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Starch digestibility of five cooked black bean (Phaseolus vulgaris L.) varieties
ARTICLE IN PRESS
JOURNAL OF FOOD COMPOSITION AND ANALYSIS
Journal of Food Composition and Analysis 17 (2004) 605–612 www.elsevier.com/locate/jfca
Original Article
Starch digestibility of five cooked black bean (Phaseolus vulgaris L.) varieties Apolonio Vargas-Torresa, Perla Osorio-D!ıaza, Jose! J. Islas-Herna! ndeza, c ! Juscelino Tovarb, Octavio Paredes-Lopez , Luis A. Bello-Pe! reza,* ! Centro de Desarrollo de Productos Bioticos del IPN, Km 8.5 carr. Yautepec-Jojutla, Colonia San Isidro, Apartado postal 24, 62731 Yautepec, Morelos, Mexico b Instituto de Biolog!ıa Experimental, Facultad de Ciencias, Universidad Central de Venezuela, Apartado Postal 47069, Caracas 1041-A, Venezuela c ! y Estudios Avanzados del IPN, Unidad Irapuato, Apartado Postal 629, 36500 Irapuato, Centro de Investigacion Guanajuato, Mexico a
Received 12 February 2003; received in revised form 26 August 2003; accepted 22 September 2003
Abstract Five common bean varieties were cooked and studied regarding starch digestibility. Cooking time of different cultivars ranged between 2.55 and 5.92 h. Available starch (AS) values decreased with the storage time and the bean sample that had the lowest AS content (control sample, without storage) showed the shortest cooking time. A similar pattern was found for resistant starch (RS); the varieties that had the longest cooking time presented the widest range in RS values, measured as the difference between the control sample and the value obtained in the sample stored during 96 h. The retrograded RS (RRS) depended on the variety and even more on the molecular structure of each starch. The in vitro a-amylolysis rate decreased with the storage time; the samples with the smallest hydrolysis percentage had the highest RS content. These results suggest that some bean varieties could be recommended depending on the specific dietetic use of beans. r 2003 Elsevier Inc. All rights reserved. Keywords: Starch; Resistant starch; Beans; Starch hydrolysis; Legumes
*Corresponding author. Fax: +52-735-394-1896. E-mail address: [email protected] (L.A. Bello-Pe! rez). 0889-1575/$ - see front matter r 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jfca.2003.09.008
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1. Introduction Common beans (Phaseolus vulgaris L.), widely grown and consumed in various regions of the world, are a rich and inexpensive source of proteins (20–25%) and carbohydrates (50–60%) for a large part of the world’s population, mainly in developing countries (Rehman et al., 2001). They are beneficial for health, with low glycemic index (GI) (Foster-Powell and Brand-Miller, 1995). Although carbohydrates are the major component of legumes, relatively little work has been carried out on this fraction (Bravo et al., 1998). Of these, starch and non-starch polysaccharides (dietary fiber) are the major constituents, with small but significant amounts of oligosaccharides (Bravo et al., 1998). In Mexico there is a great number of varieties and cultivars of black beans, which, in many cases, are cultivated for farmers’ self consumption. These varieties can have different energy value, depending on their starch bioavailability. Besides being a major plant metabolite, starch is also the dominant carbohydrate in the human . diet (Bjorck et al., 1994; Skrabanja et al., 1999). Current knowledge on nutritional features of starch indicates that the bioavailability of the polysaccharide in foods may vary widely. Hence, a nutritional classification of dietary starch has been proposed, which takes into account both the kinetic component and the completeness of its digestibility, thus comprising rapidly digestible, slowly digestible and indigestible—or resistant—fractions (Englyst et al., 1992). Several reasons explain the heterogeneity in the digestibility behavior of food starches. It is known, for instance, that structural, compositional, and fine chemical characteristics of a particular food influence the . enzymatic availability of starch and, hence, the rate of the polysaccharide digestion (Bjorck et al., 1994). Until some decades ago, starch had been considered an available carbohydrate that was completely digested and absorbed in the small intestine. However, it is now known that there exists a starch fraction that is resistant to enzyme digestion, passing through the small intestine and reaching the large bowel where it may be fermented by the colonic microflora. This fraction is