Effects of moisture content and popping method on popping characteristics of popcorn

Journal of Food Engineering 65 (04) 7 362 www.elsevier.com/locate/jfoodeng Effects of moisture content and popping method on popping characteristics of popcorn Sabri G okmen * Department of Crop Science, Agriculture Faculty, University of Gaziosmanpasa, Tasliciftlik Campus, Tokat 602, Turkey Received 9 July 03; accepted 29 January 04 Abstract In this study, the effects of moisture content and popping methods on popping quality of five popcorn genotypes were investigated. Each of the five genotypes was equilibrated to seven moisture levels ranging from 8% to % and was popped by microwave oven, hot-air popper, cooking pan with and without oil and salt. Genotype, moisture content, and popping method affected the expansion volume, flake size, and percentage of unpopped kernels. The highest popping quality, i.e. popping volume, flake size and the lowest percentage of unpopped kernels, were obtained from grains with a moisture content of 14%. The microwave oven gave the greatest flake size and percentage of unpopped kernels whereas the lowest expansion volume and flake size were obtained from the cooking pan popping with oil and salt. Genotype moisture content, genotype popping method, and moisture content popping method interactions were also significant for investigated traits. As water content deviates from the optimum, the expansion volume and flake size declined significantly. In addition, hybrids with medium kernel size had higher values for the expansion volume and flake size. These values decreased dramatically in popping with oil and salt. Ó 04 Elsevier Ltd. All rights reserved. Keywords: Popcorn; Expansion volume; Popping method 1. Introduction Popcorn is used primarily for human consumption as a favorite nutritious snack food and is becoming more and more popular over time. Like other corn types, yield is very important, but popping expansion has to be good as well. High expansion volume is associated with increased kernel palatability and tenderness (Dofing, Thomas-Compton, & Buck, 1990). Popping volume depends on many factors such as moisture content, genotype, physical properties of kernel, popping method, popping temperature, harvesting and handling practices. However, among all factors affecting expansion volume, moisture content is the most critical factor, because it affects the rate and extent of pressure build up in starch granules (Hoseney, Zeleznak, & Abdelrahman, 1983). So far, much research has been carried out in the area of moisture content and its effects on expansion volume. Studies have shown that maximum popping volume is produced at moisture ranging * Fax: +90-6-21488. E-mail address: sgokmen@gop.edu.tr (S. G okmen). from 11.0% to 15.5% (Allred-Coyle, Toma, Reiboldt, & Thaku, 00; Metzger, Hsu, Ziegler, & Bern, 1989; Pajic, 1990; Shimoni, Dirks, & Labuza, 02; Song & Eckhoff, 1994). The optimum moisture content for maximum popping volume depends on the popcorn variety (Haught, Lien, Hanes, & Ashman, 1976; Lin & Anantheswaran, 1988), popping procedures (Metzger et al., 1989), and kernel size (Song & Eckhoff, 1994). If moisture content is below or above an optimum value, expansion volume will be unsatisfactory. Metzger et al. (1989) reported that the optimum moisture content of hot-air popping for maximum expansion of popcorn was higher than that of oil popping and that hot-air popping produced a higher volume than oil popping. Physical kernel properties of popcorn such as size, shape and density also affect expansion volume. Large kernels generally give lower popping volume than small kernels, because they contain a high percentage of soft endosperm (Pajic & Babic, 1991). Dofing et al. (1990) cited that hybrid varieties produced higher expansion volumes than an open-pollinated variety. Therefore, most commercial popcorns are three-way or single cross hybrids. 0260-8774/$ - see front matter Ó 04 Elsevier Ltd. All rights reserved. doi:.16/j.jfoodeng.04.01.034

