Food Sci. Technol. Res., 18 (4), 515–524, 2012

Comparison between by Superheated Steam and by Convection on

Changes in Colour, Texture and Microstructure of Peanut (Arachis hypogaea)

* Nor Faadila M. Idrus and Tajul A. Yang

Food Technology Division, School of Industrial Technology, Universiti Sains Malaysia, 11800 Pulau Pinang, Malaysia

Received February 16, 2012; Accepted April 21, 2012

Colour, texture (hardness and fracturability) and microstructure were compared in peanuts (Arachis hypogaea) that were roasted by superheated steam oven (A-1500V, SHARP) in superheated steam mode and convection mode (normal without steam) operated at three different temperatures (250℃, 200℃ and 150℃) for durations between 5 to 45 min. Different temperatures required different heating times for the roasting to be completed. Colour was expressed in terms of brightness (L*), whiteness (WI), redness (a*) and yellowness (b*) for each combination of roasting temperature and time. We found that L* and WI val- ues decreased with increased roasting time and temperature, while a* and b* values increased when roast- ing time was extended. All the colour values (L*, a*, b*, WI) were more affected during superheated steam roasting than during convection roasting. Temperature increase by superheated steam was faster than in convection; however the roasting procedure was completed within a shorter time in convention. Hardness and fracturability, which represent the texture quality of peanuts after roasting, decreased with roasting time. This trend was, however, not statistically significant (p > 0.05). More oil was viewed to be extracted during superheated steam roasting than during convection roasting in scanning electron micrograph.

Keywords: superheated steam, convection, roasting, physical characteristics, peanut, Arachis hypogaea

