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Varietal influence on the microwave popping characteristics of sorghum
Journal of Cereal Science 65 (2015) 19e24
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Varietal influence on the microwave popping characteristics of sorghum Gayatri Mishra a, *, Dinesh C. Joshi b, Debabandya Mohapatra c, V. Bhushan Babu d a
Department of Agricultural & Food Engineering, IIT, Kharagpur, West Bengal, India College of Food Processing & Technology and Bio-Energy, AAU, Anand, Gujarat, India c Agricultural Produce Processing Division, Central Institute of Agricultural Engineering, Bhopal, Madhya Pradesh, India d Agricultural Mechanization Division, ICAR-Central Institute of Agricultural Engineering, Bhopal, Madhya Pradesh, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 December 2014 Received in revised form 1 June 2015 Accepted 8 June 2015 Available online 18 June 2015
Popped sorghums are popular snack foods and are usually prepared using dry heat. In this investigation, the effect of varieties (Nandel, local Red, Mugad, GJ 42), which were having distinct physico-chemical properties and pericarp thickness on popping properties were studied. Each of the four sorghum varieties was pretreated by moisture conditioning up to 16.5 ± 0.5% wb, 0.5% salt and 10% oil, and popped by domestic microwave oven set at 900 W for 140 s. Popping and sensory properties of popped sorghum grains were measured. The physico-chemical properties such as, diameter, sphericity, thousand grain weight, particle density, bulk density, hardness, amylose content, fat, protein, carbohydrate, ash were found to be significantly different among the four varieties. Amylose content and pericarp thickness were highly positively correlated to popping yield and volume expansion ratio. The Mugad grain variety showing the highest popping qualities, found to have small grain size (3.04 mm), lower sphericity (0.99), high bulk density (833.4 kg/m3), medium grain hardness (10.55 kg), higher pericarp thickness (56.46 mm) along with higher amylose content than the other four varieties. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Pericarp thickness Popping yield Volume expansion ratio Pearson's correlation
1. Introduction Pop sorghum, like popcorn, is a popular snack product in India. Popped sorghum is produced dry heat application, wherein grains are exposed to high temperature for short time (HTST). Popping of grain occurs when super heated vapor produced inside the grain by instantaneous heating cooks the grain and expands the starchy endosperm (Hoseney et al., 1983; Cretors, 2001). Popped sorghum being a pre-cooked ready-to-eat material can be used in snack foods, specialty foods and as a base for development of supplementary foods. There are different methods of popping viz., conventional method of dry heat, sand and salt roasting, hot air popping, gun puffing through sudden pressure differential, popping by hot oil and microwave heating (Yenagi et al., 2005a; Joshi et al., 2014a). Currently, more emphasis is given for environment friendly hygienically produced products; microwave heating fits the bill being more efficient than the traditional heating process with features of faster heating, energy efficiency, hygienic and convenience in use
* Corresponding author. E-mail address: [email protected] (G. Mishra). http://dx.doi.org/10.1016/j.jcs.2015.06.001 0733-5210/© 2015 Elsevier Ltd. All rights reserved.
(Maisont and Narkrugsa, 2009; Joshi et al., 2014b).The microwavable popcorn has become popular with the increasing availability of microwave ovens at domestic level. The mechanism of popping in case of conventional popping and microwave popping is same except for the heating mechanism. The heating mechanism is conduction in case of the traditional sand roasting method, whereas, in case of microwave popping, heating occurs due to the molecular friction of the electric dipoles and charged molecules under an oscillating field of specific frequency (Ernoult et al., 2002). In both the cases, starch expansion occurs due to the vapour pressure development inside the grain. Therefore, the grain quality traits that impact conventional popping would also impact the microwave popping process. Popped sorghum grains have good flavor and are as nutritious as popcorn (Subramanian, 1956). Moreover, popped sorghum is also considered to be superior to popped corn as it is tender and has fewer hulls. Like microwavable popcorn, sorghum can be tempered to optimum moisture content and coated with different ingredients as pretreatments in order to increase the number of popped kernels and improve the sensory quality. Though a wide range of cereals and pseudo cereals such as rice or paddy, corn, wheat, ragi, barley, amarnath seed, buck wheat, kamut,
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foxtail millet, barnyard millet, proso millet, kodo millet are used for popping; only few of them pop well (Mishra et al., 2014). The reason for this may be the factors which influence the popping qualities of cereals, such as season, variety, grain characters i.e. bran content, bran thickness, moisture content, type of endosperm, chemical composition physical characters of grains and also the method of popping (Devadas et al., 1966; Hosamani and Goud, 1978; Thorat et al., 1988; Yenagi et al., 2005b; Joshi et al., 2014a). Researchers have reported that physical properties of grain such as size, shape, 1000 grain weight, density and grain hardness (Srinivas and Desikachar, 1973; Murty et al., 1983; Pordesimo et al., 1990) along with composition of the grains such as, amylose content, protein content, and moisture content, affects the popping quality of grains significantly (Murty et al., 1983; Malleshi and Desikarhar, 1985; Thorat et al., 1988; Gokmen, 2004; Yenagi et al., 2005b; Gundboudi, 2006; Khedker et al., 2008; Joshi et al., 2014a). Pericarp thickness also affects the popping qualities of grain (Helm and Zuber, 1972; Hoseney et al., 1983; Mohamed et al., 1993) by acting as a barrier to the vapor pressure developed inside the grain during heating; so that when sufficient pressure is developed inside the kernel, it releases suddenly resulting in expansion of the starchy endosperm. Many of researchers have studied the factors affecting the popping qualities of grain during conventional popping; however, studies on popping of grains like sorghum in microwave environment are limited. Thus, the objective of this study was to evaluate the effect of variety characterized by the kernel physic-chemical properties on microwave popping performance of sorghum. 2. Materials and methods 2.1. Raw materials A bulk sample of 10 kg each of different varieties of sorghum grain, namely GJ-42, local Red, Nandel and Mugad were procured from Main Sorghum Research Station, Surat; Agricultural Produce Market Committee, Surat; Agricultural Produce Market Committee, Ahmedabad and Main Market of Mugad village, Karnataka, respectively.
(Mohapatra and Bal, 2012). All the dimensions were measured in kilogram/cubic meter. Average of the three replications was used for the calculation. True/particle density was determined by the following expression (Mohsenin, 1980):
. W a TD kg m3 ¼ Vdf
(3)
Where, Wa ¼ weight of sample in air (in kilogram); and Vdf ¼ volume of displaced fluid (in cubic meter). The bulk density (BD) expressed in kilogram/cubic meter was determined by filling an empty 10 cm 10 cm 10 cm cubic acrylic container and tare weight with the grains by pouring from a constant height, striking off the top level, and weighing the grain mass (Varnamkhasti et al., 2008; Joshi et al., 2014a).
