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Rice noodle enriched with okara: Cooking property, texture, and in vitro starch digestibility
Author’s Accepted Manuscript Rice noodle enriched with okara: Cooking property, texture, and in vitro starch digestibility Min Je Kang, In Young Bae, Hyeon Gyu Lee
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S2212-4292(17)30563-1 https://doi.org/10.1016/j.fbio.2018.02.008 FBIO277
To appear in: Food Bioscience Received date: 24 August 2017 Revised date: 30 January 2018 Accepted date: 15 February 2018 Cite this article as: Min Je Kang, In Young Bae and Hyeon Gyu Lee, Rice noodle enriched with okara: Cooking property, texture, and in vitro starch digestibility, Food Bioscience, https://doi.org/10.1016/j.fbio.2018.02.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
(To be submitted to Food Bioscience)
Rice noodle enriched with okara: Cooking property, texture, and in vitro starch digestibility 1
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Min Je Kang , In Young Bae *, and Hyeon Gyu Lee
†*
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Department of Food and Nutrition, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea
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Department of Food & Fermentation, Far East University, 76-32 Daehak-gil, Gamgok, Eumseong, Chungbuk 369-700, Republic of Korea
[email protected] [email protected] *Corresponding author: Tel: +82-43-880-3160; Fax: +82-43-879-3730 *Co-corresponding author: Tel: +82-2-2220-1202; Fax: +82-2-2281-8285 Abstract The effects of okara on in vitro starch digestibility and cooking properties of rice noodles and methods for quality improvement for their production as a functional health food were examined. Okara was added to rice noodles in proportions of 0%, 5%, 10%, and 20%. With an increasing level of okara, cooking loss, hardness, and adhesiveness of the rice noodles increased, while water absorption, swelling index, and cohesiveness decreased. The addition of
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okara reduced the in vitro starch digestibility of rice noodles, although the addition of 20% okara increased the starch digestibility. Among all samples, 10% okara showed the lowest predicted glycemic index (pGI) value. Therefore, in order to improve cooking quality of rice noodles containing 10% okara, addition of alginate or treatment with a CaCl2 coating was examined. The addition of alginate alone did not improve the cooking quality, but treatment with a CaCl 2 coating along with alginate improved cooking quality. Moreover, the combination treatment of Ca-alginate did not change the in vitro starch digestibility of rice noodles containing okara. As a result, 10% okara can be used to produce health-beneficial rice noodles with reduced in vitro starch digestibility, and the combination treatment of Ca-alginate can improve their cooking quality. Keywords rice noodle; okara; Ca-alginate; in vitro starch digestibility; cooking quality; texture 1. Introduction Recently, as consumers are becoming more concerned about their health, interest in gluten-free noodles has increased. Since wheat gluten is known to cause celiac disease, rice and buckwheat have been mainly used to produce
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gluten-free noodles (Fu, 2008; Sciarini et al., 2010). Because rice noodles account for a large portion of gluten-free noodles, many studies are being conducted on this type of noodle. There have been several previous studies aimed at improving the physical or functional quality attributes of rice noodles. One area of research is to determine the effects of various flours and starches such as hydrothermal treated rice starch (HMT), mung bean starch, legume flour, and pea and lentil starches on the quality of rice noodles (Bouasla et al., 2017; Hormdok and Noomhorm, 2007; Wang et al., 2014; Wu et al., 2015). Other studies have been conducted to improve the qualities of rice noodles with ingredients such as hydrocolloids (e.g., HPMC, xanthan gum, guar gum, and locust bean gum), transglutaminase, and emulsifiers (Lai, 2002; Lee et al., 2012; Yalcin and Basman, 2008). However, most of the studies do not include ingredients with health functionality, so research on the beneficial effects of rice noodles on health and improvements in the processing performance are needed. Dietary fiber refers to plant carbohydrate polymers that are not digested by human digestive enzymes. Dietary fiber can be divided into water-insoluble fiber (e.g., lignin, cellulose, and hemicellulose; IDF) and water-soluble fiber (e.g.,
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pectin, gum, and mucilage; SDF). Each type of fiber has different physiological effects (Dhingra et al., 2012). IDF is related to the amount of fecal bulk and gastrointestinal transit time and has a positive effect on diarrhea, constipation, and irritable bowel disease (Bosaeus, 2004). SDF is associated with reduced cholesterol level in the blood and decreased glucose absorption by the small intestine (Bosaeus, 2004). Although IDF is more abundant in food than SDF, there are limitations to its use due to its water-insolubility. Okara (gluten-free) is a dietary fiber residue mainly composed of the insoluble fraction remaining from soybeans after extraction of the water soluble fraction used to produce soymilk and tofu. Since large quantities of okara are produced worldwide with the increase in soybean consumption, the amount of okara produced annually poses a disposal problem since most of it is dumped or burned as waste (Li et al., 2012). However, it has been reported that okara has bioactivity in preventing diabetes, hypocholesterolemia, hypolipidemia, and obesity (Hosokawa et al., 2016; Hu et al., 2013; Jiménez-Escrig et al., 2008; Matsumoto et al., 2007). Thus, it could be useful as an ingredient with health functionality and has the potential to be applied in human foods. Previous studies on okara have been conducted to produce a variety of healthy okara-
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containing foods, such as ice cream cones, cakes, noodles, bread, steamed bread, yogurt, and cookies (Bedani et al., 2014; Lu et al., 2013; Mortazavi et al., 2016; Park et al., 2015; Phuenpipob et al., 2016). However, the starch digestion-retarding effect of okara in rice-based foods has not often been examined. The objective of this study was to investigate how the addition of okara in rice noodles changes the physical properties, cooking quality, and in vitro starch digestibility so as to determine the feasibility of applying okara as a healthy diet food for preventing diabetes. Furthermore, to improve the cooking quality of rice noodles, this study evaluated how the addition of alginate and a CaCl 2 coating changes the cooking quality of rice noodles containing okara.
2. Materials and methods
2.1 Materials White rice (Segoami, Oryza sativa L.) harvested in 2015 was obtained from the local market. Okara [Chewoom soy fiber, mean particle size 77.9 μm analyzed using a laser diffractometer (H3174, HELOS, Sympatec, Inc,
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Germany)] was acquired from S&Food (Seoul, Korea) and contained 67.6% total dietary fiber (TDF) and 4.8% total starch. The dietary fiber in okara was mainly 64.0% insoluble fiber and minor 3.6% soluble fiber. Rice and okara were ground with mixer (A11 basic, IKA, Berlin, Germany) and respectively sieved through 60 mesh (rice) and 100 mesh (okara) sieves. The ground flours were packed in plastic bags and stored at room temperature until the experiment. Sodium alginate (A2158), pancreatin from porcine pancreas (P7545), amyloglucosidase (A9913), and porcine bile extract (B8631) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Calcium chloride was obtained from Yakuri Pure Chemicals Co., Ltd. (Kyoto, Japan). The total dietary fiber assay (TDF-100A), total starch assay (K-TSTA-100), and GOPOD assay kit (K-GLUC) were purchased from Megazyme (Bray, Ireland). The total dietary fiber contents were determined according to the gravimetric enzymatic method, as described in a previous study (Asp et al., 1988). The amount of total starch was measured according to AACC method 76-13.01 (AACC, 2000b).