Introduction occurs when the product, initially at room temperature, is represents an important step in food prepara- exposed to the high temperature of the oven. Lower heat ab- tion as its acceptability by the consumer is dependent on the sorption occurs when the temperature difference between the heat-induced reactions affecting the colour, aroma, flavour product and the oven is reduced at the later stage of the bak- and texture quality of the final product. The correct choice ing operation. Laminar, turbulent or mixed convection are of temperature, time and humidity could trigger the desired examples of complex air flow experienced in the oven (Micael chemical reactions (protein denaturation, starch gelatinisa- et al., 2010). tion, Maillard reactions, etc.) and physical processes induced Modern superheated steam ovens bathes food in a spray by heat during this thermal process, while undesired reac- of superheated steam produced from addition of sensible heat tions are reduced (Lund, 2003). to water which raised its temperature above saturation tem- Radiation, conduction and convection are three mecha- perature at the given pressure. This technology believed to nisms involving heat transfer that differ according to the provide low-calorie cooking by removing excess fat and salt type of product and the oven chamber design and operation. from while retaining vitamins (Pronyk, 2004; Head, et Air flow, heat supply, humidity, oven load and baking time al., 2010). Superheated steam as drying medium is an energy are the major factors associated with heat distribution in the efficiency process compared to conventional hot air due to oven chamber. Heat absorption by the product affects its possibly of reuse of the latent heat of evaporation (Berghel characteristics, which vary considerably during the of and Renstrom, 2002; Fitzpatrick, 1998). Studies of Fraile the baking operation. At the beginning, high heat absorption and Burg (1997) have shown that superheated steam offers advantages during the reheating process of foodstuff with *To whom correspondence should be addressed. high initial water content. Under superheated steam treat- E-mail: [email protected] ment, food such as vegetables and meat are healthy cooked 516 N.F.M. Idrus & T.A. Yang when the excess fat and salt melts and falls away (Chen, et weight) obtained from a supplier near Georgetown, Penang al., 1992; Mujumdar, 2006) were stored in a chiller (7℃) allowed to equilibrate at room Baking has been investigated by many research teams to temperature overnight before roasting. The peanuts (100 g) improve the energy efficiency of the process as well as the were placed in a single layer in a plate and roasted by a su- physical and sensorial quality of the food product (Savoye et perheated steam oven (Healsio, AV-1500V, SHARP, Japan) al., 1992; Sablani et al., 1998; Lostie et al., 2002; Baik and in superheated steam mode and convection mode (normal Marcotte, 2002; Sakin, 2005; Sakin et al., 2007a, b). In the without steam). Roasting was carried out at 150, 200 and peanut industry, the roasting process is important as it is used 250℃. All procedures were carried out in triplicate. to enhance the flavor, colour, texture, and overall palatability Colour measurement Surface colour of raw and roasted of the finished product. Mature peanut kernels are com- peanut sample was measured using a Minolta CM-3500D monly roasted at 160℃ for 20 − 30 min (Damame, 1990). colorimeter (Light source, Pulsed xenon arc lamp; Reflec- However, the actual roasting intensities may depend upon the tance, d/8; Measuring head hole, 8 mm; Measurement time, characteristics, flavor, and applications desired. Some chemi- 2.5 s) after calibration against white and black glass stan- cal changes may occur; sugars can condense with free amino dards. Colours were expressed in CIELAB colour values (L*, acids, peptides, or proteins leading to the formation of brown a*, b*). The L* value represents the lightness to darkness Maillard reaction products that have potential antioxidant gradation, the a* value represents the greenness to redness activity (Delgado, 2005). spectrum, while the b* value represents the blueness to yel- Colour and moisture measurements are valuable indica- lowness spectrum. The colorimeter readings for L*, a*, and tors of the quality of roasted peanuts. Seed colour or changes b* values can also be converted to a whiteness index (WI) in moisture content after roasting are currently used as qual- values according to equation 1. ity standards to determine roasted peanut flavour. A number WI = 100 − ((100-L*)2 + (a*)2 + (b*)2))0.5 (Eq. 1) of other factors affect roasted peanut flavor, regardless of roast colour. These include the peanut genotype, harvest ma- Texture profile analysis Texture measurements were turity, planting date, and appropriate curing and drying pro- carried out using Universal Texture Analyser (CNS, Far- cedures (Buckholtz et al., 1980; Chiou et al., 1991; Chung et nell, UK) equipped with the Texture ProTM texture analysis al., 1993; Smyth et al., 1998). software. A 36 mm cylindrical probe P/36 R was used for With hazelnuts, the roasting process affects microstruc- measuring the texture in terms of compression force (g). The tural changes in texture that give the nuts their crispness and instrument was calibrated with a 30 kg load cell. The target crunchiness (Saklar, et al., 1999). Hence, various studies value was set at 15 mm at 1 mm/s. Samples were placed onto have been undertaken on microstructural changes to deter- the platform and the probe was allowed to compress 5 mm mine what measurable aspects of the microstructure correlate into the sample. Texture profile analysis enabled calcula- with a favourable response on the human tooth (Aguilera and tions for hardness, adhesiveness, cohesiveness, fracturability, Stanley,1990). Examining the microstructure (parenchyma springiness, chewiness, gumminess and resilience. tissue) of raw and roasted hazelnuts by SEM and TEM, Sample preparation for scanning electron microscopy Perren, et al. (1996) found that oil was released from lipid (SEM) Samples were fixed using the hexamethyldisilazane bodies during heating process. The heated tissues gradually (HMDS) technique. Cubes (3 mm square) were cut from raw lost their subcellular organization as the cytoplasmic network and roasted peanuts for each heating parameter. The sample became progressively disrupted with increasing heating time. blocks were fixed with a freshly made fixative (Mc-Dowell- At the same time, cell to cell junctions disintegrated along Trump fixative prepared in 0.1 M phosphate buffer, pH 7.0) the middle lamella and some protein bodies aggregated in the for 24 h at 4℃. After three washes with 0.1 M sodium phos- roasted cells (at 150℃ after 25 min) or combined with the phate buffer for 10 min, the samples were post-fixed for 1 h oil. in PBS buffered 1.0% osmium tetroxide at 4℃ followed by The aim of this study was to investigate a comparison three washes of distilled water. The samples were dehydrated between roasting by superheated steam and by convection at room temperature in a graduated series of ethanol (50, 75 with regard to changes in colour, texture and microstructure and 95%, changed at 15 min intervals) before dehydrating of peanut (Arachis hypogaea). in absolute ethanol (100%) three times at 20 min intervals. Dehydrated peanut samples were immersed in 2 mL hexa- Materials and Methods methyldisilazane (HMDS), which was decanted off after 10 Sample preparation Runner-type raw peanuts (average min. The samples were dried to a critical point in a SamDri length: 1.85 cm ± 0.16; moisture content: 5.05 ± 0.23 %/ 805 CPD (Tousimis, USA), mounted on aluminium speci- Colour, Texture and Microstructure Changes of Peanut (Arachis hypogaea) 517 men stubs, and coated with a layer of gold palladium to a temperatures required longer roasting times. thickness of 20 nm. The tissue was viewed with a Leo Supra The correlation coefficient (r) and p-value of each tem- 50VP Field Emission SEM (Carl-Ziess SMI, Oberkochen, perature were calculated and analysed using regression fitted Germany). line plot test. Existence of a significant correlation does not Statistical Analysis All samples for each heating param- assign dependency between variables and only quantifies eter were prepared in triplicate. Data was expressed as the the strength of association between variables. In this study, r means of measurements ± S.D. The experimental data were represents strength association value between colour analysis analyzed (p-value and r value) using MINITAB 11.12, and with time of heating. Correlation coefficients vary between the fitted regression line set at the level of α = 0.05. +1 and -1, where +1 indicates a strong correlation with both variables increasing together, −1 indicates a strong correla- Results and Discussion tion with one variable decreasing as the other increases, and Changes in colour Colour is probably the most impor- 0 indicates no correlation. In general, a correlation coefficient tant quality parameter for roasted peanuts because it is one between −0.20 and 0.20 is not strong enough to be considered of the first things a consumer would evaluate. This will affect significant (Epsteinet al., 2002; Graybosch et al., 2004). his decision to purchase the product, regardless of its textural Superheated steam roasting at 200℃ showed a better quality. Colour has been used by many researchers as a qual- relationship of lightness (L*) value with time (r = −0.903, ity control indicator of roasting process because brown pig- p > 0.05), as compared with the convection roasting where ments increase throughout the browning and caramelization r value was −0.939 (Table 1).In both instances, the value of reactions (Moss and Otten, 1989; Cammarn et al., 1990). lightness decreased over time, although the r value for both Peanuts were heated in superheated steam roasting and method was not statistically significant. convention roasting under 3 set temperatures for various time Whiteness values that were calculated based on the L*, intervals. For this experiment, each temperature required dif- a* and b* values showed a decreasing trend over time. r ferent time intervals, between 5 and 45 minutes, to complete value −1.00 indicated there was a strong negative relation- roasting (obtain a Lightness value of 58.84 ± 0.62 as deter- ship between whiteness value and time for superheated mined by a colorimeter (Model Minolta CM-3500D)), lower steam roasting at 250℃ (p < 0.05). There was a significant

