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Sweetpotato Production, Processing, and Nutritional Quality V. D. Truong1, R. Y. Avula2, K. V. Pecota3, and G. C. Yencho3

1 USDA‐ARS Food Science Research Unit, Department of Food, Bioprocessing and Nutrition Sciences, North Carolina State University, Raleigh, USA 2 Department of Food Science and Technology, University of Georgia, Athens, USA 3 Department of Horticultural Science, North Carolina State University, Raleigh, USA

­Introduction dry matter content (18–25%), high β‐carotene level, sweet and moist‐texture after cooking. Sweetpotato, Ipomoea batatas L. (Lam.), is an This sweetpotato type is imprecisely called important economic crop in many countries. In “yam,” which is not the true tropical yam of terms of annual production, sweetpotato ranks Dioscorea species. Historically, African as the fifth most important food crop in the Americans in Louisiana referred this moist‐ tropics and the seventh in the world food pro- sweetpotato as “nyami” because it reminded duction after wheat, rice, maize, potato, barley, them of the starchy tuber of that name in Africa. and cassava (FAO 2016). Sweetpotato fulfills a The Senagalese word “nyami” was eventually number of basic roles in the global food system, shortened to the trademark “yam” popular in all of which have fundamental implications for the United States. Commercial packages with meeting food requirements, reducing poverty, “yam” labels are required by the US Department and increasing food security (El‐Sheikha and of Agriculture to have the word “sweetpotato” in Ray 2017). the label to avoid confusion to the consumers Sweetpotato roots have high nutritional value (Estes 2009). and sensory versatility in terms of taste, texture, Depending on the flesh color, sweetpotatoes and flesh color (white, cream, yellow, orange, contain high levels of β‐carotene, anthocyanins, purple). The varieties with high dry matter phenolics, dietary fiber, vitamins, minerals, and (>25%), white‐cream flesh color, and mealy firm other bioactive compounds. The β‐carotene in texture after cooking are preferred by the con- orange‐fleshed sweetpotatoes can play a signifi- sumers in the tropics. These varieties are known cant role as a viable long‐term food‐based as tropical sweetpotato (e.g., “bianito,” “batiste,” ­strategy for combating vitamin A deficiency in or “camote”). The purple‐fleshed sweetpotato the world. Studies in Africa demonstrated that varieties with attractive color and high antho- increased consumption of orange‐fleshed cyanin content are the specialty type in Asia. In sweetpotatoes improved the vitamin A status of the United States, the commercially popular children, pregnant women, and lactating type is the orange‐fleshed sweetpotato with low ­mothers (Low et al. 2007; Hotz et al. 2012;

Handbook of Vegetables and Vegetable Processing, Volume II, Second Edition. Edited by Muhammad Siddiq and Mark A. Uebersax. © 2018 John Wiley & Sons Ltd. Published 2018 by John Wiley & Sons Ltd. 812 Sweetpotato Production, Processing, and Nutritional Quality

Van Jaarsveld et al. 2005). Further, polyphenolics about 21%, while the Americas (North, Central, from purple‐fleshed sweetpotatoes exhibited and South) account for about 3.6%. China was diverse biological activities, including protec- the leading producer of sweetpotatoes, with tive antioxidant, anti‐inflammatory, anticancer, 71.54 MMT or about 67% of the global produc- antidiabetic and hepatoprotective activity tion, followed by Nigeria (3.78 MMT), Tanzania effects (Hu et al. 2016; Lim et al. 2013). (3.5 MMT), Ethiopia (2.7 MMT), and Sweetpotato has a strong potential to contribute Mozambique (2.4 MMT). The United States to better nutritional quality of our diets around was the tenth largest producer, with 1.34 MMT the world. This chapter provides a contempo- production. Only two countries in Europe, rary review of production, quality, and process- Portugal and Spain, grow sweetpotatoes, with ing aspects of sweetpotatoes. 22,591 and 13,550 metric tons produced in 2014. In comparison to other major staple food crops, sweetpotatoes have good adaptability to ­Production and Consumption marginal growing conditions, short production cycle, and high yield potential. The average Sweetpotato has wide production geography, world yield of sweetpotatoes is about 14 tons from 40° north to 32° south latitude of the globe, per hectare. Under subsistence conditions in and it is cultivated in 114 countries. The world many areas of the tropics, the average sweetpo- total production of sweetpotatoes was 106.60 tato yield is about 6 metric tons/hectare, far million metric tons (MMT) in 2014. Since the below the 20–26 metric tons/hectare obtained mid‐1990s, global production has ranged from a in China, Japan, and the United States, where low of 101.28 MMT in 2007 to a high of 147.17 improved varieties, fertilizer applications, and MMT in 1999 (Figure 35.1). In 2014, about cultural managements have been introduced. three‐fourth of the global production was from The per capita consumption is highest in Asia and Pacific Islands, followed by Africa with places where sweetpotatoes are consumed as a

World China Americas Africa 150

125 106.6

tons 100

71.5 75

Million metric 50

22.6 25 3.8 0 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014

Figure 35.1 World and regional sweetpotato production for 1994–2014. Source: FAO (2016). Btn and Physiolog 813 staple food, e.g., Papua New Guinea at 550 kg analyses conducted by Huang and Sun (2000) per person per year, the Solomon Islands at 160 kg, and Zhang et al. (2000) also places Central Burundi and Rwanda at 130 kg, and Uganda at America as the region with the most genetic 85 kg. The average annual per capita consump- diversity and probable origin (Huang and Sun tion of sweetpotatoes is estimated at 18 kg in 2000; Zhang et al. 2000). Remains of dried Asia, 9 kg in Africa, 5 kg in Latin America. sweetpotato roots found in Peru have been Between 2000 and 2014, sweetpotato consump- radiocarbon dated back to 8,000–10,000 years tion in the United States increased nearly 80%, old, though it is unknown if these were col- from 1.9 kg to 3.4 kg per capita (FAO 2015; lected from the wild or were domesticated Johnson et al. 2015). Sweetpotato consumption (Engel 1970). Regardless of the center of origin, has been greatly enhanced by the wide spread sweetpotato was widely established in tropical commercial availability of frozen “French‐fried” regions of the new world around 2500 b.c. sweetpotatoes. To accommodate this recent (Austin 1988). It was established in Polynesia, growth trend, increased modern processing prior to European arrival (Roullier, et al. 2013b). capacity has been built within the southern US Europeans in the 1500s spread the sweetpotato sweetpotato growing regions. to Africa and India, with it arriving in China prior to 1600. Secondary centers of diversity include New Guinea, the Philippines, and parts ­Classification and Origin of Africa (Bohac et al. 1995; Roullier, et al. 2013a, b). The sweetpotato (I. batatas L.) is a dicotyledon- ous plant belonging to the morning glory or Convolvulaceae family. It is a new world crop, ­Botany and Physiology though there is still some confusion that exists regarding its origin, and primary and secondary The sweetpotato is a herbaceous perennial that centers of diversity. Roullier et al. (2013a,b) and is grown as an annual by stem cuttings or plant Grüneberg et al. (2015) have published thor- sprouts from storage roots (Figure 35.2). It has a ough reviews of this topic. In brief, using data predominately prostrate growth habit typically from morphology, ecology, and cytology, Austin with 1–5 m vines that grow horizontally on (1988) has postulated that cultivated sweetpo- the ground. The plant can be grouped into three tatoes originated somewhere in the region parts: between the Yucatan Peninsula of Mexico and 1) The leaves act as the photosynthetic canopy. the mouth of the Orinoco river in northeastern 2) The stems transport energy to the roots and Venezuela. Recent studies conducted by transport water and minerals from the roots. Roullier, et al. (2013 a, b) incorporating chloro- 3) The root system absorbs water and nutrients plast DNA and molecular phylogeny analyses from the soil, anchors the plant, and can act confirm this hypothesis. They also suggest that as a storage site for energy via the develop- I. batatas most likely evolved from at least two ment of fleshy storage roots. distinct autopolyploidization events in wild populations of a single progenitor species most Sweetpotatoes possess three types of roots: likely I. trifida. Secondary contact between storage, fibrous, and pencil (Kays 1985a; Firon sweetpotatoes domesticated in Central America et al. 2009). Young adventitious roots develop and in South America, from differentiated wild out of both the nodal and the internodal regions I. batatas or I. trifida populations, could have of an underground stem portion of a vine led to further introgression. Molecular marker ­cutting. Roots from the internodal regions 814 Sweetpotato Production, Processing, and Nutritional Quality

Sweetpotato plant Sweetpotato harvesting Harvested sweetpotatoes

Figure 35.2 Sweetpotato plant, harvesting and harvested tubers in the market. Source (From left): USDA Plants Database, NC State Extension, USDA‐AMS.