. W a BD kg m3 ¼ V
(4)
Where, Wa ¼ weight of sample in air (in kilogram); and V ¼ Volume measured after pouring the grain in to container (in cubic meter). The average of three replicates was used for the calculation and expressed in kilogram/cubic meter. One thousand grain weights of the sorghum grain samples of all the four varieties were determined in triplicate by weighing 1000 grains (Simonyan et al., 2007) in an electronic balance (Mettler, Switzerland; precision ¼ 0.001 g). Hardness of sorghum grains was determined using Texture Analyzer (TA-HDI texture analyzer, Stable Micro Systems, UK) following the method by Mohapatra and Bal (2012). The test was repeated for 20 times from the same sample lot, for all the four varieties. The maximum force indicated by the forceetime curve was taken as the maximum compressive force or hardness. 2.3. Determination of chemical properties
2.2. Determination of physical properties Three principal diameters viz. length (L1), width (L2) and thickness (L3) of sorghum of each variety were measured manually by vernier calliper (Mitutoyo, Japan) having least count of 0.05 mm. Measurement was made on 20 randomly drawn grains from the test samples of each variety. The equivalent diameter (De) of each variety of grain samples were calculated using the following formulas (Asoegwu et al., 2006):
De ðmmÞ ¼
ðF1 þ F2 þ F3 Þ 3
(1)
Where, F1 ¼ ðL1 þL32 þL3 Þ F2 ¼ ðL1 L2 L3 Þ1=3 1=2
F3 ¼ ðL1 L2 þL2 L33 þL1 L3 Þ
Sphericity (S) is a criterion for describing the shape of the grain. The sphericity was determined using the relationship given by Mohsenin (1980) using the following expression
S¼
ðL1 L2 L3 Þ1=3 L1
(2)
The true density (TD) of the sorghum samples of the four varieties was determined by toluene displacement method
Proximate analysis for moisture, ash, protein, crude fiber, fat, total carbohydrate was carried out by standard methods of AOAC (2000) prior to grain conditioning.Gravimetric method was adopted to determine the moisture content of different grain samples. A sample of 5 g was kept in a hot air oven (Nova Instruments, Ahmadabad) at 105 C for 24 h. The mean of three replications was calculated and used for analysis. Nitrogen (N2, %) of sorghum samples was estimated using auto Kjeldal equipment (Kel plus, Pelican system, India) and protein content of the samples were found out by multiplying factor 6.25 with N2 content. The crude fat content was estimated using Soxhlet apparatus (Socsplus, Pelican equipment, Chennai) by solvent extraction method. The crude fibre content was estimated using automatic fiber estimation system (Fibra Plus: FES 4, Pelican Equipments, India) using acid alkali digestion method. For ash content determination, sorghum flours in the crucibles were ashed at 525 C for 6 h in a muffle furnace and total ash content was found out by the standard formula (AOAC, 2000). Carbohydrate content of the grains was calculated by subtracting the moisture, protein, fat, fibre and ash content (expressed in percentage) of same proximate data of sorghum grain from 100 (AOAC, 2000). The amylose content of sample grains was estimated as per the simplified calorimetric procedure of Juliano (1989) and Joshi et al. (2014a). Whole sorghum grain samples were ground with mortar and pestle to pass through a 100 mesh sieve. Hundred miligram of the test sample was weighed accurately, in duplicate in to 50 ml
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Erlenmeyer flasks and 1ml of 95% ethanol and 9 ml of 1 N NaOH was added to it. The sample was heated for 10 min in a boiling water bath to gelatinize the starch, cooled, and transferred, with several water washing, in to a 100 ml volumetric flask, the volume was made up to 100 ml with water and was mixed well. Five ml of starch solution was pipetted in to a 100 ml volumetric flask; 1ml of 1 N acetic acid and 2 ml of iodine solution (0.2 g iodine and 2.0 g potassium iodide in 100 ml of aqueous solution) was added to it. The solution was made up to the volume with distilled water, shaken and allowed to stand for 20 min. Absorbance of the solution at 620 nm was measured with a U.V. spectrophotometer (VARIAN, Carry 50, USA). Amylose content of the sorghum samples was determined with reference to standard curve and expressed on a dry weight basis.For the standard curve, 40 mg of potato amylose (Sigma-ALDRICH, USA) was wetted with 1 ml ethyl alcohol and 9 ml 1 N NaOH, heated for five minutes in a boiling water bath, cooled and made up to 100 ml. 1,2,3,4 and 5 ml portions of the solution were placed in 100 ml volumetric flasks, acidified with 1 N acetic acid (0.2, 0.4, 0.6, 0.8 and 1.0 ml), respectively and treated further as described above. Absorbance values at 620 nm were plotted against the concentration of anhydrous amylose to determine the conversion factor. For calculation of amylose content following formula was used:
Amylose content ¼
R a 20 A r
(5)
Where, R ¼ reading of absorbance of sorghum sample A ¼ reading of amylose standard (obtained from graph for potato) a ¼ weight of potato amylose in mg. r ¼ weight of sorghum sample in mg
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uniformly coated over the conditioned grain by rubbing on the surface thoroughly. The pre-treated grains were then packed (50 g) in microwavable paper bags and sealed. 2.6. Microwave popping of pre-treated sorghum grain Microwave popping experiments were performed in a convective-microwave oven (Samsung, CF1111TL, Korea, 900 W, 2450 MHz). Sorghum grain packed in microwavable paper bags were placed on the rotary glass turn table disk in the microwave oven chamber and popped under different power densities and residence times, for all four sorghum grain varieties. After each experiment, the oven chamber was cooled off for 3 min. The popping conditions i.e. pretreatment conditions, microwave power and residence time were standardized based on the quality of popped sorghum obtained, such as higher popping yield, popping volume and sensory score as per the experimental design. 2.7. Determination of popping properties After popping, the unpopped grains were handpicked and the weight of total popped grains was recorded. The grains were considered fully popped when they did not have any unpopped part and the volume expansion ratio were 4 or more (Joshi et al., 2014b). Grains were considered semi-popped, when some part of the kernel was popped and some part still remained unpopped, and the volume expansion ratio varied between 2 and <4 (Joshi et al., 2014b). Popping yield was determined by measuring the weight of initial grain and popped grains after processing and was expressed in percentage (Joshi et al., 2014a, b) Popping yield was determined by measuring the weight of initial grain and popped grains after processing and was expressed in percentage (Sharma et al., 2014).
2.4. Determination of pericarp thickness Pericarp thickness of each variety of sorghum grains was measured by taking photograph in a scanning electron microscope (Nova, Nano SEM 450, FEI Ltd., USA). Grains of sorghum were fractured lengthwise with a razor blade (Hoseney et al., 1974). Pieces of the samples were mounted on to the aluminum stubs using silver conducting paint and scanned in the scanning electron microscope at an accelerating voltage of 5 Kv. The pericarp thickness was found out by dividing the magnification factor to the measured length in the images. The lengths were measured randomly at different sections of the cut piece image and averaged out for each variety of sorghum grain by energy dispersive X-ray analysis (EDAX) mapping system provided with TEAMTM software. 2.5. Pretreatment of sorghum grain Moisture conditioning was performed to bring the moisture content of sorghum grains to 16 ± 0.5% (wb) level by spraying the grains with pre-calculated amount of water and spreading the grain on a muslin cloth in a humidity chamber maintained at 24 C dry bulb and 22 C wet bulb temperature for 24 h (Mitchum, 2002). The grain was periodically weighed until the final weight of the grain was high enough to have reached the desired moisture level. Sorghum grain samples conditioned to desired moisture content were sprayed with 0.5% solutions of salt and stirred well for uniform coating, and then subjected to shed drying in the laboratory. The weight was taken after 30 min and through every 10 min till the salt pre-treated sorghum again attained the desired moisture level. Oil in proportions of 10 ml in 100 g of grain (10%) was
Popping yield ð%Þ ¼
Wfpg þ Wspg
W
(6)
Where, Wfpg ¼ Weight of fully popped grains Wspg ¼ Weight of semi-popped grains Wupg ¼ Weight of unpopped grains W ¼ Total weight of grains after popping ¼ Wfpg þ Wspg þ Wupg The volume expansion was measured by sand replacement method (Chinnaswamy and Bhattacharya, 1983; Joshi et al., 2014a,b). Initial volume of 50 g unpopped pre-treated sorghum grain was taken in a 500-ml cylinder and filled it with fine sand and was gently tapped, so that the fine sand can fill all the empty spaces between the grains. The volume was noted after deducting the sand volume from the total volume. Similarly, volume after popping was determined. The volume expansion ratio was expressed as:
Volume expansion ratio ¼
Vf Vi
(7)
Where, Vf ¼ Final volume of popped sorghum grain Vi ¼ Initial volume of un-popped sorghum grain
2.8. Organoleptic quality of popped sorghum An informal sensory evaluation of popped sorghum was conducted by a sensory panel consisted of 12 people familiar with the
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Table 1 Some selected physical properties of four sorghum grain varieties. Variety
De (mm)
GJ-42 Red Nandel Mugad
3.42a 3.62b 3.32c 3.04d
± ± ± ±
S
0.03 0.03 0.02 0.04
1000 GW (g)
1.13a 1.20b 1.06c 0.99d
± ± ± ±
0.01 0.01 0.01 0.00
26.6a 30.1b 24.8c 19.4d
± ± ± ±
0.2 0.1 0.1 0.25
BD (kg/m3) 806.2a 785b 800.3c 833.4d
± ± ± ±
1.1 2 2.3 2
TD (kg/m3) 1304a 1250b 1300c 1364d
± ± ± ±
1 1 0 2
Hardness (kg)
Pericarp thickness (mm)
10.8a ± 0.1 9.74b ± 0.17 10.2c ± 0.2 10.5 ab ± 0.1
29.7a 20.7b 38.7c 56.4d
± ± ± ±
2.6 1.7 2.7 1.0
All the properties are measured at 10 ± 0.5% (wb) moisture content. Mean ± Standard deviation, same letter in a column represents non-significance and different letter represents significant difference (P ¼ 0.05).