Table 1. Food temperature and moisture content of peanut during superheated steam and convection oven roasting at different temperature.

Roasting method Temperature (℃) Time (min) Peanut temperature (℃) Moisture content of the peanut (%/w) Raw 26.4 5.05 Superheated steam 250 8 103.4 4.91 10 111.6 4.38 13 115.0 3.28 15 154.8 2.95 200 8 119.4 2.59 10 120.6 2.30 18 138.4 1.93 150 15 104.5 2.49 25 116.8 2.39 30 118.0 2.00 35 120.0 1.87 40 121.2 1.81 45 122.6 1.79 Convection 250 5 124.6 2.60 7 130.2 1.72 8 141.6 1.00 200 8 109.2 1.66 10 121.2 1.38 12 129.6 1.37 150 8 102.2 3.46 13 106.8 2.79 15 107.8 2.26 25 108.8 1.93 30 112.0 1.35 40 114.6 1.30 518 N.F.M. Idrus & T.A. Yang relationship between these two variables. The lesser r value compared to during convection. Heating rate of peanut was of −0.954 for convention roasting at the same temperature based on peanut temperature measured throughout roasting indicated a lower negative relationship between these two time (Table 1). However the roasting procedure was com- variable for this method (p > 0.05). pleted within a shorter time in convection (8 min) compared Convection roasting at 150℃ gave lower positive relation- to superheated steam (15 min). Redness and yellowness val- ship between redness (a*) value with time (r value = 0.957) ues were increased as the peanuts darkened over time. compared with roasting by superheated steam, with an r value Texture analysis Hardness, which is a measure of the of 0.994 at the same temperature. Redness developed over time firmness of the peanuts, was analysed by the Texture Profile in the peanuts during roasting by both methods (p < 0.05). Analysis (TPA) method. Table 2 shows that hardness of pea- Yellowness (b*) values increased throughout roasting nuts decreased throughout the duration of heating by both time. There was a more positive relationship between yel- methods of roasting at 3 temperatures (250, 200, and 150℃). lowness value with time (r = 0.902) for the superheated The r values reflect the negative relationships between steam roasting method at 150℃, as compared with conven- hardness and roasting time value with time. For example, tion roasting at the same temperature (r = 0.859, p < 0.05). hardness decreased from 9.51 N to 9.12 N after roasting in All the colour values (L*, a*, b*, WI) were more affected superheated steam at 250℃ for 15 min. Similarly, hardness during superheated steam roasting than during convection declined from 11.16 N to 9.38 N during convection roasting roasting. Temperature increase by superheated steam was for 8 min. Generally, the rate of hardness reduction by con- faster than in convection as showed by heating rate of peanut vection roasting was higher than by superheated steam roast- throughout time of roasting in superheated steam was higher ing. According to Lin, et al. (2000), moisture content and