­normally become fibrous roots and have a and 25°C promote storage root formation and tetrarch arrangement of their primary vascular growth, while temperatures above 25°C sup- tissue. Roots from the nodes are pentarch or press storage root formation and favor shoot hexarch and have the potential to develop into growth. Night air temperatures lower than 15°C enlarged storage roots; however, unfavorable suppress storage root formation, growth, and conditions can cause many or all of these to yield. Air temperatures >30°C reduce storage develop into primary fibrous roots or to lignify root formation and growth, and promote shoot and produce pencil roots. Some of the first roots growth. Soil temperatures of 20–30°C favor to emerge from the nodal regions are the roots storage root formation, and growth while a soil that will develop into storage roots, making it temperature of 15°C favors fibrous root forma- important to minimize stress during the first tion. Soil temperatures >30°C promotes shoot month after transplanting, to ensure good growth at the expense of storage root growth. ­storage root development. Minimizing stresses Long photoperiod favors storage root growth. such as high nitrogen levels, low oxygen or Storage root bulking is determined by the dry conditions impacts many of the cultural duration and rate of storage root growth, which ­practices for sweetpotato in highly developed varies by cultivar. Growth occurs over an production systems. extended period of time and can stop due to Storage root initiation varies widely among unfavorable growing conditions and restart cultivars occurring 1–13 weeks after planting, once conditions improve. Cultivars exhibiting by which time the number of storage roots per fast initiation and rapid bulking may reach a plant is determined (Ravi and Indira 1999). maximum yield in 12–16 weeks, while long Length of the storage root is determined before duration bulking types require a >21‐week width, with shape being determined by the dif- period for maximal development. Storage root ferential rates of longitudinal and lateral growth. bulking rate is positively correlated with rainfall Root set even in the same field and cultivar is and relative humidity. highly variable in sweetpotatoes, making it The storage roots, not to be confused with ­difficult to optimize size and shape uniformity tubers, which are modified stems, range in (Firon et al. 2009). shape from spindle‐shaped to almost spherical, Air and soil temperatures affect storage root to irregular in length from a few centimeters to formation and growth. Night air temperature greater than 30 cm, and in weight from 0.1 kg to seems to be the most critical factor for storage several kilograms. Since they are perennial, they root growth. Night temperatures between 15 will continue to grow, but unless protected, will ­Breeig adFed Productio 815 often rot or be discovered by rodents. The skin (2n = 6x = 90). There is still disagreement as to and flesh contains carotenoid and anthocyanin which specie(s) are the most likely progenitors of pigments, which when combined, determine a cultivated sweetpotato (Bohac et al. 1995; Firon continuum of colors from whitish to yellow, et al. 2009). Only a few other Ipomoea species orange, and red to purple. Most cultivars have a have polyploid forms and few successful crosses uniform flesh color but landraces and breeding with diploids or other Ipomoea species have materials can be multicolored or patterned. been made. Recent molecular genetic studies Roots, when cut, ooze sticky white latex from suggest that cultiv­ ated sweetpotatoes are most laticifers present throughout the flesh. The latex likely autopolyploids, with some evidence of turns black as it dries and cannot be removed restricted recombination (Kriegner et al. 2003; from the root surface without removing the skin. Cervantes et al. 2008). Vines are usually prostrate, though some may Due to the high levels of heterozygosity twine, and form a shallow canopy. Vines are ­present in the germplasm, there is great genetic indeterminate and will root at the nodes, often diversity within the sweetpotato, which is stead- producing a secondary set of storage roots. ily being developed by breeders. This diversity Stems range from green to purple, and in thick- appears to be the result of a number of factors. ness from a few millimeters to 1.5 cm. Internode It has been domesticated for a long time and distances vary considerably and have to be con- spread through a large number of environments sidered when planting transplants. Leaves are for selection. It is at least partly autopolyploid, arranged spirally on the stem and are variable in so that there is great redundancy in the genome size and shape, even on the same plant. Leaves allowing for regions to change without compro- range from deeply lobed to entire with many mising the basic systems of the plant. Sexual plants showing a range of shapes. Most leaves reproduction in cultivated fields would allow for are green but may contain purple pigmentation. new types to be evaluated, and then clonally Recently, ornamental sweetpotatoes have been propagated, and finally, there is a fairly high rate released with solid purple leaves and stems, of somatic mutation, especially when sprouted ­others with light green foliage. Many of these from storage roots. also have compact, well‐branched, and some- Because sweetpotato is a polyploid with high what upright plant architectures. levels of heterozygosity and it is mostly an Flowering is mostly short day, but long day ­obligate out‐crossing species with numerous and day neutral clones exist. Flowering and seed mating incompatibilities, breeding in this crop set in fields cultivated by indigenous cultures is fairly difficult (Jones 1986; Collins et al. 1999). may have played a major role in the appearance Most traits of economic significance exhibit of new varieties that are then propagated via quantitative inheritance. Breeding efforts begun cuttings. Flowering is increased during stress in the 1930s in the United States and elsewhere periods and can reduce yield, though there are have significantly improved fungal, bacterial, high‐yielding flowering varieties. and nematode resistance, beta‐carotene, and anthocyanin levels, yields, storage ability, size and shape uniformity, and starch characteris- ­Breeding and Field tics. Considerable work continues on insec­t and Production virus resistance, processing qualities, and ­finding regionally adapted cultivars that match Breeding consumer preferences. Despite its worldwide importance as a food Sweetpotato is a hexaploid with a basic chromo- crop, funding for genetic and molecular genetic some number of n = 15 and 6 sets of chromosomes research have been very limited. Consequently, 816 Sweetpotato Production, Processing, and Nutritional Quality

key research to understand the inheritance of with pH from 5.0 to 7.5 considered optimal, as economically important traits that could help long as there are no mineral deficiencies developing more efficient breeding strategies (Bouwkamp 1985a). has lagged behind to that of other important Sweetpotatoes are a relatively low‐input crop crops. To date, only a few molecular genetic in terms of fertilizers. Soils with moderate fertil- studies of sweetpotato have been published, ity rarely show a yield response to additional N with most being limited to phylogenetics and or P fertilizers. In deep sandy soils with low germplasm evaluation (He et al. 1995; Prakash ­cation exchange capacity, fertilizer responses et al. 1996; Zhang et al. 2000; Hu et al. 2004; are more common especially where high‐­ Zhang et al. 2004), and genome characterization density plantings are made. Potassium usage is (Villordon and LaBonte 1995, 1996). The high and yield responses to additional K are genetic maps of sweetpotato were recently common. Nutrient deficiencies for other ­constructed by Cervantes et al. (2008) and ­elements have been described (O’Sullivan et al. Kriegner et al. (2003) represent the most com- 1997) and estimates of minimal tissue mineral prehensive genetic maps of sweetpotato. Traits concentrations reported (Bouwkamp 1985a). of economic importance or quantitative trait Response depends largely on cultivar and grow- loci (QTL) are now being placed on the map ing system. Cultivars developed in low‐input developed by Cervantes et al. (2008). systems will often show negative storage root yield responses when grown in high‐input Soil and Climate ­systems, and produce excessive vine growth.