Table 2 Some selected chemical properties of four sorghum grain varieties. Variety
Mi (%wb)
GJ-42 Red Nandel Mugad
11.5a 12.1b 9.31c 9.57d
± ± ± ±
0.1 0.0 0.06 0.08
Fat (%) 2.73b 3.22a 2.35c 3.25a
± ± ± ±
Protein (%) 0.21 0.04 0.04 0.07
11.7b 11.5b 12.0a 11.6b
± ± ± ±
Carbohydrate (%)
0.0a 0.3 0.1 0.1
71.0b 71.0b 73.8a 73.1a
± ± ± ±
Crude fiber (%)
Ash (%)
0.4 0.6 0.0 0.0
1.57a 1.15b 1.07b 0.96c
± ± ± ±
0.06 0.02 0.05 0.02
1.47a 1.07b 1.39b 1.42a
± ± ± ±
Amylose (%)
0.02 0.04 0.05 0.13
20.3a 21.1b 22.5c 23.4d
± ± ± ±
0.2 0.3 0.3 0.4
Mean ± Standard Deviation (S.D); same letter in a column represents non-significance and different letter represents significant difference (P ¼ 0.05).
Table 3 Popping and sensory property of 4 pop sorghum grain varieties. Variety
Popping yield (%)
GJ-42 Red Nandel Mugad
54.2a 63.9b 74.5c 81.2d
± ± ± ±
0.7 1.7 0.7 0.6
Volume expansion ratio 9.09a 9.89b 13.3c 14.5d
± ± ± ±
0.05 0.14 0.3 0.3
Colour
Taste
7.19bc ± 0.65 6.56c ± 1.17 7.50 ab ± 0.92 8.12a ± 0.64
7.62a 7.31a 7.25a 7.93a
Flavor ± ± ± ±
0.79 1.19 0.65 0.17
7.00b 7.31b 7.00b 8.25a
Overall acceptability ± ± ± ±
0.75 1.30 0.75 0.46
7.50b ± 0.46 6.81b ± 1.27 7.56 ab ± 0.72 8.45a ± 0.48
All the properties are measured at pretreatment combination of 16.5% (wb) moisture content, 0.5% salt and 10% oil. Mean ± Standard deviation, same letter in a column represents non-significance and different letter represents significant difference (P ¼ 0.05).
quality of grains, mainly graduate students and faculty of Food Processing and Technology, who are well versed with sensory evaluation techniques. The age of sensory panelists varied from 23 to 58 years. The experimental samples were coded using random three digit numbers and served randomly to avoid biasness due to order of presentation to the panel members in sensory laboratory, College of food processing Technology and Bio-energy, Anand Agricultural University, Anand, India. They were asked to evaluate the popped sorghum for appearance, taste and overall acceptability based on 9-point hedonic scale. 2.9. Statistical analysis The data were analyzed using one way ANOVA in Microsoft Excel 2007. Pearson's correlation coefficients (Mohapatra and Bal, 2010; Joshi et al., 2014a) and one-sample t-test were performed to find out the significance of various characteristics on the popping quality of sorghum. 3. Results and discussions 3.1. Physical, chemical and popping characteristics of sorghum Selected physical properties of four different varieties of sorghum grain, measured, were presented in Table 1. The effective diameter (De) and sphericity (S) of grain varieties varied between 3.04 and 3.62 mm and 0.99 and 1.20, respectively. This classification of the varieties is of significance in popping industry, as smaller and medium grain variety would have better yield as compared to larger grains (Murty et al., 1982). The Mugad variety had the smallest size and sphericity, while the local red variety had the largest size. Thousand grain weight of four different sorghum grain varieties varied from 19.40 to 30.17 g; true density (TD) and bulk
density (BD) varied between 1250 and 1364 kg/m3 and 785.0 and 833.4 kg/m3, respectively. This classification of the varieties is of significance in popping industry, as smaller and medium grain variety would have better yield as compared to larger grains (Murty et al., 1982). The Mugad variety had the smallest size and sphericity, while the local red variety having bold grains had the largest size. True density is of importance in this investigation, as highly dense materials have closely packed starch molecules and thus harder and denser (Juliano, 1985), which might ultimately affect the popping quality of the grains. Mugad variety was found to have the highest true and bulk density followed by GJ-42 variety. Hardness (H) of the sorghum grain samples varied between 9.74 and 10.87 kg. GJ-42 variety having the highest hardness values followed by Mugad and Nandel varieties. Grain hardness plays important role affecting the popping characteristics of sorghum grain; harder grains don't pop well, while medium hard grains pop well (Joshi et al., 2014a). Proximate composition and amylose content of four sorghum grain varieties are delineated in Table 2. Initial moisture content of different sorghum varieties varied from 9.31 to 12.17% (wb). Proximate constituents such as fat, protein, ash, crude fiber, carbohydrate varied from 2.35 to 3.25%, 11.56e12.05%, 0.96e1.57%, 1.07e1.47% and 71e73.84%, respectively for the varieties tested. Observations from Table 2 showed significant (P < 0.05) variation among all the constituents of all the four varieties of sorghum. The total amylose content of different sorghum cultivars varied from 20.35 to 23.48% and showed significant (P < 0.05) difference. The amylose content of grain plays important role in deciding the popping characteristics. Since high amylose content grains are harder to shear, there is a greater chance that higher pressure will build up during the thermal treatment (Joshi et al., 2014b). This would result in sudden expansion of endosperm making it a highly popped product compared to their low amylose content varieties.
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Fig. 1. Scanning electron microscopy of pericarps of four sorghum varieties: (a) Nandel (b) local Red (c) Mugad (d) GJ 42. (Specifications: Dwell-300nos: 1 m2 per pixel; HVaccelerating voltage:5 kV; magnification: 2000; spot size- 4.5; Scale: 1 cm ¼ 5 mm).
In this investigation, Mugad (23.48%) variety showed highest amylose content, followed by Nandel (22.5%). Popping and sensory properties such as popping yield (PY), volume expansion ratio (VER) and overall acceptability (OAA) of four sorghum varieties were evaluated (Table 3). Significant (P < 0.05) and wide variation was found in popping yield and volume expansion ratio among different sorghum cultivars ranging from 54.25 to 81.21% and 9.90 to 14.50, respectively. The higher popping yield and volume expansion ratio was found for Mugad (14.50) variety followed by Nandel (13.34). Overall acceptability of pop sorghum samples popped in domestic microwave oven varied significantly (P < 0.05) with respect to the variety. All the samples were accepted by the panel and the score varied from 7.50 to 8.45, which indicated sensory panelists liked all the pop sorghum samples prepared from pretreated four sorghum varieties. Pop sorghum prepared from Mugad cultivar was the most liked followed by the pop sorghum samples prepared from Nandel cultivar. Pearson's correlation coefficient that describes the linear relationship among various physical, chemical and popping properties of sorghum tested at 5% and 1% level of significance (LOS). Sphericity (S) was found to be significantly positively correlated (P < 0.01) with effective diameter (De) and strongly related to 1000 grain weight (P ¼ 0.01). Effective diameter of grain, sphericity and 1000 grain weight were found to be significantly negatively correlated with bulk density and true density (P < 0.01) and also with hardness (P < 0.05). Effective diameter of grain, sphericity and 1000 grain weight were found to be significantly negatively correlated with popping yield and volume expansion ratio (P < 0.01). Bulk density and true density were significantly positively correlated with the thermal expansion properties (P < 0.01).