8 S. G okmen / Journal of Food Engineering 65 (04) 7 362 There are currently different methods for popping popcorn, but conventional methods are commonly used in Turkey. In general, popcorn is popped better in conventional methods than microwave oven (Dofing et al., 1990). On the other hand, the number of household that has a microwave oven has increased in recent years and microwave popcorn has become very popular among consumers. However, there are some problems such as low expansion and a large number of unpopped kernels associated with microwave popping. Although many studies have evaluated the poppability in hot-air and oil poppers, not much published data is available on the popping performance in a microwave oven. Therefore, the objective of this study was to investigate the effects of moisture content levels, different genotypes and different popping methods on expansion volume and its components of popcorn. 2. Materials and methods 2.1. Sample preparation Four yellow pearl hybrid popcorn genotypes (P 608, 50, Exp 370, Exp 232) and an open-pollinated variety were used in this study. P 608 and 50 are commercial varieties. The Exp 370 and Exp 232 are experimental ones. Currently, the open-pollinated variety is grown in many regions of Turkey while Exp 370 and Exp 3 genotypes are grown widely in the Cß ukurova region. Kernel size classes in popcorn were defined as follows: 52 67, large; 68 75, medium and 76 5, small (Ziegler, Ashman, White, & Wysong, 1984). The number of kernels per g of P 608, 50, Exp 370, Exp 232, and open-pollinated at 14% moisture content were 72, 69, 74, 89 and 63, respectively. Each of the five varieties were tested at seven moisture content (8%, %, 12%, 14%, 16%, 18% and %). Initial moisture content of varieties was 14.2 16.5%. The samples, first distributed on the ground, were dried by natural convection at room temperature to a moisture content of about 8%, %, 12% and 14%. The samples were monitored until the desired moisture levels were obtained. In order to obtain samples with 16%, 18% and % moisture content, the samples with 14% moisture content were placed in 2 l jars and were rewetted with distilled water by using a spray gun. Later, the jars were sealed and stored at 5 C for days before popping. The jars were inverted and shaken daily to assure equal moisture distribution throughout the sample. Before popping, the final moisture content of each sample was measured by using a dry air heated oven (Elektro-mag, M 60) at 3 C for 72 h on a 50 g kernel sample. 2.2. Popping methods Fifty and 75 g samples were used for microwave and conventional methods (hot-air popper, cooking pan with and without oil and salt), respectively. Subsequently, the number of kernels for each sample was counted. Before recording the data, one sample was popped to standardize the operation of the machines for each popping test and to reduce variation among replications. The cooking pan was heated by using LPG (liquid petroleum gas) burner. Gas control knob of the burner was set at the same level to keep surface temperature of the cooking pan constant during popping. Popping procedure with the cooking pan with oil and salt consisted of the following steps: corn oil ( ml), salt (7.5 g) and the samples were combined into the cooking pan and lid was closed. The heating was continued until the popping was completed. Microwave popping was performed with a -W Arcßelik microwave oven (2 V, W). Since the capacity of microwave oven was suitable only for g, 50 g sample was split into two parts and then popped. To avoid spread of popped kernels, the sample was placed in a 00 ml thin-walled glass beaker and covered with an oven bag. Five or six holes were punched through the oven bag to release of steam. The covered beaker was put in the center of the microwave oven and heated for 2.5 3min at high power. The hot-air popping was performed with an Arcßelik corn popper (2 V, W; model, ARK 77 MP). The expansion volume, percentage of unpopped kernels (PUK), and flake size were determined by using methods described by Dofing et al. (1990). The experimental design was a completely randomized 3 3factorial with three replications. The data were analysed with analysis of variance (ANOVA) procedures using the Statistical Software Package. The comparison of the treatment means was made using the least significant difference (LSD) test. 3. Results and discussion Significant effect of moisture on expansion volume, flake size, and PUK was observed. As shown in Table 1, mean values for expansion volume varied from 21.8 to 42.1 cm 3 g 1 of dry matter, flake size varied from 2.76 to 4.81 cm 3, and PUK varied from 2.39 to 9.57 at different moisture levels. The expansion volume and flake size increased as the moisture content increased until the 14% moisture level, then these parameters decreased with further increase of moisture. These results agreed with the previous studies of Pajic (1990) and Metzger et al. (1989), who reported that the optimum moisture content for maximum expansion volume was 14%. The