Sweetpotatoes are grown from 40°N to 32°S, Cultural Practices and from sea level to 3,000 m in the tropics. Growth is negligible below 10°C and best above In tropical regions, sweetpotato planting is 24°C. Frost will kill the plants and cold tempera- ­generally done by hand with timing early in the tures damage storage roots, though the damage rainy season so that it dries out by harvest. In may not be seen until after a couple months of areas with a long rainy season, planting will be storage. Thus, cultivation is limited to temper- delayed. Some areas can produce more than one ate regions with a minimum frost‐free period of crop per season. Plants are taken from existing 4 months. Multiple crops per year can be grown plantings or nurseries used to maintain plants. in tropical regions with sufficient rainfall. In temperate regions, plants are produced in the Optimal rainfall is approximately 50 cm during spring by first presprouting storage roots at the growing season. Once established, the crop 29°C for 10–20 days and then bedding them by can handle severe drought and resume growing laying out on soil, almost touching for large when rain occurs, but drought during establish- roots and 2–4 cm between small roots, and ment can cause poor stands and poor root set. ­covering them with 2–4 cm of soil and clear or The best soils are sandy loams with permeable black plastic to keep them warm. Covers are subsoils. Sweetpotatoes do not tolerate water- removed as plants sprout, and plants may be logged soils well, especially near harvest where mowed to maintain an equal plant height. Plants roots may rot in the field or in subsequent stor- are cut above the soil line to prevent disease age. Soils with higher bulk densities or poor spread and when they are 25–30 cm tall trans- aeration cause irregular shapes and poorer root planted to the field. Plants can be stored in cool set. Cultural management using mounds or conditions, 15°C, and 85% RH for up to a week ridges can allow productive use of these soils. with no loss of yield potential. Land preparation Sweetpotatoes can tolerate a wide range of soils, usually involves producing a raised bed or ­Breeig adFed Productio 817 mound. The mounding increases drainage, and the storage. Larvae burrow throughout the temporarily lowers soil bulk density, providing ­storage root making it unmarketable, and, in more uniform root development. In tropical response, the roots produce toxic sesquiterpe- regions, planting is usually done by hand, while nes leading to a bitter flavor that is also toxic to mechanical transplanters are the norm in tem- livestock. Damage can be extensive both in the perate regions. Water will be added at trans- field and in the storage. Control through inte- planting if soil moisture is low. The vine canopy grated pest management (IPM) strategies has should cover the ground in 6–8 weeks, after been demonstrated (Talekar 1991). The combi- which minimal weeding is needed. Normally, nation of several techniques was necessary to depending on cultivar and region sweetpotato provide good control, the two most important roots can be ready to harvest 3–8 months after being preventing infestations of new fields by planting. In Papua New Guinea, where it is a using weevil free cuttings and by eliminating subsistence crop, individual roots are harvested immigration of weevils from alternate hosts and from a plant as needed, and then vines are weevil infested crops. Other techniques include ­covered with soil to encourage new storage crop rotation, sanitation, and chemical insecti- roots to develop. In tropical regions, it is com- cides, and the use of sex pheromones to monitor mon to dig only what can be marketed at that weevil populations. Timing of insecticides is time, so no storage is necessary. In temperate critical, since once weevils are present inside the regions, the harvest is timed to optimize yield of stems or storage roots they are difficult to reach. the highest value size grade and before cold Biological control and breeding for resistance weather compromises storage ability. Here roots have not been very successful to date. In areas will be dug, often mechanically or a combina- without the weevil, the use of sex pheromone tion or mechanical digging and hand harvest, traps to monitor for introduction and vigor- depending on the amount of skinning. ously enforced quarantines can be used to Japan has developed an alternative production ­prevent weevil spread. system using cut seed pieces for the production The second most damaging insect pests are of high‐starch processing lines. Storage roots are vine‐boring lepidopterans Omphisa anasto- cut into 25–50 g pieces, and planted mechani- masalis (Guenee), Megastes grandalis Guenee, cally eliminating the bedding, plant cutting, and and M. pucialis Snell causing up to 30–50% yield transplanting operations used in temperate pro- losses (Talekar and Pollard 1991) in Asia, and duction areas. Shapes are not as consistent as certain parts of South America. Virus problems roots produced from plant ­cuttings, but this are severe in certain regions, but considered does not matter since the crop is processed. This minor in other regions. Sweetpotato virus dis- type of system is being investigated in other tem- ease (SPVD), a combination of two viruses, perate regions for processing types and clones causes up to 80% yield losses in parts of Africa. suitable for ethanol production. Viruses are found in nearly all commercial plant- ings because they are vegetatively propagated Common Diseases and Pests and rapidly spread by aphid and whitefly vectors. Quarantines restrict the free movement of plant- Sweetpotato weevils (Cylas spp.) are the most ing material among countries, with most requir- important worldwide pests of sweetpotato. ing plants to be virus‐free before shipping. In the Cylas formicaries Fab. is the major weevil in 1990s, most major production regions began to most countries. They attack nearly all parts of set up virus‐indexed programs to get virus the plants with larvae developing on mature indexed plants into the hands of growers. Virus stems and storage roots both in the field and in assay kits have been developed for some viruses. 818 Sweetpotato Production, Processing, and Nutritional Quality

