Abstract

This research aimed to evaluate the effect of different varieties and proportion of yam on the resistant starch type III content (RSC) and physical properties of steamed rice bowl cake. According to the research results, RSC (9.2-11.7%) was significantly increased (p < 0.05) along with increasing the yam proportion in some experiment variables, where Taichung Sen 17 (TCS 17), Da shan line 2 (10, 20 and 30%), and Heng chun (10%) had the lowest RSC (9.2%). The 20 and 30% proportion showed the highest RSC (11.7%) in Tainung 5. Furthermore, significant differences (p < 0.05) were found in some experiment variables of texture profile analysis. Results indicated that the proportion of yam was positively related to hardness, cohesiveness and adhesiveness. Besides, color analysis results of steamed rice bowl cake with different varieties and proportion of yam were significantly different (p < 0.05). The control (TCS 17) was significantly lighter (L*=67.99) than Tainung 5 (L*=54.50, 52.67 and 51.20), and similar to Da shan line 2 (L*=69.37, 68.54 and 68.49) and Heng chun (L*=72.82, 69.97 and 71.39). Significant higher (p < 0.05) a* and b* values were observed in steamed rice bowl cake of Da shan line 2 (a*=-1.07, -0.73 and -0.60), Tainung 5 (a*=6.93, 8.59 and 9.03), Heng chun (a*=-1.63, -1.34 and -1.41) and Da shan line 2 (b*=2.41, 3.42 and 4.49), Heng chun (b*=0.45, 0.82 and 1.00), respectively. The significant differences (p < 0.05) were observed in some experiment variables of solubility and swelling power.

1. Introduction

Rice is the most important staple food crop for a large part of the world's human population, especially in Taiwan, and it plays an important energy source in people’s diets ¹. Furthermore, starch is the major component of rice ^{2, 3}, and its functional and structural properties are important.

Starch is the major source of dietary carbohydrates and is the most abundant storage polysaccharide in plants ⁴. It is present in high amounts in cereal, root and tuber ^{4, 5}. The most important sources of starch are corn, potato, wheat, tapioca, rice and yam. Starch is a polymer of glucose linked together by α-D-(1-4) and/or α-D-(1-6) glycosidic bonds ⁴. It consists different two D-glucan polymers, one is amylose, which is a primarily linear structure, and the other is amylopectin, which is a branched structure ^{6, 7}. It is also used for different textures of food ⁸. Especially, amylose content of rice is a major factor affecting the textural, functional and physicochemical properties for food, and can be related to its rheological and swelling properties ^{2, 7}.

Resistant starch (RS) is defined as the sum of starch and products of starch degradation not absorbed in the small intestine of healthy individuals ^{9, 10, 11}. Higher RS in food can reduce the blood glucose level after a meal, which is useful in diabetes and obesity controlling ^{12, 13}. Especially, resistant starch type III (RS III, retrograded starch) is particularly interesting since its functional and thermal stable properties during food processing ⁸. The higher amylose content of Indica rice is easy to retrograde, and further form RS III ^{1, 7}.

Taichung Sen 17 (TCS 17) is the variety of rice that is with the highest amylose conten, this may have a positive effect on RS III formation ^{1, 7}. TCS 17 is only planted in Taiwan, and it benefits the RS III producing ¹⁴. Besides, RS III formation is associated with retrograded amylose ⁶. The texture of rice-made food is known to easily become hard, which makes the taste worse over the time. This is because of the retrogradation resulted in RS III increasing through recrystallization of amylose. Therefore, TCS 17 has been lost the popularity due to its low acceptability in taste and mouth feel.

Steamed rice bowl cake is one of the traditional dishes in Taiwan, and the texture of steamed rice bowl cake is soft and stretchy. Traditionally, Indica rice has been used for making steamed rice bowl cake because it can provide a favorable texture and appearance. However, instead of Indica rice, other starch crops such as yam have never been utilized for making steamed rice bowl cake.

