Effect of amylose content on structure, texture and α-amylase reactivity of cooked rice

LWT - Food Science and Technology 54 (2013) 224e228

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Effect of amylose content on structure, texture and a-amylase reactivity of cooked rice Shin Lu a, b, Tan-Tiong Cik b, Cheng-yi Lii d, Phoency Lai e, Hua-Han Chen c, * a

China Grain Products Research and Development Institute, Pa-Li, Taipei 24937, Taiwan Department of Food Science, National Chung-Hsing University, Taichung 40254, Taiwan c Department of Food Science, National Penghu University, Penghu 880, Taiwan d Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan e Department of Food and Nutrition, Providence University, Taichung 43301, Taiwan b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 November 2011 Received in revised form 10 May 2013 Accepted 18 May 2013

The susceptibility of cooked rice to a-amylolysis were studied in four Taiwanese rice cultivars differing in amylose contents, i.e. Taichung Native 1(TCN1, indica), Taigung 9 (TG9, japonica), Taichung Sen Waxy 1 (TCSW1, indica waxy), and Taichung Waxy 70 (TCW70, japonica waxy). In addition, the correlation between a-amylolysis and the microstructure and textural properties of the four rice cultivars was investigated. The hardness, gumminess and chewiness of cooked rice followed the order of TCN1>TG9>TCSW1>TCW70. However the waxy rice cultivars showed a higher extent and rate of aamylolysis. Using scanning electron microscopy, the microstructure revealed that the low-amylose rice and waxy rice cultivars contained hollow in the central endosperm. The internal hollow disappeared after a-amylolysis, indicating that a-amylase had penetrated the cooked grain, resulting in the inside-out hydrolysis. These results indicated that the textural properties of cooked rice are influenced mainly by its amylose/amylopectin ratio, followed by influencing the pattern of a-amylolysis. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: a-Amylolysis Texture Cooked rice

1. Introduction Rice is the one of world’s most important cereal crops for human consumption, providing staple food for over half the world’s population. Rice variety, drying and storage conditions, rough rice moisture content, amylose content, starch type, degree of milling, water to rice ratio, cooking methods, pre-cooking and post-cooking processing are the deciding factors in the cooking and textural characteristics of cooked rice (Champagne et al., 1998, 1999; Meullenet, Marks, Griffin, & Daniels, 1999; Meullenet, Marks, Hankins, Griffin, & Daniels, 2000; Perez, Bourne, & Juliano, 1996; Perez, Juliano, Bourne, & Anzadua-Morales, 1993). In particular, a change in amylose content has a greater impact on the texture of the cooked rice than do physical attributes, such as granule morphology, crystallinity and size distribution (Ong & Blanshard, 1995). The rate and extent of starch digestibility are affected by many factors, including: starch granule structure, crystal type, granule size, amylose/amylopectin ratio, molecular structure, and the

* Corresponding author. No. 300, Liu-Ho Rd., Makung City, Penghu Hsien, Taiwan. Tel.: þ886 6 9264115x3809; fax: þ886 6 9260259. E-mail address: [email protected] (H.-H. Chen). 0023-6438/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.lwt.2013.05.028

interaction between starch and other components (Benmoussa, Moldenhauer, & Hamaker, 2007; Sasaki et al., 2009). Amylose content is known to have an obvious impact on starch hydrolysis, and high-amylose rice starch is expected to be resistant to digestion (Hu, Zhao, Duan, Linlin, & Wu, 2004). There have been few attempts to clarify the dependence of the textural properties of cooked rice on the a-amylolysis. Accordingly, this study attempts to elucidate the correlation between textural properties and microstructure and the phenomenon of a-amylolysis of cooked rice. Four Taiwanese rice cultivars differing in amylose contents: Taichung Native 1(TCN1, indica), Taigung 9 (TG9, japonica), Taichung Sen Waxy 1 (TCSW1, indica waxy), and Taichung Waxy 70 (TCW70, japonica waxy), were selected for sample.

