increasing its surface area, so that the grits and germ were more easily separated with sieves. The grits produced by the pre-break system had lower fat and ash content than did a normal break system. Roller milling tends to render meal with a specky appearance (Munck et al., 1982) while abrasive decortication provides the opportunity of abrading until the color of the grain is light and acceptable.

Thus the roller milling process is used for white food sorghum types more than for the

16 red or purple-colored sorghum grains. The percentage of bran removed, which directly influences the purity and color of flour, does not depend on the processing time for roller mill, while it does depend on time for the decortication (Merwe et al., 2013). Thus, development of an effective roller milling process could improve the production rate.

2.4.2 Wet Milling

The wet milling process includes steeping of the grain, physical separation of germ and fiber by grinding, and chemical separation of the starch from protein. Sulfur dioxide solution is used in a conventional steeping process (Wang et al., 2000). However, sorghum wet-milling results in low yields of oil and starch due to its small sized germ. In addition, the stronger starch-protein matrix makes the recovery of starch more difficult

(Watson, 1984). The sorghum pericarp is more fragile than the pericarp of corn. Thus small pericarp particles impede the separation of the starch and protein and results in off- colored starch.

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2.4.3 Tempering Methods

Tempering (or conditioning) is the process of increasing the moisture content of grains through the addition of water before dry milling. Tempering for dry milling involves controlled addition of water a different temperatures in a single or multiple stages, followed by a rest period (Eckhoff and Paulsen, 1996). Tempering toughens the bran and makes the endosperm softer and friable, and could also increase the volume of the kernels

(White, 1966), thus facilitating the structural separation of the germ, pericarp, and endosperm (Wolf et al., 1952). Water diffusivity in sorghum is of same order of

17 magnitude as it is in wheat (Fan et al., 1963). Water penetrates the outer layers of the tempered grain by capillary diffusion and is distributed throughout the endosperm and germ (Hsu, 1984).

The factors affecting proper conditioning of grain before milling are the amount and distribution of water in the kernel, the temperature, and the time allowed for water to penetrate the kernel (holding time). The moisture content and its distribution within the kernel are, in turn, affected by the amount of water added to the grain (Bradbury et al.,

1960). With a larger proportion of corneous endosperm, the kernel absorbs moisture slowly (Weinecke and Montgomery, 1965) and waxy sorghum absorb more water than non-waxy cultivars (Mustafa, 1969). The first three subsections discuss studies on tempering techniques that have been used primarily for wheat with the intention of gaining insight into techniques that may be useful for tempering sorghum.

The last subsection discusses studies on the tempering of sorghum.

2.4.3.1 Tempering with Cold Water

17

Cold conditioning is the adjustment of moisture without heating the grain or water. Water is added using fine a spray, and conditioned wheat is held in large conditioning bins to ensure even penetration of water into the bran layer (Berghofer et al., 2003). The level of moisture added to the wheat and the holding time depends on the wheat type and initial moisture content. Hard wheat is generally conditioned to 15.5-17% while soft wheat is conditioned to 14-15.5% moisture content (Butcher and Stenvert, 1973). The holding time usually ranges from 12-24 h for wheat, but sometimes could be up to 36h (Butcher

18 and Stenvert, 1973; Berghofer et al., 2003). It has been reported that tempering time and moisture content of grain affects milling yield and flour qualities. Shollenberger (1919,

1921) found that the bran portion had a distinctly higher percentage of water than the other milled product, and the moisture content of flour did not have any relation to the tempering water added to the wheat. Shorter tempering periods produce lower amounts of break flour, and the most break flour is produced by the medium tempering periods

(Swanson, 1913, 1944). McCormick (1930) found that the length of tempering did not affect the amount of middlings released from the breaks, the ash content of the break flours, or power consumption. The cold tempering method is widely used in the wheat milling industry due to its low cost and energy consumption compared to the other methods such as hot tempering. However, for some grains, such as sorghum, the effectiveness of cold water tempering may not meet the industry requirements.

2.4.3.2 Hot Tempering

Hot tempering is the application of temperatures in excess of 46°C with variable holding

18

times, thus the temperature, duration and moisture contents are the factors that are controlled to alter the kernel properties (Mentzos, 1954). Hot tempering could greatly accelerate the penetration of moisture into the wheat grain (Bradbury, 1960). Heat does not have any effect on the rapid initial moisture pickup (about 4-5% water), but affects the moisture absorption during longer tempering periods (Jones, 1948; Swanson and

Pence, 1930). Studies of hot tempering for wheat were mostly within the temperature range of 50-65°C for the holding time of 1 hour up to 24 hours (Eustace, 1962). Hot

19 tempering over 57°C can cause denaturation of protein, and improve the flour and milling quality of wheat (Swanson et al., 1916).

2.4.3.3 Steam Tempering

Heat transfer from steam to grain kernels is more rapid than the transfer from hot air or radiators (Bradbury, 1960). Thus by using steam, the conditioning time needed to achieve the target moisture content could be reduced. Several conditioners were developed for steam tempering of grains, such as Wichita Wheat Conditioner, and the MIAG laboratory conditioner, that allow the kernels to be steamed, held at high temperature/pressure for a presented time interval and then cooled (Bradbury, 1960; Wu, 1971). Steam treated wheat grain has a higher flour yield, coarser bran and requires less power consumption than normal and hot conditioning methods. Steam conditioning can harden the aleurone layer which improves the scraping efficiency of roller mill (Schafer, 1951). However, the gluten content of wheat flour increased with the increase of steam treatment time

(Berliner, 1956). However, with the many advantages, it has over the conventional

19

tempering methods, steam tempering could result in positive changes in sorghum kernel structure helping in better extraction of flour.

2.4.3.4 Sorghum Tempering Methods

Numerous attempts were made during the past decade to improve sorghum dry milling yields. The sorghum have lower moisture uptake rate than wheat due to the different pericarp structure and the present of hydrophobic kafirin proteins (Munck, 1995).Wu

20

(1971) studied the tempering of white sorghum at 21, 48 and 66 °C and then used a dehuller mill for flour extraction. He concluded that the optimum conditioning treatment was to increase the kernel moisture to 17 % at 21°C and hold the sorghum kernels for 1.5 h before milling. Wu (1971) also found that, with an increase in tempering temperature, moisture content, and holding time, the flour yield, fine grits, percent fiber content and protein content decreased, while the flour fat content increased. Abdelrahman et al.

(1981) described an optimum treatment for dry roller milling sorghum by conditioning sorghum grain for 8 hours at room temperature to 17% moisture content, and found an increase in bran and fine grit yield with the increase in moisture content. Cecil (1992) described a semi-wet milling method involving tempering sorghum to 26% moisture content at 60 °C for 6 h before roller milling. An improvement in degermination and bran separation was observed. However, this method resulted in poor flour extraction rates and high residual moisture in the flour. Gomez (1993) conditioned sorghum to 16 % moisture content at 4°C for 24 h before milling and found that in addition to improved extraction rates, the final meal residual moisture was at a microbiologically safe level. However,

20

there is still a lack of tempering technology that could be easily adopted by the industry and scaled up to a continuous method similar to those used for tempering of wheat and shelled corn grains.

2.5 Sorghum Flour Properties

The Main components of sorghum flour are starch and protein (mainly Kafirin).

Mahasukhonthachat et al., (2010) studied sorghum flour particles distributed from 5-2000

μm, and separated particles into 5-50, 50-500 and 500-2000 μm ranges. Sorghum starch

21 ranges from 5 to 30 μm in size. The particles less than 30 μm are mostly mechanically separated starch granules and damaged starch. The average sorghum grain endosperm cell size is 100-200 μm (Choi et al., 2008), particles between 500 and 2000 μm included multicellular structures (), that contained cells with complete encapsulating cell walls.

Particle size also affected the sorghum flour water absorption index (WAI), pasting properties, and water solubility index (WSI) (Mahasukhonthachat et al., 2010).

Kafirin, which differs from gluten, cannot give elastic properties to dough, but it contributes to the gluten network formation (viscoelastic properties), that leads to better quality composite doughs and (Goodall et al., 2012). Therefore to make a bread using sorghum flour, other ingredients are always required to form an elastic dough. For wheat-sorghum dough, water retention capacity and alkaline water-retention-capacity value generally increased with the addition of sorghum flour, while peak, holding and final viscosities decreased (Chang and Park, 2005). Mixing time and mixograph mixing height decreased with addition of sorghum flour.

Sorghum whole grain meal has stable pasting properties when substituted to heat

21

treatment and mechanical stirring. This stability is not obtained with barley, millet and wheat meal (Ragaee and Abdel-Aal, 2006).