A non-significant positively correlation existed between hardness of the grain and volume expansion ratio, while no conclusive relationship could be established between hardness and popping yield. It could be observed that popping yield and volume expansion ratio were significantly positively correlated with the carbohydrate content (P < 0.01), while they were non-significantly positively correlated with protein and crude fiber content. Initial moisture content and ash content was found to be significantly negatively correlated with the thermal expansion properties. Interestingly, amylose content was found to be positively correlated with both popping yield and volume expansion ratio (P < 0.01). A non-significant correlation existed between fat application and popping characteristics of sorghum grain varieties. 3.2. Pericarp thickness and popping characteristics From the scanning electron microscopic study of sorghum grains (Fig 1), the pericarp thickness of each of the four sorghum varieties was found to be 38.70, 20.70, 56.46 and 29.74 mm for Nandel, local Red, Mugad and GJ-42, respectively. From the popping characteristics study of all the grain varieties, Mugad was found to be having highest popping quality (Table 3), which had the thickest pericarp. Pericarp thickness was significantly positively correlated with both popping yield and volume expansion ratio. Thinner was the pericarp of grains, poorer was the popping qualities, while grains having thicker pericarp pop well. Presumably, a thick pericarp kernel would require a longer time and more energy to pop than one with a thin pericarp. Once popped it would give a higher volume because thick pericarp resulted in higher pressure development inside the kernel. Similar results were demonstrated by
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Hoseney et al. (1983) and Mohamed et al. (1993) while popping corn. One-way ANOVA test was conducted to find the significant difference among varieties with respect to popping yield and volume expansion ratio and was found to be significantly different (P < 0.05). One sample t-test was conducted to find the significant difference in the popping yield and volume expansion ratios of other varieties (Table 3) with respect to Mugad variety which had the highest popping yield and volume expansion ratio. In that case, the observed mean of volume expansion ratio of Mugad variety was compared with the mean values of the popping yield and volume expansion ratio of other varieties. It was found that all the popping characteristics of other varieties of sorghum grain were significantly different from Mugad variety (P < 0.05). From this investigation, considering the analysis and observed results for physical, chemical, and popping properties like popping yield and popped expansion ratios, Mugad variety may be chosen for production of popped sorghum by the popping industry, which have small grain size (3.04 mm), lower sphericity (0.99), high bulk density (833.4 kg/ m3), medium grain hardness (10.55 kg), higher pericarp thickness (56.46 mm) along with higher amylose content (23.48%) than the other four varieties. 4. Conclusion From the investigation of effect of physico-chemical properties and pericarp thickness on popping characteristics of four different grain varieties, it was found that effective diameter of grain, sphericity and 1000 grain weight were negatively correlated with popping yield and volume expansion ratio, whereas there was significant positive correlation between bulk density, true density and hardness and the popping properties. A positive correlation was found between popping properties and carbohydrate content, as well as amylose content. Initial moisture content and ash content was found to be negatively correlated with the popping qualities. Scanning electron microscopic study of sorghum grains showed that pericarp thickness plays an important role affecting popping characteristics of sorghum grain; thinner the pericarp of grains, poorer was popping, while grains having thicker pericarp pop well. Out of the four grain cultivars, Mugad variety found to have the highest popping and sensory qualities in terms of popping yield, volume expansion ratio and overall acceptability of the pop sorghum. The Mugad grain variety having the best popping qualities, found to have small grain size, lower sphericity, high bulk density, medium grain hardness; higher pericarp thickness along with higher amylose content. It may be chosen for production of popped sorghum by the popping industry. References Asoegwu, S.N., Ohanyere, S.O., Kanu, O.P., Iwueke, C.N., August 2006. Physical properties of African oil bean seed (Pentaclethra macrophylla). Agric. Eng. Int. CIGR Ejournal VIII. Manuscript FP 05 006. Association of Official Analytical Chemists (AOAC International), 2000. Official Methods of Analysis, 17th ed. Gaithersburg, MD, USA. Chinnaswamy, R., Bhattacharya, K.R., 1983. Studies on expanded rice: physicochemical basis of varietal differences. J. Food Sci. 48, 1600e1603. Cretors, C., 2001. Popcorn products. In: Lusas, E.W., Rooney, L.W. (Eds.), Snack Foods
Processing. Technomic Publishing, Lancaster, USA, pp. 385e420. Devadas, P.R., Jalaja, R., Radha Rukmani, A., 1966. Comparison of the nutritive value of sorghum. Indian J. Nutr. Dietetics 3, 47e49. Ernoult, V., Moraru, C.I., Kokini, J.L., 2002. Influence of fat on the expansion of glassy amylopectin extrudates by microwave heating. Cereal Chem. 79, 265e273. Gokmen, S., 2004. Effect of moisture content and popping method on popping characteristics of popcorn. J. Food Eng. 65, 357e362. Gundboudi, Z.A., 2006. Nutritional and Processing Qualities of Pop Sorghum Cultivars and Value Addition. Master of Home Science (foods and nutrition) Thesis (unpublished) submitted to University of Agricultural Sciences, Dharwad. Helm, J.L., Zuber, M.S., 1972. Inheritance of nericarn thickness in dent corn. Crop Sci. 12, 428. Hoseney, R.C., Davis, A.B., Herbers, L.H., 1974. Pericarp and endosperm structure of sorghum grain shown by scanning electron microscopy. Cereal Chem. 51,552e51,558. Hoseney, R.C., Zeleznak, K.A., Abdelrahman, N., 1983. Mechanism of popcorn popping. J. Cereal Sci. 1 (1), 1e11. Hosmani, S.A., Shankare Goud, B.T., 1978. ‘Roti’ (Bread) Making Qualities of Elite Sorghum Cultivars. A paper presented in All India Sorghum Workshop, held at Dharwad, 17e19, April, 1978. Joshi, N.D., Mohapatra, D., Joshi, D.C., 2014a. Varietal selection of some indica rice for production of puffed rice. Food Bioprocess Technol. 7 (1), 299e305. Joshi, N.D., Mohapatra, D., Joshi, D.C., Sutar, R.F., 2014b. Puffing characteristics of parboiled milled rice in a domestic convective-microwave oven and process optimization. Food Bioprocess Technol. 7 (6), 1678e1688. Juliano, B.O., 1989. Rice Chemistry and Technology. The American Association of Cereal Chemists, Inc, St. Paul, Minnesota, USA. Khedker, M.B., Bakane, P.H., Kambale, A.K., 2008. Effect of grain moisture on popping quality of sorghum. Int. J. Agric. Eng. 1 (2), 71e73. Maisont, S., Narkrugsa, W., 2009. Effects of some physicochemical properties of paddy varieties on puffing qualities by microwave. Kasetsart J. Nat. Sci. 43 (3), 566e575. Malleshi, N.G., Desikarhar, H.S.R., 1985. Milling, popping and malting characteristics of some minor millets. J. Food Sci. Technol. 22, 400e403. Mishra, G., Joshi, D.C., Panda, B.K., 2014. Popping and puffing of cereal grains: a review. J. Grain Process. Storage 1 (2), 34e46. Mitchum, J.A., 2002. Development of Sorghum Food Products: Popped Sorghum and Instant Sorghum Meals. M.Tech. thesis (unpublished) submitted to Oklahoma State University. Mohamed, A.A., Ashiman, R.B., Kirleis, A.W., 1993. Pericarp thickness and other kernel physical characteristics relate to microwave popping quality of popcorn. J. Food Sci. 58 (2), 342e346. Mohapatra, D., Bal, S., 2010. Optimization of polishing conditions for long grain basmati rice in a laboratory abrasive mill. Food Bioprocess Technol. 3 (3), 466e472. Mohapatra, D., Bal, S., 2012. Physical properties of indica rice in relation to some novel mechanical properties indicating grain characteristics. Food Bioprocess Technol. 