2. Experimental 2.1. Rice material Milled rice of Taichung Native (TCN1, indica variety), Taigung (TG9, japonica variety), Taichung Sen Waxy 1 (TCSW1, indica waxy variety) and Taichung Waxy 70 (TCW70, japonica waxy variety) of first crop of 980 were obtained from the Taichung District Agriculture Improvement Station. The content of apparent starch was

S. Lu et al. / LWT - Food Science and Technology 54 (2013) 224e228

measured by using amyloglucosidase/a-amylase method (Total starch assay procedure, Megazyme International Ireland Ltd., Sydney, Australia), and the quantities were 79.4, 77.13, 84.98 and 87.98 g/100 g dry weight for TCN1, TG9, TCSW1 and TCW70, respectively. 2.2. Amylose analysis Amylose content was measured by the modified method (Juliano et al., 1981). Milled rice grains were ground into flours with a cyclone sample mill (Cyclotec 1093, Tecator, Höganäs, Sweden) and passed through a 100-mesh (0.149 mm) sieve. Rice flour was defatted immediately using hexane with the Soxhlet apparatus. Then 100 mg of rice flour put in a conical flask, to which 1 mL of 95 mL/100 mL ethanol and 9 mL of 1 mol/L NaOH were added. The suspension was kept at ambient temperature for 16e24 h, and then distilled water was added to make 100 mL solution. A 5 mL aliquot of the solution was transferred to a 100 mL volumetric flask, and to adjust pH, 1 mL of 1 mol/L acetic acid was added. Then 2 mL of 0.2 g/ L iodine solution (I2: 2 g/KI: 20 g/L) and distilled water were added to make exactly 100 mL. Spectrophotometer measurements were made at 620 nm after the above starch-iodine solution was incubated for 20 min at ambient temperature. Standard curves was generated using the mixture of potato amylose (A0512, Sigma Chem. Co., St. Louis, MO, U.S.A.) and defatted TCW70 rice flour. 2.3. Cooking Rice sample of 5 grains were mixed with water (in a ratio of 2:2.33 g/g) in a beaker and was sealed with a sheet of thin aluminum foil. Thirty min later, the samples were put in an automatic rice cooker (TAC-10H Tatung Co., New Taipei City, Taiwan) and cooked for 15 min. The rice grains were kept covered for additional 30 min. After cooking, the rice grains were allowed to sit for 30 min at ambient temperature before preparation for further analysis. 2.4. Texture analysis The textural properties of cooked rice were determined by using a texture analyzer (Stable Micro System, TA-XT2i, Surrey, UK). After cooking, one kernel of cooked rice was immediately placed inside the test cylindrical probe of 100 mm diameter and compressed the kernel to 0.5 deformation at a pre-test speed of 2.0 mm/s, test and post-test speed of 5.0 mm/s. A forceetime curve was obtained from the test and the measurements of hardness, cohesiveness, gumminess, chewiness and resilience were computed using the Texture Expert software supplied with the instrument. 2.5. Enzyme hydrolysis Rice sample of 5 grains were mixed with water (in a ratio of 2:2.33 g/g) in a screw cap tubes. After the cooking procedure mentioned in section 2.3, 20 mmol/L phosphate buffer (pH 6.9,