2.6 Conclusion

Sorghum flour is often produced using dehulling methods. However, the low production rate and inconsistent flour quality remain as disadvantage. Adapting roller milling process to mill sorghum could be an alternative to improving the production rate and efficiency. Due to the unique pericarp structure, and hard endosperm texture of sorghum

22 kernels, a conditioning treatment is required before milling to assist in the separation of bran and endosperm. This thesis work deals with finding an appropriate tempering method for sorghum for developing a roller milling process methodology for sorghum.

22

23

CHAPTER 3. EFFECTS OF PRETREATMENT ON SORGHUM KERNEL CHARACTERISTICS

3.1 Introduction

Sorghum is the fifth most cultivated cereal in the world (Rooney et al., 2007) and the

United States is the second largest sorghum producer in the world, ranking after Africa, with a production of over 4 million tons in 2014 (NASS, USDA, 2015). Sorghum is mainly used as a feed in the US. (Rooney, 1992); however, it is a substantial cereal grain consumed in Africa and . Sorghum is a rich source of phytochemicals, which can reduce the risk of several types of cancer and obesity in humans (Awika and Rooney,

2004). The most common food products made from sorghum are tortillas, , , baked goods, and fermented beverages (Anglani, 1998). Sorghum is also a potential food source for celiac and gluten-sensitive consumers. Additionally, sorghum is a viable raw material for ethanol production because of its high starch content (Serna-

Saldivar and Rooney, 1995).

Sorghum is relatively low in fiber (2.7%), contains 7 to 15% protein, and up to 75%

23

starch. The sorghum kernel structure consists of pericarp, aleurone layer, endosperm, and germ (Rooney and Clark, 1968). The unique polyphenol compounds of sorghum, the tannins, are contained in some of the varieties with pigmented testa layer, located between the pericarp and the aleurone layer. Tannins decrease the nutritional value of sorghum because of their ability to bind dietary proteins, digestive enzymes, and

24 minerals, such as iron and vitamins like thiamin and (Wang and Kies, 1991).

In addition, the bran fractions of pigmented sorghum also reduce the whiteness of food products. After decades of breeding efforts, the majority of sorghum produced in the US is tannin-free; however, in many other parts of the world where pests and diseases are more common, sorghums containing tannins are still grown in significant quantities because of their high tolerance to pests (Awika and Rooney, 2004).

Sorghum kernels are primarily decorticated and milled into flour, or flaked for further processing. Removal of the bran fraction is critical for sorghum milling (Rooney, 2003).

Sorghum have extremely hard endosperm and the pericarp is brittle compared to wheat

(Hahn, 1969). Milling sorghum using a roller mill, following a similar procedure to that for wheat and corn, results in gritty and speckled flour with an astringent taste (Cecil,

1992). For this reason, many processors dehull the sorghum kernels and subsequently grind them using a hammer mill or a roller mill to break the intact kernel and reduce the endosperm particle size to avoid high bran contamination in the flour (Wondra et al.

1995; Svihus et al., 2004). However, dehullers developed for industrial milling are rather

24

small in capacity and result in huge losses (up to 30%) during milling; moreover, the flour quality obtained with this process is not consistent (Taylor and Dewar, 2001).

Researchers have attempted to mill sorghum using roller mills without decorticating; however, the separation efficiency of grain constituents has remained very low (Kebakile et al., 2007). This low efficiency is because of the extremely friable pericarp, the large integral germ, and the highly variable endosperm texture.

The pericarp of sorghum is divided into four parts: epicarp, mesocarp, cross-cell layer, and tube cell layer (Rooney and Miller, 1981). The efficiency of the tempering method

25 depends on moisture absorption through these layers and the resultant toughening of the pericarp. Standardized tempering methods to condition cereal grain kernels for effective separation of bran from endosperm are available for the majority of cereal grains and are based on their physical and mechanical attributes.

Water uptake by cereal grains during tempering is known to occur in two distinct phases: water first adheres to the surface of the kernel, and subsequently diffuses to the center of the kernel through the germ and the pericarp (Hsu, 1984; Zinn, 1990; Ruan et al., 1992).

The addition of moisture toughens the bran, mellows the endosperm (MacMaster, 1961), and results in an increase in kernel volume due to swelling (White, 1966). Differential swelling of parts of the kernel facilitates the structural separation of germ, pericarp, and endosperm (Wolf et al., 1952). Tempering methods for wheat have been standardized for effective flour separation. When the moisture content of wheat endosperm increases, compressive strength and elasticity decrease (Glenn et al., 1991). Single kernel characteristics (SKCS) hardness of wheat kernels decreased after conditioning, and shell stiffness and endosperm strength decreased following conditioning (Edwards et al.,

25

2007). However, the bran becomes more compliant and resilient with increasing moisture content (Mabille et al., 2001; Hemery et al., 2010).

Moreover, cold water tempered sorghum with high moisture content was found to have a higher rate of weight loss over time upon dehulling, compared with that of untempered sorghum kernels (Lawton and Faubion, 1989). Weaker endosperms break into small pieces during grinding, whereas the bran is more resistant and yields larger particles that can be easily separated by sieving (Posner and Hibbs, 1997).

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Higher temperature and steam treatment were found to be more appropriate than cold water treatment because of their capability to accelerate the diffusion of moisture into grain kernels and attainm a uniform distribution of moisture (Bradbury, 1960). Although hot water tempering and steam tempering hasten the penetration of water into the kernel, the addition of heat can also change biochemical properties of the grain (Bradbury, 1960) that could affect the end-use qualities of flour (Kathuria and Sidhu, 1984). Compared with cold water and hot water tempering, steam tempering is an energy saving and rapid conditioning method, which results in high flour yield (Galter, 1951). Steam tempering does not alter the external pericarp structure (McDonough et al., 1998). Many studies have reported an increased yield of break flour and bran by steam treatment for wheat

(Mozzone, 1971; Kathuria and Sidhu, 1984).

Tempering of sorghum, which has a harder pericarp than other common grains such as wheat and corn, for efficient flour extraction remains a challenge. Typically, soft are conditioned to 15–15.5%, hard wheats to 16–16.5%, and wheats to 18% (w.b.) moisture (Kathuria and Sidhu, 1984). Cecil (1992) tempered sorghum to 26% moisture

26

content at 60 °C for 6 h; however, the moisture content of the flour was excessively high.

Preconditioning sorghum to 16% moisture at 4°C and subsequent milling using a roller mill could produce better sorghum meals with a higher flour extraction, and slightly lower ash and fat content compared to meals obtained by dehulling and hammer milling

(Gomez, 1993). However, tempering at low temperatures is a challenge. Contrastingly,

Hemery at al. (2010) found that bran layers are more brittle at very low temperatures, and result in fine particles during grinding. Therefore, it is important to develop scalable tempering methods for efficient sorghum flour extractions with minimum bran friability.

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As a prelude to the development of a scalable tempering method, this study evaluated the effects of cold water, hot water, and steam tempering on the sorghum kernel physical and mechanical properties.

3.2 Materials and Methods

3.2.1 Samples

Sorghum Partner 217 (with mid-level tannin, red color) sorghum kernels were obtained from Nu Life Market (Scott City, KS, US). The average initial moisture content of kernels was 12.13% (w.b., wet basis). Moisture content, before and after conditioning, was measured using the ASABE Standard S352.2 for drying 10 g of sample in an air oven at 130°C for 18 h (ASABE, 2012).

3.2.2 Tempering Methods

Cold water tempering: Calculated amounts of distilled water were added to 500 g of sorghum samples to achieve a target moisture content of 18% (w.b.). This moisture

27

content was selected based on preliminary studies (Appendix) on bran yield conducted at

16–18% moisture contents. Amounts of water to be added were calculated using

Equation 1. After mixing, samples were stored at 21°C for equilibration before conducting physical and mechanical analyses.

푊(푀 −푀 ) 푄 = 푓 푖 (1) 100−푀푓

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Eq. 1 parameters are as follows: Q is the amount of water added, g; W is the weight of sorghum kernels (1000 g); Mf is the final desired moisture content on dry basis %; Mi is the initial moisture content of samples on dry basis %.

Hot water tempering: Calculated amounts of water were added to sorghum kernels for conditioning to a moisture content of 18% (w.b.). Moisture content was the same as in cold water tempering. After adding water, sealed glass containers with the kernels were kept in a water bath at 60 °C for 12, 18, and 24 h (Cecil, 1992). The bottles with the samples were shaken every 30 min for better moisture equilibration. After conditioning, samples were cooled to room temperature before the subsequent measurements.

Steam tempering: A MIAG laboratory conditioner (MIAG North America, Inc.,

Minneapolis, MN, USA) was used for steam tempering. Sorghum kernels were loaded into the conditioner drum and steamed for 1, 1.5, 2, and 2.5 min. Steam pressure was maintained at 40 psi. Steam pressure and time values were selected based on preliminary studies (not reported in this manuscript) targeting moisture content in the range of 15–

20% (w.b.). After steam treatment, kernels were surface-dried at 60 °C with 10 m3/hr

28

airflow for 30 min. Dried sorghum kernels were subsequently cooled to room temperature and stored in sealed Ziploc bags until the experimental measurements.