5 (6), 2111e2119. Mohsenin, N.N., 1980. Physical Properties of Plant and Animal Materials. Gordon and Breach Science, New York, USA. Murty, D.S., Patil, H.D., Prasad Rao, K.E., House, L.R., 1982. A note on screening the India sorghum collection for popping quality. J. Food Sci. Nutr. 19, 79e80. Murty, D.S., Nicodemus, K.D., Patil, H.D., Mukur, S.Z., House, L.R., 1983. Studies on popping quality in sorghum. Sorghum Newsl. 20, 97e99. Pordesimo, L.O., Anantheswaran, R.C., Fleischmann, A.M., Lin, Y.E., Hanna, M.A., 1990. Physical properties as indicators of popping characteristics of microwave pop corn. J. Food Sci. 55 (5), 1352e1355. Simonyan, K.J., EL-Okene, A.M., Yiljep, Y.D., August, 2007. Some physical properties of Samaru sorghum 17 grains. Agric. Eng. Int. IX. Manuscript FP 07-008. Srinivas, T., Desikachar, H.S.R., 1973. Factors affecting the puffing quality of paddy. J. Sci. Food Agric. 24, 883e891. Subramanian, A., 1956. Jowar pops are a real delicacy. Indian Farm 6, 25e26. Thorat, S.S., Satwadhar, P.N., Kulkarni, D.N., Choudhari, S.D., Ingle, U.M., 1988. Effect of various grain parameters on popping quality of sorghum. J. Food Sci. Technol. 25 (8), 361e363. Varnamkhasti, M.G., Mobli, H., Jafari, A., Keyhani, A.R., Soltanabdi, M.H., Rafiee, S., Kheiralipur, K., 2008. Some physical properties of rough rice (Oryza Sativa L.) grain. J. Cereal Sci. 47, 496e501. Yenagi, N.B., Chittapur, B.M., Kachapur, M.D., Nadagoud, V.V., 2005a. Suitable process for popping of sorghum cultivars for value addition. Maharashtra J. Agric. Ext. Educ. 16, 366e368. Yenagi, N.B., Kachapur, M.D., Bhuvaneshwari, G., 2005b. Popping quality and proximate composition of pop sorghum cultivars. Research highlights. JADU 15, 119e124.
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Varietal influence on the microwave popping characteristics of sorghum Gayatri Mishra a, *, Dinesh C. Joshi b, Debabandya Mohapatra c, V. Bhushan Babu d a
Department of Agricultural & Food Engineering, IIT, Kharagpur, West Bengal, India College of Food Processing & Technology and Bio-Energy, AAU, Anand, Gujarat, India c Agricultural Produce Processing Division, Central Institute of Agricultural Engineering, Bhopal, Madhya Pradesh, India d Agricultural Mechanization Division, ICAR-Central Institute of Agricultural Engineering, Bhopal, Madhya Pradesh, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 December 2014 Received in revised form 1 June 2015 Accepted 8 June 2015 Available online 18 June 2015
Popped sorghums are popular snack foods and are usually prepared using dry heat. In this investigation, the effect of varieties (Nandel, local Red, Mugad, GJ 42), which were having distinct physico-chemical properties and pericarp thickness on popping properties were studied. Each of the four sorghum varieties was pretreated by moisture conditioning up to 16.5 ± 0.5% wb, 0.5% salt and 10% oil, and popped by domestic microwave oven set at 900 W for 140 s. Popping and sensory properties of popped sorghum grains were measured. The physico-chemical properties such as, diameter, sphericity, thousand grain weight, particle density, bulk density, hardness, amylose content, fat, protein, carbohydrate, ash were found to be significantly different among the four varieties. Amylose content and pericarp thickness were highly positively correlated to popping yield and volume expansion ratio. The Mugad grain variety showing the highest popping qualities, found to have small grain size (3.04 mm), lower sphericity (0.99), high bulk density (833.4 kg/m3), medium grain hardness (10.55 kg), higher pericarp thickness (56.46 mm) along with higher amylose content than the other four varieties. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Pericarp thickness Popping yield Volume expansion ratio Pearson's correlation
1. Introduction Pop sorghum, like popcorn, is a popular snack product in India. Popped sorghum is produced dry heat application, wherein grains are exposed to high temperature for short time (HTST). Popping of grain occurs when super heated vapor produced inside the grain by instantaneous heating cooks the grain and expands the starchy endosperm (Hoseney et al., 1983; Cretors, 2001). Popped sorghum being a pre-cooked ready-to-eat material can be used in snack foods, specialty foods and as a base for development of supplementary foods. There are different methods of popping viz., conventional method of dry heat, sand and salt roasting, hot air popping, gun puffing through sudden pressure differential, popping by hot oil and microwave heating (Yenagi et al., 2005a; Joshi et al., 2014a). Currently, more emphasis is given for environment friendly hygienically produced products; microwave heating fits the bill being more efficient than the traditional heating process with features of faster heating, energy efficiency, hygienic and convenience in use
* Corresponding author. E-mail address: [email protected] (G. Mishra). http://dx.doi.org/10.1016/j.jcs.2015.06.001 0733-5210/© 2015 Elsevier Ltd. All rights reserved.
(Maisont and Narkrugsa, 2009; Joshi et al., 2014b).The microwavable popcorn has become popular with the increasing availability of microwave ovens at domestic level. The mechanism of popping in case of conventional popping and microwave popping is same except for the heating mechanism. The heating mechanism is conduction in case of the traditional sand roasting method, whereas, in case of microwave popping, heating occurs due to the molecular friction of the electric dipoles and charged molecules under an oscillating field of specific frequency (Ernoult et al., 2002). In both the cases, starch expansion occurs due to the vapour pressure development inside the grain. Therefore, the grain quality traits that impact conventional popping would also impact the microwave popping process. Popped sorghum grains have good flavor and are as nutritious as popcorn (Subramanian, 1956). Moreover, popped sorghum is also considered to be superior to popped corn as it is tender and has fewer hulls. Like microwavable popcorn, sorghum can be tempered to optimum moisture content and coated with different ingredients as pretreatments in order to increase the number of popped kernels and improve the sensory quality. Though a wide range of cereals and pseudo cereals such as rice or paddy, corn, wheat, ragi, barley, amarnath seed, buck wheat, kamut,
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foxtail millet, barnyard millet, proso millet, kodo millet are used for popping; only few of them pop well (Mishra et al., 2014). The reason for this may be the factors which influence the popping qualities of cereals, such as season, variety, grain characters i.e. bran content, bran thickness, moisture content, type of endosperm, chemical composition physical characters of grains and also the method of popping (Devadas et al., 1966; Hosamani and Goud, 1978; Thorat et al., 1988; Yenagi et al., 2005b; Joshi et al., 2014a). Researchers have reported that physical properties of grain such as size, shape, 1000 grain weight, density and grain hardness (Srinivas and Desikachar, 1973; Murty et al., 1983; Pordesimo et al., 1990) along with composition of the grains such as, amylose content, protein content, and moisture content, affects the popping quality of grains significantly (Murty et al., 1983; Malleshi and Desikarhar, 1985; Thorat et al., 1988; Gokmen, 2004; Yenagi et al., 2005b; Gundboudi, 2006; Khedker et al., 2008; Joshi et al., 2014a). Pericarp thickness also affects the popping qualities of grain (Helm and Zuber, 1972; Hoseney et al., 1983; Mohamed et al., 1993) by acting as a barrier to the vapor pressure developed inside the grain during heating; so that when sufficient pressure is developed inside the kernel, it releases suddenly resulting in expansion of the starchy endosperm. Many of researchers have studied the factors affecting the popping qualities of grain during conventional popping; however, studies on popping of grains like sorghum in microwave environment are limited. Thus, the objective of this study was to evaluate the effect of variety characterized by the kernel physic-chemical properties on microwave popping performance of sorghum. 2. Materials and methods 2.1. Raw materials A bulk sample of 10 kg each of different varieties of sorghum grain, namely GJ-42, local Red, Nandel and Mugad were procured from Main Sorghum Research Station, Surat; Agricultural Produce Market Committee, Surat; Agricultural Produce Market Committee, Ahmedabad and Main Market of Mugad village, Karnataka, respectively.