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with 0.02 g/100 mL Na N3) was added to make 5 mL solution. A aamylase solution (0.5 mL, 600U, EC3.2.1.1 Bacillus sp., Sigma Chemicals Co., St. Louise, MO, U.S.A.) was added to the cooked rice suspension. The sealed tube was then placed in a shaking water bath 37  C for 10, 20, 30, 40, 50, 60, 180, 360, 540, 720, 1080, 1440 min. Next, a-amylase was inactivated immediately by placing the tubes containing the aliquots in boiling water for 5 min. The amount of hydrolyzable carbohydrate and reducing sugar were then quantified according the phenol-H2SO4 method (Dubois, Gilles, Hamilton, Rebers, & Smith, 1956) and Somogyi-Nelson method (Somogyi, 1945), respectively. All measurements were performed in triplicate. 2.6. Scanning electron microscopy (SEM) The morphological changes of cooked rice grains were observed using a scanning electron microscopy (JSM-5400, JEOL, Tokyo, Japan) at 10 kV. The cooked rice grains were freeze-dried by liquid nitrogen. After cutting using scalpel, the cross section of cooked grains was observed. Samples were attached to an SEM stub using a double-backed cellophane tape. The stub and sample were coated with goldepalladium by an SPI-Module Sputter Coater and then examined and photographed. 2.7. Statistical analysis Analysis of variance and the significance of differences among samples were analysed with the ANOVA procedure and Duncan’s multiple range test of SAS for Windows R 8.0 (SAS Institute Inc., Cary, NC), respectively. 3. Results and discussion 3.1. Amylose content and textural properties The cultivar TCN1 had high amylose content (31.9195 g/100 g dry weight), and the cultivar TG9 had an intermediate amylose content (17.78 g/100 g dry weight). The remaining two cultivars, TCSW1 and TCW70, were waxy, with starch consisting exclusively of amylopectin (Table 1). Table 1 shows some of the textural properties of the cooked rice grains. TCN1, with the highest amylose content, had the highest hardness, whereas japonica waxy, TCW70, was the least one. This clearly indicates that the higher the amylose content, the higher hardness will be. TCN1 also had the higher values for cohesiveness, gumminess, chewiness and resilience: 0.688, 362, 360 and 0.859, respectively. All of the textural parameters demonstrated a strong positive correlation with amylose content (Table 1), indicating that amylose content is the most important influence on each of these parameters. Previous finding concerning the influence of amylose content on rice texture were confirmed: cooked rice with low amylose content is soft and sticky, while rice with high amylose is firm and fluffy (Perdon, Siebenmorgen, Buescher, & Gbur, 1999).

Table 1 Amylose contents and textural parameters of freshly cooked rice by textural profile analysis (TPA). Variety

Amylose content (g/100 g)

TCN1 TG9 TCSW1 TCW70

31.91 17.78 0.92 1.00

   

0.18a,a 0.22b 0.09c 0.09c

Hardness (g)

Cohesiveness (dimensionless)

526  17a 347  11b 1754c 114  13d

0.688 0.574 0.436 0.480

   

0.003a 0.020b 0.025c 0.023c

Gumminess (g)

Chewiness (g)

Resilience (dimensionless)

362  13a 1999b 765c 554d

360  14a 1977b 735c 534c

0.859 0.672 0.503 0.588

   

0.034a 0.030b 0.054c 0.052c

a Means  standard deviations, n ¼ 3 and 5 for amylose content and TPA analysis, respectively; means within the same column with different letters are different significantly at p < 0.05.

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3.2. Kinetics of in vitro starch digestion There were some soluble compounds leached from the rice kernels after cooking that were assumed to be mainly carbohydrates. The amounts of total carbohydrates and reducing sugars that leached into the buffer are provided in Table 2, in weight percentages compared with cooked rice solids. The total amount of carbohydrate leached was in the order of TCW70 (2.86 g/100 g dry weight)>TCSW1 (1.28 g/100 g dry weight)>TG9 (0.79 g/100 g dry weight)>TCN1 (0.32 g/100 g dry weight). As to the total reducing sugars, TCW70 had 0.16 g/100 g dry weight, the other cultivars only had 0.02e0.05 g/100 g dry weight. The initial susceptibility of cooked rice to a-amylase attack was in the order of TCSW1 > TCW70 > TCN1 ¼ TG9 and TCSW1 > TCN1 > TCW70 > TG9 for total carbohydrates and reducing power, respectively. But after the long-term reaction time, the susceptibility was changed in the order of TCW70 ¼ TCSW1 ¼ TG9 > TCN1 and TCW70 > TCSW1 > TG9 ¼ TCN1, for total carbohydrates and reducing power, respectively. Amylolysis activities were determined by measuring the amount of reducing sugars produced from cooked rice. The amylolysis curves for cooked samples are shown in Fig. 1(A). The initial rates of amylolysis, recorded within 0e180 min, were linear by linear regression. The slopes were: 0.0941, 0.079, 0.1168 and 0.1272 for TCN1, TG9, TCSW1 and TCW70, respectively. The non-waxy cultivars, TCN1 and TG9, reached equilibrium conditions after 540 and 720 min, respectively. The amylolysis curves of the waxy cultivars continue and do not seem to be terminated after 24 h. The amounts of carbohydrates soluble during a-amylolysis are shown in Fig. 1(B). Higher extents of solubilization were observed for the waxy cultivars than for the non-waxy cultivars. This same tendency was noted in the initial rate recorded within 0e180 mm, wherein the waxy cultivars revealed the higher values of 0.3863 and 0.2885 for TCSW1 and TCW70, respectively. The extent and rate of amylolysis in the waxy cultivars, TCSW1 and TCW70, are higher than in the non-waxy cultivars. In particular, the indica cultivars, TCN1 and TCSW1, reached equilibrium conditions after 540 min, but the japonica cultivars, TK9 and TCW70, continued and did not appear to be terminated after 24 h. Previous reports indicated that the amount of native starch hydrolysis by amylase is inversely related to the amylose content (CarrÈ, 2004; Cone & Wolters, 1990; Evans & Thompson, 2004; Li, Vasanthan, Hoover, & Rossnagel, 2004; Rendleman, 2000; Riley et al., 2004), with high amylose starches being especially resistant (Gallant, Bouchet, Buleon, & Perez, 1992). Table 2 summarizes the parameters in the starch hydrolysis model, the k value of total carbohydrate and reducing power are about 2.5e7 and 1.6e3.1, respectively. Indica cultivars (TCN1 and TCSW1) showed the higher k value than Japonica cultivars (TG9 and TCW70) from the hydrolysis pattern of total carbohydrates. But