S. G okmen / Journal of Food Engineering 65 (04) 7 362 9 Table 1 Effects of moisture content on expansion volume, flake size and percentage of unpopped kernels (PUK) Moisture content (%) Expansion volume (cm 3 g 1 ) Flake size (cm 3 ) PUK 8 22.2e 2.76f 3.86c 28.0d 3.47e 2.84c 12 32.2c 3.78d 3.08c 14 42.1a 4.81a 2.39c 16 38.2b 4.63b 4.17c 18 29.8d 4.22c 6.62b 21.8e 3.41e 9.57a Different letters in the same column indicate significant difference (LSD a ¼ 0:01). Each data is an average of five genotypes and four popping methods. maximum popping volume was obtained from moisture content of 14% because it had the highest flake size, but the lowest PUK. The popping volumes at 14% moisture content were approximately 24% and % higher than those of the 12% and 16% moisture content samples, respectively. To describe the relationship between expansion volume and moisture content, a stepwise polynomial regression analysis was conducted. Results revealed that the second degree polynomial regression analysis successfully described this relationship (Fig. 1). Decrease in the popping volume and the flake size beyond a critical moisture content can be explained by the rupture of the pericarp at a temperature when the pressure inside the kernel is too low. As moisture content of kernel increases, the melting temperature of the pericarp decreases, thus when the water content is high, the pressure in the kernel at popping moment is lower, causing less expansion and lower final popped volume (Shimoni et al., 02). The pericarp acts as a vessel cap, hence the high mechanical resistance of the popcorn pericarp allows it to sustain high pressure, favoring high Expansion volume (cm 3 /gr) 45 6 8 12 14 16 Moisture content (%) Fig. 1. Expansion volume versus moisture content. The least squares quadratic equation was y ¼ 56:98 þ 12:45x 0:475x 2, R 2 ¼ 0:88, SD ¼ 3.24 cm 3 g 1. 18 22 popping volume in popcorn varieties (Hoseney et al., 1983). Also, glass transition (T g ) of amorphous polymers in the kernel may influence the pressure build-up inside the kernels. It is known that increased moisture content of polymeric materials such as starch, protein, and nonstarch polysaccharides, reduces their T g (Hoseney, 1994). It is possible that at elevated moisture contents, the amorphous regions of starch granules in the endosperm transform from glassy to rubbery state. Similarly, the non-starch polysaccharides in the pericarp may undergo such a transition and reduce moisture build-up in the kernel. On the other hand, the sharp drop in the popped volume and the flake size at lower moisture content could be due to lack of enough pressure to burst the pericarp. Differences in research findings imply that the optimum moisture content for maximum popping volume depends on popcorn genotype (Allred-Coyle et al., 00; Dofing et al., 1990; Haught et al., 1976), popping methods (Dofing et al., 1990; Metzger et al., 1989), and kernel size (Allred-Coyle et al., 00; Song & Eckhoff, 1994). The popping volume, flake size and PUK of the genotypes were significantly different from one another (Table 2). The popping volumes of genotypes varied from 24.4 to 34.4 cm 3 g 1 of dry matter, and the PUK ranged from 2.95% to 7.78%. The variety 50 had the highest popping volume, flake size and the lowest PUK among genotypes. All hybrids had higher popping volumes than the open-pollinated variety. This may be explained by the fact that the open-pollinated variety had non-uniform but large kernels (Song, Eckhoff, Paulsen, & Litchfield, 1991). The larger flake size and smaller PUK associated with large kernel samples did not necessarily result in increased expansion volume because of a corresponding decrease in the number of kernels per g, such as in the open-pollinated variety. The popping volumes of 50, Exp 370, P 608, and Exp 232 genotypes at 14% moisture content were approximately 29%, %, 23% and 15% higher than that of open-pollinated variety, respectively (data not shown). These results are in agreement with the earlier findings Table 2 Expansion volume, flake size and percentage of unpopped kernels (PUK) of the five popcorn genotypes popped by four popping methods at seven moisture contents Genotype Expansion volume (cm 3 g 1 ) Flake size (cm 3 ) PUK 50 34.4a 4.62a 2.95e P 608 31.7b 3.79c 4.72b Exp 370 33.1a 4.17b 3.67d Exp 232 29.4c 3.d 7.78a Open-pollinated 24.4d 3.57c 4.47c Different letters in the same column indicate significant difference (LSD a ¼ 0:01).