2.2 Noodle preparation . The rice noodles were prepared by a method slightly modified from
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previous studies (Choi et al., 2012). Rice flour and okara flour (0, 5, 10, and 20% w/w, rice flour basis) were separately mixed with distilled water (150% w/w, total flour basis) and stirred for 10 min. Each suspension was combined and then spread on a stainless steel tray (15 × 16 cm) and steamed in a steamer (Stainless Steel Pot, Seshin Queensense Co., Ltd., Gyeonggi-do, Korea) for 1 min. The steamed rice noodle sheet was peeled from the tray and dried in a dry oven at 70℃ for 15 min. The dried rice noodle sheet was covered with cotton and packed in a plastic bag, then rested at 37℃ for 3 h and cut into 5-mm-wide strands. The rice noodle strands were further dried in a dry oven at 40℃ for 3 h. To improve the cooking quality of rice noodles containing 10% okara, which has the most effective in vitro starch digestibility-retarding effect, alginate (0.5% w/w, total flour basis) and 0.2 M CaCl2 solution were used according to a previously described method (Koh et al., 2009). Rice flour, 10% okara flour (w/w, rice flour basis), and 0.5% alginate (w/w, total flour basis) were mixed with distilled water (150% w/w, total flour basis) and stirred for 10 min. Each suspension was combined and then spread on a stainless steel tray (15 × 16 cm) and steamed in a steamer (Stainless Steel Pot, Seshin Queensense Co., Ltd., Gyeonggi-do, Korea) for 1 min. The steamed rice noodle sheet containing
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10% okara was peeled from the tray and dried in a dry oven at 70℃ for 15 min. The dried rice noodle sheet containing 10% okara was covered with cotton and packed in a plastic bag, then rested at 37℃ for 3 h and cut into 5-mm-wide strands. The rice noodle strands containing 10% okara were soaked in 0.2 M CaCl2 solution for 30 min or were not soaked, followed by drying in a dry oven at 40℃ for 3 h. Finally, all rice noodle strands were packed in plastic bags and stored at room temperature until the experiment. In this study, the samples are referred to as follows: CON, control; SF5, soy fiber 5% replacement; SF10, soy fiber 10% replacement; SF20, soy fiber 20% replacement; ASF10, 0.5% alginate added to SF10; Ca-ASF10, ASF10 soaked in a 0.2 M CaCl2 solution.
2.3 Experimental methods 2.3.1 Cooking property. The cooking properties of rice noodles were determined as depicted in AACC-approved method 66-50.01 (AACC, 2000a). Rice noodles (2 g) were added to boiling distilled water (100 mL). The cooked rice noodles were then removed at 30 second time intervals and placed into cold water. When the white
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and hard cores of the rice noodles disappeared, the time was considered to be the optimum cooking time. After boiling for the optimum time (3 min), the cooked rice noodles were added to cold distilled water and then removed to prevent overcooking, allowed to stand for 5 min to drain, and then weighed. To dry the rice noodles, cooked rice noodles were kept at 40℃ overnight. The cooking water and rinse water were put into a beaker and dried to a constant weight in a 105℃ oven. The cooking loss was indicated by the percentage of solid loss during cooking. Rice noodle samples were prepared in triplicate. The cooking quality parameters were calculated as follows:
Cooking loss (%) =
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑑𝑟𝑖𝑒𝑑 𝑟𝑒𝑠𝑖𝑑𝑢𝑒 𝑖𝑛 𝑐𝑜𝑜𝑘𝑖𝑛𝑔 𝑤𝑎𝑡𝑒𝑟 𝑎𝑛𝑑 𝑟𝑖𝑛𝑠𝑒 𝑤𝑎𝑡𝑒𝑟 × 100 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑟𝑎𝑤 𝑛𝑜𝑜𝑑𝑙𝑒𝑠
Water absorption (%) =
Swelling index =
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑐𝑜𝑜𝑘𝑒𝑑 𝑛𝑜𝑜𝑑𝑙𝑒𝑠 − 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑟𝑎𝑤 𝑛𝑜𝑜𝑑𝑙𝑒𝑠 × 100 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑟𝑎𝑤 𝑛𝑜𝑜𝑑𝑙𝑒𝑠
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑐𝑜𝑜𝑘𝑒𝑑 𝑛𝑜𝑜𝑑𝑙𝑒𝑠 − 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑐𝑜𝑜𝑘𝑒𝑑 𝑛𝑜𝑜𝑑𝑙𝑒𝑠 𝑎𝑓𝑡𝑒𝑟 𝑑𝑟𝑦𝑖𝑛𝑔 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑑𝑟𝑖𝑒𝑑 𝑛𝑜𝑜𝑑𝑙𝑒𝑠 𝑎𝑓𝑡𝑒𝑟 𝑑𝑟𝑦𝑖𝑛𝑔
2.3.2 Texture. The textural properties of the rice noodles were measured by texture
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profile analysis (TPA) with a texture analyzer (TAXT2i, Stable Micro Systems, UK). The texture analysis of rice noodles was based on a former study (Bhattacharya et al., 1999) with slight modifications. A single strand of each cooked rice noodle was cut into a 2.5 cm length. An aluminum cylinder probe with a 35 mm diameter compressed the rice noodle at a speed of 1 mm/s for the pretest, test, and post-test with a constant rate of deformation to 75% of the original rice noodle thickness. The probe was retracted and kept stationary for 1 s before conducting the next compression measurement. The three textural parameters recorded from the TPA were hardness, adhesiveness, and cohesiveness.
2.3.3 In vitro starch digestion. In vitro starch digestibility was analyzed according to the method described in a previous study (Minekus et al., 2014) with slight modifications. The cooked rice noodles were dried in a freeze dryer for 3 days, then ground with a mixer (A11 basic, IKA, Berlin, Germany) and sieved through a 100 mesh sieve. Simulated intestinal fluid (SIF) was composed of 0.5 M KCl (6.8 mL), 0.5 M KH2PO4 (0.8 mL), 1 M NaHCO3 (42.5 mL), 2 M NaCl (9.6 mL), and 0.15 M
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MgCl2(H2O)6 (1.1 mL) in distilled water (400 mL). Pancreatin was diluted 1:5 (w/v) with SIF and centrifuged at 1,952 x g for 20 min. The enzyme solution was prepared by dilution in 5 mL pancreatin supernatant, 0.435 g bile extract, and 2 mL SIF. The mixture (5 g) of rice noodle powder and distilled water (10.71%, w/v) was stirred with SIF (26 mL), 0.3 M CaCl2 (0.04 mL), 1 M NaOH (0.15 mL), and distilled water (1.31 mL) at 500 rpm for 30 min. The enzyme solution (7.5 mL) and amyloglucosidase (0.2 mL/g of starch in the sample) were added. The sample was adjusted to pH 6.0 with 1 N HCl and incubated in a 37℃ shaking water bath. Aliquots (0.1 mL) were taken at 0, 30, 60, 90, 120, and 180 min for incubation and mixed with 1.4 mL of 99% ethanol. These solutions were centrifuged at 600 x g for 3 min, and the released glucose content of the supernatant was measured by a spectrophotometer (Biomate 3S, Thermo Scientific, Waltham, MA, USA) at 510 nm using the GOPOD assay kit. The levels of rapidly digestible starch (RDS) and slowly digestible starch (SDS) were measured after intestinal digestion for 30 min and 120 min, respectively, and resistant starch (RS) was measured as the starch remaining after 180 min of incubation.