3.2.3 Physical Property Measurements

Sorghum kernel hardness indexes, kernel weights, and diameters were measured using a single kernel characteristics system (SKCS; 4100, Perten Instruments, Hagersten,

Sweden). Glumes, broken kernels, and foreign matter were removed by hand before the measurement (Pedersen et al., 1996). The hardness index was measured as the force

29 required to crush sorghum kernels as they passed through a wedge-shaped cavity between a smooth crescent surface and a coarse-toothed rotor in the SKCS (Gaines et al., 1996).

A Tangential Abrasive Dehulling Device (TADD, Venebles Machine Works, Saskatoon,

Canada) was used to measure the abrasive hardness index (AHI) (Oomah et al., 1981).

The TADD uses an 80-grit abrasive disk. For each measurement, 10 g of sample was loaded into the TADD cups (12-cup plates). The time (seconds) taken to abrade 1% of the weight of the grain was considered as the AHI (Johnson et al., 2010).

Texture analysis was carried out as described by Sirisomboon et al. (2007), using a TA-

XT plus texture analyzer (Texture Technologies Corp., Scarsdale, NY, US). During each single kernel measurement, kernels were aligned horizontally on the measuring platform.

A stainless steel probe (10 mm diameter) was used to compress each sample at a speed of

0.5 mm/s until the breaking point of kernels, up to 60% of the kernel height. Thirty kernels were analyzed with each tempering method. From the force-deformation curve, rupture force, deformation at rupture point, and rupture energy were measured using the

TA-XT installed software. The rupture force is the minimum force required to break a

29

sample. Deformation at rupture point is described as the time needed to rupture a sample.

Energy used for rupture is the energy needed to break a kernel, and is determined from the area under the curve between the initial point and the rupture point.

The effect of tempering on the internal structure of sorghum kernels was evaluated using a cryo-scanning electron microscope (Cryo-SEM, Ramos, 2009). Sorghum kernels were cut into thick slices (of approximately 2 mm) and frozen in liquid nitrogen. Samples were cut vertically and kept at -120°C for sublimation for approximately 3 min. Thick sorghum slices were subsequently imaged at -90°C with a JEOL JSM-840 SEM (Jeol

30

USA Inc., Peabody, MA, US) using 5 kV of accelerating voltage, after sputter coating with nitrogen under freezing conditions. Three kernels images for each treatment were acquired under 1000X magnification and pericarp thickness were measured at three pericarp position for each kernel using the Adobe Photoshop CS software (Adobe, San

Jose, CA, US). Actual thickness of pericarp was calculated based on the scale shown on the image.

All tempering treatments and measurements were performed in triplicate unless otherwise mentioned. Statistical analysis was conducted using PROC GLM with the SAS software

(SAS Institute, Cary, North Carolina, US) to determine the whither differences was significant when comparing tempering processes (α = 0.05) based on ANOVA. PROC

CORR was used to find the Pearson correlation between moisture content, AHI, HI, diameter, pericarp thickness, rupture force, rupture energy, and deformation.

3.3 Results and Discussion

The kernel physical characteristics, including moisture content, SKCS-kernel weight, and

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SKCS-diameter after tempering pretreatment, are shown in Table I. With an increase in duration of hot water tempering, moisture content slightly decreased, mainly due to the loss of moisture as vapor during prolonged conditioning at 60 °C. In addition, after hot water tempering, samples were removed from the containers and surface dried; for this reason, there was no re-absorption of vapor and this resulted in a decrease in moisture content. However, with an increase in duration of steam treatment, the moisture content of the samples increased significantly from 15.20% (1 min) to 19.64% (w.b., 2.5 min).

31

Kathuria and Sidhu (1984) reported an increase in moisture content after steaming of wheat kernels for extended steam duration and at different pressures.

Tempering method and moisture content did not have a significant influence on kernel size and weight (Table I). Only a slight swelling was noticed after tempering, when compared with untreated sorghum; this finding is in agreement with the results described by White (1966). Germs always have higher moisture uptake capacity than the other components of grain (Shelef and Mohsenin, 1966); the swelling force caused by differential moisture distribution can make germ and pericarp readily separable from the endosperm with minimum damage (White, 1966).

3.3.1 Kernel Hardness

Our results indicate that sorghum endosperm hardness was greatly influenced by moisture content and heat treatment (Table 3.1). The hardness index of untreated sorghum kernels was significantly higher than that of tempered kernels. As described by

Galter (1951), the grain resistance to crushing decreases with an increase in moisture

31

content. A similar trend was noticed for steam tempered kernels, where the hardness index decreased with an increase in moisture content caused by prolonged steam duration. The hardness index of steam tempered kernels (1 min) was slightly higher than the one of cold water tempered kernels; this is likely to be related to the low moisture content in steam treated kernels. Thus, moisture content is the main factor affecting the hardness of the kernel endosperm. However, cold water tempering resulted in a higher hardness index compared with steam (1.5 to 2.5 min) and hot water tempering. Steam tempering displays higher flour and bran yields compared with that by hot and cold water

32 tempering (Schafer, 1952). Kathuria and Sidhu (1984) reported that heat treatment results in a higher break flour yield, confirming that steam tempering mellows the grain, as reported by Gehle (1935).

3.3.2 Abrasive Hardness Index

The abrasive hardness index (AHI) of kernels is a parameter that can be used to predict the dehullling performance for grains. In this study, we found that the tempering process has a significant influence on the AHI of sorghum kernels (Table 3.1). However, the duration of hot water treatment did not have a significant effect on the AHI of sorghum.

Steam tempering increased the AHI values, that were higher than the ones of cold and hot water tempered kernels. The increase in AHI values indicates that pericarp strength increased after steam treatment, probably due to a toughening of the bran. This trend shows that the pericarp can be made brittle by steam treatment and this could result in the efficient separation of the bran from the endosperm during roller milling processes.

Untreated sorghum also has a higher value of AHI, but, if untempered, a higher hardness index value indicates that the endosperm remained hard and predicts that a clean

32

separation of the bran from the flour particles is hard to obtain.

33

Table 3.1Kernal physical characteristics Moisture Diameter Abrasive Treatment content Weight (g) Hardness index (mm) hardness index (% w.b.) Untreated 12.13 (0.03)a 20.50 (5.60) a 23.20 (0.23) a 75.41 (0.21)a 16.70 (0.59)a Cold water 17.56 (0.13)bc 20.64 (6.28) a 24.54 (0.89) a 64.20 (1.02)bc 14.27 (0.68)b Hot water-12 h 17.33 (0.74)bc 20.14 (6.10) a 25.22 (0.24) a 59.71 (1.75)d 10.94 (1.16)c Hot water-18 h 17.13 (0.58)bc 20.49 (5.80) a 24.58 (0.24) a 62.78 (1.44)cd 11.13 (0.97)c Hot water-24 h 16.90 (0.12)bc 20.42 (6.42) a 24.83 (0.24) a 62.44 (1.74)cd 11.12 (1.20)c Steam-1.0 min 15.20 (1.40)ab 21.70 (6.41) a 24.50 (0.24) a 65.05 (4.26)b 14.61 (1.60)b Steam-1.5 min 16.52 (2.50)abc 20.07 (6.64) a 24.74 (0.24) a 62.83 (2.64)cd 19.43 (1.04)d Steam-2.0 min 18.78 (1.35)c 18.68 (6.60) a 25.27 (0.24) a 62.06 (1.35)cd 20.64 (0.83)e Steam-2.5 min 19.64 (4.97)c 20.49 (7.30) a 26.01 (0.27) a 53.50 (9.24)e 22.99 (2.62)f Values between parentheses are standard deviations. Within the same column, values with different letters are significantly different (P < 0.05).

3.3.3 Structural Changes in Sorghum Kernels

The sorghum kernels used in this research had a starchy-mesocarp and consisted of several layers of starch-filled cells (Figure 1). In comparison with untreated kernels, cold water did not alter the pericarp structure (Figure 1A, 1B). Hot water treatment removed

the starch granules present between the mesocarp layers; this is probably due to the 33

partial gelatinization of starch granules and the dissolution of amylose chains at 60 °C.

Steam treated sorghum kernels did not show a starchy mesocarp and cell layers were compact compared with that by other treatments; this is likely due to the high temperature and high pressure during steam treatment. The steam conditioner side wall temperature reached up to 120°C, and this temperature is significantly higher than the starch granule pasting temperature range of 67–71°C (Wootton and Bamunuarachchi, 1979). In

34 addition, the pressure applied during steam treatment was 40 psi; this might have resulted in the compaction of cell layers.