Yam (Family Dioscoraeceae) is a high economic value food crop, and an important source of energy ^{15, 16} in people’s diets. Starch is the major component of yam and provides large proportion of daily caloric intake. Since yam tubers contain about 70–82% starch ^{17, 18}, the cooking, processing characteristics and storage quality of yam-containing products, and perhaps the physiological effectiveness of the bioactive ingredients involved will be greatly dependent on starch properties of yam ¹⁹. According above mentions, starch properties of yam affect the physicochemical, functional, and textural characteristics for yam-containing products ²⁰. The yam is also rich in carbohydrate, dietary fibers, vitamin B-complex, and is an important source of the β-carotene, mineral, such as manganese. Oluwanmukomi and Akinsoka ²¹ further reported higher amounts of phosphorus in yam (0.022%) ^{17, 21}. Particularly, the yam has a higher amylose content when compared to potato, rice and cassava ¹⁷, and it benefits the RS III producing ¹⁴. The yam is becoming increasingly popular for various products manufacturing such as steaming, boiling, roasting, and frying ²⁰. That is to say, the yam is becoming widely accepted in the daily diet as a cheap source of carbohydrate ¹⁸ and yam is becoming an alternative of rice.

Steamed rice bowl cake is the one of Taiwanese traditional rice-made food, but as the impact of instant food, the consumption of rice is decreasing gradually. Therefore, promoting Taiwanese traditional rice-made food is becoming an important task. As a matter of fact, the yam has been used as a food crop. Moreover, the yam is not only cheaper but also can be processed into higher quality products. Different varieties of yam differ in many of their characteristics, ranging from the physicochemical, fuuctional, textural properties and mouth feel. However, nowadays, the main application of yam has so far less utilized for industry in Taiwan. Besides, research regarding the combination of both yam and rice for the development of Taiwanese traditional rice-made food is so far limited.

Nowadays, natural and health food without chemical processing is the major demand for consumers. Overall, in order to improve the quality and functional demands of steamed rice bowl cake and to broaden the utility of rice. The objectives of this research were to evaluate the effect of different varieties (Da shan line 2, Tainung 5 and Heng chun) and proportion (0, 10, 20, 30%) of yam on the RSC, texture, color, solubility and swelling power of steamed rice bowl cake, and finally to evaluate the optimal conditions for the production of steamed rice bowl cake with high RSC.

2. Materials and Methods

Indica rice kernal, Taichung Sen 17 (TCS 17, grown in Taiwan) had been harvested in 2017. The total starch and amylose content of TCS 17 was analyzed by the method described by Chou and Li ⁷. The total starch and amylose content of TCS 17 had been determined as 77.99 and 28.66%, respectively.

Three varieties of yam harvested in Taiwan namely yellow flesh color (Da shan line 2), purple flesh color (Tainung 5) and white flesh color (Heng chun) were purchased from a local market (Pingtung, Taiwan). The freshly harvested material of rice and yam were collected and stored at 7°C for further processing. All used chemicals and reagents were in analytical grade.

Three varieties of yam (Da shan line 2, Tainung 5 and Heng chun) were allowed 2 h to equilibrate at room temperature. Then, these yam were manually cleaned by hand followed with clean water to remove soils. The cleaned yam were cut into 1 cm3 volume and steamed by a steamer (Model No KS-980, Quickly Food Machinery Co., Ltd., Taoyuan, Taiwan) for 30 min. The mashed yam were then allowed 2h to equilibrate at the room temperature and used for further processes.

First, the TCS 17 rice kernal and mashed yam were then allowed 2 h to equilibrate at the room temperature before the preparation. Second, 100 g TCS 17 rice kernel were soaked in 350 mL distilled water for 30 min. Each group of TCS 17 rice kernel and mashed yam was weighed according to Table 1 and steamed by a steamer (Model No KS-980, Quickly Food Machinery Co., Ltd., Taoyuan, Taiwan) for 40 min.