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2.3.4 Predicted glycemic index (pGI). The starch digestion kinetics and pGI of the rice noodles were analyzed according to the theory established by a former study (Goñi et al., 1997). The kinetics of starch hydrolysis were calculated using the equation: [C = C ∞(1-e-kt)], where C, C∞, and k represent the hydrolysis degree at each time point, the maximum hydrolysis extent, and the kinetic constant, respectively. The hydrolysis index (HI) was calculated by dividing the area under the hydrolysis curve of each sample by the area of fresh bread as the reference sample. The predicted glycemic index (pGI) was obtained using the equation pGI = 39.71 + 0.549HI.
2.4 Statistical analysis. Statistical analyses were conducted using statistical software version 21.0 (SPSS, IBM Inc., Chicago, IL, USA). One-way analyses of variance (ANOVA) were performed (p<0.05) by Duncan’s multiple range test. The cooking property and in vitro starch digestibility measurements were performed in triplicate, and the texture analysis was conducted with at least 10 measurements. All results are expressed as the mean ± standard deviation (SD).
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3. Results and discussion
3.1 Cooking properties of the rice noodles with okara. As shown in Table 1, increasing the level of okara from 0 to 20% led to an increase in cooking loss of the rice noodles (3.88-4.87%). In particular, the cooking loss of SF20 was increased by 25.52% compared to CON. This finding might be related to weakening of the rice noodle structure caused by the addition of okara (Shiau et al., 2012). These results are in agreement with previous reports that found that the incorporation of durum bran and pollard (Aravind et al., 2012; Rakhesh et al., 2015) or rajma bean (Kumar and Prabhasankar, 2015) into noodles increased the cooking loss. Cooking loss is an indicator of the cooking quality of noodles. An increase in cooking loss is not desirable for noodles because it is related to the stickiness of the noodles (Bhattacharya et al., 1999). All rice noodles with okara exhibited significant concentration-dependent decrease (p<0.05) in water absorption (99.37-110.19%) and swelling index (1.13-1.24). Water absorption and the swelling index are correlated with the cooking qualities of starch-based noodles (Lee et al., 2005). The reduced water
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uptake caused by the decrease in water absorption results in noodles with a hard and coarse texture. These results are in agreement with reports showing that insoluble dietary fiber decreased the water absorption of noodle samples because the binding of water competes with starch binding (Aravind et al., 2012).
3.2 Texture of the rice noodles with okara. The textural properties of rice noodles produced with 5 to 20% of okara are presented in Table 2. As the level of okara increased, the hardness and adhesiveness
significantly increased
(p<0.05),
while
the
cohesiveness
significantly decreased (p<0.05). For SF20, the hardness and adhesiveness were respectively increased by 14.45% and 53.19%, and the cohesiveness was decreased by 6.67% compared with the CON. In general, the addition of insoluble dietary fiber to noodles enhanced their hardness values. The high hardness values indicate that hard and brittle noodles were produced. The stickiness of noodles are denoted by the adhesiveness in the texture profile analyses, and higher stickiness is reported to be undesirable (Guo et al., 2003). Cohesiveness affects the cooking quality and
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chewiness of noodles since it is considered as an indicator of the degree of rupture of the noodle structure during mastication (Singh et al., 2002; Tan et al., 2009). In this study, it was confirmed that the increase in the amount of okara added had an adverse effect on the quality of rice noodles. Therefore, it is necessary to select the proper amount of okara to add in consideration of its functional aspects.
3.3 In vitro starch digestibility of rice noodles with okara. The effects of okara on in vitro starch digestion in rice noodles were investigated by measuring the released glucose content during starch digestion. Fig. 1-(a) shows the effect of rice noodles enriched with okara on starch digestibility in comparison with rice noodles without okara. Glucose release was measured as the reducing sugars hydrolyzed from starch by digestive enzymes. Overall, the starch hydrolysis sharply increased after 30 min and then gradually increased at a slow, steady rate for 180 min. As the proportion of okara in rice noodles increased, the amount of released glucose significantly decreased (p<0.05). The amounts of rapidly digestible starch (RDS), slowly digestible starch
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(SDS), and resistant starch (RS) in the rice noodles with okara are shown in Fig. 1-(b). Compared to the CON, SF10 contained the lowest level of RDS and the highest level of RS. However, the amount of SDS did not significantly change (p>0.05) in any of the rice noodles containing okara. The pGI values of the rice noodles containing okara compared to the CON are presented in Fig. 1-(c). Compared with the CON, the pGI value was significantly reduced (p<0.05) in the presence of okara and was lowest in SF10. The pGI of SF10 was 4.73% lower than that of the CON. The increased pGI observed in rice noodles containing 20% okara could be due to the ability of okara to disrupt the integrity of the rice noodle matrix. According to similar former studies, when oat fiber was added above a level of 10%, the starch structure was disrupted (Aravind et al., 2012; Bustos et al., 2011a) so that starch granules become more accessible and hence more susceptible to enzyme degradation (Hormdok and Noomhorm, 2007; Tudorica et al., 2002). Also, other studies have reported that in vitro starch digestion in cooked pasta and spaghetti was respectively affected by oat bran (Bustos et al., 2011b) and durum bran and pollard (Aravind et al., 2012). Based on the overall results, an increase in the amount of okara led to
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excessive increase in hardness and adhesiveness and a decrease in cohesiveness of the rice noodles. However, from a functional point of view, the addition of 10% okara resulted in a significant decrease in pGI. Based on these findings, the level of okara was set at 10% for subsequent experiments.
3.4 Effect of the addition of alginate and a CaCl2 coating on cooking properties and texture of the rice noodles with okara. Table 3 shows the effect of alginate and CaCl2 on the cooking properties of rice noodles containing 10% okara. Compared to SF10, the cooking loss of ASF10 was increased by 50.11%, and the water absorption and swelling index were respectively decreased by 18.81% and 6.84% with the addition of alginate and a CaCl2 coating. Likewise, the water absorption of Ca-ASF10 was decreased by 3.36% compared to SF10, but was increased by 19.04% compared to ASF10. On the other hand, the swelling index of Ca-ASF10 was increased by 8.55%, and cooking loss was reduced by 8.74% compared to SF10. In particular, the cooking loss and swelling index of Ca-ASF10 were similar to those of the CON (p>0.05). These results indicate that, although 0.5% alginate alone did not have a positive effect, the quality of the rice noodles was
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improved when a 0.2 M CaCl2 coating was applied together with alginate. Some former studies suggested that the presence of hydrocolloids in rice starch paste results in a weaker structure than starch alone because of the increased loss in the tangent value (tan δ) on the rheological properties (Rosell et al., 2011) and the different network structure formed by the interaction of hydrocolloids with amylose and amylopectin (Shi and BeMiller, 2002). Our results are in agreement with reports showing that the incorporation of xanthan gum (Jarnsuwan and Masubon, 2012) or alginate (Oishi et al., 2009) into wheat noodles increased the cooking loss. Likewise, the presence of hydrocolloids might lead to lower water absorption and reduced swelling index than SF10 alone. In agreement with this, a former study concluded that hydrocolloids interact with starch granules through hydrogen bonding to lower the swelling properties (Liu et al., 2003). The effects of alginate and CaCl2 on texture are shown in Table 4. The addition of 0.5% alginate and a 0.2 M CaCl2 coating significantly changed (p<0.05) all textural parameters except the cohesiveness value. Compared to SF10, the hardness and adhesiveness of ASF10 were respectively increased by 16.22% and 21.64%, whereas the cohesiveness did not significantly change
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(p>0.05). Previous studies (Jang et al., 2015; Lee et al., 2012; Lee et al., 2008) have reported that the addition of alginate in noodles enhanced the hardness and adhesiveness. This increase in adhesiveness might be due to melting of some of the alginate after cooking (Lee et al., 2012). The CaCl2 coating affected both the hardness and adhesiveness of ASF10. Compared to ASF10, the hardness and adhesiveness were respectively decreased by 28.92% and 34.34% in the Ca-ASF10 rice noodles, but the cohesiveness did not significantly change (p>0.05). These results indicate that the Ca-alginate treatment improved the textural qualities.
