Effect of the cooking on physicochemical and starch digestibility properties of two varieties of common bean (Phaseolus vulgaris L.) grown under different water regimes

Food Chemistry 129 (2011) 358–365

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Effect of the cooking on physicochemical and starch digestibility properties of two varieties of common bean (Phaseolus vulgaris L.) grown under different water regimes Maribel Ovando-Martínez a, Perla Osorio-Díaz a, Kristin Whitney b, Luis A. Bello-Pérez a, Senay Simsek b,⇑ a b

Centro de Desarrollo de Productos Bióticos del IPN, Apartado postal 24 C.P., 62731 Yautepec, Morelos, Mexico Department of Plant Sciences, North Dakota State University, P.O. Box 6050, Department 7670, Fargo, ND 58108-6050, USA

a r t i c l e

i n f o

Article history: Received 23 August 2010 Received in revised form 5 April 2011 Accepted 25 April 2011 Available online 4 May 2011 Keywords: Cooking Starch Digestibility Glycemic index

a b s t r a c t Growing and cooking conditions influence the quality and nutritional value of beans. The objective of this research was to determine the effect of cooking on digestibility and physicochemical properties of two varieties of bean grown under different water regimes. Black 8025 and Pinto Durango varieties were grown in irrigated and temporal (rain fed) conditions in two locations of Guanajuato, Mexico. The pasting profiles of the cooked beans showed a significant decrease in viscosity. The enthalpy of the raw and cooked beans ranged from 2.75 to 3.95 and 0.62 to 0.97 J/g, respectively. The percentage of rapidly digestible starch and slowly digestible starch increased, while the percentage of resistant starch was lower in cooked samples. Black 8025 beans had lower glycemic index than Pinto Durango, but no significant difference (P < 0.05) was noted between water regimes. The variety of bean had a more pronounced effect on digestibility properties than the water regime. Published by Elsevier Ltd.

1. Introduction The common bean (Phaseolus vulgaris L.) is the main legume consumed in many Latin American countries. Beans have high nutritional quality; they are an excellent source of protein, complex carbohydrates and a good source of vitamins and minerals (Guzmán-Maldonado & Paredes-López, 1998). However, the nutritional value of the bean is affected by the variety and environmental conditions. Kigel (1999) reported that drought stress during seed development decreased starch content and increased the soluble sugar content. Protein content diminishes when beans are grown at high temperatures, which is associated with water stress (Hosfield, Uebersax, & Isleib, 1984). The nutritional and culinary quality of bean seeds will be affected by the variety and abiotic factors present during plant growth and seed development (Kigel, 1999). The properties of raw beans are important, however, the application of heat, such as in cooking, can change the nutritional and physiochemical composition of beans. Beans are consumed after cooking as whole seeds together with the cooking water (Serrano & Goñi, 2004). The most common cooking method in Mexico is cooking at atmospheric pressure without soaking because this process does not affect the taste of beans (Castellanos et al., 1997). Physicochemical changes in the bean oc-

⇑ Corresponding author. Tel.: +1 701 231 7737; fax: +1 701 231 8474. E-mail address: [email protected] (S. Simsek). 0308-8146/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.foodchem.2011.04.084