called resistant starch (RS) and is defined as the sum of starch and the products of starch degradation not absorbed in the small intestine of healthy individuals (Asp, 1992). Recently, there has been a considerable interest in the possibility of improving diabetic control by altering the glycemic impact of the carbohydrates ingested. A tool for ranking foods with respect to their blood glucose raising potential is the GI concept (Jenkins et al., 1981). A nutritional variable frequently linked to low GI properties is RS. The types of RS identified in foods are physically entrapped starch within whole or partly milled grains or seeds (RS1), native, ungelatinized granules or B-type starches (RS2), and retrograded starch (RS3) (Englyst et al., 1992). Raw and processed legumes have been shown to contain significant amounts of RS in comparison with other products such as cereals, tubers and unripe fruits (Bravo et al., 1998; Jenkins et al., 1982; Tovar et al., 1992b; Tovar and Melito, 1996; Velasco et al., 1997; Bravo et al., 1999). For this reason, the starch digestion rate and therefore the release of glucose into the blood stream are slower after the ingestion of legumes, resulting in reduced glycemic and insulinemic post-prandial responses in comparison with cereal grains or potatoes (Jenkins et al., 1982; Tovar et al., 1992a). In addition to starch, legumes contain high amounts of dietary fiber in a form that gives cell walls high resistance toward disintegration during cooking (Tovar et al., 1992a; Melito and Tovar, 1995; Wursch . et al., 1986). This, along with the presence of certain antinutrients, may account for the low digestibility of starch in pulses.
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The objective of the present study was to investigate the influence of cooking and storage time on the available starch (AS) content, RS level and in vitro rate of starch in different bean varieties. In view of the common Mexican household practice of cooking large batches of beans and keeping them in cold-store for several days before consumption, particular attention was paid to the impact of such a procedure on the digestibility of the legume starch constituent.
2. Materials and methods 2.1. Sample preparation Black seeds from five different common bean cultivars (Tacana, Zitlala, Huasteco, TLP-19 and Veracruz) were generated by breeding through a special improvement program of INIFAPIguala, Mexico. The non-stored beans were cooked using a Mattson cooker type and the cooking ! time was determined for each bean variety (Reyes-Moreno and Paredes-Lopez, 1993). Using the cooking time determined for each material in the previous experiment, 100 g of beans were cooked in 200 mL water, the samples, cooked seeds plus cooking water, were cooled down at room temperature (control sample) and stored during 24, 48, 72 and 96 h at 4 C, simulating cooking and store conditions applied in Mexican households. After each storage time, the samples were equilibrated at room temperature (25 C) during 15 min before analysis. The control sample (without storage) also was analyzed. Three replicates were prepared. All cooked samples were mechanically homogenized (Polytron PT 1200 Kinematica AG, Switzerland) for further analysis. 2.2. Digestibility tests Potentially AS content was assessed following the multienzymatic protocol of Holm et al. (1986) using Termamyls (Novo A/S, Copenhagen) and amyloglucosidase (Boehringer, Mannheim). The method proposed by Gon˜i et al. (1996) was employed to estimate the amount of indigestible starch (comprising part of RS1 plus RS2 and RS3 fractions). Retrograded resistant starch (RS3) content was measured as starch remnants in dietary fiber residues, according to the so-called Lund method as modified by Saura-Calixto et al. (1993). The in vitro rate of hydrolysis was measured using hog pancreatic amylase according to Holm et al. (1985); each assay was run with 500 mg AS. In all these measurements a fraction of the cooked bean was weighed and placed into a test tube or a beaker and homogenized with the corresponding solution (depending on the technique) under controlled conditions: first step (speed level 2, 1 min) and second step (speed level 2.5, 1 min) using a polytron homogenizer (Polytron PT 1200 Kinematica AG, Switzerland). 2.3. Statistics Results were expressed by means7standard deviation of the three separate determinations. Comparison of means was performed by one-way analysis of variance (ANOVA) followed by Tukey’s multiple comparison tests using minitab-Windows computer system, Version 2.1, 1995.