3.5 Effect of the addition of alginate and a CaCl2 coating on in vitro starch digestibility of the rice noodles with okara. Fig. 2-(a) shows the effects of the addition of alginate and a CaCl2 coating in rice noodles containing 10% okara on glucose release in comparison with control noodles. Overall, the starch hydrolysis sharply increased after 30 min and then gradually increased at a slow, steady rate for 180 min. Regardless of alginate addition and CaCl2 coating, the amount of released glucose did not significantly change (p>0.05) compared to SF10.
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Changes in the RDS, SDS, and RS content of rice noodles containing 10% okara by the addition of alginate and a CaCl2 coating are shown in Fig. 2-(b). Compared to the SF10, no changed significantly (p>0.05) regardless of alginate addition and CaCl2 coating. Compared to SF10, the effects of alginate addition and CaCl2 coating on the pGI values are presented in Fig. 2-(c). Regardless of alginate addition and CaCl2 coating, the pGI values did not significantly change (p>0.05). These results indicate that alginate addition and CaCl2 coating do not affect starch digestibility. Consequently, the addition of alginate and CaCl2 coating are considered to improve the cooking quality of rice noodles enriched with okara without affecting the starch digestibility.
4. Conclusions The cooking properties and textural characteristics of rice noodles were negatively affected by okara. Nonetheless, adding okara at 10% in rice noodles positively decreased the pGI values. The addition of alginate and a CaCl 2 coating to the rice noodles containing okara improved the cooking properties
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and textural characteristics to be similar to those of the control noodles. In addition, the pGI of rice noodles containing okara was maintained regardless of the addition of alginate and a CaCl2 coating. These findings indicate that alginate and CaCl2 can be used to improve the cooking properties and textural characteristics of rice noodles containing okara without affecting starch digestibility. Furthermore, the possibility of applying okara to produce rice noodles with health functional properties with reduced starch digestion was confirmed.
Acknowledgments This work was supported by the Far East University Research Grant (FEU2017S03).
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International Journal of Food Science & Technology. 43, 1637-1644.
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Table 1. Cooking properties of rice noodles containing okara of 5, 10, and 20% (w/w, flour basis)
Sample
Cooking loss (%)
Water absorption (%)
Swelling index
CON
3.88 ± 0.11d
110.19 ± 1.77a
1.24 ± 0.03a
SF5
4.13 ± 0.10c
105.46 ± 1.60b
1.18 ± 0.01b
SF10
4.35 ± 0.06b
103.38 ± 1.41c
1.17 ± 0.02b
SF20
4.87 ± 0.09a
99.31 ± 1.27d
1.13 ± 0.02c
Means with the different letter in the same column are significantly different at p<0.05. CON, control; SF5, soy fiber 5% replacement; SF10, soy fiber 10% replacement; SF20, soy fiber 20% replacement.
Table 2. Texture characteristics of rice noodles containing okara of 5, 10, and 20% (w/w, flour basis)
Sample
Hardness (g)
Adhesiveness (g∙s)
Cohesiveness
CON
6,760 ± 58a
-39.69 ± 0.67d
0.75 ± 0.00a
SF5
6,971 ± 89b
-45.84 ± 0.42c
0.73 ± 0.00b
30
SF10
7,189 ± 73c
-53.15 ± 0.41b
0.72 ± 0.00c
SF20
7,737 ± 73d
-60.80 ± 0.26a
0.70 ± 0.00d
Means with the different letter in the same column are significantly different at p<0.05. CON, control; SF5, soy fiber 5% replacement; SF10, soy fiber 10% replacement; SF20, soy fiber 20% replacement.
Table 3. Effect of 0.2% alginate addition (w/w, flour basis) and 0.2 M CaCl 2 coating on cooking properties of rice noodles containing level of okara at 10% (w/w, flour basis)
Sample
Cooking loss (%)
Water absorption (%)
Swelling index
CON
3.88 ± 0.11c
110.19 ± 1.77a
1.24 ± 0.03a
SF10
4.35 ± 0.06b
103.38 ± 1.41b
1.17 ± 0.02b
ASF10
6.53 ± 0.15a
83.93 ± 1.64d
1.09 ± 0.01c
Ca-ASF10
3.97 ± 0.05c
99.91 ± 1.88c
1.27 ± 0.02a
Means with the different letter in the same column are significantly different at p<0.05. CON, control; SF10, soy fiber 10% replacement; ASF10, added 0.5% alginates in SF10; Ca-ASF10, soaked ASF10 in 0.2 M CaCl2 solution.
31
Table 4. Effect of 0.2% alginate addition (w/w, flour basis) and 0.2 M CaCl 2 coating on texture characteristics of rice noodles containing level of okara at 10% (w/w, flour basis)
Sample
Hardness (g)
Adhesiveness (g∙s)
Cohesiveness
CON
6,760 ± 58c
-39.69 ± 0.67c
0.75 ± 0.00a
SF10
7,189 ± 73b
-53.15 ± 0.41b
0.72 ± 0.00b
ASF10
8,355 ± 71a
-64.65 ± 4.42a
0.73 ± 0.00b
Ca-ASF10
5,939 ± 47d
-42.45 ± 3.64c
0.72 ± 0.00b
Means with the different letter in the same column are significantly different at p<0.05. CON, control; SF10, soy fiber 10% replacement; ASF10, added 0.5% alginates in SF10; Ca-ASF10, soaked ASF10 in 0.2 M CaCl2 solution.
Fig. 1.
32
Fig. 2.
33
34
Fig. 1. Changes in the glucose release (a), starch digestion fraction (rapidly digestible starch, RDS; slowly digestible starch, SDS; resistant starch, RS) (b), and predicted glycemic index (pGI) (c) of rice noodles with okara of 5, 10, and 20% (w/w, flour basis) under in vitro starch digestion. Means with the different letter in the same column are significantly different at p<0.05. CON, control; SF5, soy fiber 5% replacement; SF10, soy fiber 10% replacement; SF20, soy fiber 20% replacement.
Fig. 2. Changes in the glucose release (a), starch digestion fraction (rapidly digestible starch, RDS; slowly digestible starch, SDS; resistant starch, RS) (b), and predicted glycemic index (pGI) (c) of 0.2 M CaCl2 coated rice noodles with 0.5% alginates and 10% okara (w/w, flour basis) under in vitro starch digestion. Means with the different letter in the same column are significantly different at p<0.05. CON, control; SF10, soy fiber 10% replacement; ASF10, added 0.5% alginate in SF10; Ca-ASF10, soaked ASF10 in 0.2 M CaCl2 solution.
35
Highlights
Okara effect on starch digestibility and cooking property of rice noodle was evaluated.
Alginate addition and CaCl2 coating for quality improvement was examined.