A method previously described by Chou and Li ⁷ for the RSC analysis was used in this research. 0.4 g sample powders (dry basis) were weighed in a centrifuge tube, 20 mL pH 6.0 phosphate buffer (55.6 mM) and 0.16 g α-amylase (Sigma A-3176, Steinheim, Germany) were added and incubated at 37°C for 16 h. The sample was adjusted to pH 4.5 using phosphoric acid solution (2 mL/100 mL), 0.4 mL amyloglucosidase (Sigma A-7095, Steinheim, Germany) was added and incubated at 60°C for 30 min. After incubation, the sample was centrifuged at 4000 ×g for 15 min by a high-speed micro centrifuge (CF15R, Hitachi, Koki, Ltd., Tokyo, Japan), the residue was resuspended in 20 mL pH 7.5 phosphate buffer (0.08 M), 0.4 mL protease (Sigma P-2143, Steinheim, Germany) was added and incubated at 42°C for 4h. Finally, the sample was centrifuged at 6000×g for 15 min, dried to constant weight at 60°C by a constant-temperature oven and weighed for the RS weight. The analysis was carried out in triplicate.

The RSC was calculated by the following equation:

(1)

The textural properties of the steamed rice bowl cake were analyzed using the EZ Test-500N texture analyzer (TAXTZ-5, Shimadzu Co., Kyoto, Japan) attached to a cylindrical plunger (10 mm diameter, depression speed = 30 mm/min) and a 500 N load cell, by performing texture profile analysis (TPA). The freshly prepared samples were prepared by cutting the steamed rice bowl cake to cube shaped samples (2 cm³ volume). The samples were compressed twice to 50% of the original height, texture profile analyses were determined by two compression cycles. Texture profile parameters measured include hardness, cohesiveness, adhesiveness, gumminess, springiness and chewiness. The analysis was performed with triplicates.

Colors of steamed rice bowl cake were measured using Hunter Lab Color Measuring System (Color Quest XE, Hunter Associates Laboratory, Reston, USA). The analysis was performed with triplicate.

The solubility and swelling power of the steamed rice bowl cake determine was carried out by a modification of the previous method ¹⁹. 0.6 g of sample powders (dry basis) was dissolved with distilled water to a total volume of 40 mL using a 50 mL graduated centrifuge tube. The suspension was stirred since it might cause fragmentation of the starch granules. The slurry in the tube was heated at 65℃ for 30 min. This tube was then removed, wiped dry on the outside and cooled to room temperature. It was then centrifuged at 6000 ×g for 20 min by a high-speed micro centrifuge (CF15R, Hitachi, Koki, Ltd., Tokyo, Japan). The supernatant was decanted into a pre-weighed moisture can. The solubility was determined by a constant-temperature oven at 105°C and the residue weighted. The sedimented paste was weighed and swelling power as well as percent solubility calculated using equations (2) and (3), respectively.

(2)

(3)

The data obtained were first calculated using Microsoft Office Excel 2010 (Microsoft Corporation, Redmond, WA). Analysis of variance (ANOVA) for the effect of varieties (Da shan line 2, Tainung 5 and Heng chun) and proportion (10, 20 and 30%) on the resistant starch content and physical characteristics of steamed rice bowl cake was accomplished using the Statistical Analysis System (SAS, USA). The differences among mean values were processed by Duncan’s Multiple Range Test, and significance was defined at p < 0.05.

3. Results and Discussion

Table 2 shows the effects of different varieties of yam (Da shan line 2, Tainung 5 and Heng chun) and proportion (10, 20 and 30%) on the RSC of steamed rice bowl cake. The measured RSC of steamed rice bowl cake ranged between 9.2 and 11.7%, which agreed with previously reported values of 10.00-12.01% ⁷. This may be explained that the starch relapses to a structure during gelatinization that could be highly resistant to amylase hydrolysis which is referred to as RS III ²². The amylose content, processing and storage conditions may affect the gelatinization and retrogradation, further affecting the RS III formation ²³.