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3. Results and discussion 3.1. Cooking time The non-stored samples showed variation in cooking time, because Tacana (5.9270.2 h) and Zitlala (4.5070.5 h) had the highest values, and Veracruz the lowest ones (2.5570.04 h). The TLP-19 (3.2870.13 h) and Huasteco (3.2670.12 h) did not show differences in cooking time. 3.2. Available starch Appreciable variation was recorded among samples for AS content (Table 1). The values for control samples ranged between 21.7% (Veracruz) and 32.16% (Tacana). Only the Huasteco and Zitlala varieties were not statistically different (a ¼ 0:05). The lowest value of AS determined for Veracruz variety and the highest for Tacana could be associated with the cooking time determined in both samples. Tovar and Melito (1996) reported AS content of 37.4% and 39.1% in two varieties of raw and cooked beans, and a value of 37.57% was obtained in beans cooked for 36 min after soaking in water at 4 C for 18 h (Rosin et al. 2002). However, Bravo et al. (1999) reported lower AS value in five different raw beans (between 26.6% and 35.7%), similar to those found in this study. These results suggest that some cultivars, such as Veracruz, may have low digestibility, whereas Tacana has high digestibility. The TLP-19 sample after 72 h of storage presented a decrease in AS values and Tacana, after 24 h. However, Huasteco and Veracruz did not change their AS values (a ¼ 0:05) after 48 h of storage, and Zitlala did not show differences in its AS values after 72 h. 3.3. Resistant starch The control samples of Tacana and TLP-19 had the highest values, and Huasteco and Zitlala the lowest (Table 2). In general RS values increased with storage time, and the highest difference in the values between control sample and the sample stored during 96 h was shown by Zitlala and Tacana varieties, which also presented high cooking time. The destruction of cotyledon in the seed produced higher enzyme accessibility and a decrease in RS amounts. Bravo et al. (1999) found RS values that ranged between 3.40% and 8.31% for five different bean varieties using boiled samples Table 1 Available starch in cooked beans according to different storage times and varieties (%)a,b Storage time (h)
Tacana
Zitlala
TLP19
Huasteco
Veracruz
Control 24 48 72 96
32.2+0.57a 31.4+0.70a 24.0+0.31b 24.0+0.44b 20.7+0.48c
27.0+0.20a 23.2+0.60b 19.0+0.30c 14.5+0.30d 14.1+0.31d
28.4+0.70a 28.3+0.45a 28.2+0.59a 24.2+0.38b 21.7+0.70c
27.5+0.24a 24.2+0.29b 21.4+0.30c 21.9+0.30c 21.8+1.39c
21.7+0.50a 20.0+0.31b 13.8+0.29c 14.2+0.62c 13.5+0.61c
Means in columns not sharing the same letter are significantly different (Po0:001). a Values are means of three replicates7s.d., dry matter basis. b Cooking times were: Tacana 5.92 h, Zitlala 4.50 h, TLP-19 3.28 h, Huasteco 3.26 and Veracruz 2.55 h.
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of whole grains, the same method employed here (Gon˜i et al., 1996). They found that differences between their samples were mainly due to agronomic variety. For samples evaluated in this investigation, lower variations in RS values were found among varieties as compared to the work reported by Bravo et al. (1999). 3.4. Retrograded resistant starch (RRS) The values of RRS in the control samples of Huasteco and Veracruz varieties were not statistically different (a ¼ 0:05). Tacana bean had the highest RRS value and Zitlala the lowest one (Table 3). The type of variety played an important role in this behavior, which is related with the molecular starch structure (Yuan et al., 1993; Shi and Seib, 1995). In general, RRS values tended to increase with the storage time; the retrogradation tendency, calculated as the difference between RRS values for the sample stored for 96 h and the control sample, showed that TLP19 variety had the highest value and Veracruz the lowest. 3.5. Hydrolysis rate The in vitro a-amylolysis reaction for the various beans is represented in Fig. 1. All samples analyzed showed the highest hydrolysis percentage at 90 min, except Tacana stored for 96 h that Table 2 Resistant starch content in cooked beans according to different storage times and varieties (%)a,b Storage time (h)
Tacana
Zitlala
TLP-19
Huasteco
Veracruz
Control 24 48 72 96
5.1+0.05a 5.5+0.21b 6.3+0.15c 6.4+0.31c 7.7+0.31d
3.5+0.09a 3.4+0.04b 4.0+0.10c 5.6+0.18d 6.6+0.38e
4.9+0.17a 6.0+0.22b 6.3+0.13c 6.4+0.18b,c 7.4+0.40d
3.6+0.17a 3.8+0.17a 4.8+0.36b 5.1+0.33b 5.8+0.18c
4.0+0.07a,c 4.7+0.14b 4.2+0.20b,c 5.0+0.22d 6.3+0.13e
Means in columns not sharing the same letter are significantly different (Po0:001). a Values are means of three replicates7s.d., dry matter basis. b Cooking times were: Tacana 5.92 h, Zitlala 4.50 h, TLP-19 3.28 h, Huasteco 3.26 and Veracruz 2.55 h.