cur during cooking, such as, denaturation of proteins, solubilisation of polysaccharides, softening and breakdown of the middle lamella and gelatinisation of starch (Wang, Hatcher, Warkentin, & Toews, 2010). These changes improve the texture, nutritional value and digestibility of the bean starch (Serrano & Goñi, 2004). However, bean starch digestibility is relatively low compared to starch from other sources. The decreased digestibility of bean starch is due to intrinsic factors (crystalline structure, structural characteristics of amylopectin) as well as extrinsic factors (dietary fibre) that increase the viscosity in the small intestine, decreasing the accessibility of the enzyme to the starch (Tovar & Melito, 1996). Digestibility of the bean starch is an important characteristic when evaluating the nutritional quality of beans. Starch is classified in three categories depending on the rate and extent of digestion as rapidly digestible starch (RDS), slowly digestible starch (SDS) and resistant starch (RS) (Englyst, Kingman, & Cummings, 1992). Raw beans have low contents of RDS and SDS and are high in the content of RS; the amount of these types of starch in beans can be changed by cooking and can be influenced by the variety of bean (Chung, Liu, Pauls, Fan, & Yada, 2008). The starch digestibility is also influenced by the physicochemical and structural properties of the starch granules (Englyst & Englyst, 2005; Hoover & Zhou, 2003). During cooking, the microstructure of the starch granules changes as gelatinisation occurs. Gelatinisation results in damage to the starch granules which allows for faster enzymatic hydrolysis (Singh, Dartois, & Kaur, 2010). In the case of legumes, the starch granule is enclosed in a matrix of fibre and protein affecting the

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digestibility and physicochemical properties. The aim of the present research was to evaluate the effect of cooking on physicochemical properties and digestibility of two varieties of bean, Black 8025 and Pinto Durango, grown under irrigated and rain fed conditions in Celaya and Ocampo, Mexico, respectively.

Tucson, AZ, USA). Images were obtained using a JEOL JSM-6300 scanning electron microscope (JEOL USA, Peabody, MA, USA) while using an accelerating voltage of 10 kV.

2. Materials and methods

Thermal characteristics of raw and cooked beans were studied with a Perkin–Elmer Differential Scanning Calorimeter, DSC-7 using the method described by Kim, Wiesenborn, and Grant (1997) with minor modifications. Samples (3.5 mg) were weighed into aluminium pans, and deionised water (8 ll) was added. The pans were sealed hermetically and kept at room temperature overnight. The samples were heated at 10 °C/min from 20 to 120 °C. An empty aluminium pan was used as a reference. Enthalpy of gelatinisation (DH), onset (To), peak (Tp) and end (Tc) temperatures were obtained using the data processing software supplied with the DSC instrument. A second DSC scan of these gelatinised flours was performed after storage for 7 days at 4 °C to characterise the extent of retrogradation. Samples taken from storage were held at room temperature for 2 h prior to analysis. During the second DSC scan the samples were heated at the same interval of temperature used for gelatinisation properties. All analyses were carried out in triplicate.

2.1. Materials Two dry bean varieties (Black 8025 and Pinto Durango) were used in this experiment. Both varieties were grown in irrigated and rain fed (‘‘temporal’’) conditions in two localities of Guanajuato, Mexico. Black 8025 and Pinto Durango grown under rain fed conditions in Ocampo, were planted in July and harvested in October 2008. While the same varieties grown under irrigation conditions in Celaya were planted in February and were harvested in June of the same year. Samples were donated by the National Institute of Research Forest, Agricultural and Livestock (INIFAP) of Celaya, Guanajuato, Mexico. Samples were cleaned to remove any foreign material and damaged seeds prior to analyses. 2.2. Preparation of raw and cooked bean samples Raw and cooked bean samples were analysed in the form of flour. The raw bean flour of each variety was prepared using the method described by Chung et al. (2008). The raw bean seeds were ground using a hand mill and then were ground to flour using a cyclone mill (A-10 Analytical mill, Tekmar). Finally, the flour was passed through a 450 lm sieve. Bean samples were cooked according to their cooking optimum time. Cooked beans were lyophilised together with cooking water. Finally, samples were ground and passed through a 450 lm sieve. Samples were stored at 4 °C in sealed plastic bags until analysed.

2.5. Thermal properties

2.6. Pasting properties Pasting properties of raw and cooked bean samples were determined using a Newport Scientific (Perten Instruments, Springfield IL, USA) Rapid Visco-Analyzer (RVA). Raw and cooked bean flours (4 g, 14% moisture basis) were added to 25 ml deionised water in a RVA canister. The flour slurries were held at 50 °C for 1 min before heating from 50 to 95 °C at a rate of 12 °C/min and held at 95 °C for 2 min. The slurry was then cooled at a rate of 12 °C/min to 50 °C and held for there 2 min.