Most starchy food, such as cereals, beans and tuber root are necessary processed by heating with or without water addition before consumption ²⁰. The temperature of steaming method is about 100°C, and gelatinization temperatures of yam starch is reported ranging between 65°C and 75°C ²⁴, caused physical modifications or rupture of starch granular structure due to gelatinization after hydration and then dissolved ²⁰, and these fragments can combine with other molecules creating a new compounds resistant to amylase hydrolysis ⁵. Previous research reported that steam cooking at atmospheric pressure formed higher RSC than steam cooked at higher pressure ^{7, 22}. In addition, recrystallization during retrogradation could be formed by the association between amylose and amylopectin through the cooling of steamed rice bowl cake preparation ^{10, 20}, resulting in similar RSC between different varieties of yam. Furthermore, processing raw materials in most food destroys RS I and RS II, but it can produce RS III (retrograded amylose) ²⁵. RS III is produced by gelatinization, which is a disruption of granular structure by heating starch, and then retrogradation during cooling.

Compared with different varieties and proportion of yam, the control (TCS 17), Da shan line 2 (10, 20 and 30%), and Heng chun (10%) have the lowest RSC (9.2%). Previous research ²⁸ reported that various factors such as milling degree, heating treatment, moisture content during processing, and other components in food (proteins or lipids) are known to influence the formation of RS III in processed foods. In addition, the higher RSC in processing food may attribute to the encapsulation of starch molecule by protein matrix, further forms new compounds in food ²⁶, and the interaction between amylose and amylopectin ²⁷, increase the realignment of starch chains, leading to it less susceptible to amylase hydrolysis and also limit the action of amylolytic enzymes on substrate, further influencing RS III formation.

It was observed in Table 2 that the 20 and 30% proportion showed the highest RSC (11.7 and 11.7%) in Tainung 5. Higher RSC could be observed in this research when compared with previous researches ²⁸, and this may be attributed to the higher amylose content of TCS 17 and yam ²⁹. Especially, yam has higher amylose content when comparing to potato, rice and cassava ¹⁷. Previours research further reported the yam with 21.69-31.56% amylose content ¹⁶, which amylose retrogrades quickly during the first few hours of cooling after gelatinization during steamed rice bowl cake preparation ⁵.

Furthermore, various factors are known to affect the RS III formation in processed foods. TCS 17 has higher amylose content than Japonica rice. The high degree of amylose polymerization, formation of amylolytic enzyme-resistant double helices stabilized by hydrogen bonds ³⁰, and about 30-60% moisture content during steamed rice bowl cake preparation may be attributed to the main reasons. It is known that water acts as a plasticizer for retrogradation of amylose, and this can be maximized in about 30-60% moisture content ³¹.

Texture is one of the major factors which influence food acceptability and mouth feel, especially for rice-made food. The texture profile analysis of steamed rice bowl cake based on different varieties of yam (Da shan line 2, Tainung 5 and Heng chun) and proportion (10, 20 and 30%) was shown in Table 3. Significant differences (p < 0.05) were found in all experiment variables. Compared with different varieties and proportion of yam, the 30% proportion of Tainung 5 showed the highest hardness and cohesiveness. This result indicates that the higher proportion of the yam, the better acceptability and mouth feel of the steamed rice bowl cake.

The texture properties of rice starch gel depend upon structure of starch granule and other components. This may be explained by higher crude fiber content of Tainung 5 which absorbs more water. This result was also observed by Chou and Li ⁷, who observed that steamed rice bowl cake containing sweet potato and rice flour displayed higher hardness and cohesiveness.