Table 3 Retrograded resistant starch content in cooked beans according to different storage times and varieties (%)a,b Storage time (h)
Tacana
Zitlala
TLP-19
Huasteco
Veracruz
Control 24 48 72 96
3.1+0.01a 3.1+0.12a 3.8+0.22b 4.6+0.11c 4.6+0.18c
1.6+0.18a 2.4+0.22b 2.3+0.08b 3.3+0.20c 3.3+0.03c
2.8+0.06a 3.3+0.18b 4.9+0.29c 5.5+0.35d 5.8+0.18d
2.5+0.07a 2.5+0.11a 2.6+0.25a 4.0+0.14b 4.1+0.29b
2.3+0.15a 2.5+0.15a 2.5+0.14a 3.3+0.16b 3.7+0.10b
Means in columns not sharing the same letter are significantly different (Po0:001). a Values are means of three replicates7s.d., dry matter basis. b Cooking times were: Tacana 5.92 h, Zitlala 4.50 h, TLP-19 3.28 h, Huasteco 3.26 and Veracruz 2.55 h.
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Hydrolysis (%)
25
20
15
10
5
0 0
10
20
30
40
50
60
70
80
90
100
Time (min)
Fig. 1. In vitro starch hydrolysis of cooked beans. E, Tacana (control); , Tacana (96 h of storage); ’, Veracruz (control); , Veracruz (96 h of storage); m, Zitlala (control); K, Zitlala (96 h of storage).
presented a tendency to increase the hydrolysis percentage, perhaps due to the fact that this starch is of slow digestibility. The control samples at 15 min showed hydrolysis between 7% and 10%, without statistical differences between Tacana and Zitlala samples. However, at 90 min, Zitlala showed the highest hydrolysis (approximately 28%) and Tacana the lowest (approximately 17%). Values between 10% and 30% of hydrolysis at 90 min were obtained in cooked legumes by Bravo et al. (1998). Several factors are involved in the reduced bioavailability of legume starches. The presence of intact tissue/cell structures enclosing starch granules hinders the swelling and solubilization of starch resulting in reduced in vitro digestion rate (Tovar et al., 1990). Permanence of intact starch granules trapped within cells in precooked legume flours was observed even after extensive homogenization and pepsin treatment (Tovar et al., 1991). The samples stored fpr 96 h had lower hydrolysis values than their respective control samples. At 90 min, rates of around 12% were obtained for Tacana and Veracruz, and 17% for Zitlala. The highest hydrolysis showed by Zitlala and the lowest one by TLP-19 (data not shown) agree with the RRS because the former had the lowest RRS value (Table 3).
4. Conclusions The cooking and storage times, in addition to the variety may influence digestibility of starch in common beans. Therefore, some cultivars could be recommended for specific dietetic uses. For instance, in rural Mexican areas where beans and tortillas are the main staple, a variety leading to the highest starch digestibility should be used, whereas diabetic and hyperlipidemic subjects may be advised to consume beans with decreased starch digestibility.
Acknowledgements We appreciate the financial support from OMNILIFE-CONACYT, CGPI-IPN, IPICS, LANFOOD, COSNET and COFAA-IPN. The supply of the samples by Eng. Reza of INIFAPIguala, Mexico is gratefully acknowledged.
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Velasco, Z.I., Rascon, A., Tovar, J., 1997. Enzymic availability of starch in cooked black beans (Phaseolus vulgaris L) and cowpeas (Vigna spp.). Journal of Agricultural and Food Chemistry 45, 1548–1551. Wursch, . P., Del Vedovo, S., Koellreuter, B., 1986. Cell structure and starch nature as key determinants of the digestion rate of starch in legumes. American Journal of Clinical Nutrition 43, 25–29. Yuan, R.C., Thompson, D.B., Boyer, C.D., 1993. Fine structure of amylopectin in relation to gelatinization and retrogradation behavior of maize starches from three wx-containing genotypes in two inbred lines. Cereal Chemistry 70, 81–89.