Mosier et al. (2005) indicated that hot water and steam treatment increase the pore volume of lignocellulosic materials. Dissolved amylose chains could have penetrated into the endosperm through the porous cell walls. In addition, it has been reported that the outermost layer of kernels gelatinizes to a greater extent and that the degree of gelatinization depends on moisture content (Fang and Chinnan, 2004). The extent of gelatinization of starch is mainly dependent on the severity of the heat treatment and the availability of water (Srivastava et al., 2002). Earp and Rooney (1982) reported that pericarp thickness and structure depend on grain variety. In addition, they found that thick starchy mesocarps, characterized by a layered structure with abundant starch granules, could easily be peeled off the endosperm by abrasive methods compared with thin non-starchy pericarps. Thus, non-starchy pericarps show longer decorticating times through abrasion, compared with kernels with a starchy-pericarp. AHI results corroborate the structural analysis of grain kernels. In addition, the pericarp thickness of hot water

34

and steam tempered kernels (Table 3.2) was significantly reduced compared with cold water tempered and untreated sorghum kernels.

A) Untreated sorghum B) Cold water tempered C) Hot water tempered-12 h

35

D) Hot water tempered-18 h E) Hot water tempered-24 h F) Steam tempered 1.0min

G) Steam tempered-1.5min H) Steam tempered-2.0min I) Steam tempered 2.5min

Figure 3.1. Cross sectional SEM images of differently tempered sorghum kernels. (The yellow arrows indicate the thickness of the pericarp)

35

36

Table 3.2 Kernel pericarp thickness Treatment Pericarp thickness (m) Untreated 57.98 (29.70)b Cold water 58.49 (30.77)b Hot water -12 h 31.44 (9.93)a Hot water -18 h 37.60 (9.53)a Hot water - 24 h 28.66 (5.60)a Steam - 1.0min 24.49 (4.56)a Steam - 1.5min 26.75 (5.18)a Steam - 2.0min 32.87 (13.80)a Steam - 2.5min 33.40 (11.88)a Values between parentheses are standard deviations. Within the same column, values with different letters are significantly different (P < 0.05).

3.3.4 Texture Analysis

The force-deformation curve of sorghum kernels exhibits several points of inflection

(Figure 3.2), indicating the kernel rupture points during compression. Approximately, at

1 mm displacement, where the major rupture occurred, kernels were crushed into smaller pieces. Before being crushed and broken into smaller pieces, kernels exhibited elastic

36

properties; the force-deformation curve appears first concave, and subsequently convex.

From the first derivative of the force-deformation curve, several inflection points were identified at the initial compression of the kernels; the change in slope suggests that these are the rupture points of the bran and aleurone layers (Figueroa et al., 2011). The first inflection is approximately at 0.05 mm, as our SEM image analysis suggested.

37

120

100 Rupture points 80

Deformation 100 60 Force (N) 80 A 60 40 40 A Rupture 20 20 force 0 Energy dF/dx(N/mm) 0 0.05 0.1 0.15 0.2 0.25 Displacement (mm) 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Displacement (mm)

Figure 3.2. Typical compression pattern of sorghum kernels (steam tempered for 2.0 min).

The rupture force of untreated sorghum is significantly lower than the one of cold water tempered sorghum, and its deformation is higher (Table 3.3). Cold water tempering shows the highest rupture force and lowest deformation, indicating that the kernel is hard and brittle, even compared to untreated sorghum. The increase in rupture force and 37

energy can be observed with an increase of steam duration. The resistance to deformation decreases with an increase in kernel moisture content (Haddad et al., 2001; Altuntas and

Yildiz, 2005). SKCS hardness tests showed a decrease in hardness with an increase in moisture content, therefore the main factor increasing rupture force is likely to be kernel surface deformation. With the increase in deformation, force increased rapidly, resulting in an increased energy. Hot water tempering treatment resulted in a relatively low rupture force and the deformation value increased slightly with an extended hot water tempering

38 time. This indicates that hot water treatments result in soft kernels and that an increase in treatment time can reduce the brittleness of kernels. Cold water tempering had fewer effects on the softening of the endosperm and the kernel was still friable. Steam tempered and hot water tempered (for 24 h) kernels were less susceptible to breakage, resulting in a better separation of the bran and endosperm of sorghum kernels.

Table 3.3 Texture of pretreated sorghum kernels Treatment Displacement (mm) Rupture force (N) Energy (N*mm) Untreated 0.62 (0.32)ab 60.81 (14.82)a 35.40 (15.10)ab Cold water 0.49 (0.19)a 82.43 (21.92)c 38.87 (14.89)ab Hot water - 12 h 0.61 (0.28)ab 61.52 (13.42)a 37.41 (18.26)a Hot water - 18 h 0.62 (0.30)abc 75.20 (16.51)ab 44.70 (20.04)abc Hot water - 24 h 0.87 (0.42)abc 66.46 (19.50)a 54.08 (24.92)abcd Steam - 1.0 min 1.06 (0.66)abc 66.43 (17.08)ab 63.41 (31.66)bcd Steam - 1.5 min 0.91 (0.37)c 71.41 (19.33)a 62.66 (26.15)cd Steam - 2.0 min 1.01 (0.46)bc 73.67 (16.25)bc 73.45 (37.55)d Steam - 2.5 min 1.03 (0.58)bc 74.13 (18.89)ab 70.04 (30.80)cd Values between parentheses are standard deviations. Within the same column, values with different letters are significantly different (P < 0.05).

38

3.3.5 Correlation Analysis

Moisture content statistically correlated with kernel diameter and hardness index (Table

3.4). This is probably due to a direct relationship between moisture content and swelling, softening of sorghum kernels. AHI, pericarp thickness and compressive properties may additionally be influenced by conditioning methods rather than properties. Rupture energy is highly related to displacement (r = 0.94). Such relationships indicate that tough

39 bran and thin pericarp lead to high displacement and energy required to break sorghum kernels.

Table 3.4 Correlation between sorghum kernel characteristics

Pericarp Rupture Rupture MC Diameter HI AHI Displacement thickness force energy MC 1 0.93** -0.92** 0.27 -0.37 0.29 0.60 0.51

Diameter 1 -0.98** 0.33 -0.57 0.47 0.34 0.60

HI 1 -0.23 0.56 -0.39 -0.38 -0.51

AHI 1 -0.04 0.58 0.25 0.68*

Pericarp thickness 1 -0.73* 0.22 -0.65

Displacement 1 -0.07 0.94**

Rupture force 1 0.23

Rupture energy 1 **significant at p = 0.01; * significant at p = 0.05.

3.4 Conclusions

This study explains and quantifies the effects of tempering on the physical and

mechanical properties of sorghum kernels. Due to a low hardness index value and a high 39

AHI (at similar moisture content levels) under hot and steam conditioning, treatment with heat can soften kernel endosperms and increase bran resistance; these kernels are less friable (with high deformation values) compared with those that are cold water tempered.

Steam treated sorghum has a tougher pericarp than hot water tempered sorghum, owing to the structural changes of sorghum kernels during pretreatment. High temperature results in the gelatinization of starch granules inside the mesocarp; the pressure inside the container compressed the pericarp layers. Increasing steam tempering duration increases

40 the moisture content of sorghum kernels, thus reducing endosperm hardness while toughening the pericarp by changing its structure. These data on pericarp toughness and endosperm hardness of sorghum, as influenced by pretreatment, will help optimize scalable tempering and milling processes.

40

41

CHAPTER 4. EFFECTS OF TEMPERING METHODS ON SORGHUM MILLING PROPERTIES

4.1 Introduction

Sorghum, one of the most cultivated cereals in the world, is highly resistant to severe drought conditions (Doggett, 1988). In semi-arid zones of Africa and Asia, sorghum represents a major source of energy for humans (Taylor, 2001). Additionally, sorghum, a gluten-free cereal, contains phytochemicals that reduce the risk of certain types of cancer and obesity (Awika and Rooney, 2004). In sorghum, condensed tannins, present in the pigmented testa and pericarp, protect the plant from bird predation and insect and fungal infestation (Waniska et al., 1989). Tannins confer a bitter taste and affect protein digestibility; therefore, bran removal is critical during sorghum milling.

The sorghum grain has a similar structure to that of ; therefore, the processing

41

technologies of maize are applicable to sorghum (Taylor, 2001). Even though the kernel size of sorghum is similar to that of wheat, the pericarp of sorghum is more friable than that of other cereals due to its unique starchy pericarp, which makes the milling process of sorghum considerably difficult (Rooney, 1973). The endosperm of sorghum comprises both a hard and soft part, which is also disadvantageous in sorghum milling (Maxson et al., 1971).