(Mohapatra and Bal, 2012). All the dimensions were measured in kilogram/cubic meter. Average of the three replications was used for the calculation. True/particle density was determined by the following expression (Mohsenin, 1980):
. W a TD kg m3 ¼ Vdf
(3)
Where, Wa ¼ weight of sample in air (in kilogram); and Vdf ¼ volume of displaced fluid (in cubic meter). The bulk density (BD) expressed in kilogram/cubic meter was determined by filling an empty 10 cm 10 cm 10 cm cubic acrylic container and tare weight with the grains by pouring from a constant height, striking off the top level, and weighing the grain mass (Varnamkhasti et al., 2008; Joshi et al., 2014a).
. W a BD kg m3 ¼ V
(4)
Where, Wa ¼ weight of sample in air (in kilogram); and V ¼ Volume measured after pouring the grain in to container (in cubic meter). The average of three replicates was used for the calculation and expressed in kilogram/cubic meter. One thousand grain weights of the sorghum grain samples of all the four varieties were determined in triplicate by weighing 1000 grains (Simonyan et al., 2007) in an electronic balance (Mettler, Switzerland; precision ¼ 0.001 g). Hardness of sorghum grains was determined using Texture Analyzer (TA-HDI texture analyzer, Stable Micro Systems, UK) following the method by Mohapatra and Bal (2012). The test was repeated for 20 times from the same sample lot, for all the four varieties. The maximum force indicated by the forceetime curve was taken as the maximum compressive force or hardness. 2.3. Determination of chemical properties
2.2. Determination of physical properties Three principal diameters viz. length (L1), width (L2) and thickness (L3) of sorghum of each variety were measured manually by vernier calliper (Mitutoyo, Japan) having least count of 0.05 mm. Measurement was made on 20 randomly drawn grains from the test samples of each variety. The equivalent diameter (De) of each variety of grain samples were calculated using the following formulas (Asoegwu et al., 2006):
De ðmmÞ ¼
ðF1 þ F2 þ F3 Þ 3
(1)
Where, F1 ¼ ðL1 þL32 þL3 Þ F2 ¼ ðL1 L2 L3 Þ1=3 1=2
F3 ¼ ðL1 L2 þL2 L33 þL1 L3 Þ
Sphericity (S) is a criterion for describing the shape of the grain. The sphericity was determined using the relationship given by Mohsenin (1980) using the following expression
S¼
ðL1 L2 L3 Þ1=3 L1
(2)
The true density (TD) of the sorghum samples of the four varieties was determined by toluene displacement method
Proximate analysis for moisture, ash, protein, crude fiber, fat, total carbohydrate was carried out by standard methods of AOAC (2000) prior to grain conditioning.Gravimetric method was adopted to determine the moisture content of different grain samples. A sample of 5 g was kept in a hot air oven (Nova Instruments, Ahmadabad) at 105 C for 24 h. The mean of three replications was calculated and used for analysis. Nitrogen (N2, %) of sorghum samples was estimated using auto Kjeldal equipment (Kel plus, Pelican system, India) and protein content of the samples were found out by multiplying factor 6.25 with N2 content. The crude fat content was estimated using Soxhlet apparatus (Socsplus, Pelican equipment, Chennai) by solvent extraction method. The crude fibre content was estimated using automatic fiber estimation system (Fibra Plus: FES 4, Pelican Equipments, India) using acid alkali digestion method. For ash content determination, sorghum flours in the crucibles were ashed at 525 C for 6 h in a muffle furnace and total ash content was found out by the standard formula (AOAC, 2000). Carbohydrate content of the grains was calculated by subtracting the moisture, protein, fat, fibre and ash content (expressed in percentage) of same proximate data of sorghum grain from 100 (AOAC, 2000). The amylose content of sample grains was estimated as per the simplified calorimetric procedure of Juliano (1989) and Joshi et al. (2014a). Whole sorghum grain samples were ground with mortar and pestle to pass through a 100 mesh sieve. Hundred miligram of the test sample was weighed accurately, in duplicate in to 50 ml
G. Mishra et al. / Journal of Cereal Science 65 (2015) 19e24
Erlenmeyer flasks and 1ml of 95% ethanol and 9 ml of 1 N NaOH was added to it. The sample was heated for 10 min in a boiling water bath to gelatinize the starch, cooled, and transferred, with several water washing, in to a 100 ml volumetric flask, the volume was made up to 100 ml with water and was mixed well. Five ml of starch solution was pipetted in to a 100 ml volumetric flask; 1ml of 1 N acetic acid and 2 ml of iodine solution (0.2 g iodine and 2.0 g potassium iodide in 100 ml of aqueous solution) was added to it. The solution was made up to the volume with distilled water, shaken and allowed to stand for 20 min. Absorbance of the solution at 620 nm was measured with a U.V. spectrophotometer (VARIAN, Carry 50, USA). Amylose content of the sorghum samples was determined with reference to standard curve and expressed on a dry weight basis.For the standard curve, 40 mg of potato amylose (Sigma-ALDRICH, USA) was wetted with 1 ml ethyl alcohol and 9 ml 1 N NaOH, heated for five minutes in a boiling water bath, cooled and made up to 100 ml. 1,2,3,4 and 5 ml portions of the solution were placed in 100 ml volumetric flasks, acidified with 1 N acetic acid (0.2, 0.4, 0.6, 0.8 and 1.0 ml), respectively and treated further as described above. Absorbance values at 620 nm were plotted against the concentration of anhydrous amylose to determine the conversion factor. For calculation of amylose content following formula was used:
Amylose content ¼
R a 20 A r
(5)
Where, R ¼ reading of absorbance of sorghum sample A ¼ reading of amylose standard (obtained from graph for potato) a ¼ weight of potato amylose in mg. r ¼ weight of sorghum sample in mg
21
uniformly coated over the conditioned grain by rubbing on the surface thoroughly. The pre-treated grains were then packed (50 g) in microwavable paper bags and sealed. 2.6. Microwave popping of pre-treated sorghum grain Microwave popping experiments were performed in a convective-microwave oven (Samsung, CF1111TL, Korea, 900 W, 2450 MHz). Sorghum grain packed in microwavable paper bags were placed on the rotary glass turn table disk in the microwave oven chamber and popped under different power densities and residence times, for all four sorghum grain varieties. After each experiment, the oven chamber was cooled off for 3 min. The popping conditions i.e. pretreatment conditions, microwave power and residence time were standardized based on the quality of popped sorghum obtained, such as higher popping yield, popping volume and sensory score as per the experimental design. 2.7. Determination of popping properties After popping, the unpopped grains were handpicked and the weight of total popped grains was recorded. The grains were considered fully popped when they did not have any unpopped part and the volume expansion ratio were 4 or more (Joshi et al., 2014b). Grains were considered semi-popped, when some part of the kernel was popped and some part still remained unpopped, and the volume expansion ratio varied between 2 and <4 (Joshi et al., 2014b). Popping yield was determined by measuring the weight of initial grain and popped grains after processing and was expressed in percentage (Joshi et al., 2014a, b) Popping yield was determined by measuring the weight of initial grain and popped grains after processing and was expressed in percentage (Sharma et al., 2014).