JOURNAL OF FOOD COMPOSITION AND ANALYSIS
Journal of Food Composition and Analysis 17 (2004) 605–612 www.elsevier.com/locate/jfca
Original Article
Starch digestibility of five cooked black bean (Phaseolus vulgaris L.) varieties Apolonio Vargas-Torresa, Perla Osorio-D!ıaza, Jose! J. Islas-Herna! ndeza, c ! Juscelino Tovarb, Octavio Paredes-Lopez , Luis A. Bello-Pe! reza,* ! Centro de Desarrollo de Productos Bioticos del IPN, Km 8.5 carr. Yautepec-Jojutla, Colonia San Isidro, Apartado postal 24, 62731 Yautepec, Morelos, Mexico b Instituto de Biolog!ıa Experimental, Facultad de Ciencias, Universidad Central de Venezuela, Apartado Postal 47069, Caracas 1041-A, Venezuela c ! y Estudios Avanzados del IPN, Unidad Irapuato, Apartado Postal 629, 36500 Irapuato, Centro de Investigacion Guanajuato, Mexico a
Received 12 February 2003; received in revised form 26 August 2003; accepted 22 September 2003
Abstract Five common bean varieties were cooked and studied regarding starch digestibility. Cooking time of different cultivars ranged between 2.55 and 5.92 h. Available starch (AS) values decreased with the storage time and the bean sample that had the lowest AS content (control sample, without storage) showed the shortest cooking time. A similar pattern was found for resistant starch (RS); the varieties that had the longest cooking time presented the widest range in RS values, measured as the difference between the control sample and the value obtained in the sample stored during 96 h. The retrograded RS (RRS) depended on the variety and even more on the molecular structure of each starch. The in vitro a-amylolysis rate decreased with the storage time; the samples with the smallest hydrolysis percentage had the highest RS content. These results suggest that some bean varieties could be recommended depending on the specific dietetic use of beans. r 2003 Elsevier Inc. All rights reserved. Keywords: Starch; Resistant starch; Beans; Starch hydrolysis; Legumes
*Corresponding author. Fax: +52-735-394-1896. E-mail address: [email protected] (L.A. Bello-Pe! rez). 0889-1575/$ - see front matter r 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jfca.2003.09.008
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1. Introduction Common beans (Phaseolus vulgaris L.), widely grown and consumed in various regions of the world, are a rich and inexpensive source of proteins (20–25%) and carbohydrates (50–60%) for a large part of the world’s population, mainly in developing countries (Rehman et al., 2001). They are beneficial for health, with low glycemic index (GI) (Foster-Powell and Brand-Miller, 1995). Although carbohydrates are the major component of legumes, relatively little work has been carried out on this fraction (Bravo et al., 1998). Of these, starch and non-starch polysaccharides (dietary fiber) are the major constituents, with small but significant amounts of oligosaccharides (Bravo et al., 1998). In Mexico there is a great number of varieties and cultivars of black beans, which, in many cases, are cultivated for farmers’ self consumption. These varieties can have different energy value, depending on their starch bioavailability. Besides being a major plant metabolite, starch is also the dominant carbohydrate in the human . diet (Bjorck et al., 1994; Skrabanja et al., 1999). Current knowledge on nutritional features of starch indicates that the bioavailability of the polysaccharide in foods may vary widely. Hence, a nutritional classification of dietary starch has been proposed, which takes into account both the kinetic component and the completeness of its digestibility, thus comprising rapidly digestible, slowly digestible and indigestible—or resistant—fractions (Englyst et al., 1992). Several reasons explain the heterogeneity in the digestibility behavior of food starches. It is known, for instance, that structural, compositional, and fine chemical characteristics of a particular food influence the . enzymatic availability of starch and, hence, the rate of the polysaccharide digestion (Bjorck et al., 1994). Until some decades ago, starch had been considered an available carbohydrate that was completely digested and absorbed in the small intestine. However, it is now known that there exists a starch fraction that is resistant to enzyme digestion, passing through the small intestine and reaching the large bowel where it may be fermented by the colonic microflora. This fraction is called resistant starch (RS) and is defined as the sum of starch and the products of starch degradation not absorbed in the small