­Postharvest Handling and allow for heat transfer. Cultivars vary Practices ­tremendously as to how long they will store and maintain the necessary quality. Curing also Storage ­produces changes in the culinary characteristics increasing moistness and sweetness (Walter In temperate regions where production is lim- 1987). ited to a summer season and marketing is Sweetpotatoes continue to respire during ­continuous, sweetpotatoes are stored year- storage, converting starch to sugar, which is round. Varieties have been selected for both low then oxidized to carbon dioxide and water respiration and low water loss, giving a storage ­providing energy for the living cells. Over time, life up to 13 months or until the next crop is the loss of dry matter will cause pithiness, a harvested. Careful handling of sweetpotatoes is ­textural defect caused by an increase in intercel- critical to ensure long‐term storage. Bruising lular space, up to the point where there are air and skinning in the field are minimized by hand pockets in the root tissue. This is greatly accel- harvest or by using a combination of mechani- erated by warmer temperatures. Once tempera- cal and hand harvesting. Roots exposed to tures go above 16°C, the roots will begin to bright sun for more than 30 minutes may have a sprout which greatly increases the respiration darkening of skin called sun scalding, which is a rate and weight loss (Edmunds et al. 2008). cosmetic defect but can also be a site for post- Large commercial storage facilities in developed harvest decay. Roots should not be harvested nations can maintain very precise conditions to when the weather is too cold. Chilling injury is a optimize root storability and quality. In devel- function of temperature and duration of expo- oping countries, storage of sweetpotatoes has sure. Temperatures below 10°C will cause been done for hundreds of years and is still ­chilling, though cooler temperatures will cause practiced using various pit, or underground more damage. Chilling injury may not be seen storage structures. The success of these struc- for weeks after the chilling occurs and can be tures depends on how close they come to main- expressed by various symptoms including taining the ideal temperature, moisture, and increased respiratory rate, greater susceptibility oxygen levels as described. Storage losses due to to decay, surface pitting, internal breakdown, rodents, weevils, and rots tend to be high, and hardcore and reduced culinary quality. the length of time often limited to a few months. After harvest, roots are immediately “cured” at 29–33°C and 85–90% RH with proper ventila- Packing and Shipping tion for 4–7 days. Curing heals wounds that occur during the harvest, first by a lignification Market requirements, especially shape and size beneath cells damaged at harvest, and second by requirements, for sweetpotatoes vary by region. the formation of a wound periderm beneath the Where it is a subsistence food, shape, and size lignified cells in a process called suberization. are not as important, but where it is a luxury The healing provides a pathogen barrier and item, appearance is very important. In the reduces desiccation at the wound site resulting United States, highly mechanized packing lines in less weight loss during storage. Uncured are used to grade for strict size and shape roots do not store well but properly cured roots parameters. Lines typically start with a tank of stored at 13–15°C and 85–95% relative humid- water into which roots are dumped. This wets ity will be marketable for up to 12 months the roots for washing and allows roots to be (Edmunds et al. 2008). Good airflow is essential metered onto a conveyor system. Roots go to maintain oxygen and carbon dioxide exchange through water rinse to remove soil, followed by Ntiinl Cmoiin of Sweetpotatoe 819 an eliminator to remove trash and small, unmar- sweetpotato leaves are eaten as green vegeta- ketable roots, usually accomplished by going bles. The nutrient content of sweetpotato leaves across a set of rollers at a specified width. Roots varies among the varieties, harvest dates, crop are then sorted, usually by hand, to remove years and cooking methods. On dry weight decaying or otherwise unmarketable roots. basis, sweetpotato leaves contain 25–37% pro- Roots that will be shipped for retail are then tein, 42–61% carbohydrate, 2–5% crude fat, generally treated with a fungicide to reduce 23–38% total dietary fiber, 60–200 mg/100 g decay. This is followed by sizing into various ascorbic acid, and 60–120 mg/100 g carotene classes, some by diameter, or with electronic (Almazan et al. 1997, Sun et al. 2014). They are sizers measuring both length and diameter. also rich in calcium (230–1,958 mg/100 g), iron Roots are put into boxes, and boxes onto pallets (2–22 mg), potassium (479–5,230 mg), and for efficient handling. magnesium (220–910 mg). The high level of Bruising on packing lines can greatly affect phenolics (1.4–17.1 mg/100 g dry weight), shelf‐life of the sweetpotatoes, and care should anthocyanins, and radical‐scavenging activities be taken in design and setup of the packing lines in sweetpotato leaves indicates their potential to reduce any impacts. The dump tank drop‐ benefits on human health and nutrition (Islam offs and onto conveyors, turns, and packing‐line 2006, Truong et al. 2007). Sweetpotato greens speed and length account for much of the damage are very rich in lutein, 38–51 mg/100 g in fresh and should be minimized (Edmunds et al. 2008). leaves, which are even higher than the lutein Market life, which begins when roots are levels in the vegetables that are known as a removed from bulk storage bins, of a sweetpo- source for lutein, such as kale (38 mg/100 g) and tato is generally 2–3 weeks. The most common spinach (12 mg/100 g) (Menelaou et al. 2006). disease in storage and packed sweetpotatoes is Novel galactolipids were recently isolated and Rhizopus soft rot caused by the fungus Rhizopus characterized from sweetpotato leaves stolonifer. Present in most stored sweetpotatoes, (Napolitano et al. 2007), indicating that this it will contaminate packing lines and enter leafy vegetable can be a potential source of through wounds produced during packing. omega‐3 polyunsaturated fatty acid. Health Sanitation and minimizing wounds on packing benefits and disease prevention of bioactive lines is the most effective control, and the main compounds in sweetpotato leaves have been reason for the fungicide treatment. Care must reported (Johnson and Pace 2010). be taken to ensure that shipping containers are The nutrient composition of sweetpotato maintained at 13°C to prevent excessive respira- roots varies widely, depending on the cultivar, tion or chilling damage. growing conditions, maturity, and storage. Overall, sweetpotato roots have a high moisture level with an average dry matter content of ­Nutritional Composition 25–30%. A wide range of dry matter content of of Sweetpotatoes 13–45% from a sweetpotato germplasm collec- tion was reported by Tsou and Hong (1992) and All the plant parts, roots, vines, and young Brabet et al. (1998). Sweetpotato roots are good leaves of sweetpotatoes are used as foods, ani- source of carbohydrates and generally low in mal feeds and traditional medicine around the protein and fat. Protein content ranged from world (Mohanraj and Sivasankar 2014). The 1.73–9.14% on dry weight with substantial levels nutritional values of sweetpotato roots and of nonprotein nitrogen (Yeoh and Truong 1996). leaves and selected processed products are Sweetpotato protein overall, however, is of good shown in Table 35.1. In Asia and Africa, the quality, and the levels of essential amino acids 820 Sweetpotato Production, Processing, and Nutritional Quality

Table 35.1 Nutritional profile content in sweetpotato roots and leaves (per 100g fresh weight).

Sweetpotato Root

Boiled, Without Canned, Snacks/Chips, Leaves, Nutrient Unit Raw Skin Mashed Unsalted Raw

Proximate Water g 77.28 80.13 73.88 4.51 86.81 Energy kcal 86 76 101 532 42 Protein g 1.57 1.37 1.98 2.94 2.49 Total lipid (fat) g 0.05 0.14 0.2 32.35 0.51 Carbohydrate, by difference g 20.12 17.72 23.19 56.82 8.82 Fiber, total dietary g 3 2.5 1.7 8.8 5.3 Sugars, total g 4.18 5.74 5.45 8.82 na1 Minerals Calcium mg 30 27 30 59 78 Iron mg 0.61 0.72 1.33 2.12 0.97 Magnesium mg 25 18 24 65 70 Phosphorus mg 47 32 52 145 81 Potassium mg 337 230 210 925 508 Sodium mg 55 27 75 35 6 Zinc mg 0.3 0.2 0.21 0.53 na Vitamins Vitamin C, total ascorbic acid mg 2.4 12.8 5.2 0 11 Thiamin mg 0.078 0.056 0.027 0.088 0.156 Riboflavin mg 0.061 0.047 0.09 0.161 0.345 Niacin mg 0.557 0.538 0.955 2.088 1.130

Vitamin B6 mg 0.209 0.165 0.235 0.535 1 Folate, DFE µg 11 6 11 37 1

Vitamin B12 µg 0 0 0 0 0 Vitamin A, IU IU 14187 15740 8699 23675 3378 Vitamin E (α‐tocopherol) mg 0.26 0.94 1.09 9.82 na Vitamin D IU 0 0 0 0 0 Vitamin K (phylloquinone) µg 1.8 2.1 2.4 24.5 302.2

Source: USDA (2016), Standard Reference 28, revised May 2016. 1 not analyzed. Ntiinl Cmoiin of Sweetpotatoe 821 compare significantly to the FAO reference greatest concentration in sweetpotato, with an ­protein (Maloney et al. 2014; Walter et al. 1983). average of 396 mg/100 g fwb. Phosphorous, Most of the dry matter in sweetpotatoes ­calcium, magnesium, iron, copper, and magne- ­consists of carbohydrates, primarily starch and sium are also present in significant amounts sugars and to a lesser extent pectins, cellulose, (Woolfe 1992). and hemicellulose. Dietary fiber in sweetpotato Sweetpotato roots also contain vitamins such roots range from 2 to 4% of fresh weight. as ascorbic acid, thiamin (B1), riboflavin (B2), Residues from sweetpotato starch and juice pro- niacin (B6), pantothenic acid (B5), folic acid, and cessing of commercial varieties are good sources vitamin E. Bradbury and Singh (1986) reported of dietary fiber, 16–36% of dry weight (Mei et al. values between 9.5 and 25.0 mg/100 g (fwb) for 2010; Truong et al. 2012a). Starch comprises ascorbic acid and 7.3–13.6 mg/100 g (fwb) for 60–70% of the total dry matter, but the values dehydroascorbic acid resulting in a total vitamin vary for different types of cultivars. As with C range of 17.3–34.5 mg/100 g for the sweetpo- other starches, sweetpotato starch granules are tato roots. Orange‐fleshed sweetpotatoes are made up of amylose (20%) and amylopectin and rich in β‐carotene (Table 35.2). A wider range of pasting temperatures are usually in a range of β‐carotene content in cooked orange‐fleshed 60–76°C (Zhu and Wang, 2014). A special sweet- sweetpotatoes, 6.7–16.0 mg/100 g fwb, has been potato cultivar in Japan named Quick Sweet has reported by different investigators (Bovell‐ starch gelatinization temperature of < 50°C and Benjamin 2007). The sweetpotato carotenoids short cooking time. Short amylopectin chain exist in an all trans configuration, which exhib- length and cracking on the hilum of starch gran- its the highest provitamin A activity among the ules contribute to the lower pasting temperature carotenoids. van Jaarsveld et al. (2005) advocate of the Quick Sweet cultivar (Takahata et al. the increased consumption of orange‐fleshed 2010). Much variability in sugars exists between sweetpotatoes as an effective approach to sweetpotato types. Truong et al. (1986) found improve the vitamin A nutrition in the develop- total sugars to vary from 5.6% in a Filipino culti- ing countries. Total carotenoid content is cor- var to 38% in a Louisiana cultivar on a dry weight related with the dry matter content and sensory basis (db). Sucrose, glucose, and fructose make attributes involving visual, odor, taste and tex- up the majority of the total sugars in raw sweet- tural characteristics of cooked sweetpotatoes. potato roots. During cooking, amylases act on Doubling in carotenoid content would result in the gelatinized starch resulting in the formation a decrease of about 1.2% of dry matter content of maltose in cooked sweetpotatoes. There is in sweetpotato varieties (Tomlins et al. 2012) substantial genetic diversity within the sweetpo- Epidemiological studies indicated the beneficial tato genotypes collected around the world in effects of high carotene diets in reducing the term of sugar content and degree of sweetness risks of cancer, age‐related macular degenera- that contribute to the consumer preferences tion, and heart diseases (Tanumihardjo 2008). of processed products (Kays et al. 2005; Purple‐fleshed sweetpotato roots have attrac- Leksrisompong et al. 2012). The glycemic ­indices tive reddish‐purple color with high levels of of cooked sweetpotatoes were about 63–66, anthocyanins and total phenolics (Table 35.2). indicative of moderate glycemic index food The flowable purées with a solids content of 18% (Allen et al. 2012). processed from this sweetpotato type had total Ash content of sweetpotatoes is approxi- phenolic and anthocyanin contents of 314 mg mately 3% of the dry weight or between 0.3% chlorogenic acid equivalent/100 g fwb and 58 and 1.0% of the fresh weight basis (fwb) mg cyanidin‐3‐glucosdie equivalent/100 g fwb, (Table 35.1). Potassium is the mineral with the respectively. The 2, 2‐diphenyl‐1‐picrylhydrazyl 822 Sweetpotato Production, Processing, and Nutritional Quality