2.3. Chemical composition of raw and cooked bean 2.7. In vitro starch digestibility Moisture and ash were determined gravimetrically, using approved methods 44-15A and 08-01, respectively (AACC, 2000). Nitrogen was analysed using a Leco (LECO Corp. St. Joseph, MI, USA) combustion nitrogen analyser (approved method 46-30, AACC, 2000). Crude protein content was calculated as N  6.25. Fat content was determined by extraction of the ground sample with hexane for 3 h using a Soxhlet apparatus according to the AOCS (1998) method Ba 3-38. A total starch assay kit (Megazyme International, Co. Wicklow, Ireland) was used to analyse total starch content on a dry weight basis for each of the samples following the approved method 76-13 (AACC, 2000). The amount of starch damage was determined using a starch damage assay kit (Megazyme International, Co. Wicklow, Ireland) (Method 76-31, AACC, 2000). Sucrose and galactosyl–sucrose oligosaccharide content were determined using an assay kit (Megazyme International, Co. Wicklow, Ireland). All analyses were carried out in duplicate. Megazyme assay kit was used for the quantitative estimation of amylose/amylopectin. The samples were dispersed by heating in dimethyl sulfoxide and the lipids were removed with ethanol. The amylopectin was precipitated with concanavalin A and eliminated through centrifugation. The amylose was hydrolysed to Dglucose and analysed with glucose oxidase/peroxidase reagent. 2.4. Granular morphology Raw and cooked bean samples were mounted on aluminium mounts using colloidal silver or carbon adhesive tabs and coated with gold using a Balzers SCD 030 sputter coater (BAL-TEC RMC,

In vitro starch digestibility of raw and cooked beans was analysed using the method described by Englyst et al. (1992). Amyloglucosidase (140 AGU/ml, Megazyme International Ireland) (1.07 ml) was brought to 25 ml with deionised water. Invertase (Sigma I-4504) (60 mg) was added to deionised water (8 ml). Pancreatin (Sigma P-7545) was dispersed in deionised water (18 g in 120 ml), stirred for 10 min at 4 °C and centrifuged. One hundred eight millilitres of supernatant were collected and mixed with the invertase and 12 ml of amyloglucosidase. The solution was freshly prepared for the digestion analysis. For the analysis, samples (0.3 g) with 0.1 M acetate buffer (20 ml, pH 5.2) were placed in a water bath (37 °C) with agitation (200 strokes/min) to equilibrate. Guar gum (50 mg) and glass beads were added to each tube. Blank and glucose standard tubes were prepared. Five millilitres of enzyme solution were added to each tube at 1 min intervals. Every 20 min for 3 h aliquots (0.5 ml) were taken and added to 5 ml of absolute ethanol and centrifuged. The glucose content was measured using glucose oxidase and peroxidase assay kit (Megazyme International, Co. Wicklow, Ireland). A glucose standard curve was used to calculate the amount of glucose released, and then the rapidly digestible starch (RDS), slowly digestible starch (SDS) and resistant starch (RS) were determined from the amounts of glucose determined at 20 and 120 min. The hydrolysis index (HI) was obtained by dividing the area under the hydrolysis curve of the sample by the area obtained for white bread (hydrolysis curve 0–180 min). The glycemic index of