intestine of healthy individuals (Asp, 1992). Recently, there has been a considerable interest in the possibility of improving diabetic control by altering the glycemic impact of the carbohydrates ingested. A tool for ranking foods with respect to their blood glucose raising potential is the GI concept (Jenkins et al., 1981). A nutritional variable frequently linked to low GI properties is RS. The types of RS identified in foods are physically entrapped starch within whole or partly milled grains or seeds (RS1), native, ungelatinized granules or B-type starches (RS2), and retrograded starch (RS3) (Englyst et al., 1992). Raw and processed legumes have been shown to contain significant amounts of RS in comparison with other products such as cereals, tubers and unripe fruits (Bravo et al., 1998; Jenkins et al., 1982; Tovar et al., 1992b; Tovar and Melito, 1996; Velasco et al., 1997; Bravo et al., 1999). For this reason, the starch digestion rate and therefore the release of glucose into the blood stream are slower after the ingestion of legumes, resulting in reduced glycemic and insulinemic post-prandial responses in comparison with cereal grains or potatoes (Jenkins et al., 1982; Tovar et al., 1992a). In addition to starch, legumes contain high amounts of dietary fiber in a form that gives cell walls high resistance toward disintegration during cooking (Tovar et al., 1992a; Melito and Tovar, 1995; Wursch . et al., 1986). This, along with the presence of certain antinutrients, may account for the low digestibility of starch in pulses.
ARTICLE IN PRESS A. Vargas-Torres et al. / Journal of Food Composition and Analysis 17 (2004) 605–612
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The objective of the present study was to investigate the influence of cooking and storage time on the available starch (AS) content, RS level and in vitro rate of starch in different bean varieties. In view of the common Mexican household practice of cooking large batches of beans and keeping them in cold-store for several days before consumption, particular attention was paid to the impact of such a procedure on the digestibility of the legume starch constituent.
2. Materials and methods 2.1. Sample preparation Black seeds from five different common bean cultivars (Tacana, Zitlala, Huasteco, TLP-19 and Veracruz) were generated by breeding through a special improvement program of INIFAPIguala, Mexico. The non-stored beans were cooked using a Mattson cooker type and the cooking ! time was determined for each bean variety (Reyes-Moreno and Paredes-Lopez, 1993). Using the cooking time determined for each material in the previous experiment, 100 g of beans were cooked in 200 mL water, the samples, cooked seeds plus cooking water, were cooled down at room temperature (control sample) and stored during 24, 48, 72 and 96 h at 4 C, simulating cooking and store conditions applied in Mexican households. After each storage time, the samples were equilibrated at room temperature (25 C) during 15 min before analysis. The control sample (without storage) also was analyzed. Three replicates were prepared. All cooked samples were mechanically homogenized (Polytron PT 1200 Kinematica AG, Switzerland) for further analysis. 2.2. Digestibility tests Potentially AS content was assessed following the multienzymatic protocol of Holm et al. (1986) using Termamyls (Novo A/S, Copenhagen) and amyloglucosidase (Boehringer, Mannheim). The method proposed by Gon˜i et al. (1996) was employed to estimate the amount of indigestible starch (comprising part of RS1 plus RS2 and RS3 fractions). Retrograded resistant starch (RS3) content was measured as starch remnants in dietary fiber residues, according to the so-called Lund method as modified by Saura-Calixto et al. (1993). The in vitro rate of hydrolysis was measured using hog pancreatic amylase according to Holm et al. (1985); each assay was run with 500 mg AS. In all these measurements a fraction of the cooked bean was weighed and placed into a test tube or a beaker and homogenized with the corresponding solution (depending on the technique) under controlled conditions: first step (speed level 2, 1 min) and second step (speed level 2.5, 1 min) using a polytron homogenizer (Polytron PT 1200 Kinematica AG, Switzerland). 2.3. Statistics Results were expressed by means7standard deviation of the three separate determinations. Comparison of means was performed by one-way analysis of variance (ANOVA) followed by Tukey’s multiple comparison tests using minitab-Windows computer system, Version 2.1, 1995.