360 S. G okmen / Journal of Food Engineering 65 (04) 7 362 Table 3 Effect of popping methods on expansion volume, flake size and percentage of unpopped kernels (PUK) for five popcorn genotypes at seven moisture contents Popping methods Expansion volume (cm 3 g 1 ) Flake size (cm 3 ) PUK Hot air 32.0a 3.76b 3.58b Cooking pan without 31.7a 3.93b 2.95b oil and salt Cooking pan with oil 28.5c 3.59c 4.02b and salt Microwave oven.2b 4.18a 8.33a Different letters in the same column indicate significant difference (LSD a ¼ 0:01). that hybrids produced higher expansion volumes than open-pollinated varieties (Dofing et al., 1990). The Exp 232 genotype, which had the smallest kernels among hybrids, resulted in significantly lower average expansion volume and flake size but significantly higher PUK than the other hybrid genotypes with medium kernel samples (Table 3). However, previous findings indicated that genotypes with a high number of kernels per g tended to have greater expansion volumes than those with a low number of kernels per g (Babic & Pajic, 1992; Dofing et al., 1990; Ziegler et al., 1984). The Exp 232 variety had significantly lower average flake size (16 %) and expansion volume (7 15%), but significantly higher PUK ( 63%) than that of other hybrid genotypes. This may be due to high number of unpopped kernels and low flake sizes. Song et al. (1991) reported that smallest-sized fraction (4.36 4.76 mm kernels) had the greatest number of unpopped kernels. Similarly, Dofing et al. (1990) reported that large kernel samples had larger flake size than small kernel samples, and high expansion volume was negatively associated with number of unpopped kernels. Allred-Coyle et al. (00) also mentioned that medium-sized kernels produced the greatest expansion volume. Although the P 608, 50, and Exp 370 popcorn hybrids have medium sized kernels, significant differences were observed among them for all the traits studied (Table 3). The reasons for differences in the popping volume, flake size, and the PUK in different genotypes are not fully understood. However, different genotypes may have different ratios of soft and hard endosperm or different starch structures, which directly affect the popping volume (Pajic & Babic, 1991; Pordesimo, Anatheswaran, & Mattern, 1991). In addition, the differences in chemical composition and structure may be partly reflected as differences in equilibrium moisture content (Song et al., 1991). Therefore, genotypes of similar kernel size but different kernel densities due to endosperm composition could differ in their expansion volumes (Dofing et al., 1990). Park, Allen, Stermitz, and Maga (00) noted that physical characteristics of popcorn hybrids varied from hybrid to hybrid. The popping volume, flake size, and PUK were significantly affected by the popping methods. The popping with oil and salt resulted in 8 % lower average expansion volume and 6 14% lower flake size than those of other methods (Table 3). This can probably be explained by the fact that the popping of popcorn in the presence of oil and salt is a complex process because these ingredients have different heat conductivity and loss factors (Singh & Singh, 1999). The microwave popping had the highest PUK value (8.33%) among methods for all genotypes tested. In fact, PUK for microwave oven was determined significantly higher than the conventional popping methods (Table 3). The same observation was obtained by Dofing et al. (1990), who reported that the percent unpopped kernels for conventional popping was only 1.1, compared with.2 for microwave popping. Significantly higher PUK compared to other methods could result from the facts that popped kernels attenuate the intensity of the electromagnetic waves to reach unpopped kernels at the bottom and that in a certain time, temperature goes beyond a critical level above which kernels do not pop. However, the maximum flake size, in contrast, was obtained with microwave popping. This may be because electromagnetic waves quickly and simultaneously transfer the heat to every part of the kernel and internal pressure of the kernel for popping is achieved in a short time. Moisture content popping method interaction was significant for expansion volume, flake size, and PUK, indicating that the effect of popping method for these parameters varied with moisture content. For example, the hot-air method produced a higher popping volume than other methods at 8%, % and 12% moisture, but it had a lower expansion volume than the cooking pan without oil and salt at 14%, 16%, 18% and % moisture contents (Fig. 2). Our results demonstrated the presence of a genotype moisture content interaction for expansion vol- Expansion volume(cm3/g) 50 45 15 5 0 Hot air w/o salt&oil w/ salt/oil Microwave 8 12 14 16 18 Moisture levels (%) Fig. 2. Effects of different popping methods on expansion volume of popcorn kernels at seven moisture levels (average of five genotypes).