Tempering, prior to milling, may improve the efficiency of bran separation by strengthening the bran and softening the endosperm in the kernel. Based on the

42 temperature, conditioning techniques can be classified as cold, warm, or hot. Tempering of sorghum for milling through dehulling process has been widely discussed (Wu, 1976;

Beta et al., 2000), but only a few studies focused on roller milling of sorghum. Cecil

(1992) described a milling method using a standard wheat mill in which the sorghum kernels were tempered to 26% moisture at 60 °C for 6 h. However, the moisture content of the resulting flour was very high. Pre-conditioning sorghum to 16% moisture at 4°C followed by roller milling yielded sorghum meal with higher flour extraction rates and slightly lower ash and fat contents compared to the dehulling and hammer milling process (Gomez, 1993). However, tempering at low temperatures presents a significant challenge for adoption by the industry. Abdelrahman and Farrell (1981) found an increase in the tempering moisture content increased the yield of bran and fines grits, but reduced the coarse grit yield. They concluded that 17% moisture content and 8 h of tempering were optimum for sorghum for a three breaks roll milling system.

The roller milling process involves break rolls, reduction rolls, and sifting and purifying systems. A gradual scraping of endosperm material off bran particles with minimal bran

42

damage is preferred to avoid bran dust, which cannot be separated from flour (Posner and

Hibbs, 2005). Campbell (2007, 2003) observed that during wheat roller milling, breakage from the first break roll is dependent on the milling process and the grain physical characteristics including moisture content, kernel shape, and hardness. The first break roll opens the kernel so that the bran remains relatively intact, while the endosperm is shattered into small particles, thereby facilitating the separation of endosperm from bran.

The scraping of endosperm particles, which is dependent on the roll gap, is independent of the size of the bran particles, because the bran particles pass through the roll gap along

43 their longest dimensions (Mateos-Salvador et al., 2013). The roller mill parameters of break roll corrugations, roll gap, speed differential, and roll disposition affects the milling process. But, the physical and mechanical properties of sorghum kernels are different than wheat, including its size, shape and hardness. So the milling process needs to be modified and designed specifically for sorghum kernel.

Sorghum flour, with 98% of particles passing through 212 µm sieve (CFR 137.105-

Flour), is mainly composed of starch and protein. The effect of flour particle size on dough development and bread quality have been studied by several researchers (Wang and Flores, 2000; Hera, et al., 2014). Flour particle size and composition affect the pasting properties of sorghum flour, and further affects product quality such as noodle cooking time and bread qualities. The particle size is usually associated with the surface area available for enzymatic action (Mahasukhonthachat et al., 2011). Low pasting viscosity is characteristic of A-type granules (> 15 μm). B-type (5–15 μm) and C-type (<

5 μm) starch granules have high pasting viscosity, breakdown viscosity, and holding strength (Wilson et al., 2006; Wilson et al., 2008; Liu et al., 2011). Starch is the major

43

component that controls pasting properties in cereal flour; however, pasting properties are affected by other components also. Interactions among food components play an important role on the rheological properties of foods (Shibanuma et al., 1996). Three- component interactions among starch, protein, and free fatty acids lead to the formation of unusual and notably high cooling-stage viscosity peaks (Zhang and Hamaker, 2005).

The presence of fiber, fat, and protein (with starch) may lower peak viscosity. A reduction in peak viscosity is obtained with increasing content of damaged starch (Leon et al., 2006).

44

High temperature treatments affect the structure of the sorghum pericarp thereby altering the bran properties and milling performance of sorghum (Chapter.3). In this Chapter, milling performance of sorghum kernels as affected by tempering methods (steam, hot water, and cold water) and their effects on flour quality are presented.

4.2 Materials and Methods

Sample preparation and tempering methods (cold water tempering, hot water tempering and steam tempering) were described in Chapter.3; Section 3.2, Material and methods section.

4.2.1 Milling Experiment

Pretreated sorghum was milled in a Ross Table-top flour mill (Ross, OK, USA) in the milling lab at the Department of Grain Science and Industry, Kansas State University,

Manhattan, KS. The milling process (Fig. 4.1) consisted of five break rolls and two smooth rolls. The break rolls had a speed differential of 2.5:1, and the roll disposition used was Dull-to-Dull. The smooth rolls speed differential was maintained at 1.25:1.

44

Based on preliminary experiments, the roll gap was fixed as described in Fig. 4.1. This typical roller milling process was specifically designed for sorghum to obtain high flour extraction rates with minimum flour contamination. Sorghum (1 kg) was milled and the streams separated using laboratory sifters. Stocks over each sieve were weighed and the bran, flour, coarse grits, fine grits, and shorts from different streams were combined to calculate the yield, as described in Fig. 4.1. The mass yields (%) of flour, bran, coarse grits, fine grits and red dogs were calculated from weight of stocks collected at each sieve and from the total sorghum used in the milling experiment.

Figure 4.1 Laboratory scale sorghum milling process (BK=break, M=middling, c=coarse, f=fine, C=corrugations W=wire, GG=grit gauze, XX=flour silk; Mesh opening: 8W=2464 µm, 10W=2030 µm, 12W=1659 µm, 16W=1358 µm, 20W=1041 µm, 36W=682 µm, 23GG=900 µm, 30GG=900µm, 8XX=193 µm, 9XX=150 µm.)

45

46

4.2.2 Flour Properties

Flour particle size and shape distribution were determined using a Morphologi G3-ID

(Malvern Instrument, Malvern, United Kingdom). A total of 10,000 particles were analyzed to obtain the number distribution of circle equivalent (CE) diameter, convexity, and high sensitivity (HS) circularity. CE-diameter is the measurement of particle size, expressed as the diameter of a circle with the same area as the particle image. Convexity is the measurement of the surface roughness of a particle and it is calculated by dividing the convex hull perimeter by the actual particle perimeter. The value of convexity ranges from 0 to 1. HS-circularity quantified how close the shape is to a perfect circle by the ratio of the particle area to the square of the perimeter of the object.

Sorghum flour pasting characteristics were examined using a Rapid Visco Analyzer

(RVA; Newport Scientific, Jessup, Maryland, USA) following the AACC standard flour method (AACC, 2010). Sorghum flour (2.5 g) was dissolved in 25 ml distilled water and heated from 50°C to 95°C at 5°C/min, held at 95°C for 2 min, and cooled to 50°C at

5°C/min. The flours viscosity profile was obtained during the process. RVA parameters

46

measured were pasting temperature, peak viscosity (maximum viscosity during heating cycle), peak time, trough (minimum viscosity), breakdown (peak viscosity minus trough), final viscosity, and setback (final viscosity minus trough).

4.2.3 Flour Composition

The flour samples were analyzed for ash (AACC method 08-01.01), fiber (AACC method

32-10.01), protein (AACC method 46-30.01), fat content (AACC method 30-25.01), starch damage (AACC 76-30.02), and total starch content (AOAC 996.11).

47

Statistical analysis: All measurements were performed in triplicate and statistical analysis was conducted using SAS software (SAS Institute, Cary, North Carolina). Significant difference for comparison between tempering was determined based on ANOVA (α =

0.05).

4.3 Results and Discussion

4.3.1 Milling Properties of Tempered Sorghum

Tempering affects the physical properties of kernels including their hardness, size, moisture content, and bran strength. Therefore, different tempering methods could lead to differences in their milling performance. During milling, endosperm, germ, and bran are separated, and the endosperm is reduced to fine particles. The milling process consisted of five pairs of break rolls with their corresponding sifters to separate different sizes of grits and bran. Grits were classed into coarse and fine endosperm grits based on their size and purity. The coarse and fine endosperm grits obtained at each stage were combined and transferred to the smooth roll to reduce them into flour. Stocks passing through 9 XX

47

sieve (150 μm) are considered as flour.

The first break roll contributes to different particle size distribution after breaking open the kernels and determines the flow balance through the rest of the milling process

(Campbell et al., 2007). Therefore, the first break roll represents a critical control point for the entire milling process. The stocks over each sieve after the first break roll are presented in Fig. 2A. Stocks retained on 8 W mesh (2,464 μm opening size) represent the opened kernels, and stocks greater than 12 W (opening 1,358 μm) represent large bran fractions still attached to the endosperm. After passing through the first break roll, steam-

48 tempered kernels had higher proportion of stocks larger than 2,464 μm and with smaller bran pieces > 1,358 μm, compared to hot water- and cold water-tempered sorghum kernels (Fig. 4.2a). This indicates that the bran of steam-tempered sorghum has the higher resistance to grinding than hot water- or cold water-tempered sorghum. With increased steam treatment from 1.0 to 2.0 min, proportion of stocks > 2,464 μm size increased. But, after 2.5 min of steam treatment, proportion of stocks > 2,464 μm decreased, with no significant effect on coarse grits > 30 GG (900 μm), fine grits > 9 XX (150 μm), or flour mass.