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the samples was estimated using the equation described by Granfeldt, Bjorck, Drews, and Tovar (1992): pGI = 8.198 + 0.862 HI. 2.8. Statistical analysis The DSC and in vitro starch digestibility analysis were done in triplicate. All other analyses were done in duplicate. The mean and SEM (standard error of the mean) were reported for each of the results. Analysis of variance was done using Sigma Stat (version 2.03, Jandel Corporation, San Rafael, CA). Tukey multiple comparison test was used to determine significant differences among means. 3. Results and discussion 3.1. Chemical composition of raw and cooked beans The chemical composition of raw and cooked beans is presented in Table 1. The ash content was highest in beans grown under rain fed conditions. Campos-Vega et al. (2009) reported similar ash contents for the same varieties of raw beans cultivated in Celaya, Mexico (4.7% and 4.2% for Black 8025 and Pinto Durango, respectively). The ash content in the cooked beans increased in Pinto Durango, but decreased in Black 8025. The lipid contents of Pinto Durango beans grown in different growing conditions were not different. However, Black 8025 grown under rain fed conditions had higher lipid content compared to the same variety cultivated under irrigation. CamposVega et al. (2009) reported that the fat content for Black 8025 was 2% and 1.3% for Pinto Durango, which is comparable to the values reported in this research. In general, after cooking the samples under irrigation conditions had similar fat content than its uncooked counterpart, while the samples under rain fed conditions showed a decrease in fat level after cooking. The protein content of Pinto Durango was higher in the beans grown under rain fed conditions and Black 8025 had higher protein content when grown in Celaya. However, Pinto Durango showed the greatest difference in protein content between growing locations. According to Florez et al. (2009), the presence of sufficient water and warm temperatures during bean growth result in decreased protein content and increased starch content. The protein content of beans grown in both water regimes decreased after thermal processing. Proteins undergo

changes in their structure when subjected to thermal processing that cause alterations in nutritional, physicochemical and functional properties (Carbonaro, Vecchini, & Carnovale, 1993). Nitrogen solubility decreases because of thermal processing, making the nitrogen unavailable for interaction with other compounds in the bean seeds. Perez-Hidalgo, Guerra-Hernandez, and Garcia-Villanova (1997) analysed the nitrogen content of the insoluble dietary fibre of legumes and deduced that cooking lead to formation of tannin–protein complexes and Maillard products. The proportion of protein that is associated with insoluble dietary fibre may also explain the decrease of protein content in the cooked beans. Analysis of the carbohydrates of the cooked and raw beans showed significant differences. Total starch, ranging from 27.55% to 39.84%, was found to be higher in Pinto Durango than in Black 8025. Both bean varieties grown in rain fed conditions showed a decrease in total starch content compared to the beans cultivated under irrigation. This may be due to water deficiency, which reduces starch synthesis and increases protein synthesis (Florez et al., 2009). The starch content is low in several legumes species compared to cereals or tubers because the starch is utilised during seed maturation to provide carbon skeletons for synthesis of other compounds (Gallardo, Thompson, & Burstin, 2008). The total starch obtained in the samples was comparable to those reported by Chung et al. (2008) for Majesty, Red Kanner and AC Nautica (40.3%, 39.8% and 36.8% respectively). The total starch content decreased in cooked samples when compared to raw beans except in Black 8025 grown under rain fed conditions, which had increased total starch content after cooking. Milling or processing of seeds damages the starch granule affecting the physicochemical and functional properties of the starch. Starch damage (Table 1) was similar in all beans except Black 8025 grown in rain fed conditions, which had the highest values, 1.01% and 4.04% for raw and cooked beans, respectively. The starch damage more than doubled for all beans after cooking. During cooking of bean seeds the bean starch granules gelatinize, swell, disrupt (leaching of amylose) and lose their crystalline structure. This phenomenon could contribute to the increase of damaged starch in cooked beans. Another carbohydrate component in beans is low molecular weight soluble carbohydrates, which include sucrose and raffinose family oligosaccharides (RFOs). The RFOs undergo anaerobic fermentation and cause gas to form in the lower intestine leading to flatulence. Consequently, people may limit their intake of beans.

Table 1 Chemical composition of raw and cooked beans A.