2.4. Determination of pericarp thickness Pericarp thickness of each variety of sorghum grains was measured by taking photograph in a scanning electron microscope (Nova, Nano SEM 450, FEI Ltd., USA). Grains of sorghum were fractured lengthwise with a razor blade (Hoseney et al., 1974). Pieces of the samples were mounted on to the aluminum stubs using silver conducting paint and scanned in the scanning electron microscope at an accelerating voltage of 5 Kv. The pericarp thickness was found out by dividing the magnification factor to the measured length in the images. The lengths were measured randomly at different sections of the cut piece image and averaged out for each variety of sorghum grain by energy dispersive X-ray analysis (EDAX) mapping system provided with TEAMTM software. 2.5. Pretreatment of sorghum grain Moisture conditioning was performed to bring the moisture content of sorghum grains to 16 ± 0.5% (wb) level by spraying the grains with pre-calculated amount of water and spreading the grain on a muslin cloth in a humidity chamber maintained at 24 C dry bulb and 22 C wet bulb temperature for 24 h (Mitchum, 2002). The grain was periodically weighed until the final weight of the grain was high enough to have reached the desired moisture level. Sorghum grain samples conditioned to desired moisture content were sprayed with 0.5% solutions of salt and stirred well for uniform coating, and then subjected to shed drying in the laboratory. The weight was taken after 30 min and through every 10 min till the salt pre-treated sorghum again attained the desired moisture level. Oil in proportions of 10 ml in 100 g of grain (10%) was
Popping yield ð%Þ ¼
Wfpg þ Wspg
W
(6)
Where, Wfpg ¼ Weight of fully popped grains Wspg ¼ Weight of semi-popped grains Wupg ¼ Weight of unpopped grains W ¼ Total weight of grains after popping ¼ Wfpg þ Wspg þ Wupg The volume expansion was measured by sand replacement method (Chinnaswamy and Bhattacharya, 1983; Joshi et al., 2014a,b). Initial volume of 50 g unpopped pre-treated sorghum grain was taken in a 500-ml cylinder and filled it with fine sand and was gently tapped, so that the fine sand can fill all the empty spaces between the grains. The volume was noted after deducting the sand volume from the total volume. Similarly, volume after popping was determined. The volume expansion ratio was expressed as:
Volume expansion ratio ¼
Vf Vi
(7)
Where, Vf ¼ Final volume of popped sorghum grain Vi ¼ Initial volume of un-popped sorghum grain
2.8. Organoleptic quality of popped sorghum An informal sensory evaluation of popped sorghum was conducted by a sensory panel consisted of 12 people familiar with the
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Table 1 Some selected physical properties of four sorghum grain varieties. Variety
De (mm)
GJ-42 Red Nandel Mugad
3.42a 3.62b 3.32c 3.04d
± ± ± ±
S
0.03 0.03 0.02 0.04
1000 GW (g)
1.13a 1.20b 1.06c 0.99d
± ± ± ±
0.01 0.01 0.01 0.00
26.6a 30.1b 24.8c 19.4d
± ± ± ±
0.2 0.1 0.1 0.25
BD (kg/m3) 806.2a 785b 800.3c 833.4d
± ± ± ±
1.1 2 2.3 2
TD (kg/m3) 1304a 1250b 1300c 1364d
± ± ± ±
1 1 0 2
Hardness (kg)
Pericarp thickness (mm)
10.8a ± 0.1 9.74b ± 0.17 10.2c ± 0.2 10.5 ab ± 0.1
29.7a 20.7b 38.7c 56.4d
± ± ± ±
2.6 1.7 2.7 1.0
All the properties are measured at 10 ± 0.5% (wb) moisture content. Mean ± Standard deviation, same letter in a column represents non-significance and different letter represents significant difference (P ¼ 0.05).
Table 2 Some selected chemical properties of four sorghum grain varieties. Variety
Mi (%wb)
GJ-42 Red Nandel Mugad
11.5a 12.1b 9.31c 9.57d
± ± ± ±
0.1 0.0 0.06 0.08
Fat (%) 2.73b 3.22a 2.35c 3.25a
± ± ± ±
Protein (%) 0.21 0.04 0.04 0.07
11.7b 11.5b 12.0a 11.6b
± ± ± ±
Carbohydrate (%)
0.0a 0.3 0.1 0.1
71.0b 71.0b 73.8a 73.1a
± ± ± ±
Crude fiber (%)
Ash (%)
0.4 0.6 0.0 0.0
1.57a 1.15b 1.07b 0.96c
± ± ± ±
0.06 0.02 0.05 0.02
1.47a 1.07b 1.39b 1.42a
± ± ± ±
Amylose (%)
0.02 0.04 0.05 0.13
20.3a 21.1b 22.5c 23.4d
± ± ± ±
0.2 0.3 0.3 0.4
Mean ± Standard Deviation (S.D); same letter in a column represents non-significance and different letter represents significant difference (P ¼ 0.05).
Table 3 Popping and sensory property of 4 pop sorghum grain varieties. Variety
Popping yield (%)
GJ-42 Red Nandel Mugad
54.2a 63.9b 74.5c 81.2d
± ± ± ±
0.7 1.7 0.7 0.6
Volume expansion ratio 9.09a 9.89b 13.3c 14.5d
± ± ± ±
0.05 0.14 0.3 0.3
Colour
Taste
7.19bc ± 0.65 6.56c ± 1.17 7.50 ab ± 0.92 8.12a ± 0.64
7.62a 7.31a 7.25a 7.93a
Flavor ± ± ± ±
0.79 1.19 0.65 0.17
7.00b 7.31b 7.00b 8.25a
Overall acceptability ± ± ± ±
0.75 1.30 0.75 0.46
7.50b ± 0.46 6.81b ± 1.27 7.56 ab ± 0.72 8.45a ± 0.48
All the properties are measured at pretreatment combination of 16.5% (wb) moisture content, 0.5% salt and 10% oil. Mean ± Standard deviation, same letter in a column represents non-significance and different letter represents significant difference (P ¼ 0.05).