Kernels were opened by the first break roll, and the sorghum grits with both bran and endosperm (stocks retained on 8 W and 16 W) were milled in subsequent break rolls.

These break rolls helped scraping of the endosperm material off of the bran particles.

Major differences in milled stock proportion were observed at 2,030 μm (10W) and 1,695

μm (12 W) mesh size sieves at the 2nd (Fig. 4.2b) and 3rd (Fig. 4.2c) break roll. After passage through second and third break rolls, the amount of bran collected from hot water- and cold water-tempered sorghum was less than the amount collected from steam-

48

tempered sorghum. This indicates that the steam-tempered sorghum contained more intact bran, while the bran from hot water- and cold water-tempered sorghum were easily broken into small pieces. After passing through the second break roll, stocks from hot water- and cold water-tempered sorghum had a higher proportion of coarse grits than steam-tempered sorghum. This suggests that more endosperm particles were scraped off from bran; therefore, there were fewer particles > 2,030 μm. However, after passing through the third break roll, grits (< 1,695 μm) proportion did not show any significant difference.

49

100 90 80 70 60 50

40 Mass Mass yield(%) 30 20 10 0 Pan (<9XX) 150 (9XX) 900 (30GG) 1358 (16W) 2464 (8W) Mesh opening size (μm)

a) 1st Break 100 90 80 70 60 50 40 Mass Mass yield(%) 30 20 10 49

0 0 (<9XX) 150 (9XX) 670 1385 2030 (30GG) (16W) (10W) Mesh opening size (μm) b) 2nd Break

50

100 90 80 70 60 50

40 Mass Mass yield(%) 30 20 10 0 0 (<9XX) 150 (9XX) 670 1385 1659 (30GG) (16W) (12W) Mesh opening size (μm) c) 3rd Break 100 90 80 70 60 50 40 Mass Mass yield(%) 30 20 10 50

0 0 (<9XX) 150 (9XX) 670 900 1041 (30GG) (23GG) (20W) Mesh Opening size (μm) d) 4th Fine break

51

100 90 80 70 60 50 40 Mass Mass yield(%) 30 20 10 0 Pan 150 (9XX) 670 900 1041 (<9XX) (30GG) (23GG) (20W) Mesh opening size (μm) e) 4th Coarse Break

Figure 4.2 Effects of different tempering methods on the mill stream stock distribution at different stages of milling.

(—+—: Cold water tempering; —◊—: Hot water-12h; —∆—: Hot water-18h; —x—: Hot water-24h; —■—: Steam-1.0min; —♦—: Steam-1.5min; —▲—: Steam-2.0min; — ●—: Steam-2.5min.)

As illustrated in Figure 4.1, the coarse break sample is obtained from the material

51

retained on the top of the 12W screen in the 3rd break while the fine break sample was obtained from the material retained on the 16W in the third break. The coarse break material often has larger pieces of bran attached to the endosperm particles while the particles in the fine break have smaller pieces of endosperm attached. The 4th coarse and

4th fine break rolls had different roll gaps due to the difference in the size of incoming stock, but the sieve openings were the same. Large bran with endosperm was milled with the fourth coarse break roll (Fig. 4.2d), while small stocks (> 1,385 μm) were milled by

52 the fourth fine break roll (Fig. 4.2e) to further scrape endosperm particles off from bran.

The high yield of sorghum meals over 20 W (1051 μm) from the fourth coarse break roll revealed that a large fraction of bran remained as bigger sized particles. The kernel have same proportion of bran contents, so a high bran weight over 20 W (1051 μm) indicates a reduced bran contamination in flour. Therefore, steam tempering is preferable because it minimizes the production of bran dust and reduces flour contamination.

4.3.2 Yield of Sorghum Milling Stock

Kweon et al. (2009) reported that grain moisture content has a significant effect on bran breakage and flour yield during roller milling. At high moisture content, the bran is not easily broken into smaller pieces and the flour yield is low. In addition, the proportion of coarse fraction increases with increasing tempering moisture. However, based on the results presented in Table 4.1, the tempering methods (hot water, cold water, and steam) did not show significant influence on flour yields. The effect of tempering could be seen on the differences in bran, coarse grits and fine grits yield. Increasing steam duration

52

from 1 to 2 min significantly increased the bran yield from 18.37 to 26.42% (Fig. 4.3a).

Steam treatment for 1.5 and 2.0 min contributed to a significantly higher bran yield compared to hot water and cold water tempering, associated with low flour yield. This indicates that steam tempering increased the bran strength, and the bran remain as larger pieces that could be separated from grits and flour with ease.

Addition of more break rolls could increase the flour yield of steam tempered sorghum, hence get different grades of sorghum flour. Coarse grit (Fig. 4.3c) yield was significantly lower after steam treatment for 1.5, 2.0, and 2.5 min than the hot water and

53 cold water tempered samples. Similarly, the fine grit (Fig. 4.3d) yield after steam treatment for 2 min was the lowest (28.31%). The low mass yield of coarse grits in steamed-tempered sorghum could be due to inefficient scraping of endosperm grits from bran during break mill and reduced bran fraction contamination, which corresponded to high bran yields obtained with steam tempering of sorghum.

There were no significant differences in bran, flour coarse grit, fine grit, or red dogs yields between hot water and cold water tempered sorghum. Even though the hot water treatment was found having a significant effect on sorghum kernel SKCS-hardness and abrasive hardness (Chapter. 3 Section 3.3.1 and 3.3.2) compared with cold water tempered kernels, where the kernel endosperm softened and the bran resistance increased, the milling performance did not show any significant difference. This indicates that the hot water tempering (at 60 oC) was not effective enough to affect the milling properties of sorghum.

Table 4.1 Sorghum milling stock yields Coarse grits Red dogs Treatment Bran (%) Flour (%) Fine grits (%)

(%) (%) 53

Cold water 10.93 (1.56)ab 73.85 (3.23)a 25.66 (1.08)a 38.77 (1.44)ab 1.23 (0.79) a Hot water-12 h 11.49 (0.66)a 72.12 (0.75) ab 27.77 (1.12)a 38.45 (2.19)ab 2.79 (0.12) a Hot water-18 h 10.03 (1.59)a 71.59 (2.88) ab 25.28 (3.25)a 38.54 (5.07)ab 1.78 (1.23) a Hot water-24 h 10.62 (5.76)a 71.31 (2.89) ab 27.21 (1.78)a 41.66 (2.21)a 3.77 (2.97) a Steam-1.0 min 18.37 (3.28)ab 60.08 (9.63)c 19.49 (5.76)ab 38.38 (4.78)ab 3.26 (1.27) a Steam-1.5 min 23.47 (5.45)b 49.64 (7.30)d 15.04 (3.73)b 34.15 (4.11)ab 3.76 (1.27) a Steam-2.0 min 26.42 (14.06)c 62.40 (5.25)bc 20.67 (1.97)b 28.31 (14.00)b 4.10 (0.52) a Steam-2.5 min 20.92 (0.33)ab 55.61 (5.75)bcd 14.42 (0.79)b 34.68 (2.62)ab 4.27 (0.21) a Values in parenthesis are standard deviations. Within the same column, values with different letters are significantly different (P < 0.05)

54

a. b.

c. d.

54

Figure 4.3 Sorghum milling stocks (a. bran; b. shorts; c. fine grits; d. coarse grits)

4.3.3 Sorghum Flour Composition

The chemical composition of differently tempered sorghum kernel flour is presented in

Table 4.2. There were no significant differences in ash content among the three different tempering methods (Table 4.2). In this study, ash content (0.331–0.427 %) was remarkably lower than that reported (1.25–1.65%) reported by Frederick (2009), Schober et al., (2005) and Fernholz (2008). At higher extraction rate, where the bran yield is

55 lower, sorghum flour contain a higher concentration of the aleurone layer and the peripheral endosperm, which are the main sources of fat, ash, protein, and fiber (Rooney and Clark, 1968). Ash content of wheat flour has a linear relationship with flour extraction (Li and Posner, 1989). In comparison with published studies on sorghum milling, that utilized hammer milling, this study indicates that the use of roller milling could reduce the ash content in the sorghum flour. Similarly, the fiber content ranged between 0.87 and 1.28% (Table 4.2), which was lower than the values reported by Liu et al. (2011) in the range of 1.30–1.69%.

There were no significant differences in total starch, protein, and fat (Table 4.2) contents among the flour from different tempering methods. Total starch contents were in agreement with the values (66–73%) reported by Liu (2011), Schober et al. (2005), and

Fernholz (2008). Protein and fat contents were lower than those previously reported (8–

13% and 2–4%, respectively; Liu, 2011; Schober et al., 2005; Fernholz, 2008), possibly due to the low concentration of aleurone and peripheral endosperm in the flour.

The damaged starch content of flour from steam tempered kernels (3.69–4.12%) was

55

notably lower than that obtained after hot water or cold water tempering (4.81–5.96%).