Raw bean Celaya: Irrigation Black 8025 Pinto Durango Ocampo: Rain fed Black 8025 Pinto Durango Cooked bean Celaya: Irrigation Black 8025 Pinto Durango Ocampo: Rain fed Black 8025 Pinto Durango

Ash (%)B

Lipid (%)

B

TSC (%)B

SDD (%)

RFOsE (g/100 g flour)

Amylose (%)

4.20 ± 0.03b 3.96 ± 0.03a

1.93 ± 0.03b 1.56 ± 0.08a

23.93 ± 0.06f 22.65 ± 0.18c

35.27 ± 0.37e 39.84 ± 0.49g

0.72 ± 0.03a 0.86 ± 0.02b

3.56 ± 0.11c,b 3.24 ± 0.14b,a

37.22 ± 1.27b 36.03 ± 0.59b

4.49 ± 0.03d 4.59 ± 0.01e

2.15 ± 0.04c 1.64 ± 0.09a

23.14 ± 0.05e 27.32 ± 0.08h

27.55 ± 0.44a 36.08 ± 0.36f

1.01 ± 0.13c 0.93 ± 0.07c

3.33 ± 0.17b,a 3.80 ± 0.25c

30.72 ± 1.15a 32.57 ± 0.98a

3.96 ± 0.01a 4.00 ± 0.08a

2.04 ± 0.11d 1.66 ± 0.06a

22.98 ± 0.03d 21.91 ± 0.08a

28.73 ± 0.49b 31.84 ± 0.34d

2.81 ± 0.04e 2.30 ± 0.06d

3.68 ± 0.05c 2.91 ± 0.11a

39.43 ± 0.93c 39.59 ± 1.03c

4.43 ± 0.02c 4.65 ± 0.02f

1.60 ± 0.16a 1.51 ± 0.10a

22.18 ± 0.04b 26.12 ± 0.10g

30.92 ± 0.52c 32.43 ± 0.73d

4.04 ± 0.08f 2.74 ± 0.16e

3.23 ± 0.06b,a 3.12 ± 0.26a

38.27 ± 0.76c,b 34.99 ± 1.99b,a

Protein (%)

B

Data with the same superscript alphabets in the same column are not significantly different (P < 0.05) by Tukey test. A Values are mean ± SEM, n = 2. B Values are in dry weigh basis. C Total starch. D Starch damage. E Raffinose family oligosaccharides.

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Soaking and cooking beans can reduce these undesirable carbohydrates (Abdel-Gawad, 1993).The amount of RFOs present in raw and cooked beans is shown in Table 1. The RFOs content in uncooked beans was affected by water regimes; however, the variety plays an important role, because RFOs content of Pinto Durango was higher under rain fed conditions than under irrigation, and inverse pattern was observed in Black 8025 bean variety. The RFOs level in cooked samples of Black 8025 bean was not affected by water regimes, but in Pinto Durango RFOs amount decrease under rain fed conditions. Starch, composed of amylose and amylopectin, makes up the majority of the carbohydrate content in beans. The amylose content of raw and cooked beans is presented in the Table 1. In the case of raw beans, Black 8025 and Pinto Durango grown under irrigation conditions had higher amylose content than the same varieties grown under rain fed conditions. Between the varieties of bean grown in the same water regime, there were no significant differences (P < 0.05). Black 8025 and Pinto Durango grown under irrigation are in the range of high amylose starches (35–70%), while the same bean varieties grown under rain fed conditions are in the range of normal starches (15–30%) (Hoover, Hughes, Chung, & Liu, 2010). After cooking, the amylose content of the beans increased (Table 1), indicating that the thermal process caused depolymerisation of some long chains of amylopectin during gelatinisation. After cooking, the amylose content is more affected by water regime than the bean variety. The relationship between these components in the starch and their structural characteristics might determine the physicochemical and digestibility properties of the starch. 3.2. Granular morphology of raw and cooked bean Scanning electron microscopy (SEM) images from raw and cooked beans are shown in Fig. 1. SEM images of raw bean are located in the left panel and images of the cooked beans are located in the right panel (550). Starch granules with round or oval shapes and different sizes could be seen for both Black 8025 and Pinto Durango varieties grown in Celaya (irrigation) and Ocampo (rain fed). Marconi, Ruggeri, Cappelloni, Leonardi, and Carnovale (2000) found the cell wall forms a regular structure which encloses the starch granule when viewing cotyledon cross sections of raw common beans using SEM. This indicated that the cell wall from the samples in this research was fragmented due to grinding. In the SEM images of cooked beans (Fig. 1B, D, F and H), all bean varieties showed critical morphological changes. The starch granules of cooked beans are larger in size compared to starch granules from raw beans (Fig. 1A, C, E and G). The cooked starch granules maintained their integrity; however, the surface of the granules has become rough and flattened. Marconi et al. (2000) explained that this morphological change was due to the cell wall being broken and shattered during cooking. Fragments of the protein matrix which might have disrupted during milling were observed in SEM images of raw beans but not observed in cooked beans. During cooking, protein bodies may undergo changes in their structure and denature. The remnants of the protein matrix might adhere to the starch granules. The degree of morphological changes during the thermal process varies according to the legume tested (Aguilera, Esteban, Benítez, Mollá, & Martín-Cabrejas, 2009). 3.3. Thermal properties Gelatinisation describes the irreversible changes that disrupt the starch granule structure during heating in the presence of water. The gelatinisation transition temperatures (To, Tp and Tc), gelatinisation temperature range (Tc–To) and enthalpy gelatinisation (DHG) from raw beans are exhibited in the Table 2. The transition temperatures To, Tp and Tc ranged from 67.12 to 77.47 °C, 77.00 to 81.78 °C, and