Table 35.2 Phytonutrients in orange‐ and purple‐fleshed sweetpotato roots.

Dry Matter β‐carotene (fwb) Varieties Flesh Color (g/100g) (mg/100g) Anthocyanins1 Total Phenolics2

Beauregard Orange 20.5 9.4 n.a. 88.9 Covington Orange 20.3 9.1 3.8 58.4 Stokes purple Dark purple 36.43 n.a. 80.2 401.6 NC 415 Dark purple 29.03 n.a. 69 652.5 Okinawa Light purple 30.03 n.a. 21.1 458.3

Sources: Truong et al. (2007); Steed and Truong (2008); Yencho et al. (2008). 1 mg cyanidin‐3‐glucoside/100g fw. 2 mg chlorogenic acid/100g fw. 3 Dry matter adjusted to 18‐20% for flowable purées; n.a. = not analyzed.

(DPPH) radical scavenging activity was 47 µmol ultraviolet light protection, and reduction in trolox equivalent/g fwb and oxygen radical memory impairment effects and colorectal absorbance capacity (ORAC) of 26 µmol trolox ­cancer (Lim et al. 2013; Wu et al. 2008). A study equivalent/g fwb (Steed and Truong 2008). on healthy adult men with borderline hepatitis The purple‐fleshed sweetpotato varieties indicated that purple‐fleshed sweetpotato have anthocyanin content up to 348 mg/100 g ­beverage intake (400 mg anthocyanins/day) fwb) and antioxidant activities in a competitive may have a potential capacity for protection of level with other food commodities known to be the liver against oxidative stress (Suda et al. a good source of antioxidants such as black 2008). bean, red onion, black berries, cultivated blue- berries, sweet cherries, and strawberries. Seventeen anthocyanins were identified by ­Processing and Utilization HPL‐MS/MS. The major anthocyanidins, cya- nidin and peonidin, contributors to the blue and Sweetpotato roots and other plant parts are used red hues of purple‐fleshed sweetpotatoes, can as human food, animal feed, and processing be simply quantified by acid hydrolysis of the industry. Various processing technologies that extracted anthocyanins. This method has been convert sweetpotatoes into functional ingredients, adapted in the breeding programs to select food, and industrial products are summarized in clones with various levels of cyanidin or peoni- Figure 35.3. For industrial processing, starch, din for targeted reddish‐purple flesh colors sugars, and natural colorants are the major inter- (Truong et al. 2010, 2012b; Xu et al. 2015). mediate products that can be used in both food Research on nutraceutical properties of pur- and nonfood processing industry. ple‐fleshed sweetpotato indicated that the Sweetpotato varieties with high levels of dry extracted anthocyanins exhibited strong radical matter (35–41%), total starch (25–27%), and scavenging activity, antimutagenic activity, and extractable starch (20–23%) are available for significantly reduced high blood pressure and starch processing (Brabet et al. 1998). There are liver injury in rats (Zhang et al. 2009). Other many small and medium factories in Asia pro- physiological functions of anthocyanins include ducing about 26% of starch production (Bovell‐ anti‐inflammatory activity, antimicrobial activity, Benjamin 2007). The process for manufacturing ­Processn and Utilizatio 823

Human food Animal (young tips, leaves) feed

Vines

Sweetpotatoes

Roots

Fresh root markets Processing

Food Industrial Home cooking products products

Starch Frozen Dried Fermented Alcohol Sugars *Cubes *Flakes *Beverages *Chunks *Flour *Vinegar Chemicals *Strips *Pickle, curd Puree/Juice Canned Fried *Yoghurt Natural *Cubes *Chips food *Chunks *Strips colorants Aseptic puree/Juice

Formulated products

Figure 35.3 Different processing and utilization streams of sweetpotatoes. sweetpotato starch is basically similar to the Centrifugation and mechanical drying, such as starch extraction from other sources. The roots flash dryer, are commonly used for medium‐ are ground in limewater (pH 8.6–9.2) to prevent scale factories. browning due to polyphenol oxidase, to dissolve Sweetpotato starch is used in the production pigments, and to flocculate the impurities. The of traditional noodles, vermicelli, thickening extracted starch is separated from the pulp by agents, or converted into sugar syrups, which thoroughly washing over a series of screens, are used in many processed food products. The bleaching with sodium hypochlorite, and then sweetpotato starch and sugars are also utilized settling by gravity or centrifugation. In small‐ in the production of fuel alcohol, monosodium scale establishments, starch is stored wet in glutamate, microbial enzymes, citric acid, lactic concrete tanks or sun‐dried to a moisture con- acid, and other chemicals (Kotecha and Kadam tent of about 12%, pulverized and screened. 1998; Padmaja 2009). In Japan, the orange‐ and 824 Sweetpotato Production, Processing, and Nutritional Quality