Table 2. Relationship of lightness and colour with the duration of roasting in a superheated steam and in a convection.

Analysis Roasting method Temperature (℃) Time (min) p-value r value Lightness Superheated steam 250 8 0.043 −0.998 10 13 15 200 8 0.097 −0.903 10 18 150 15 0.220 −0.780 25 30 35 40 45 Convection 250 5 0.188 −0.957 7 8 200 8 0.224 −0.939 10 12 150 8 0.000 −0.998 13 15 25 30 40 Whiteness Superheated steam 250 8 0.007 −1.000 10 13 15 200 8 0.022 −0.978 10 18 150 15 0.083 −0.755 25 30 35 Colour, Texture and Microstructure Changes of Peanut (Arachis hypogaea) 519

Table 2. (Continued) 40 45 Convection 250 5 0.193 −0.954 7 8 200 8 0.166 −0.966 10 12 150 8 0.002 −0.984 13 15 25 30 40 Redness Superheated steam 250 8 0.098 0.988 10 13 15 200 8 0.002 0.984 10 18 150 15 0.006 0.994 25 30 35 40 45 Convection 250 5 0.217 0.942 7 8 200 8 0.062 0.938 10 12 150 8 0.001 0.957 13 15 25 30 40 Yellowness Superheated steam 250 8 0.055 0.945 10 13 15 200 8 0.009 1.000 10 18 150 15 0.086 0.902 25 30 35 40 45 Convection 250 5 0.260 0.918 7 8 200 8 0.316 0.684 10 12 150 8 0.013 0.859 13 15 25 30 40 520 N.F.M. Idrus & T.A. Yang cooking temperature were significant factors affecting the extruded and crunchy products such as puffed and texture of the food. This results shows the rate of decrease cereals. Deformation properties and texture of food can be in moisture content of peanut during convection roasting affected even at the low level of moisture due to changes in was higher compared during superheated steam. Hence, this the distribution of fracture intensities (Barret and Kaletunc, means that hardness of the peanut was more affected during 1998). In this study, the results showed fracturability de- convection roasting. creased with the duration of heating in both the convection Fracturability is a defining textural characteristic of and superheated steam roasting, as reflected in the r values

Table 3. Hardness and Fracturability values of peanuts roasted by superheated steam or by a convection roasting at different temperatures and duration.