Addition of okara at 10% showed the lowest predicted glyce mic index.
Combination treatment of Ca-alginate can improve noodle m aking property.
36
www.elsevier.com/locate/sdj
PII: DOI: Reference:
S2212-4292(17)30563-1 https://doi.org/10.1016/j.fbio.2018.02.008 FBIO277
To appear in: Food Bioscience Received date: 24 August 2017 Revised date: 30 January 2018 Accepted date: 15 February 2018 Cite this article as: Min Je Kang, In Young Bae and Hyeon Gyu Lee, Rice noodle enriched with okara: Cooking property, texture, and in vitro starch digestibility, Food Bioscience, https://doi.org/10.1016/j.fbio.2018.02.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
(To be submitted to Food Bioscience)
Rice noodle enriched with okara: Cooking property, texture, and in vitro starch digestibility 1
2
Min Je Kang , In Young Bae *, and Hyeon Gyu Lee
†*
1
Department of Food and Nutrition, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea
2
Department of Food & Fermentation, Far East University, 76-32 Daehak-gil, Gamgok, Eumseong, Chungbuk 369-700, Republic of Korea
[email protected] [email protected] *Corresponding author: Tel: +82-43-880-3160; Fax: +82-43-879-3730 *Co-corresponding author: Tel: +82-2-2220-1202; Fax: +82-2-2281-8285 Abstract The effects of okara on in vitro starch digestibility and cooking properties of rice noodles and methods for quality improvement for their production as a functional health food were examined. Okara was added to rice noodles in proportions of 0%, 5%, 10%, and 20%. With an increasing level of okara, cooking loss, hardness, and adhesiveness of the rice noodles increased, while water absorption, swelling index, and cohesiveness decreased. The addition of
1
okara reduced the in vitro starch digestibility of rice noodles, although the addition of 20% okara increased the starch digestibility. Among all samples, 10% okara showed the lowest predicted glycemic index (pGI) value. Therefore, in order to improve cooking quality of rice noodles containing 10% okara, addition of alginate or treatment with a CaCl2 coating was examined. The addition of alginate alone did not improve the cooking quality, but treatment with a CaCl 2 coating along with alginate improved cooking quality. Moreover, the combination treatment of Ca-alginate did not change the in vitro starch digestibility of rice noodles containing okara. As a result, 10% okara can be used to produce health-beneficial rice noodles with reduced in vitro starch digestibility, and the combination treatment of Ca-alginate can improve their cooking quality. Keywords rice noodle; okara; Ca-alginate; in vitro starch digestibility; cooking quality; texture 1. Introduction Recently, as consumers are becoming more concerned about their health, interest in gluten-free noodles has increased. Since wheat gluten is known to cause celiac disease, rice and buckwheat have been mainly used to produce
2
gluten-free noodles (Fu, 2008; Sciarini et al., 2010). Because rice noodles account for a large portion of gluten-free noodles, many studies are being conducted on this type of noodle. There have been several previous studies aimed at improving the physical or functional quality attributes of rice noodles. One area of research is to determine the effects of various flours and starches such as hydrothermal treated rice starch (HMT), mung bean starch, legume flour, and pea and lentil starches on the quality of rice noodles (Bouasla et al., 2017; Hormdok and Noomhorm, 2007; Wang et al., 2014; Wu et al., 2015). Other studies have been conducted to improve the qualities of rice noodles with ingredients such as hydrocolloids (e.g., HPMC, xanthan gum, guar gum, and locust bean gum), transglutaminase, and emulsifiers (Lai, 2002; Lee et al., 2012; Yalcin and Basman, 2008). However, most of the studies do not include ingredients with health functionality, so research on the beneficial effects of rice noodles on health and improvements in the processing performance are needed. Dietary fiber refers to plant carbohydrate polymers that are not digested by human digestive enzymes. Dietary fiber can be divided into water-insoluble fiber (e.g., lignin, cellulose, and hemicellulose; IDF) and water-soluble fiber (e.g.,
3
pectin, gum, and mucilage; SDF). Each type of fiber has different physiological effects (Dhingra et al., 2012). IDF is related to the amount of fecal bulk and gastrointestinal transit time and has a positive effect on diarrhea, constipation, and irritable bowel disease (Bosaeus, 2004). SDF is associated with reduced cholesterol level in the blood and decreased glucose absorption by the small intestine (Bosaeus, 2004). Although IDF is more abundant in food than SDF, there are limitations to its use due to its water-insolubility. Okara (gluten-free) is a dietary fiber residue mainly composed of the insoluble fraction remaining from soybeans after extraction of the water soluble fraction used to produce soymilk and tofu. Since large quantities of okara are produced worldwide with the increase in soybean consumption, the amount of okara produced annually poses a disposal problem since most of it is dumped or burned as waste (Li et al., 2012). However, it has been reported that okara has bioactivity in preventing diabetes, hypocholesterolemia, hypolipidemia, and obesity (Hosokawa et al., 2016; Hu et al., 2013; Jiménez-Escrig et al., 2008; Matsumoto et al., 2007). Thus, it could be useful as an ingredient with health functionality and has the potential to be applied in human foods. Previous studies on okara have been conducted to produce a variety of healthy okara-
4
containing foods, such as ice cream cones, cakes, noodles, bread, steamed bread, yogurt, and cookies (Bedani et al., 2014; Lu et al., 2013; Mortazavi et al., 2016; Park et al., 2015; Phuenpipob et al., 2016). However, the starch digestion-retarding effect of okara in rice-based foods has not often been examined. The objective of this study was to investigate how the addition of okara in rice noodles changes the physical properties, cooking quality, and in vitro starch digestibility so as to determine the feasibility of applying okara as a healthy diet food for preventing diabetes. Furthermore, to improve the cooking quality of rice noodles, this study evaluated how the addition of alginate and a CaCl 2 coating changes the cooking quality of rice noodles containing okara.
2. Materials and methods
2.1 Materials White rice (Segoami, Oryza sativa L.) harvested in 2015 was obtained from the local market. Okara [Chewoom soy fiber, mean particle size 77.9 μm analyzed using a laser diffractometer (H3174, HELOS, Sympatec, Inc,
5
Germany)] was acquired from S&Food (Seoul, Korea) and contained 67.6% total dietary fiber (TDF) and 4.8% total starch. The dietary fiber in okara was mainly 64.0% insoluble fiber and minor 3.6% soluble fiber. Rice and okara were ground with mixer (A11 basic, IKA, Berlin, Germany) and respectively sieved through 60 mesh (rice) and 100 mesh (okara) sieves. The ground flours were packed in plastic bags and stored at room temperature until the experiment. Sodium alginate (A2158), pancreatin from porcine pancreas (P7545), amyloglucosidase (A9913), and porcine bile extract (B8631) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Calcium chloride was obtained from Yakuri Pure Chemicals Co., Ltd. (Kyoto, Japan). The total dietary fiber assay (TDF-100A), total starch assay (K-TSTA-100), and GOPOD assay kit (K-GLUC) were purchased from Megazyme (Bray, Ireland). The total dietary fiber contents were determined according to the gravimetric enzymatic method, as described in a previous study (Asp et al., 1988). The amount of total starch was measured according to AACC method 76-13.01 (AACC, 2000b).