Fig. 1. Changes in the degradation ratio of cooked rice with a-amylolysis time (A: degradation percentage (%) ¼ RSt/wt. of rice (d.b.)*100, RSt ¼ the reducing sugar contents on hydrolysis for t min; B: degradation ratio ¼ TCt/wt. of rice (d.b.)*100, TCt ¼ the total carbohydrate contents on hydrolysis for t min). Data are means  standard deviation of triplicates. TCN1(C), TG9(,), TCSW1(þ), TCW70(7).

from the hydrolysis pattern of reducing power, non-waxy rice cultivars (TCSW1 and TCW70) revealed the less k value than waxy rice cultivars (TCN1 and TG9). Generally, cooked-grain resistance to disintegration is also related to amylose content, with high-amylose rice being the most resistant and waxy rice the least resistant (Juliano, 1979). The tendency to disintegrate may increase the contact area of aamylase, which would in turn be followed by an improved the extent and rate of amylolysis. Therefore, the rice cultivar (Indica or Japonica) affects the hydrolysis pattern of total carbohydrates, and amylose content of rice shows the influence in the hydrolysis pattern of reducing power.

Table 2 Compositions of soluble starch materials of cooked rice before and after a-amylolysis and first-order kinetic parameters for a-amylolysis. Variety Soluble materials

Kinetic parameters

Blank

0h

24 h

Sugar (g/100 g DW)

Reducing sugar Total carbohydrates Reducing sugar Total carbohydrates Reducing sugar Total carbohydrates k (g/100 g DW) (g/100 g DW) (g/100 g DW) (g/100 g DW) (g/100 g DW) (g/100 g DW) TCN1 TG9 TCSW1 TCW70 a

0.03 0.05 0.02 0.16

   

0.01c,a 0.00b 0.00d 0.00a

0.32 0.79 1.28 2.86

   

0.03d 0.09c 0.07b 0.06a

1.20 0.50 2.48 0.98

   

0.22b 0.18d 0.14a 0.16c

6.72 6.46 14.07 8.29

   

0.70c 0.16c 0.62a 0.94b

32.57 32.65 41.04 45.79

   

2.01c 1.49c 1.68b 1.76a

70.25 84.85 88.67 92.22

   

5.13b 5.92a 4.12a 2.18a

3.1 3 1.7 1.6

Means  standard deviations, n ¼ 3; means within the same column with different letters are different significantly at p < 0.05.

Total carbohydrate (g/100 g DW)

R2

k

R2

0.951 0.958 0.936 0.921

6.3 2.5 7 2.7

0.971 0.704 0.832 0.742

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Fig. 2. Scanning electron micrograms of entire fracture face in the endosperm of cooked rice after just cooking and subsequent hydrolysis for 1 h and 24 h by Bacillus sp. a-amylase.