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3. Results and discussion 3.1. Cooking time The non-stored samples showed variation in cooking time, because Tacana (5.9270.2 h) and Zitlala (4.5070.5 h) had the highest values, and Veracruz the lowest ones (2.5570.04 h). The TLP-19 (3.2870.13 h) and Huasteco (3.2670.12 h) did not show differences in cooking time. 3.2. Available starch Appreciable variation was recorded among samples for AS content (Table 1). The values for control samples ranged between 21.7% (Veracruz) and 32.16% (Tacana). Only the Huasteco and Zitlala varieties were not statistically different (a ¼ 0:05). The lowest value of AS determined for Veracruz variety and the highest for Tacana could be associated with the cooking time determined in both samples. Tovar and Melito (1996) reported AS content of 37.4% and 39.1% in two varieties of raw and cooked beans, and a value of 37.57% was obtained in beans cooked for 36 min after soaking in water at 4 C for 18 h (Rosin et al. 2002). However, Bravo et al. (1999) reported lower AS value in five different raw beans (between 26.6% and 35.7%), similar to those found in this study. These results suggest that some cultivars, such as Veracruz, may have low digestibility, whereas Tacana has high digestibility. The TLP-19 sample after 72 h of storage presented a decrease in AS values and Tacana, after 24 h. However, Huasteco and Veracruz did not change their AS values (a ¼ 0:05) after 48 h of storage, and Zitlala did not show differences in its AS values after 72 h. 3.3. Resistant starch The control samples of Tacana and TLP-19 had the highest values, and Huasteco and Zitlala the lowest (Table 2). In general RS values increased with storage time, and the highest difference in the values between control sample and the sample stored during 96 h was shown by Zitlala and Tacana varieties, which also presented high cooking time. The destruction of cotyledon in the seed produced higher enzyme accessibility and a decrease in RS amounts. Bravo et al. (1999) found RS values that ranged between 3.40% and 8.31% for five different bean varieties using boiled samples Table 1 Available starch in cooked beans according to different storage times and varieties (%)a,b Storage time (h)
Tacana
Zitlala
TLP19
Huasteco
Veracruz
Control 24 48 72 96
32.2+0.57a 31.4+0.70a 24.0+0.31b 24.0+0.44b 20.7+0.48c
27.0+0.20a 23.2+0.60b 19.0+0.30c 14.5+0.30d 14.1+0.31d
28.4+0.70a 28.3+0.45a 28.2+0.59a 24.2+0.38b 21.7+0.70c
27.5+0.24a 24.2+0.29b 21.4+0.30c 21.9+0.30c 21.8+1.39c
21.7+0.50a 20.0+0.31b 13.8+0.29c 14.2+0.62c 13.5+0.61c
Means in columns not sharing the same letter are significantly different (Po0:001). a Values are means of three replicates7s.d., dry matter basis. b Cooking times were: Tacana 5.92 h, Zitlala 4.50 h, TLP-19 3.28 h, Huasteco 3.26 and Veracruz 2.55 h.
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of whole grains, the same method employed here (Gon˜i et al., 1996). They found that differences between their samples were mainly due to agronomic variety. For samples evaluated in this investigation, lower variations in RS values were found among varieties as compared to the work reported by Bravo et al. (1999). 3.4. Retrograded resistant starch (RRS) The values of RRS in the control samples of Huasteco and Veracruz varieties were not statistically different (a ¼ 0:05). Tacana bean had the highest RRS value and Zitlala the lowest one (Table 3). The type of variety played an important role in this behavior, which is related with the molecular starch structure (Yuan et al., 1993; Shi and Seib, 1995). In general, RRS values tended to increase with the storage time; the retrogradation tendency, calculated as the difference between RRS values for the sample stored for 96 h and the control sample, showed that TLP19 variety had the highest value and Veracruz the lowest. 3.5. Hydrolysis rate The in vitro a-amylolysis reaction for the various beans is represented in Fig. 1. All samples analyzed showed the highest hydrolysis percentage at 90 min, except Tacana stored for 96 h that Table 2 Resistant starch content in cooked beans according to different storage times and varieties (%)a,b Storage time (h)
Tacana
Zitlala
TLP-19
Huasteco
Veracruz
Control 24 48 72 96
5.1+0.05a 5.5+0.21b 6.3+0.15c 6.4+0.31c 7.7+0.31d
3.5+0.09a 3.4+0.04b 4.0+0.10c 5.6+0.18d 6.6+0.38e
4.9+0.17a 6.0+0.22b 6.3+0.13c 6.4+0.18b,c 7.4+0.40d
3.6+0.17a 3.8+0.17a 4.8+0.36b 5.1+0.33b 5.8+0.18c
4.0+0.07a,c 4.7+0.14b 4.2+0.20b,c 5.0+0.22d 6.3+0.13e
Means in columns not sharing the same letter are significantly different (Po0:001). a Values are means of three replicates7s.d., dry matter basis. b Cooking times were: Tacana 5.92 h, Zitlala 4.50 h, TLP-19 3.28 h, Huasteco 3.26 and Veracruz 2.55 h.