Analysis Roasting Method Temperature (℃) Time (min) Mean ± SD (N) p-value r value Hardness Superheated steam 250 8 9.506 ± 0.140 0.074 −0.926 10 9.472 ± 0.016 13 9.086 ± 0.043 15 9.117 ± 0.009 200 8 8.842 ± 0.020 0.240 −0.930 10 9.369 ± 0.032 18 6.684 ± 0.031 150 15 9.737 ± 0.009 0.102 −0.727 25 8.918 ± 0.012 30 7.757 ± 0.042 35 7.744 ± 0.039 40 7.061 ± 0.023 45 8.428 ± 0.023 Convection 250 5 11.164 ± 0.014 0.410 −0.800 7 8.562 ± 0.005 8 9.375 ± 0.029 200 8 10.920 ± 0.020 0.165 −0.967 10 8.457 ± 0.025 12 7.542 ± 0.038 150 8 9.335 ± 0.020 0.066 −0.781 13 8.275 ± 0.002 15 8.261 ± 0.042 25 7.496 ± 0.096 30 7.253 ± 0.034 40 7.743 ± 0.036 Fracturability Superheated steam 250 8 9.526 ± 0.011 0.140 −0.976 10 9.065 ± 0.017 15 2.765 ± 0.029 200 12 11.350 ± 0.025 0.493 −0.715 15 11.863 ± 0.029 18 9.875 ± 0.016 150 15 10.821 ± 0.019 0.522 −0.331 20 10.261 ± 0.032 25 9.184 ± 0.010 35 10.349 ± 0.037 40 9.301 ± 0.108 45 10.233 ± 0.091 Convection 250 5 11.456 ± 0.025 0.616 −0.568 7 7.762 ± 0.043 8 9.962 ± 0.004 200 8 10.061 ± 0.028 0.231 −0.935 10 9.865 ± 0.026 12 8.914 ± 0.016 150 8 11.357 ± 0.034 0.008 −0.927 20 10.380 ± 0.011 25 9.373 ± 0.042 30 9.179 ± 0.007 35 9.633 ± 0.022 40 8.554 ± 0.017 Colour, Texture and Microstructure Changes of Peanut (Arachis hypogaea) 521

( a ) ( b )

( c ) ( d )

( e ) ( f )

( g ) ( h )

( i ) ( j )

Fig. 1. (a) Scanning electron micrograph of raw peanut cotyledon (Calibration bar = 20 µm), (b) Scanning electron micrograph of raw pea- nut cotyledon (Calibration bar = 10 µm), (c) Scanning electron micrograph of peanut roasted with superheated steam at 250℃ (Calibration bar = 20 µm), (d) Scanning electron micrograph of peanut roasted with superheated steam at 250℃ (Calibration bar = 10 µm), (e) Scan- ning electron micrograph of peanut roasted with superheated steam at 150℃ (Calibration bar = 20 µm), (f) Scanning electron micrograph of peanut roasted with superheated steam at 150℃ (Calibration bar = 10 µm), (g) Scanning electron micrograph of peanut roasted with convection oven at 250℃ (Calibration bar = 20 µm), (h) Scanning electron micrograph of peanut roasted with convection oven at 250℃ (Calibration bar = 10 µm), (i) Scanning electron micrograph of peanut roasted with convection oven at 150℃ (Calibration bar = 20 µm), (j) Scanning electron micrograph of peanut roasted with convection oven at 150℃ (Calibration bar = 10 µm). 522 N.F.M. Idrus & T.A. Yang