3.3. Microstructure of cooked rice with amylolysis Fig. 2 shows SEM cross sections of cooked grains with and without amylolysis treatment. The cooked grain of TCN1 appears to demonstrate the most even and compact structure. The other cultivars contain hollows in the central endosperm, and the waxy rice cultivars show the larger hollows than is found in TG9. In addition, TCSW1 exhibited the largest hollow and a more compact structure in the center. Voids or “internal hollows” in cooked rice grains were previously observed by NMR microimaging (Horigane et al., 1999, 2000), and by SEM (Ogawa, Glenn, Orts, & Wood, 2003). The appearance of hollows can be explained by the fact, which the microenvironment of the grain periphery is different from that of the grain center. The peripheral cells are smaller in diameter and contain more protein than those in the center of the grain. Additionally, during cooking a buildup of pressure occurs in the center of the grain that does not happen at the periphery. The periphery therefore remains intact during cooking, whereas the grain center contains both intact and voided areas (Ogawa et al., 2003). However, the hollows are not present in the center of TCN1, which have high amylose content. The high amylose content results in the strong physical structure of TCN1, and makes it more difficult to disrupt the central part of the grain during cooking. In addition, the appearance of hollows could be

predicted to affect the texture properties during compression processing. During amylolysis, the hollow cavities of TG9, TCSW1 and TCW70 disappeared gradually with increasing treatment time. This phenomenon explains the idea that a-amylase disrupted the structure near the hollow cavity, which was followed by a shrinking of the cooked grain, especially in waxy rice cultivars. In addition, a-amylase resulted in a loose structure of cooked rice. After 24 h of amylolysis, the hydrolysis extents reaches 70e92 g/100 g dry weight, so the resistant grains can still retain the shape of cooked rice. 4. Conclusion This study revealed that amylose content affects the textural properties, starch digestibility and morphology of the cooked rice. High-amylose rice appeared harder and less sticky than waxy rice cultivars. This corresponded to the observed morphology, where internal hollows in low-amylose rice and waxy rice cultivars resulted in a soft texture. Disruption in the structure of these internal hollows illustrated that a-amylase penetrated into the grain during amylolysis. Therefore, waxy rice with a soft texture revealed a higher extent and rate of amylolysis, whereas the high-amylose indica cultivar, TCN1, had a significantly higher resistant content.

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References Benmoussa, M., Moldenhauer, K. A. K., & Hamaker, B. R. (2007). Rice amylopectin fine structure variability affects starch digestion properties. Journal of Agricultural and Food Chemistry, 55(4), 1475e1479. CarrÈ, B. (2004). Causes for variation in digestibility of starch among feedstuffs. World’s Poultry Science Journal, 60(01), 76e89. Champagne, E. T., Bett, K. L., Vinyard, B. T., McClung, A. M., Barton, F. E., Moldenhauer, K., et al. (1999). Correlation between cooked rice texture and rapid visco analyser measurements. Cereal Chemistry, 76(5), 764e771. Champagne, E. T., Lyon, B. G., Bong, K. M., Vinyard, B. T., Bett, K. L., Barton, F. E., et al. (1998). Effects of postharvest processing on texture profile analysis of cooked rice. Cereal Chemistry, 75(2), 181e186. Cone, J. W., & Wolters, M. G. E. (1990). Some properties and degradability of isolated starch granules. Starch e Stärke, 42(8), 298e301. Dubois, M., Gilles, K. A., Hamilton, J. K., Rebers, P. A., & Smith, F. (1956). Colorimetric method for determination of sugars and related substances. Analytical Chemistry, 28, 350e356. Evans, A., & Thompson, D. B. (2004). Resistance to alpha-amylase digestion in four native high-amylose maize starches. Cereal Chemistry, 81(1), 31e37. Gallant, D. J., Bouchet, B., Buleon, A., & Perez, S. (1992). Physical characteristics of starch granules and susceptibility to enzymatic degradation. European Journal of Clinical Nutrition, 46(Suppl. 2), S3eS16. Horigane, A. K., Engelaar, W., Toyoshima, H., Ono, H., Sakai, M., Okubo, A., et al. (2000). Differences in hollow volumes in cooked rice grains with various amylose contents as determined by NMR micro imaging. Journal of Food Science, 65(3), 408e412. Horigane, A. K., Toyoshima, H., Hemmi, H., Engelaar, W. M. H. G., Okubo, A., & Nagata, T. (1999). Internal hollows in cooked rice grains (Oryza sativa cv. Koshihikari) observed by NMR micro imaging. Journal of Food Science, 64(1), 1e5. Hu, P., Zhao, H., Duan, Z., Linlin, Z., & Wu, D. (2004). Starch digestibility and the estimated glycemic score of different types of rice differing in amylose contents. Journal of Cereal Science, 40(3), 231e237. Juliano, B. O. (1979). The chemical basis of rice grain quality. In N. C. Brady (Ed.), Chemical aspects of rice grain quality (pp. 74). Manila, Philippines: International Rice Research Institute.