Previous research ³² reported the hardness of a starch gel increased linearly with retrogradation rate during storage, and it is highly correlated with the starch content of gel, maximum rate of retrogradation was observed at a 50–55% solid content ³², similar to the steamed rice bowl cake in this study, the amylose retrogradation rate might continuously and quickly increase.

When the starch granule is heated up to the gelatinization temperature in excess water, heat and moisture transfer phenomena occur. In addition, rigidity of starch gel is highly correlated with retrogradation of starch granules after gelatinization, syneresis and crystallisation of amylopectin ³³. Though the starch molecules in starch-rich food is enhanced during retrogradation leading to an increase in the hardness of steamed rice bowl cake ³⁴.

Therefore, presence of soluble carbohydrates from the combination of both yam and rice with relatively high amylose content materials could enhance retrogradation of starch gel formation that would gave rise to harder gels. Puncha-arnon and Uttapap ³⁵ further indicated that the formation of starch gel mainly depends on hold water in the network within the swollen starch granules. Amylose leaks out of the swollen starch granules also plays an important role, but it becomes significant when the swollen starch granules disrupt ⁷.

Hardness, cohesiveness, adhesiveness, gumminess, springiness and chewiness of all steamed rice bowl cake were in the range of 231.5-307.4 gf, 425.5-501.9, -149.6-(-118.1) gf.sec, 148.5-160.5 gf, 0.9-1.1 and 1.1-1.3 gf, respectively. These observation are agreement with the previously reported by Chou and Li ⁷. This indicates that the steamed rice bowl cake with different varieties and proportion (10, 20 and 30%) of yam (Da shan line 2, Tainung 5 and Heng chun) are with good acceptability. From these results, we concluded that the 30% proportion in Tainung 5 was the most suitable for making the steamed rice bowl cake.

The results of color evaluation presented in Table 4 showed that color characteristics of steamed rice bowl cake with different varieties (Da shan line 2, Tainung 5 and Heng chun) and proportion (10, 20 and 30%) of yam were significantly different (p < 0.05). The measured L* value of steamed rice bowl cake ranged between 51.20 and 72.82. Compared with different varieties and proportions of yam, the control (TCS 17) was significantly lighter (L* = 67.99) than Tainung 5 (L*=54.50, 52.67 and 51.20). This research revealed that L* value increased with increasing yam (Da shan line 2 and Heng chun) proportion, which may be attributed to the deep color of the yam. Previous research indicated some varieties of yam were rich in different colored pigments, such as β-carotene, a precursor of Vitamin A, are responsible for different colors in yam ²¹.

The chromaticity coordinate a* measures red when positive and green when negative, while b* measures yellow when positive and blue when negative. Starch, which is colourless, the deep red and yellow color of yam were confirmed by their higher a* and b* values. This may be possibly explained by the fact that Da shan line 2, Tainung 5 and Heng chun were the varieties of yellow, purple and white, respectively. The higher a* and b* values indicated yellow-orange color in steamed rice bowl cake with different varieties of yam (Da shan line 2, Tainung 5 and Heng chun) may due to the presence of polyphenols, carotene, and other compounds ³⁶.

Moreover, it has been reported that β-carotene is the precursor of Vitamin A ²¹, serves as an important nutrient in food as it has functional properties due to their antioxidative activity.

The solubility is the percentage of starch solubilized in water during gelatinization. The solubility of steamed rice bowl cake with different varieties (Da shan line 2, Tainung 5 and Heng chun) and proportion (10, 20 and 30%) of yam is between 4.4 and 10.0% at 65℃ (Table 5). These observation are agreement with the previously reported by Alex and Georges ³⁷. Compared with different varieties and proportions of yam, the control (TCS 17) was significantly higher (10.0%) than Da shan line 2 (8.3, 8.3%), Tainung 5 (4.4, 6.6 and 5.0%) and Heng chun (6.7, 8.3 and 8.3). This research revealed that solubility decreased with addition of yam, which may be attributed to the difference of yam varieties ^{19, 37}.