2.2 Noodle preparation . The rice noodles were prepared by a method slightly modified from
6
previous studies (Choi et al., 2012). Rice flour and okara flour (0, 5, 10, and 20% w/w, rice flour basis) were separately mixed with distilled water (150% w/w, total flour basis) and stirred for 10 min. Each suspension was combined and then spread on a stainless steel tray (15 × 16 cm) and steamed in a steamer (Stainless Steel Pot, Seshin Queensense Co., Ltd., Gyeonggi-do, Korea) for 1 min. The steamed rice noodle sheet was peeled from the tray and dried in a dry oven at 70℃ for 15 min. The dried rice noodle sheet was covered with cotton and packed in a plastic bag, then rested at 37℃ for 3 h and cut into 5-mm-wide strands. The rice noodle strands were further dried in a dry oven at 40℃ for 3 h. To improve the cooking quality of rice noodles containing 10% okara, which has the most effective in vitro starch digestibility-retarding effect, alginate (0.5% w/w, total flour basis) and 0.2 M CaCl2 solution were used according to a previously described method (Koh et al., 2009). Rice flour, 10% okara flour (w/w, rice flour basis), and 0.5% alginate (w/w, total flour basis) were mixed with distilled water (150% w/w, total flour basis) and stirred for 10 min. Each suspension was combined and then spread on a stainless steel tray (15 × 16 cm) and steamed in a steamer (Stainless Steel Pot, Seshin Queensense Co., Ltd., Gyeonggi-do, Korea) for 1 min. The steamed rice noodle sheet containing
7
10% okara was peeled from the tray and dried in a dry oven at 70℃ for 15 min. The dried rice noodle sheet containing 10% okara was covered with cotton and packed in a plastic bag, then rested at 37℃ for 3 h and cut into 5-mm-wide strands. The rice noodle strands containing 10% okara were soaked in 0.2 M CaCl2 solution for 30 min or were not soaked, followed by drying in a dry oven at 40℃ for 3 h. Finally, all rice noodle strands were packed in plastic bags and stored at room temperature until the experiment. In this study, the samples are referred to as follows: CON, control; SF5, soy fiber 5% replacement; SF10, soy fiber 10% replacement; SF20, soy fiber 20% replacement; ASF10, 0.5% alginate added to SF10; Ca-ASF10, ASF10 soaked in a 0.2 M CaCl2 solution.
2.3 Experimental methods 2.3.1 Cooking property. The cooking properties of rice noodles were determined as depicted in AACC-approved method 66-50.01 (AACC, 2000a). Rice noodles (2 g) were added to boiling distilled water (100 mL). The cooked rice noodles were then removed at 30 second time intervals and placed into cold water. When the white
8
and hard cores of the rice noodles disappeared, the time was considered to be the optimum cooking time. After boiling for the optimum time (3 min), the cooked rice noodles were added to cold distilled water and then removed to prevent overcooking, allowed to stand for 5 min to drain, and then weighed. To dry the rice noodles, cooked rice noodles were kept at 40℃ overnight. The cooking water and rinse water were put into a beaker and dried to a constant weight in a 105℃ oven. The cooking loss was indicated by the percentage of solid loss during cooking. Rice noodle samples were prepared in triplicate. The cooking quality parameters were calculated as follows:
Cooking loss (%) =
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑑𝑟𝑖𝑒𝑑 𝑟𝑒𝑠𝑖𝑑𝑢𝑒 𝑖𝑛 𝑐𝑜𝑜𝑘𝑖𝑛𝑔 𝑤𝑎𝑡𝑒𝑟 𝑎𝑛𝑑 𝑟𝑖𝑛𝑠𝑒 𝑤𝑎𝑡𝑒𝑟 × 100 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑟𝑎𝑤 𝑛𝑜𝑜𝑑𝑙𝑒𝑠
Water absorption (%) =
Swelling index =
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑐𝑜𝑜𝑘𝑒𝑑 𝑛𝑜𝑜𝑑𝑙𝑒𝑠 − 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑟𝑎𝑤 𝑛𝑜𝑜𝑑𝑙𝑒𝑠 × 100 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑟𝑎𝑤 𝑛𝑜𝑜𝑑𝑙𝑒𝑠
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑐𝑜𝑜𝑘𝑒𝑑 𝑛𝑜𝑜𝑑𝑙𝑒𝑠 − 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑐𝑜𝑜𝑘𝑒𝑑 𝑛𝑜𝑜𝑑𝑙𝑒𝑠 𝑎𝑓𝑡𝑒𝑟 𝑑𝑟𝑦𝑖𝑛𝑔 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑑𝑟𝑖𝑒𝑑 𝑛𝑜𝑜𝑑𝑙𝑒𝑠 𝑎𝑓𝑡𝑒𝑟 𝑑𝑟𝑦𝑖𝑛𝑔
2.3.2 Texture. The textural properties of the rice noodles were measured by texture
9
profile analysis (TPA) with a texture analyzer (TAXT2i, Stable Micro Systems, UK). The texture analysis of rice noodles was based on a former study (Bhattacharya et al., 1999) with slight modifications. A single strand of each cooked rice noodle was cut into a 2.5 cm length. An aluminum cylinder probe with a 35 mm diameter compressed the rice noodle at a speed of 1 mm/s for the pretest, test, and post-test with a constant rate of deformation to 75% of the original rice noodle thickness. The probe was retracted and kept stationary for 1 s before conducting the next compression measurement. The three textural parameters recorded from the TPA were hardness, adhesiveness, and cohesiveness.
2.3.3 In vitro starch digestion. In vitro starch digestibility was analyzed according to the method described in a previous study (Minekus et al., 2014) with slight modifications. The cooked rice noodles were dried in a freeze dryer for 3 days, then ground with a mixer (A11 basic, IKA, Berlin, Germany) and sieved through a 100 mesh sieve. Simulated intestinal fluid (SIF) was composed of 0.5 M KCl (6.8 mL), 0.5 M KH2PO4 (0.8 mL), 1 M NaHCO3 (42.5 mL), 2 M NaCl (9.6 mL), and 0.15 M
10
MgCl2(H2O)6 (1.1 mL) in distilled water (400 mL). Pancreatin was diluted 1:5 (w/v) with SIF and centrifuged at 1,952 x g for 20 min. The enzyme solution was prepared by dilution in 5 mL pancreatin supernatant, 0.435 g bile extract, and 2 mL SIF. The mixture (5 g) of rice noodle powder and distilled water (10.71%, w/v) was stirred with SIF (26 mL), 0.3 M CaCl2 (0.04 mL), 1 M NaOH (0.15 mL), and distilled water (1.31 mL) at 500 rpm for 30 min. The enzyme solution (7.5 mL) and amyloglucosidase (0.2 mL/g of starch in the sample) were added. The sample was adjusted to pH 6.0 with 1 N HCl and incubated in a 37℃ shaking water bath. Aliquots (0.1 mL) were taken at 0, 30, 60, 90, 120, and 180 min for incubation and mixed with 1.4 mL of 99% ethanol. These solutions were centrifuged at 600 x g for 3 min, and the released glucose content of the supernatant was measured by a spectrophotometer (Biomate 3S, Thermo Scientific, Waltham, MA, USA) at 510 nm using the GOPOD assay kit. The levels of rapidly digestible starch (RDS) and slowly digestible starch (SDS) were measured after intestinal digestion for 30 min and 120 min, respectively, and resistant starch (RS) was measured as the starch remaining after 180 min of incubation.