quality of grains, mainly graduate students and faculty of Food Processing and Technology, who are well versed with sensory evaluation techniques. The age of sensory panelists varied from 23 to 58 years. The experimental samples were coded using random three digit numbers and served randomly to avoid biasness due to order of presentation to the panel members in sensory laboratory, College of food processing Technology and Bio-energy, Anand Agricultural University, Anand, India. They were asked to evaluate the popped sorghum for appearance, taste and overall acceptability based on 9-point hedonic scale. 2.9. Statistical analysis The data were analyzed using one way ANOVA in Microsoft Excel 2007. Pearson's correlation coefficients (Mohapatra and Bal, 2010; Joshi et al., 2014a) and one-sample t-test were performed to find out the significance of various characteristics on the popping quality of sorghum. 3. Results and discussions 3.1. Physical, chemical and popping characteristics of sorghum Selected physical properties of four different varieties of sorghum grain, measured, were presented in Table 1. The effective diameter (De) and sphericity (S) of grain varieties varied between 3.04 and 3.62 mm and 0.99 and 1.20, respectively. This classification of the varieties is of significance in popping industry, as smaller and medium grain variety would have better yield as compared to larger grains (Murty et al., 1982). The Mugad variety had the smallest size and sphericity, while the local red variety had the largest size. Thousand grain weight of four different sorghum grain varieties varied from 19.40 to 30.17 g; true density (TD) and bulk
density (BD) varied between 1250 and 1364 kg/m3 and 785.0 and 833.4 kg/m3, respectively. This classification of the varieties is of significance in popping industry, as smaller and medium grain variety would have better yield as compared to larger grains (Murty et al., 1982). The Mugad variety had the smallest size and sphericity, while the local red variety having bold grains had the largest size. True density is of importance in this investigation, as highly dense materials have closely packed starch molecules and thus harder and denser (Juliano, 1985), which might ultimately affect the popping quality of the grains. Mugad variety was found to have the highest true and bulk density followed by GJ-42 variety. Hardness (H) of the sorghum grain samples varied between 9.74 and 10.87 kg. GJ-42 variety having the highest hardness values followed by Mugad and Nandel varieties. Grain hardness plays important role affecting the popping characteristics of sorghum grain; harder grains don't pop well, while medium hard grains pop well (Joshi et al., 2014a). Proximate composition and amylose content of four sorghum grain varieties are delineated in Table 2. Initial moisture content of different sorghum varieties varied from 9.31 to 12.17% (wb). Proximate constituents such as fat, protein, ash, crude fiber, carbohydrate varied from 2.35 to 3.25%, 11.56e12.05%, 0.96e1.57%, 1.07e1.47% and 71e73.84%, respectively for the varieties tested. Observations from Table 2 showed significant (P < 0.05) variation among all the constituents of all the four varieties of sorghum. The total amylose content of different sorghum cultivars varied from 20.35 to 23.48% and showed significant (P < 0.05) difference. The amylose content of grain plays important role in deciding the popping characteristics. Since high amylose content grains are harder to shear, there is a greater chance that higher pressure will build up during the thermal treatment (Joshi et al., 2014b). This would result in sudden expansion of endosperm making it a highly popped product compared to their low amylose content varieties.
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Fig. 1. Scanning electron microscopy of pericarps of four sorghum varieties: (a) Nandel (b) local Red (c) Mugad (d) GJ 42. (Specifications: Dwell-300nos: 1 m2 per pixel; HVaccelerating voltage:5 kV; magnification: 2000; spot size- 4.5; Scale: 1 cm ¼ 5 mm).
In this investigation, Mugad (23.48%) variety showed highest amylose content, followed by Nandel (22.5%). Popping and sensory properties such as popping yield (PY), volume expansion ratio (VER) and overall acceptability (OAA) of four sorghum varieties were evaluated (Table 3). Significant (P < 0.05) and wide variation was found in popping yield and volume expansion ratio among different sorghum cultivars ranging from 54.25 to 81.21% and 9.90 to 14.50, respectively. The higher popping yield and volume expansion ratio was found for Mugad (14.50) variety followed by Nandel (13.34). Overall acceptability of pop sorghum samples popped in domestic microwave oven varied significantly (P < 0.05) with respect to the variety. All the samples were accepted by the panel and the score varied from 7.50 to 8.45, which indicated sensory panelists liked all the pop sorghum samples prepared from pretreated four sorghum varieties. Pop sorghum prepared from Mugad cultivar was the most liked followed by the pop sorghum samples prepared from Nandel cultivar. Pearson's correlation coefficient that describes the linear relationship among various physical, chemical and popping properties of sorghum tested at 5% and 1% level of significance (LOS). Sphericity (S) was found to be significantly positively correlated (P < 0.01) with effective diameter (De) and strongly related to 1000 grain weight (P ¼ 0.01). Effective diameter of grain, sphericity and 1000 grain weight were found to be significantly negatively correlated with bulk density and true density (P < 0.01) and also with hardness (P < 0.05). Effective diameter of grain, sphericity and 1000 grain weight were found to be significantly negatively correlated with popping yield and volume expansion ratio (P < 0.01). Bulk density and true density were significantly positively correlated with the thermal expansion properties (P < 0.01).
A non-significant positively correlation existed between hardness of the grain and volume expansion ratio, while no conclusive relationship could be established between hardness and popping yield. It could be observed that popping yield and volume expansion ratio were significantly positively correlated with the carbohydrate content (P < 0.01), while they were non-significantly positively correlated with protein and crude fiber content. Initial moisture content and ash content was found to be significantly negatively correlated with the thermal expansion properties. Interestingly, amylose content was found to be positively correlated with both popping yield and volume expansion ratio (P < 0.01). A non-significant correlation existed between fat application and popping characteristics of sorghum grain varieties. 3.2. Pericarp thickness and popping characteristics From the scanning electron microscopic study of sorghum grains (Fig 1), the pericarp thickness of each of the four sorghum varieties was found to be 38.70, 20.70, 56.46 and 29.74 mm for Nandel, local Red, Mugad and GJ-42, respectively. From the popping characteristics study of all the grain varieties, Mugad was found to be having highest popping quality (Table 3), which had the thickest pericarp. Pericarp thickness was significantly positively correlated with both popping yield and volume expansion ratio. Thinner was the pericarp of grains, poorer was the popping qualities, while grains having thicker pericarp pop well. Presumably, a thick pericarp kernel would require a longer time and more energy to pop than one with a thin pericarp. Once popped it would give a higher volume because thick pericarp resulted in higher pressure development inside the kernel. Similar results were demonstrated by
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Hoseney et al. (1983) and Mohamed et al. (1993) while popping corn. One-way ANOVA test was conducted to find the significant difference among varieties with respect to popping yield and volume expansion ratio and was found to be significantly different (P < 0.05). One sample t-test was conducted to find the significant difference in the popping yield and volume expansion ratios of other varieties (Table 3) with respect to Mugad variety which had the highest popping yield and volume expansion ratio. In that case, the observed mean of volume expansion ratio of Mugad variety was compared with the mean values of the popping yield and volume expansion ratio of other varieties. It was found that all the popping characteristics of other varieties of sorghum grain were significantly different from Mugad variety (P < 0.05). From this investigation, considering the analysis and observed results for physical, chemical, and popping properties like popping yield and popped expansion ratios, Mugad variety may be chosen for production of popped sorghum by the popping industry, which have small grain size (3.04 mm), lower sphericity (0.99), high bulk density (833.4 kg/ m3), medium grain hardness (10.55 kg), higher pericarp thickness (56.46 mm) along with higher amylose content (23.48%) than the other four varieties. 4. Conclusion From the investigation of effect of physico-chemical properties and pericarp thickness on popping characteristics of four different grain varieties, it was found that effective diameter of grain, sphericity and 1000 grain weight were negatively correlated with popping yield and volume expansion ratio, whereas there was significant positive correlation between bulk density, true density and hardness and the popping properties. A positive correlation was found between popping properties and carbohydrate content, as well as amylose content. Initial moisture content and ash content was found to be negatively correlated with the popping qualities. Scanning electron microscopic study of sorghum grains showed that pericarp thickness plays an important role affecting popping characteristics of sorghum grain; thinner the pericarp of grains, poorer was popping, while grains having thicker pericarp pop well. Out of the four grain cultivars, Mugad variety found to have the highest popping and sensory qualities in terms of popping yield, volume expansion ratio and overall acceptability of the pop sorghum. The Mugad grain variety having the best popping qualities, found to have small grain size, lower sphericity, high bulk density, medium grain hardness; higher pericarp thickness along with higher amylose content. It may be chosen for production of popped sorghum by the popping industry. References Asoegwu, S.N., Ohanyere, S.O., Kanu, O.P., Iwueke, C.N., August 2006. Physical properties of African oil bean seed (Pentaclethra macrophylla). Agric. Eng. Int. CIGR Ejournal VIII. Manuscript FP 05 006. Association of Official Analytical Chemists (AOAC International), 2000. Official Methods of Analysis, 17th ed. Gaithersburg, MD, USA. Chinnaswamy, R., Bhattacharya, K.R., 1983. Studies on expanded rice: physicochemical basis of varietal differences. J. Food Sci. 48, 1600e1603. Cretors, C., 2001. Popcorn products. In: Lusas, E.W., Rooney, L.W. (Eds.), Snack Foods
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