(Table 2). roasting (Figures 1g and 1h), while they were swollen and For example, fracturability during superheated steam loosely packed after superheated steam roasting (Figures 1c roasting at 200℃ dropped from 11.35 N to 9.88 N whilst the and 1d). The cytoplasmic network was almost fully disrupted decrease was from 10.06 N to 8.91 N during convection oven during superheated steam roasting and the organelles became heating at the same temperature. The moisture content of disorganized (Figures 1c and 1d). Oil was obviously more most nuts was decreased during roasting process causes the thoroughly extracted during superheated steam roasting, with texture became more fracture (crispy) and crumbly (Emily, the appearance of more light coloured cellular constituents et al., 2009; Vincent, 2004). Generally, peanut fracturability (Figures 1c and 1d). was more affected during superheated steam roasting than Roasting at 150℃ by the two methods (superheated during convection roasting. This is based on moisture con- steam roasting and convection roasting) showed significant tent of the peanut after superheated steam roasting at each microstructure thermal modifications. Superheated steam temperature was lower than in convection roasting (Table 1). roasting at 150℃ caused more disruption of the cytoplas- Hardness and fracturability values were positively correlated, mic network in the tissue area (Figures 1e and 1f) compared with decreasing hardness occurring in tandem with decreas- with convection roasting at 150℃ that left the cytoplasmic ing fracturability. network mainly intact (Figures 1i and 1j). The organelles Microstructure analysis The microstructure images of became swollen after both of roasting process. Temperature the mid-region of peanut samples were studied in transverse increase in both roasting process (superheated steam roasting section under 500X and 1000X magnification for the follow- and convection roasting) resulted in more disruption of cyto- ing selected experimental parameters: Superheated steam plasmic network and increased oil extraction. roasting at 250℃ (8 min), superheated steam roasting at 150℃ (30 min), convection roasting at 250℃ (12 min) and Conclusion convection roasting at 150℃ (40 min). A comparison was Peanut colour was affected both by superheated steam made of the microstructure with regard to the effect between roasting and convection roasting. Redness (a*) and yellow- two roasting methods (Superheated Steam and Convection ness (b*) were properties that were enhanced with longer roasting) and two temperatures (250℃ and 150℃). durations of roasting. On the other hand, Lightness (L*) and Larger parenchyma cells subtending the epidermal cells whiteness (WI) decreased with roasting time. There was a are seen in transverse sections of the rounded outer surface significant relationship of L* with roasting time, and of WI of raw peanuts (Young and Schadel, 1993). Figure 1a shows with roasting time during superheated steam roasting. Red- the parenchyma cells of the mid-region containing a cyto- ness (a*) was correlated with duration of roasting in both plasmic network that surrounds the subcellular organelles methods. The relationships between convection roasting (starch grains, protein bodies, etc.). Spaces left vacant by time and L* or WI were not statistically significant while the lipid bodies after the lipids had been removed during alco- change in yellowness (b*) value with time was significant hol dehydration are also visible. Under higher magnification during superheated steam roasting. All the colour values (L*, (Figure 1b) there appear to be small lipid bodies (0.1 − 2.0 a*, b*, WI) were more affected during superheated steam µm), aluerone (protein bodies) and starch grains (3.0 − 10.0 roasting than during convection roasting. Colour develop- µm) that are mainly intact and unmodified. They appear as ment is an extremely important and major feature of the distinct spherical bodies that are difficult to distinguish from extent of Maillard reaction, browning reaction of the foods one another in the SEM image. (Nursten, 1986). Colour formation was also founded to be The heating process resulted in cellular organelles be- due to sugar caramelisation in milk and milk-based product coming swollen. Pockmark artefacts were visible when mois- (Walstra and Jenness, 1984). ture escaped and oil was released from lipid bodies during Textural properties of hardness and fracturability were heating. Traces of oil from the lipid granules in the form of generally affected by both methods of roasting. Hardness de- light coloured constituents were also seen. Parenchyma cells creased over roasting time, although the relationship was not gradually lost subcellular organization as the cytoplasmic statistically significant (p > 0.05). Increasing roasting time network became increasingly disrupted with increase in heat- decreased fracturability, this relationship being again non- ing time. significant (p > 0.05). Hardness and fracturability of peanuts Superheated steam roasting at 250℃ caused similar were more affected during superheated steam roasting, as thermal modification in peanut cotyledon microstructure compared with convection roasting. compared with convection roasting at the same temperature. The microstructure of peanuts was affected by both su- Cellular organelles appeared more intact after convection perheated steam roasting and convection roasting. An exami- Colour, Texture and Microstructure Changes of Peanut (Arachis hypogaea) 523 nation of electron micrographs revealed that more oil was Delgado-Andrade, C. and Morales, F.J. (2005). Unravelling the extracted during superheated steam roasting than during con- contribution of melanoidins to the antioxidant activity of coffee vection roasting. This suggests that superheated steam treat- brews. J. Agr. Food Chem., 53, 1403-1407. ment may be suitable for the preparation of food products Emily, L.B., Terri, D.B. and Lester, A.W. (2009). Effect of cultivar with relatively lower fat content. Organelle and cytoplasmic and roasting methods on composition of roasting methods on network disruptions were also seen after both methods of composition of roasted soybeans. J. Food Sci. Agr., 89, 821-826. roasting, with such disruption more pronounced in super- Epstein, J., Morris, C.F. and Huber, K.C. (2002). 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