S. G okmen / Journal of Food Engineering 65 (04) 7 362 361 ume, flake size, and PUK in popcorn. This shows that the effect of moisture content for these variables is different according to genotype. The 50 hybrid, for example, produced the highest expansion volume at %, 12%, 14%, 18% and % moisture contents among genotypes while the Exp 370 had the highest value at 8% and 16% (Fig. 3). All hybrids, on the other hand, had higher expansion volumes at 16% whereas the openpollinated variety had a higher value at 12%. This may imply that optimum moisture content of the larger kernels is lower than that of the smaller kernels. These results are in agreement with those of Song and Eckhoff (1994), who reported that smaller kernels required slightly higher moisture to obtain maximum expansion volume. Genotype popping method interaction was also significant for three popping characteristics, showing that the effect of method on these variables varied depending on genotype. For example, the 50 hybrid had a higher expansion volume than other genotypes at hot-air, cooking pan with and without oil and salt methods; however, it had a lower expansion volume than Exp 370 in microwave popping (Fig. 4). Similarly, Ziegler and Ashman (1994) reported that performance Expansion volume (cm 3 /g) 50 45 15 5 0 50 P 608 Exp 370 Exp 232 Open-poll. 8 12 14 16 18 Moisture levels (%) Fig. 3. Effects of different genotypes on expansion volume of popcorn kernels at seven moisture levels (average of four popping methods). Expansion volume(cm 3 /g) 15 5 0 Hot air w/o salt&oil w/ salt&oil Microwave Popping methods 50 P608 Exp370 Exp232 Open-poll. Fig. 4. Expansion volume of five popcorn genotypes by four popping methods (average of seven moisture levels). of popcorn genotypes varied according to popping methods. 4. Conclusions Moisture content significantly affects the popping volume, flake size and PUK. The popping volume and flake size increased with an increase in moisture content up to 14%, and then decreased with any additional moisture. The study shows that when the moisture content deviates by ±2% from the optimum moisture level (14%), expansion volume could be reduced as much as 28% depending upon the genotypes. Performances of all genotypes were poor in terms of expansion volume and flake size in the cooking pan with oil and salt than those of the other methods. The hybrids produced higher expansion volume than the openpollinated variety. Also, genotypes with high expansion volumes produce more popped corn than genotypes with low expansion volumes. Microwave oven had the greatest flake size and PUK in all genotypes tested. Because of the genotype popping method interaction, breeders who attempt to develop such varieties should evaluate expansion volume using different methods of popping. References Allred-Coyle, T. A., Toma, R. B., Reiboldt, W., & Thaku, M. (00). Effects of moisture content, hybrid variety, kernel size, and microwave wattage on the expansion volume of microwave popcorn. International Journal of Food Sciences and Nutrition, 51, 389 394. Babic, M., & Pajic, Z. (1992). Effect of genotype environment interaction on expansion volume in popcorn hybrids (Zea mays L.). Genetika, 24, 27 32. Dofing, S. M., Thomas-Compton, M. A., & Buck, J. S. (1990). Genotype popping method interaction for expansion volume in popcorn. Crop Science,, 62 65. Haught, C. G., Lien, R. M., Hanes, R. E., & Ashman, R. B. (1976). Physical properties of popcorn. Transactions of the ASAE, 19, 168. Hoseney, C. R. (1994). Principles of cereal science and technology (2nd ed.). American Association of Cereal Chemists, Inc. (pp. 312 318). Hoseney, C. R., Zeleznak, K., & Abdelrahman, A. (1983). Mechanism of popcorn popping. Journal of Cereal Science, 1, 43 52. Lin, Y. E., & Anantheswaran, R. C. (1988). Studies in popping of popcorn in a microwave oven. Journal of Food Science, 53, 1746 1749. Metzger, D. D., Hsu, K. H., Ziegler, K. E., & Bern, C. J. (1989). Effect of moisture content on popcorn popping volume for oil and hot-air popping. Cereal Chemistry, 66, 247 248. Pajic, Z. (1990). Popcorn and sweet corn breeding. In International advanced course maize breeding, production, processing and marketing in Mediterranean Countries MAIZE 90, September 17 to October 13, 1990, Belgrade, Yugoslavia. Pajic, Z., & Babic, M. (1991). Interrelation of popping volume and some agronomic characteristics in popcorn hybrides. Genetica, 23, 137 144. Park, D., Allen, K. G. D., Stermitz, F. R., & Maga, J. A. (00). Chemical composition and physical characteristics of unpopped

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