11
2.3.4 Predicted glycemic index (pGI). The starch digestion kinetics and pGI of the rice noodles were analyzed according to the theory established by a former study (Goñi et al., 1997). The kinetics of starch hydrolysis were calculated using the equation: [C = C ∞(1-e-kt)], where C, C∞, and k represent the hydrolysis degree at each time point, the maximum hydrolysis extent, and the kinetic constant, respectively. The hydrolysis index (HI) was calculated by dividing the area under the hydrolysis curve of each sample by the area of fresh bread as the reference sample. The predicted glycemic index (pGI) was obtained using the equation pGI = 39.71 + 0.549HI.
2.4 Statistical analysis. Statistical analyses were conducted using statistical software version 21.0 (SPSS, IBM Inc., Chicago, IL, USA). One-way analyses of variance (ANOVA) were performed (p<0.05) by Duncan’s multiple range test. The cooking property and in vitro starch digestibility measurements were performed in triplicate, and the texture analysis was conducted with at least 10 measurements. All results are expressed as the mean ± standard deviation (SD).
12
3. Results and discussion
3.1 Cooking properties of the rice noodles with okara. As shown in Table 1, increasing the level of okara from 0 to 20% led to an increase in cooking loss of the rice noodles (3.88-4.87%). In particular, the cooking loss of SF20 was increased by 25.52% compared to CON. This finding might be related to weakening of the rice noodle structure caused by the addition of okara (Shiau et al., 2012). These results are in agreement with previous reports that found that the incorporation of durum bran and pollard (Aravind et al., 2012; Rakhesh et al., 2015) or rajma bean (Kumar and Prabhasankar, 2015) into noodles increased the cooking loss. Cooking loss is an indicator of the cooking quality of noodles. An increase in cooking loss is not desirable for noodles because it is related to the stickiness of the noodles (Bhattacharya et al., 1999). All rice noodles with okara exhibited significant concentration-dependent decrease (p<0.05) in water absorption (99.37-110.19%) and swelling index (1.13-1.24). Water absorption and the swelling index are correlated with the cooking qualities of starch-based noodles (Lee et al., 2005). The reduced water
13
uptake caused by the decrease in water absorption results in noodles with a hard and coarse texture. These results are in agreement with reports showing that insoluble dietary fiber decreased the water absorption of noodle samples because the binding of water competes with starch binding (Aravind et al., 2012).
3.2 Texture of the rice noodles with okara. The textural properties of rice noodles produced with 5 to 20% of okara are presented in Table 2. As the level of okara increased, the hardness and adhesiveness
significantly increased
(p<0.05),
while
the
cohesiveness
significantly decreased (p<0.05). For SF20, the hardness and adhesiveness were respectively increased by 14.45% and 53.19%, and the cohesiveness was decreased by 6.67% compared with the CON. In general, the addition of insoluble dietary fiber to noodles enhanced their hardness values. The high hardness values indicate that hard and brittle noodles were produced. The stickiness of noodles are denoted by the adhesiveness in the texture profile analyses, and higher stickiness is reported to be undesirable (Guo et al., 2003). Cohesiveness affects the cooking quality and
14
chewiness of noodles since it is considered as an indicator of the degree of rupture of the noodle structure during mastication (Singh et al., 2002; Tan et al., 2009). In this study, it was confirmed that the increase in the amount of okara added had an adverse effect on the quality of rice noodles. Therefore, it is necessary to select the proper amount of okara to add in consideration of its functional aspects.
3.3 In vitro starch digestibility of rice noodles with okara. The effects of okara on in vitro starch digestion in rice noodles were investigated by measuring the released glucose content during starch digestion. Fig. 1-(a) shows the effect of rice noodles enriched with okara on starch digestibility in comparison with rice noodles without okara. Glucose release was measured as the reducing sugars hydrolyzed from starch by digestive enzymes. Overall, the starch hydrolysis sharply increased after 30 min and then gradually increased at a slow, steady rate for 180 min. As the proportion of okara in rice noodles increased, the amount of released glucose significantly decreased (p<0.05). The amounts of rapidly digestible starch (RDS), slowly digestible starch
15
(SDS), and resistant starch (RS) in the rice noodles with okara are shown in Fig. 1-(b). Compared to the CON, SF10 contained the lowest level of RDS and the highest level of RS. However, the amount of SDS did not significantly change (p>0.05) in any of the rice noodles containing okara. The pGI values of the rice noodles containing okara compared to the CON are presented in Fig. 1-(c). Compared with the CON, the pGI value was significantly reduced (p<0.05) in the presence of okara and was lowest in SF10. The pGI of SF10 was 4.73% lower than that of the CON. The increased pGI observed in rice noodles containing 20% okara could be due to the ability of okara to disrupt the integrity of the rice noodle matrix. According to similar former studies, when oat fiber was added above a level of 10%, the starch structure was disrupted (Aravind et al., 2012; Bustos et al., 2011a) so that starch granules become more accessible and hence more susceptible to enzyme degradation (Hormdok and Noomhorm, 2007; Tudorica et al., 2002). Also, other studies have reported that in vitro starch digestion in cooked pasta and spaghetti was respectively affected by oat bran (Bustos et al., 2011b) and durum bran and pollard (Aravind et al., 2012). Based on the overall results, an increase in the amount of okara led to
16
excessive increase in hardness and adhesiveness and a decrease in cohesiveness of the rice noodles. However, from a functional point of view, the addition of 10% okara resulted in a significant decrease in pGI. Based on these findings, the level of okara was set at 10% for subsequent experiments.
3.4 Effect of the addition of alginate and a CaCl2 coating on cooking properties and texture of the rice noodles with okara. Table 3 shows the effect of alginate and CaCl2 on the cooking properties of rice noodles containing 10% okara. Compared to SF10, the cooking loss of ASF10 was increased by 50.11%, and the water absorption and swelling index were respectively decreased by 18.81% and 6.84% with the addition of alginate and a CaCl2 coating. Likewise, the water absorption of Ca-ASF10 was decreased by 3.36% compared to SF10, but was increased by 19.04% compared to ASF10. On the other hand, the swelling index of Ca-ASF10 was increased by 8.55%, and cooking loss was reduced by 8.74% compared to SF10. In particular, the cooking loss and swelling index of Ca-ASF10 were similar to those of the CON (p>0.05). These results indicate that, although 0.5% alginate alone did not have a positive effect, the quality of the rice noodles was
17
improved when a 0.2 M CaCl2 coating was applied together with alginate. Some former studies suggested that the presence of hydrocolloids in rice starch paste results in a weaker structure than starch alone because of the increased loss in the tangent value (tan δ) on the rheological properties (Rosell et al., 2011) and the different network structure formed by the interaction of hydrocolloids with amylose and amylopectin (Shi and BeMiller, 2002). Our results are in agreement with reports showing that the incorporation of xanthan gum (Jarnsuwan and Masubon, 2012) or alginate (Oishi et al., 2009) into wheat noodles increased the cooking loss. Likewise, the presence of hydrocolloids might lead to lower water absorption and reduced swelling index than SF10 alone. In agreement with this, a former study concluded that hydrocolloids interact with starch granules through hydrogen bonding to lower the swelling properties (Liu et al., 2003). The effects of alginate and CaCl2 on texture are shown in Table 4. The addition of 0.5% alginate and a 0.2 M CaCl2 coating significantly changed (p<0.05) all textural parameters except the cohesiveness value. Compared to SF10, the hardness and adhesiveness of ASF10 were respectively increased by 16.22% and 21.64%, whereas the cohesiveness did not significantly change
18
(p>0.05). Previous studies (Jang et al., 2015; Lee et al., 2012; Lee et al., 2008) have reported that the addition of alginate in noodles enhanced the hardness and adhesiveness. This increase in adhesiveness might be due to melting of some of the alginate after cooking (Lee et al., 2012). The CaCl2 coating affected both the hardness and adhesiveness of ASF10. Compared to ASF10, the hardness and adhesiveness were respectively decreased by 28.92% and 34.34% in the Ca-ASF10 rice noodles, but the cohesiveness did not significantly change (p>0.05). These results indicate that the Ca-alginate treatment improved the textural qualities.