Juliano, B. O., Perez, C. M., Blakeney, A. B., Castillo, T., Kongseree, N., Laignelet, B., et al. (1981). International cooperative testing on the amylose content of milled rice. Starch e Stärke, 33(5), 157e162. Li, J. H., Vasanthan, T., Hoover, R., & Rossnagel, B. G. (2004). Starch from hull-less barley: V. In-vitro susceptibility of waxy, normal, and high-amylose starches towards hydrolysis by alpha-amylases and amyloglucosidase. Food Chemistry, 84(4), 621e632. Meullenet, J. F. C., Marks, B. P., Griffin, V. K., & Daniels, M. J. (1999). Effects of rough rice drying and storage conditions on sensory profiles of cooked rice. Cereal Chemistry, 76(4), 483e486. Meullenet, J. F. C., Marks, B. P., Hankins, J. A., Griffin, V. K., & Daniels, M. J. (2000). Sensory quality of cooked long-grain rice as affected by rough rice moisture content, storage temperature, and storage duration. Cereal Chemistry, 77(2), 259e263. Ogawa, Y., Glenn, G. M., Orts, W. J., & Wood, D. F. (2003). Histological structures of cooked rice grain. Journal of Agricultural and Food Chemistry, 51(24), 7019e7023. Ong, M. H., & Blanshard, J. M. V. (1995). Texture determinants in cooked, parboiled rice. I: rice starch amylose and the fine structure of amylopectin. Journal of Cereal Science, 21(3), 251e260. Perdon, A. A., Siebenmorgen, T. J., Buescher, R. W., & Gbur, E. E. (1999). Starch retrogradation and texture of cooked milled rice during storage. Journal of Food Science, 64(5), 828e832. Perez, C. M., Bourne, M. C., & Juliano, B. O. (1996). Measuring hardness distribution of cooked rice by single-grain puncture. Journal of Texture Studies, 27, 1e13. Perez, C. M., Juliano, B. O., Bourne, M. C., & Anzadua-Morales, A. (1993). Hardness of cooked milled rice by instrumental and sensory methods. Journal of Texture Studies, 24, 81e94. Rendleman, J. A., Jr. (2000). Hydrolytic action of alpha-amylase on high-amylose starch of low molecular mass. Biotechnology and Applied Biochemistry, 31(Pt 3), 171e178. Riley, C. K., Wheatley, A. O., Hassan, I., Ahmad, M. H., Morrison, E. Y. S. A., & Asemota, H. N. (2004). In vitro digestibility of raw starches extracted from five Yam (Dioscorea spp.) species grown in Jamaica. Starch e Stärke, 56(2), 69e73. Sasaki, T., Kohyama, K., Suzuki, Y., Okamoto, K., Noel, T. R., & Ring, S. G. (2009). Physicochemical characteristics of waxy rice starch influencing the in vitro digestibility of a starch gel. Food Chemistry, 116(1), 137e142. Somogyi, M. (1945). A new reagent for the determination of sugars. The Journal of Biological Chemistry, 160, 61e68.