Starch is insoluble in water at ambient temperature, however if increasing the temperature, starch granules could be swollen up to a certain degree ²⁰. Therefore, swelling power and solubility is the evidence of the interaction between starch chains within the amorphous and crystalline domains, and also provide an evidence of association bonding within the granules of yam starch ¹⁹. Swelling power reflecting the hydration capacity of the water-insoluble starch fraction ³⁷. The swelling power of steamed rice bowl cake with different varieties (Da shan line 2, Tainung 5 and Heng chun) and proportion (10, 20 and 30%) of yam is between 6.9 and 11.4 g/g at 65°C (Table 5). These observation are agreement with the previously research ¹⁹, who reported that the higher swelling power have the lowest associative forces. These may be attributed to the difference of yam varieties ^{19, 37}. Therefore, the heating treatment slightly affected starch granule size. As the temperature higher than 50°C, there was a progressive increase in swelling power of yam ²⁰. In this research, the starch granules gelatinization during steaming about 100°C, starch granule hydrate progressively, hydrogen bonds are ruptured resulting in crystalline regions being converted into amorphous regions and granules continue to imbibe water and swell ^{16, 20}, caused starch granules swelled and collapse ¹⁶. It was observed in Table 5 that Da shan line 2 showed the higher swelling power (11.4, 8.5 and 8.6 g/g) than Tainung 5 (8.3, 7.6 and 6.9 g/g), but similar to the results of Heng chun (9.2, 9.1 and 8.1 g/g). These results are agreement with the observations in swelling power of purple yam, which may be attributed to its higher amylose content ²⁰.

In this research, the greater extent of starch granule swelling was because of the lower protein and lipid content. Sanful and Engmann ³⁶ reported the swelling power of starch gel decreased linearly with protein and lipid content during retrogradation. Especially, the retrogradation rate is highly correlated with the starch content of gel. In a starch gel, maximum rate of retrogradation was observed at a 50–55% solid content ²⁰, similar to the steamed rice bowl cake in this research.

Other components in starch gel, including protein, lipid, and non-starch polysaccharide, would also facilitate or impede the network formation within the swollen starch granules (Chou and Li 2018).

4. Conclusion

Results of this research revealed the effects of different varieties (Da shan line 2, Tainung 5 and Heng chun) and proportion (10, 20 and 30%) of yam on the RSC, texture, color, solubility and swelling power of steamed rice bowl cake. According to the results, the texture profile analysis and color evaluation were significantly affected by different varieties and proportion of yam. The effects of different varieties and proportion of yam on RSC of steamed rice bowl cake showed that control (TCS 17), Da shan line 2 (10, 20 and 30%), and Heng chun (10%) had the lowest RSC. Some similar RSC among different varieties of yam (Tainung 5 and Heng chun) was observed. The above results suggest that firms should consider this method to further develop the Taiwanese traditional rice-made food combination of both yam and rice. Moreover, significant differences (p < 0.05) were found in all experiment variables of texture profile analysis. Compared with other experiment variables, the hardness, cohesiveness and adhesiveness of steamed rice bowl cake were positively related to the proportion of yam. The 30% proportion in Tainung 5 is the most suitable for making the steamed rice bowl cake.

Furthermore, the L*, a* and b* value of steamed rice bowl cake were significantly affected by the different flesh color of yam varieties and proportion. Higher a* and b* values indicate that yellow-orange color in steamed rice bowl can be attributed to different varieties of yam. The solubility and swelling power were significantly affected by the addition of yam. Thus, this suggests that the food industry should consider the application of yam in order to further develop rice-made food. In summary, results of this research has revealed that the yam is not only a potentially useful but also a natural material for high RSC in the manufacturing of rice-made food with better acceptability in texture and color characteristics.

Acknowledgments

We thank the Meiho University in Pingtung supporting excellent equipments and technology. All of the authors participated in this study, and read and approved the final manuscript.

References