3.5 Effect of the addition of alginate and a CaCl2 coating on in vitro starch digestibility of the rice noodles with okara. Fig. 2-(a) shows the effects of the addition of alginate and a CaCl2 coating in rice noodles containing 10% okara on glucose release in comparison with control noodles. Overall, the starch hydrolysis sharply increased after 30 min and then gradually increased at a slow, steady rate for 180 min. Regardless of alginate addition and CaCl2 coating, the amount of released glucose did not significantly change (p>0.05) compared to SF10.
19
Changes in the RDS, SDS, and RS content of rice noodles containing 10% okara by the addition of alginate and a CaCl2 coating are shown in Fig. 2-(b). Compared to the SF10, no changed significantly (p>0.05) regardless of alginate addition and CaCl2 coating. Compared to SF10, the effects of alginate addition and CaCl2 coating on the pGI values are presented in Fig. 2-(c). Regardless of alginate addition and CaCl2 coating, the pGI values did not significantly change (p>0.05). These results indicate that alginate addition and CaCl2 coating do not affect starch digestibility. Consequently, the addition of alginate and CaCl2 coating are considered to improve the cooking quality of rice noodles enriched with okara without affecting the starch digestibility.
4. Conclusions The cooking properties and textural characteristics of rice noodles were negatively affected by okara. Nonetheless, adding okara at 10% in rice noodles positively decreased the pGI values. The addition of alginate and a CaCl 2 coating to the rice noodles containing okara improved the cooking properties
20
and textural characteristics to be similar to those of the control noodles. In addition, the pGI of rice noodles containing okara was maintained regardless of the addition of alginate and a CaCl2 coating. These findings indicate that alginate and CaCl2 can be used to improve the cooking properties and textural characteristics of rice noodles containing okara without affecting starch digestibility. Furthermore, the possibility of applying okara to produce rice noodles with health functional properties with reduced starch digestion was confirmed.
Acknowledgments This work was supported by the Far East University Research Grant (FEU2017S03).
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Table 1. Cooking properties of rice noodles containing okara of 5, 10, and 20% (w/w, flour basis)
Sample
Cooking loss (%)
Water absorption (%)
Swelling index
CON
3.88 ± 0.11d
110.19 ± 1.77a
1.24 ± 0.03a
SF5
4.13 ± 0.10c
105.46 ± 1.60b
1.18 ± 0.01b
SF10
4.35 ± 0.06b
103.38 ± 1.41c
1.17 ± 0.02b
SF20
4.87 ± 0.09a
99.31 ± 1.27d
1.13 ± 0.02c
Means with the different letter in the same column are significantly different at p<0.05. CON, control; SF5, soy fiber 5% replacement; SF10, soy fiber 10% replacement; SF20, soy fiber 20% replacement.
Table 2. Texture characteristics of rice noodles containing okara of 5, 10, and 20% (w/w, flour basis)
Sample
Hardness (g)
Adhesiveness (g∙s)
Cohesiveness
CON
6,760 ± 58a
-39.69 ± 0.67d
0.75 ± 0.00a
SF5
6,971 ± 89b
-45.84 ± 0.42c
0.73 ± 0.00b
30
SF10
7,189 ± 73c
-53.15 ± 0.41b
0.72 ± 0.00c
SF20
7,737 ± 73d
-60.80 ± 0.26a
0.70 ± 0.00d
Means with the different letter in the same column are significantly different at p<0.05. CON, control; SF5, soy fiber 5% replacement; SF10, soy fiber 10% replacement; SF20, soy fiber 20% replacement.
Table 3. Effect of 0.2% alginate addition (w/w, flour basis) and 0.2 M CaCl 2 coating on cooking properties of rice noodles containing level of okara at 10% (w/w, flour basis)
Sample
Cooking loss (%)
Water absorption (%)
Swelling index
CON
3.88 ± 0.11c
110.19 ± 1.77a
1.24 ± 0.03a
SF10
4.35 ± 0.06b
103.38 ± 1.41b
1.17 ± 0.02b
ASF10
6.53 ± 0.15a
83.93 ± 1.64d
1.09 ± 0.01c
Ca-ASF10
3.97 ± 0.05c
99.91 ± 1.88c
1.27 ± 0.02a
Means with the different letter in the same column are significantly different at p<0.05. CON, control; SF10, soy fiber 10% replacement; ASF10, added 0.5% alginates in SF10; Ca-ASF10, soaked ASF10 in 0.2 M CaCl2 solution.
31
Table 4. Effect of 0.2% alginate addition (w/w, flour basis) and 0.2 M CaCl 2 coating on texture characteristics of rice noodles containing level of okara at 10% (w/w, flour basis)
Sample
Hardness (g)
Adhesiveness (g∙s)
Cohesiveness
CON
6,760 ± 58c
-39.69 ± 0.67c
0.75 ± 0.00a
SF10
7,189 ± 73b
-53.15 ± 0.41b
0.72 ± 0.00b
ASF10
8,355 ± 71a
-64.65 ± 4.42a
0.73 ± 0.00b
Ca-ASF10
5,939 ± 47d
-42.45 ± 3.64c
0.72 ± 0.00b
Means with the different letter in the same column are significantly different at p<0.05. CON, control; SF10, soy fiber 10% replacement; ASF10, added 0.5% alginates in SF10; Ca-ASF10, soaked ASF10 in 0.2 M CaCl2 solution.
Fig. 1.
32
Fig. 2.
33
34
Fig. 1. Changes in the glucose release (a), starch digestion fraction (rapidly digestible starch, RDS; slowly digestible starch, SDS; resistant starch, RS) (b), and predicted glycemic index (pGI) (c) of rice noodles with okara of 5, 10, and 20% (w/w, flour basis) under in vitro starch digestion. Means with the different letter in the same column are significantly different at p<0.05. CON, control; SF5, soy fiber 5% replacement; SF10, soy fiber 10% replacement; SF20, soy fiber 20% replacement.
Fig. 2. Changes in the glucose release (a), starch digestion fraction (rapidly digestible starch, RDS; slowly digestible starch, SDS; resistant starch, RS) (b), and predicted glycemic index (pGI) (c) of 0.2 M CaCl2 coated rice noodles with 0.5% alginates and 10% okara (w/w, flour basis) under in vitro starch digestion. Means with the different letter in the same column are significantly different at p<0.05. CON, control; SF10, soy fiber 10% replacement; ASF10, added 0.5% alginate in SF10; Ca-ASF10, soaked ASF10 in 0.2 M CaCl2 solution.
35
Highlights
Okara effect on starch digestibility and cooking property of rice noodle was evaluated.
Alginate addition and CaCl2 coating for quality improvement was examined.
Addition of okara at 10% showed the lowest predicted glyce mic index.
Combination treatment of Ca-alginate can improve noodle m aking property.
36