Table 3 Retrograded resistant starch content in cooked beans according to different storage times and varieties (%)a,b Storage time (h)
Tacana
Zitlala
TLP-19
Huasteco
Veracruz
Control 24 48 72 96
3.1+0.01a 3.1+0.12a 3.8+0.22b 4.6+0.11c 4.6+0.18c
1.6+0.18a 2.4+0.22b 2.3+0.08b 3.3+0.20c 3.3+0.03c
2.8+0.06a 3.3+0.18b 4.9+0.29c 5.5+0.35d 5.8+0.18d
2.5+0.07a 2.5+0.11a 2.6+0.25a 4.0+0.14b 4.1+0.29b
2.3+0.15a 2.5+0.15a 2.5+0.14a 3.3+0.16b 3.7+0.10b
Means in columns not sharing the same letter are significantly different (Po0:001). a Values are means of three replicates7s.d., dry matter basis. b Cooking times were: Tacana 5.92 h, Zitlala 4.50 h, TLP-19 3.28 h, Huasteco 3.26 and Veracruz 2.55 h.
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Hydrolysis (%)
25
20
15
10
5
0 0
10
20
30
40
50
60
70
80
90
100
Time (min)
Fig. 1. In vitro starch hydrolysis of cooked beans. E, Tacana (control); , Tacana (96 h of storage); ’, Veracruz (control); , Veracruz (96 h of storage); m, Zitlala (control); K, Zitlala (96 h of storage).
presented a tendency to increase the hydrolysis percentage, perhaps due to the fact that this starch is of slow digestibility. The control samples at 15 min showed hydrolysis between 7% and 10%, without statistical differences between Tacana and Zitlala samples. However, at 90 min, Zitlala showed the highest hydrolysis (approximately 28%) and Tacana the lowest (approximately 17%). Values between 10% and 30% of hydrolysis at 90 min were obtained in cooked legumes by Bravo et al. (1998). Several factors are involved in the reduced bioavailability of legume starches. The presence of intact tissue/cell structures enclosing starch granules hinders the swelling and solubilization of starch resulting in reduced in vitro digestion rate (Tovar et al., 1990). Permanence of intact starch granules trapped within cells in precooked legume flours was observed even after extensive homogenization and pepsin treatment (Tovar et al., 1991). The samples stored fpr 96 h had lower hydrolysis values than their respective control samples. At 90 min, rates of around 12% were obtained for Tacana and Veracruz, and 17% for Zitlala. The highest hydrolysis showed by Zitlala and the lowest one by TLP-19 (data not shown) agree with the RRS because the former had the lowest RRS value (Table 3).
4. Conclusions The cooking and storage times, in addition to the variety may influence digestibility of starch in common beans. Therefore, some cultivars could be recommended for specific dietetic uses. For instance, in rural Mexican areas where beans and tortillas are the main staple, a variety leading to the highest starch digestibility should be used, whereas diabetic and hyperlipidemic subjects may be advised to consume beans with decreased starch digestibility.
Acknowledgements We appreciate the financial support from OMNILIFE-CONACYT, CGPI-IPN, IPICS, LANFOOD, COSNET and COFAA-IPN. The supply of the samples by Eng. Reza of INIFAPIguala, Mexico is gratefully acknowledged.
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