purple‐fleshed sweetpotatoes have been used in ­disposal. The peeled sweetpotatoes are then commercial production of natural beta‐carotene washed thoroughly to remove all disintegrated and anthocyanin pigments in beverages and peel, followed by trimming, cutting into slices other food products. The following sections or dices. The purées can be simply produced by describe recent developments in processing of steam cooking of the chunks, slices, strips, sweetpotatoes into functional ingredients and cubes, or ground particles, and passing the common food products. cooked materials through a pulp finisher. Hoover and Harmon (1967) developed an Purées and Juices enzyme activation technique using the endoge- nous amylolytic enzymes for starch hydrolysis Processing in sweetpotato purée processing, and this The use of sweetpotatoes in the food industry ­process is now commonly used in the food often involves processing of the roots into industry. purées that can be subsequently frozen, canned, As shown in Figure 35.4, the peeled sweetpota- or packaged in aseptic conditions to produce toes either can be cut into cubes of 2 cm, strips of shelf‐stable products for year‐round availability. 2 × 2 × 6 cm, and slices of 0.5–0.95 cm thickness In purée processing, roots of all sizes and shapes or mashed using a hammer mill with rotating can be utilized and, therefore, the entire blades to chop and push the materials through a ­harvested crop is utilized including the 30–40% 1.5–2.3 mm mesh screen (Walter and Schwartz off‐grade from the fresh root markets (Kays 1993). Next, the materials are steamed blanched 1985b; Walter and Schwartz 1993). The chal- at 65–75°C that activates the amylases and gelati- lenges in purée processing industry are: (1) the nizes the starch for hydrolysis. For the process difficulty in adjusting the process to account for with slices, strips, and cubes, comminuting the differences in cultivar types, root handling, blanched materials into purée is ­carried out at ­curing, and storage; and processing techniques this point using a hammer mill. The blanched in order to produce consistent and high quality purée is pumped into a surge tank and hold at purée; and (2) the preservation technology that 65–75°C for further starch hydrolysis depending could produce shelf‐stable product for conveni- on the targeted maltose levels. Raw sweetpotato ent incorporation in processed foods. mash as a source of amylases can be optionally Several techniques have been developed for added at this stage to increase starch conversion. purée processing in order to produce purées α‐ and β‐amylases hydrolyze the starch produc- with consistent quality, despite the variations ing maltose, maltotriose, ­glucose, and dextrins. due to cultivar differences in carbohydrate ­content, starch degrading enzyme activities, Packaging and Preservation and postharvest handling practices (Kays 1985b; The finish‐cooked purée can be packaged in Collins and Walter 1992). Process operations cans and retorted to produce shelf‐stable product. for puréeing of sweetpotatoes include washing, The purée can also be filled in plastic containers peeling, hand‐trimming, cutting, steamed for refrigerated or frozen storage (Kays 1985b; blanching or cooking, and grinding into purées Collins and Walter 1992; Walter and Wilson which can be subjected to canning or freezing 1992; Pérez‐Díaz et al. 2008). for preservation (Figure 35.3). Raw sweetpota- Preservation by canning for low‐acid food toes can be peeled by abrasive rollers, lye such as sweetpotato purées (pH, 5.8–6.3) usu- ­solution, or steam flashing. Lye peeling is no ally involves high thermal treatment of the longer a common method in the industry due to product because heat transfer in the purée is the issues on equipment corrosion and waste mainly by conduction. High thermal treatment ­Processn and Utilizatio 825

Peeled sweetpotatoes

Cut Grind with hammer mill

Slices, strips, or cubes Raw mash

Blanch at 70°C Steam inject to 70°C; for 10–15 minutes hold for 20–30 minutes to hydrolyze starch Steamed cook Mix steamed-raw Steam inject puree (80:20) at 100°C Puree with to 110°C pulp finisher Raw mash for amylases Puree with Hold at 70°C (optional) Steam inject to 70°C; pulp finisher for 10–60 minutes hold for 20–30 minutes for more starch to hydrolyze starch Steam inject to 110°C conversion to inactivate amylases

Steam inject to 110°C Steam inject to 110°C to inactivate amylases to inactivate amylases

Cooked puree

Freezing, canning, aseptic processing, dehydrating

Figure 35.4 Different processes for sweetpotato purée production (Truong and Avula 2010).

(e.g., 165 minutes at 121°C for an institutional and storage as well as space, energy, time, and #10 can size) also results in severe degradation requires defrosting before use. Currently, only of color, flavor, texture, and nutrients. The slow limited amount of canned and frozen sweetpo- rate of heat transfer from the wall to the center tato purées are commercially produced by a few of the can to attain commercial sterilization of companies in the United States and Japan. the product limits the maximum can size of Aseptic processing is considered as a poten- number 10 for canned sweetpotato purées. This tial alternative to overcome the stated problems size limitation is another obstruction for the associated with canning and low‐temperature wider uses of sweetpotato purées as a food preservation. As opposed to conventional ingredient in the food industry. Nevertheless, ­canning, the use of high temperatures (≥125°C) canning does not have the need for special stor- for a short period of time in aseptic processing age; lower capital investment and unit of pro- can produce a higher quality product with equal duction is less when compared to refrigerated or better level of microbiological safety as that and frozen purée. On the other hand, frozen in a conventional canning system. A process for purée is an established method for preservation rapid sterilization and aseptic packaging of the and provides least degradation of nutritional orange‐fleshed sweetpotato purées using a and sensory quality as compared to canning. ­continuous flow microwave system operated at However, preservation by freezing requires 915 MHz has been successfully developed by ­considerable investment in frozen distribution Coronel et al. (2005). This process has the 826 Sweetpotato Production, Processing, and Nutritional Quality

advantage of avoiding long retort processing The uses of sweetpotato purées in processing schedules, maintaining high quality retention, into flakes and powders, and various fermented and producing shelf‐stable products. The result- food products are described in a section below. ing product packed in flexible plastic containers With the recent commercial development of the had color and viscosity comparable to the non- microwave‐assisted processing and aseptic sterilized purée and was shelf‐stable for at least packaging of sweetpotato purées, it is expected 12 months. that more processed food products from the Purple‐fleshed sweetpotato purées were also purée will be developed. successfully processed into high quality aseptic Sweetpotato purées have also been used as product using the continuous flow microwave ingredient in various formulated fruit and veg- system (Steed et al. 2008). With this technology, etable juices (Padmaja, 2009; Truong, 1994). shelf‐stable purées with consistently high qual- However, heat treatment in sweetpotato purée ity can be packaged into various container sizes processing gelatinizes starch and produces (up to 300 gallons) for conveniently utilizing as thick slurry with cooked flavor, which may not food ingredients in the food processing indus- be preferred in several juicy products. These try. This technology can be extended to highly problems can be overcome by alternative pro- viscous biomaterials and purées from other cesses involving grinding raw sweetpotatoes fruits and vegetables (Kumar et al. 2008). In this and acid treatment to inactivate oxidizing new ­process, sweetpotato purée is loaded into a enzymes ­during juice extraction. Aside from hopper, and pumped through the system. the juice, which is subjected to thermal pro- Microwaves from a generator are delivered to cessing for packaging and preservation as sterilize the purée at 130–135°C, to retain in the described above, the ungelatinized starch and holding tube for 30 seconds, to rapidly cool in a flour with high dietary fiber can be recovered tubular heat exchanger, and then to aseptically as co‐products from this alternative process package in aluminum polyethylene laminated (Truong et al. 2012a). Polyphenols in purple‐ bags (Simunovic et al. 2014). The first commer- fleshed sweetpotato juice can be sorbed in pro- cial venture on aseptically packaged sweetpo- tein‐rich matrices such as soy flour, light roast tato purée using this microwave‐assisted peanut flour, and rice protein concentrate to sterilization technology has been carried out. create functional food ingredients rich in both With rapid heating, high retention of carotene proteins and bioactive compounds from sweet- and anthocyanins (>85%) in the purées can be potatoes. Using the same strategy, sweetpotato achieved, and this development opens up a flour efficiently sorbed, concentrated, and new market opportunity for the sweetpotato ­stabilized antioxidant polyphenols in fruit industry. juices such as blueberries, blackcurrant, and Sweetpotato purées has been used as an muscadine grape (Grace et al. 2015). ingredient in numerous formulated food prod- ucts, including baby food, casseroles, puddings, Frozen Products pies, cakes, ice cream, leather, bread, patties, and soups (Collins and Washam‐Hutsell 1986; Sweetpotatoes can be frozen in different forms Collins and Walter 1992; Truong and Walter such as whole roots, halves, quarters, slices, 1994). The sweetpotato purées are also used in cubes, French fries, paste, or as purée. The pro- fruit/vegetable based beverages and restruc- cessing steps include peeling, sizing, cutting, tured products (Truong 1992; Truong et al. blanching or cooking, packaging, and freezing. 1995). Other commercial utilization of sweet- Sizing is important to assure appropriate potato purée includes jam and ketchup (Truong blanching or cooking and freezing time when 1994; Fawzia et al. 1999). the roots are to be frozen whole (Bouwkamp ­Processn and Utilizatio 827