Starch damage occurs when intact starch granules are fractured, shattered, or chopped when grain endosperm is gradually reduced into flour (Chen and D’Appolonia, 1986).

The higher shear encountered during milling, the higher the quantity of damaged starch produced (Liu, 2011; Gąsiorowski et al., 1999). Steam tempering could have reduced the bonding strength between starch granules and protein, and resulted in a lower damaged starch than the flour obtained from hot water or cold water tempering. Starch damage during milling increases the water holding capacity of cereal flours (Dendy and

56

Dobraszczyk, 2001) and improves starch digestibility (Rahman, et al., 2000; Sadowska et al., 1999). However, excessive starch damage leads to sticky dough, strong proofing, and undesirable red crust color (Bettge et al., 1995).

Table 4.2 Chemical composition of sorghum flour Treatment/ Damaged Ash Protein Crude fat Crude fiber Total starch Component % starch Cold water 0.333(0.002) a 5.96(0.43) a 0.95(0.17) a 0.87(0.19)a 74.33(5.40) a 5.96(0.24)a Hot water-12 h 0.331(0.090) a 5.79(0.48) a 0.79(0.08) a 0.85(0.03)a 71.30(4.36) a 5.20(0.23)ab Hot water-18 h 0.390(0.026) a 5.83(0.49) a 0.75(0.10) a 1.04(0.15)ab 70.40(2.46) a 4.81(0.58)bc Hot water-24 h 0.404(0.003) a 5.64(0.59) a 0.87(0.05) a 0.90(0.15)a 70.40(0.75) a 5.11(0.64)ab Steam-1.0min 0.365(0.005) a 4.94(0.30) a 0.92(0.52) a 1.28(0.09)b 69.43(4.74) a 3.63(0.24)d Steam-1.5min 0.358(0.025) a 5.12(0.50) a 0.73(0.11) a 0.95(0.15)ab 70.10(3.80) a 3.91(0.47)cd Steam-2.0min 0.324(0.018) a 5.49(0.56) a 0.95(0.20) a 0.92(0.12)a 71.20(3.00) a 4.18(0.41)bcd Steam-2.5min 0.427(0.014) a 5.62(0.45) a 0.68(0.09) a 1.02(0.04)ab 73.17(2.46) a 4.18(0.00)bcd Values in parenthesis are standard deviations. Within the same column, values with different letters are significantly different (P < 0.05)

4.3.4 Particle Size and Shape Distribution of Sorghum Flours 56

Sorghum flour particle CE diameter and HS circularity distributions are presented in Fig.

4.4. Kernel physical properties and milling methods affect particle size distribution of flours (Dexter et al., 1994; Gaines, 1985). Most flour particles diameter ranged in between 3–40 μm (Figure 4a). Sorghum starch granules are polyhedral with diameters that ranged between 5 and 30 μm (Tester et al., 2006; Choi et al., 2008), which suggests that the flour samples contained separated starch granules. But, grinding intensity might increase starch damage (Fan et al., 1976), resulting in smaller particle sizes. Compared to

57 steam and hot water tempering, cold water tempering contributed to smaller particles

(Fig. 4.4a), due to a higher damaged starch content. Steam treatments for 1 and 2 min had resulted in a larger proportion of smaller particles (5–10 μm), indicates that the bond between starch granules were weakened by steam and easily separated during milling.

The convexity of starch granules were higher because they were separate and less shape- distorted. On the other hand, bran fractions have less uniform shapes, with low convexity and circularity (Saad et al., 2011). Irrespective of using the same milling procedure, the

HS-circularity (Fig. 4.4b) and convexity (Fig. 4.4c) of sorghum flours were highly different. Steam treatments (1 and 2 min) had resulted in flours with more convex and circular shaped particles compared to the other flours. Steam treatments could have weakened the bond between bran and endosperm and between starch and protein, resulting in a better separation of bran from the endosperm cells and also the flour particle separation.

The average sorghum grain endosperm cell size is 100–200 μm (Mahasukhonthachat et al., 2011). The particles in the range of 125–165 μm represent bran and cell wall fractions

57

(Jane et al., 1994), while particles > 165 μm represent albumen fractions (i.e., starch granules embedded in the protein matrix). Figure 4.5 shows some particles of sorghum flour. The particle length over 20 μm are generally in irregular shape, indicates that the larger particles are composed with several starch granules, protein matrix, or bran fractions. The small particles are mostly rounded (length around 20 μm or below), which could be starch granules.

58

60 Cold water 1.6 Steam-1.0min 1.4 50 Steam-1.5min 1.2 Steam-2.0min 1 40 Steam-2.5min 0.8 Hot water-12h 0.6 Hot water-18h 30 0.4 Hot water-24h Percent 0.2 20 0 0 10 20 30 40 50 10 CE-diameter

0 0 50 100 150 200 CE-diameter (μm)

a) Number distribution of sorghum flour particle circle-equivalent (CE) diameter

9

8 Cold water Steam-1.0min 7 Steam-1.5min 6 Steam-2.0min Steam-2.5min 5 Hot water-12h 4 Hot water-18h Percent Hot water-24h 3

2

1

58 0

0 0.2 0.4 0.6 0.8 1 HS Circularity

b) Number distribution of sorghum flour particle high-sensitivity (HS) circularity

59

35 Cold water 30 Steam-1.0min Steam-1.5min 25 Steam-2.0min Steam-2.5min 20 Hot water-12h Hot water-18h

Percent 15 Hot water-24h

10

5

0 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 Convexity

c) Number distribution of convexity of sorghum flour particle

Figure 4.4 Sorghum flour particle size and shape distribution

59

Figure 4.5 Selected sorghum flour particle images (steam-2.0 min)

60

4.3.5 Sorghum Flour Pasting Properties

All flour samples attained their peak viscosity at 95°C and 300s holding time (Fig. 4.6).

Significantly higher peak viscosity values (Table 4.3) were obtained for flour samples produced with 12-h of hot water tempering (1,456.00 cP), and a high break down value of

1, 503.00 cP was obtained for flour samples produced after 2 min steam treatment of kernels (P > 0.05). Trough, an indication of holding strength, ranged between 987.67 cP

(hot water treatment for 24 h) and 1,099.33 cP (steam treatment for 1.5 min). Breakdown viscosity ranged from 279.67 cP (hot water treatment for 18 h) to 447.33 cP (hot water treatment for 12 h). Setback viscosity ranged from 370.00 cP (steam treatment for 1.0 min) to 576.00 cP (hot water treatment for 18 h). Setback occurs during the cooling process, when starch molecules re-order and form gels. Low setback values are indicative of low rates of starch retrogradation (Varavinit et al., 2003). Hot water tempering of sorghum kernels for 18 h had resulted in a significantly higher setback viscosity value after cooling than the other two treatments. Even though significant differences were obtained among the different treatments, the RVA results of the tempered sorghum flour

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were in agreement with reported sorghum flour RVA profiles (Tharanathan, 2003).

Milling process, or grinding mechanism influence the particle size distribution, shape, surface damaged starch, and all these characteristics of flour particle affect the RVA profile (Becher, et al., 2001). RVA profile quantify the pasting properties of flour during heating, and provide a reference for food process. The amount of starch conversion or the degree of cook that has occurred during the processing of starchy materials could be

61 determined according to RVA profile (Becher, et al., 2001).

2500 120

100 2000

80 ) 1500 ℃

60

1000 Viscocity(cP) Cold water Visc(cP)

Hot water-12H Visc(cP) 40 Temperature ( Hot water-18H Visc(cP) Hot water-24H Visc(cP) 500 Steam-1.0min Visc(cP) 20 Steam-1.5min Visc(cP) Steam-2.5min Visc(cP) 0 Steam-2.0min Visc(cP) 0 0 100 200 300 Time(s)400 500 600 700 800

Figure 4.6 Sorghum flour pasting properties

Table 4.3 Sorghum flour pasting properties Peak Trough Break Final Total Setback Treatments Viscosity Viscosity Down Viscosity Setback Region (Cp) (Cp) (Cp) (Cp) (Cp) (Cp) 1424.33 1037.00 387.33 2032.33 995.33 608.00 raw (87.96)abc (48.88)ab (44.81)bc (69.33)c (26.08)c (19.16)a