1985b). Packaging may precede freezing such as minutes is done to drive out gases, maintain can frozen purées or may follow freezing when the vacuum, and increase the initial temperature of roots or cut pieces are individually quick frozen the contents of the cans (Bouwkamp 1985b; (IQF). In Japan, sweetpotato slices/crushed Padmaja 2009). However, low‐temperature roots mixed with 35% sugar are packed in plastic blanching at 62°C increases firmness and intact- bags and blast frozen at −40°C (Woolfe 1992). In ness retention of canned sweetpotatoes as large‐scale production of French fries, partially ­compared to the unblanched samples or samples fried products are frozen for distribution to blanched at higher temperatures (Truong et al. institutional and retailed consumers. Good‐ 1998). Immediately after blanching, the material quality fries could be produced by blanching the is packed in cans and covered with syrup at 95°C strips in 60% sucrose solution for 4.5 minutes or to prevent discoloration. Sugar (20–40%) or for 3 minutes in boiling water containing 0.25% water is used, depending on consumer prefer- sodium acid pyrophosphate (SAPP) and 0.25% ences. Cans should be exhausted long enough for calcium chloride (Padmaja 2009). Textural the internal temperature to reach 77°C to ensure properties of the frozen French fries are affected a good vacuum of the finished cans (Bouwkamp by root storage, and the problem can be over- 1985b). After closing, the cans should be retorted come by calcium treatment and low‐tempera- according to the processing schedules and ture blanching. Loss in ascorbic acid and color quickly cooled after retorting to an internal tem- score were reported for French fries stored perature of 35°C to avoid “stackburn” and slow ­frozen for 1 year (Schwartz et al. 1987). drying of the can, which may lead to rusting. Discoloration is a major problem for frozen Firmness is one of the most important attrib- sweetpotato products. Enzymatic discoloration utes determining the quality and marketability of caused by polyphenol oxidase is characterized canned sweetpotato roots. Firmness was slightly by a brown, dark gray, or black color. This dis- greater for sweetpotatoes packed with sucrose coloration can be minimized or prevented by than with corn syrup, and canning in syrups with heat inactivation of the enzymes prior to high sugar concentrations produced firmer roots. ­peeling, soaking the cut pieces in solutions con- Variations between the same cultivars grown in taining sulfites and acidulants. The nonenzy- different locations, application of fertilizers, and matic ­discoloration is caused by phenolics irrigation influence firmness (Bouwkamp 1985b). complexing with iron and other metals, which Sweetpotatoes canned immediately after harvest can be ­prevented by pyrophosphates in the are firmer than those previously cured or stored. blanching medium or added directly into ­several Changes in pectic fractions are responsible for products (Walter and Wilson 1992). the decreased ­firmness of previously stored, canned roots. Adjustment of pH in sweetpotato Canned Products tissue by acidification or alkali treatment and ­calcium treatments improved the firmness of Canned sweetpotatoes are widely consumed canned products and French fries processed among the sweetpotato products available to from cured and stored roots (Walter et al. 1998). consumers in the United States. Sweetpotatoes can be canned whole, halved, or cut into chunks, Dehydrated Forms: Slices, Granules, in either syrup or water. Sweetpotatoes can also Flakes, and Flour be puréed and canned as a solid pack. The unit operations leading to the production of canned Sweetpotato roots are processed into dehydrated sweetpotato roots include peeling, cutting, forms such as dried chips, cubes, gran­ ules, ­sizing, blanching, filling, syruping, exhausting, flakes, and flour for storage and use in food and retorting. Blanching in water at 77°C for 1–3 preparations, including soups, bakery products, 828 Sweetpotato Production, Processing, and Nutritional Quality

vermicelli, noodles, extruded snack foods, and season, to produce dried chips and flour from breakfast cereals (Peters and Wheately 1997; both white‐ and colored‐fleshed sweetpotato Padmaja 2009; Truong and Avula 2010; varieties. Sweetpotato roots are cut into 2–3 Waramboi et al. 2014). Drying pro­ duces a light, mm thick slices and optionally blanched in boil- compact, relatively inexpensive, easily stored, ing water for several minutes. The slices may be and transported material. Processing methods subjected to metabisulfite treatment before or vary in sophistication from simple slicing and during blanching to prevent browning. The field sun‐drying of roots as practiced at the blanched or unblanched slices are sun‐dried ­village levels in many tropical countries to the until the slices reached a moisture content of large‐scale, multistage production of dehydrated about 6–10%. Drying times vary from 4 hours to products by large food companies (Figure 35.5). 5 days, depending on climatic conditions, and Functionality, nutrient retention, and product the dried slices are ground into flour. Owori and storability of dehydrated products of sweetpo- Hagenimana (2000) developed processes for tato roots are important to provide competitive- small‐scale production of flour with the desired ness of these ingredients in food processing. degree of odor, color, and nutritional and micro- biological quality. In Indonesia, sun‐dried chips Sun and Solar Drying are fried and packed in polyethylene for retail Sun‐drying has long been practiced in develop- sale (Kotecha and Kadam 1998). Sweetpotato ing countries, where there is a pronounced dry flour has been produced for decades in Peru to

Sweetpotatoes

Peeling

Slicing/dicing

Anti-browning agents

Blanching/cooking

Sun-drying Hot-air-drying Mashing Pureeing (Cabinet/oven drying) Amylase Amylase Maltodextrin

Drum-drying Spray-drying

Figure 35.5 Drying technologies applied to dehydrated sweetpotato products. ­Processn and Utilizatio 829 use in wheat–sweetpotato bread (van Hal 2000). and should have high dry matter with white However, poor control of energy input and color (Bovell‐Benjamin 2007). The dehydrated product quality; interruption of drying caused product should be suitably packaged in by cloud, rain, nightfall; and frequent contami- ­aluminum‐laminated packages or plastic con- nation by microorganisms, dust, and insects are tainers to exclude air and moisture for good the disadvantages of sun‐drying (Woolfe 1992). storage stability (Avula et al. 2006; Hathorne Microbial evaluation of sun‐dried sweetpotato et al. 2008). A high‐temperature, short‐time slices showed the presence of 12 fungal species, drying process, 150°C for 10 minutes, was whereas the oven‐dried slices had no fungal developed by Antonio et al. (2008) for osmotic growth (Okungbowa and Osagie 2009). These dehydration of sweetpotatoes. problems can be overcome by using modern Drum‐drying of sweetpotato purées is com- solar‐assisted dryers that effectively utilize solar mercially practiced in the United States for energy for control drying, resulting in good‐ ­producing sweetpotato flakes/powder, which quality products. Several types of solar dryers can be reconstituted into mashed sweetpotatoes with external means (e.g., fans, furnace), for or incorporated into a variety of other products moving solar energy in the form of heated air such as pies, pastries, cakes, casseroles, and from the solar collector to the drying bed that other food preparations. The cooked and com- have been used for fruits and other vegetables minuted sweetpotatoes are dried in a double‐ (Lopez‐Malo and Rios‐Cass 2009), can be drum dryer heated with steam. The flakes were applied in drying sweetpotatoes. milled into <60 mesh particles and stored under nitrogen at −20°C (Valdez et al. 2001). Szyperski Mechanical Drying et al. (1986) developed an alternative drum‐drying Mechanical dryers such as cabinet, tunnel, process to produce a consistent product inde- drum, or spray dryers are used in large com- pendent of raw material variations. A commer- mercial enterprises. Cabinet and tunnel drying cial α‐amylase was used to hydrolyze a part of are based on the same principle as solar dry- the pregelatinized purée, which is then blended ing, except that the air is heated by fuel. The with the untreated portion. SAPP or citric acid is dehydration conditions such as drying temper- added to the purée before drying to control non- ature, drying time, and air velocity can be con- enzymatic browning, which causes discoloration trolled in these dryers. As in purée processing, of the reconstituted flakes. Avula et al. (2006) raw sweetpotatoes can be peeled by abrasive prepared drum‐dried flour by subjecting sweet- rollers or steam flashing, followed by washing, potato mash to a double‐drum dryer of 60 cm trimming, cutting into slices/dices, soaking in width and 35 cm diameter. The speed of the solution containing antibrowning substances drum was maintained at 3 rpm with a clearance such as sulfite, SAPP, and steam‐blanching for of 0.3 mm and at a steam pressure of 6 kg/cm2. about 7 minutes. The slices/dices are spread on The sheets of dried sweetpotato were collected, trays and dried in the cabinet or tunnel dryers crushed, and milled into flour in a hammer mill at about 50–80°C for 4–12 hours to a moisture provided with a 500‐µm sieve. Drum‐drying content of less than 7%. The drying ratio of caused a reaction of the ε‐amino group of lysine fresh to finished product is about 3:1 to 5:1, with reducing groups of carbohydrates, which depending on the dry matter content in caused the lysine to be destroyed irreversibly, sweetpotatoes. To produce good quality flour, and formation of browning compounds (Walter sweetpotato roots should be low in total free et al. 1983). sugars, reducing sugars (<2%), ash content, Spray‐drying of sweetpotato purées was amylase activity, polyphenol oxidase content, reported by several investigators (Grabowski 830 Sweetpotato Production, Processing, and Nutritional Quality