1424.00 1049.67 374.33 1904.00 854.33 480.00 61 Cold water abc abc abc b bc

(27.51) (17.21)ab (12.42) (33.41) (19.35) (7.00) 1456.00 1008.67 447.33 1923.67 915.00 467.67 Hot water-12h (32.51)bc (28.71)ab (8.50)cd (38.07)abc (13.23)bc (5.69)c 1305.67 1026.00 279.67 1881.67 855.67 576.00 Hot water-18h (17.01)a (55.46)ab (56.86)a (76.56)ab (56.01)b (89.02)ab 1306.67 987.67 319.00 1813.33 825.67 506.67 Hot water-24h (41.04)a (23.63)a (48.38)ab (48.19)ab (51.01)b (10.50)bc 1384.00 1088.00 296.00 1754.00 666.00 370.00 Steam-1.0min (29.72)ab (49.00)ab (19.67)ab (80.72)a (44.44)a (54.46)c 1387.33 1099.33 288.00 1791.33 692.00 404.00 Steam-1.5min (45.65)abc (30.07)b (15.62)ab (65.31)ab (36.29)a (21.66)c 1503.00 1062.33 440.67 1963.33 901.00 460.33 Steam-2.0min (36.10)c (13.87)ab (43.55)cd (30.07)bc (34.12)bc (12.42)c 1342.67 1005.67 337.00 1771.33 765.67 428.67 Steam-2.5min (58.73)ab (37.58)ab (21.38)ab (59.08)ab (21.78)ab (0.58)c Values in parenthesis are standard deviations. Within the same column, values with different letters are significantly different (P < 0.05)

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4.4 Conclusions

Steam-tempered sorghum had significantly higher bran yield and had a higher proportion of larger bran pieces than hot water- or cold water-tempered sorghum. Steam treatment for 1 min resulted in higher bran fraction contamination, which was indicated by the high fiber content in flour. Steam treatment for 2 min resulted in flour with lower content of fiber, damaged starch, and large flour particles. In addition, this treatment resulted helped obtaining flour with a higher proportion of convex and circular shaped particles indicating smoother separation of flour particles. Therefore, steam treatment of sorghum kernels for 2 min could help extraction of pure sorghum flour with higher separation of bran and endosperm. Cold water tempering of sorghum kernels resulted in flour that had the highest content of damaged starch. In comparison with cold water tempering, hot water tempering produced less damaged starch which indicates that hot water tempering loosen the links between starch granule and protein. The pasting qualities of flour were not affected by the steam or hot water tempering of sorghum kernels.

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CHAPTER 5. SUMMARY OF CONCLUSION

5.1 Restatement of Research Objectives and Goals

The market segment of sorghum flour as human food is expanding in recent years and the demand for high-quality sorghum flour. Sorghum kernel has a brittle pericarp and contains both floury and corneous endosperm that makes the milling process difficult.

The widely used sorghum milling process, which constitutes of dehulling and milling, has the disadvantages of inconsistent flour quality and low production rate. Using roller milling process could increase the production rate. But in roller milling process the separation efficiency of endosperm and bran is low due to unsuitable tempering method.

To minimize the bran speckle contamination by roller milling process, a proper tempering method for sorghum must be developed. The separation of endosperm and bran during roller milling is directly dependent on bran strength and endosperm texture

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properties. The aim of this thesis was to study the effect of different tempering methods on sorghum milling, and the specific objectives as stated in Chapter 1 were,

1. To determine the effect of different tempering methods on sorghum kernel characteristics.

2. To study the effect of tempering on percent flour extraction and flour properties.

This thesis work explores the effect of cold water tempering, hot water tempering, and steam tempering methods on both kernel and milling properties in order to develop a

64 proper tempering method for sorghum milling. In this part of the thesis, a project overview is described in section 6.2, and the discussion of major findings from the study is described in section 6.3. Potential future work based on this study and the questions evolved from the study are stated in section 6.4.

5.2 Project Overview

The challenges of sorghum milling, project hypothesis and objectives were discussed in

Chapter 1. In Chapter 2, the studies and results available in published literature on sorghum production and utilization, sorghum structure and chemical composition, and sorghum nutrition value were presented. In addition, cereal grain tempering methods, currently practiced sorghum milling process were also discussed. Different tempering methods used in the wheat milling process and some of the tempering methods studied for sorghum milling were also addressed for understanding the effect of tempering on grain kernels and milling process.

Chapter 3 highlights the sorghum kernel physical characteristics under different

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tempering methods. Cold water, 3 levels of hot water (12, 18, 24h) and four levels of steam (1.0, 1.5, 2.0, 2.5min) tempering were studied. This chapter discussed the sorghum kernels bran and endosperm hardness and texture after tempering treatments. The SKCS, abrasive hardness and texture analysis were conducted, and kernels structural changes were studied using SEM-imaging. Chapter 4 focused on the effect of tempering methods on sorghum milling process and flour properties. A milling flow was developed to perform the milling experiments. Sorghum stock yields, flour composition, and flour pasting properties after milling were studied to evaluate tempering methods.

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5.3 Discussion of Major Findings

The moisture contents of steam tempered sorghum kernels were positively correlated to the duration of steam treatment. The addition of moisture softens the sorghum kernels.

Abrasive hardness and texture analysis of differently tempered sorghum kernels showed that tempering using water at a high temperature could further increase the bran’s resistance to abrasion. Hot water tempering and steam treatment could reduce the brittleness of kernels. With a increase in duration of steam, the time needed to remove the bran using a dehuller increased. Hot water tempering and steam tempering changed the pericarp structure of the sorghum kernels. The sorghum pericarp lost the starch present between the layers of bran and its thickness decreased after steam tempering, which led to the increase in abrasive resistance of steam tempered sorghum kernels.

High bran yields from steam-tempered sorghum indicate that the bran was strengthened by steaming, which allows gradually scraping of endosperm from large bran pieces during roller milling, and made separation of bran and endosperm easier. The low fiber content after 2 min of steam tempering shows that this tempering method produced high-

65

quality flour with less bran contamination. In addition, the high particle circularity indicated the steam weakened the protein-starch bond. Cold water tempering had minimal effect on the weakening of protein-starch matrix and led to increased damaged starch content. Steam tempering for 2.5 min resulted in increased fiber content in the flour and this could be due to the heat damage to the pericarp. The bran yield and fiber content were similar for kernels tempered with cold and hot water, but there was less damaged starch obtained from hot water tempered kernels. Hot water tempering did not significantly improve the pericarp resistance to the roller milling process, but it softened

66 the endosperm causing easier separation of flour particles. Thus, steam tempering could be a preferred tempering method for sorghum that is processed using a roller mill because it could strengthen the pericarp and soften the endosperm. The steam treatment time should be kept to less than 2.0 min to avoid heat damage to the kernels.

5.4 Future Work

Cold water, hot water, and steam tempering were evaluated as a pretreatment process for sorghum roller milling. Additional work that needs to be conducted to meet the industry requirement are listed below:

1. Energy cost for steam tempering

The purpose of this study is to develop an efficient roller milling process, however, steam tempering evaluated in this study might be energy intensive. Even though steam tempering produced a pure flour than other tempering methods, the balance of cost and performance is important for the process design in industrial scale. Thus, the energy cost evaluation of different tempering methods is required.

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2. Flow diagram development for industrial scale

The current study was conducted using laboratory roller mill, with a maximum capacity of 1000 kg per batch. For higher capacity sorghum milling capabilities, this process has to be evaluated in pilot scale and industrial scale roller mills for refining the process flow and industry adoption.

3. Bread baking tests

Pasting properties of differently treated sorghum flour did not change significantly. But heat treatment could denature the protein and could make a difference during dough

67 development, fermentation or on the color of bread. Unlike wheat or corn flour, due to the lack of sorghum flour quality standards, specific quality requirements for sorghum flour that would be processed into bread, muffins and other products are also required to increase industry adoption.

67

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APPENDICES

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Appendix A Preliminary Milling Studies on Sorghum at Different Moisture

Contents

Table A. 1 Stocks yields over 8W of cold tempered sorghum through First break roll Moisture content (% w.b.) Roll gap (in) Stocks over 8W (%) 16 0.058 12.9 16 0.048 8.7 16 0.045 5.9 16 0.031 2.7 18 0.051 40.7 18 0.048 23.8 18 0.045 12.5

More stocks over 8W of first break is preferred to allow subsequent breaks to scrape endosperm material from bran particles. Thus 18% moisture content is chosen as the tempering target moisture for this research.

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Appendix B Example Calculation on Moisture Addition During Tempering

The water amount (g) for cold water and hot water tempering calculation:

푊(푀 −푀 ) 푄 = 푓 푖 (1) 100−푀푓

Eq. 1 parameters are as follows: Q is the amount of water added, g; W is the weight of sorghum kernels, g; Mf is the final desired moisture content on dry basis %; Mi is the initial moisture content of samples on dry basis %.

For sorghum at initial moisture content of 12.13%, target moisture content of 18%. To tempering 1000 g sorghum kernels, the water amount need to add to the grain is calculated:

푊(푀 −푀 ) 1000∗(18−12.13) 푄 = 푓 푖 = = 71.59 푔 100−푀푓 100−18

71.59 g water needed to temper the sorghum from 12.13% to 18% moisture content.

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