et al. 2006; Peng et al. 2013). The purée was For chip processing, unpeeled or peeled roots ­subjected to pretreatment with α‐amylase to are sliced into 0.8–2.0 mm thin chips, which are reduce viscosity and maltodextrin was used to blanched for 2 minutes at 93°C, then drained aid in spray‐drying. Maltodextrin facilitates and partially dehydrated using heated forced air product recovery by raising the glass transi- at 119°C. The thickness of the chip is important, tion temperature of the product, thereby since it affects the length of cooking and the reducing stickiness and partially encapsulat- quality of the finished product. Partial drying ing the material. The purée was spray‐dried has a pronounced effect on the appearance, using a dryer equipped with a rotary atomizer ­flavor, and texture of the finished product. and a ­mixed‐flow air‐product pattern. The Optimum frying temperature was between 143 final characteristics and functionality of the and 154°C (Hoover and Miller 1973). Frying at spray‐dried sweetpotato powders are affected lower temperature under vacuum (130°C) and by predrying treatments and spray‐drying deoiling by centrifuging system can produce temperature. Rheological properties of the good‐quality chips with low fat content (Ravli reconstituted slurries from spray‐dried sweet- et al. 2013). Picha (1986) reported that color of potato powders behaved similarly to the the chips was positively related to reducing sug- pregelatizied starch (Grabowski et al. 2008). It ars. However, recent screening on various was demonstrated that good quality sweetpo- sweetpotato genotypes with a wide range of tato powder produced by spray‐drying has reducing sugar content indicated that other potential applications in food and nutraceuti- substances such as amino acids might signifi- cal products. cantly contribute to the browning of fried chips (Lim et al. 2014). Following frying, the chips are Fried Products: Chips, French Fries drained and salted/sugared. After cooling, the chips will be packaged immediately to exclude Sweetpotato chips and French fries are popular water and oxygen. For French fries, sweetpotato in many countries. In the past few years, several roots are cut into strips 1.9 cm thick × 6.4 cm food companies in the United States have ven- thick, blanched in boiling water containing 1% tured into processing of sweetpotato chips and SAPP to inhibit polyphenolic discoloration, French fries with high beta‐carotene content ­followed by partial drying at 120°C for 5 min- from orange‐fleshed sweetpotatoes in response utes, frozen, and stored at −34°C until the slices to the growing demands of the consumers on are fried for consumption (Schwartz et al. 1987). healthy foods. For commercial success, the Partial drying reduces oil absorption and product should be of consistent quality regard- increases sensory quality of French fries (Walter less of root storage duration. Consistent quality­ and Hoover 1986). Coating of sweetpotato SPFF can be produced year round from SP strips with starch‐based materials improved roots stored under appropriate conditions. appearance and textural properties of sweetpo- Sweetpotato French fries have relatively low fat tato French fries (Truong and Pascua 2009, content, 10% fresh weight (fw), and high caro- unpublished). tene content, 10 mg/100 g fw (Truong et al. The quality of fried chips and French fries are 2014). Reconstituted sweetpotato chips were affected by sweetpotato varieties, postharvest developed in China, and extruded snack products handling, and storage conditions. Changes in with alternative shapes to those of conventional reducing sugars, amino acids, and other sub- chips were produced in Japan, with characteris- stances involved in the discoloration of sweet- tics similar to those of extruded potato snacks potatoes affect the color of this product type. (Woolfe 1992). Textural properties and oil content of the fried ­Summar 831 products are influenced by dry matter and starch sweetpotatoes (Yamakawa 2000). Red vinegar content. Aside from quality, acrylamide forma- with high antioxidant activity and antihypergly- tion could be a potential health concern. Lim cemic effect made from purple‐fleshed sweetpo- et al. (2014) reported acrylamide levels of tatoes was developed (Matsui et al. 2004). 296‐2849 µg/kg in sweetpotato chips fried in Sweetpotatoes are also used as substrates in soy various oil types and unsaturated oils with lipid sauce fermentation (Data et al. 1986). Other fer- oxidation tend to cause high acrylamide forma- mented sweetpotato products rich in carotene tion. Integrating common processing treatments and anthocyanins that have been developed in such as water blanching and soaking sweetpo- recent years include yogurt (Collins et al. 1990), tato strips in sodium acid pyrophosphate could curd (Mohapatra et al. 2007), fermented bever- reduce acrylamide levels by 85% or <100 µg/kg ages (Saigusa et al. 2005), lacto‐pickle (Panda in sweetpotato French fries, the desired level for et al. 2009a), lacto‐juice (Panda et al. 2009b), and white potato French fries (Truong et al. 2014). probiotic milk‐sweetpotato drink (Perez and An integrated approach including selection of Tan 2006). The application of bioprocessing suitable varieties, growing conditions, and technology and the progress in the development appropriate postharvest handling and storage of ­fermented products from sweetpotatoes has conditions should be considered in order to pro- been reviewed by Ray et al. (2010). duce sweetpotato chips and French fries with consistent quality during long‐term storage. ­Summary Fermented Products Sweetpotato, an important economic crop in Being rich in starch, sugars, and other nutrients, many countries, ranks the fifth most important sweetpotatoes have been used in the production food crop in the tropics and the seventh in world of many fermented products. In Japan, high‐ food production after wheat, rice, maize, potato, starch sweetpotato varieties are used in shochu barley, and cassava. All the plant parts, roots, fermentation. Shochu is traditional distilled vines, and young leaves of sweetpotatoes are ­liquor from sweetpotatoes or other sources used as foods, animal feeds, and traditional such as rice, barley, corn, or potato (Sakamoto medicine around the world. Most of the dry and Bouwkamp 1985). Sweetpotato shochu is matter in sweetpotatoes consists of carbohy- very popular, especially in southern Japan. The drates, primarily starch and sugars, and to a process involves the inoculation of steamed lesser extent pectins, cellulose, and hemicellu- sweetpotato slurry with a starter Koji contain- lose. Residues from sweetpotato starch and ing Aspergillus niger or A. kawachii as an juice processing of commercial varieties are enzyme source for starch conversion to sugars, good sources of dietary fiber. Sweetpotatoes are followed by fermentation to alcohol by yeast processed into purée, juice, and canned, frozen, Saccharomyces cerevisiae. The whole process dried, and snack products. Development of usually takes 12–14 days to yield a broth having high yield sweetpotato varieties with high 13–15% alcohol, which is then distilled and nutritional values and good eating quality, along blended to produce shochu with 20–40% with improving processing technologies for the alcohol. Recently, sweetpotato vodka with 40% development of food products and functional alcohol has been produced and commercialized ingredients from sweetpotatoes would meet the by companies in the United States. consumer demands for healthy foods and even- Wine and beer are the recent alcoholic tually increase the consumption of this nutri- ­beverages from the orange‐ and purple‐fleshed tious vegetable in the human diet. 832 Sweetpotato Production, Processing, and Nutritional Quality

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