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86.80 to 89.79 °C, respectively. Black 8025 had higher To and Tp, than Pinto Durango grown at both locations. The Tc showed no significant differences between raw bean varieties but there were significant differences (p < 0.05) between growing locations. There was an increase in To and decrease in Tc for raw beans grown under rain fed conditions (Ocampo) compared to beans grown under irrigation conditions in Celaya. Raw bean varieties showed significant differences in Tp between varieties and water regimes. Transition temperatures are influenced by the, size, form and distribution of starch granules, amylose content, internal arrangement of amylose and amylopectin in the starch granule and amylose–lipid complex (Jayakody et al., 2007; Kaur, Sandhu, & Lim, 2009). Chung et al. (2008) reported that the difference in Tp in legume flours is attributed to the protein content and starch structure. The gelatinisation temperature range was significantly different between bean varieties and growing locations. Pinto Durango grown under irrigation had the widest gelatinisation temperature range (22.67 °C); while Black 8025 grown under rain fed conditions had the most narrow gelatinisation temperature range (9.64 °C). The extent of the gelatinisation temperature range indicated the presence of crystallites with different stability within the starch crystalline domains (Chung et al., 2008). The gelatinisation enthalpy corresponds to the amylopectin’s loss of double helical order and crystallinity (Jayakody et al., 2007). There were not significant differences (P < 0.05) observed in the DHG among raw bean samples. In raw beans, a second thermal transition occurs as a result of melting of the amylose lipid complex. The thermal transition of the amylose–lipid complex in raw bean was observed between 94 and 104 °C (Table 2). There were not significant differences in the thermal transition between bean varieties grown at the same location. The enthalpy values of the amylose–lipid complex of bean varieties grown under rain fed condition were higher than bean varieties grown under irrigation conditions. This could be attributed to the higher lipid content present in the raw bean grown in rain fed conditions. In cooked beans the thermal transition occurred at much lower temperatures compared to raw beans; the temperatures were approximately 20 °C lower (Table 2). The To was significantly higher (P < 0.05) for black 8025 grown under rain fed conditions. Significant differences (P < 0.05) were not observed for cooked beans in Tp and Tcbetween bean varieties and water regimes. The lower melting temperatures could be attributed to pre-gelatinisation of starch granules during cooking. The gelatinisation temperature range showed no differences between bean varieties or water regimes. However, the cooked beans had a larger gelatinisation temperature range compared to most of the raw beans. The thermal transition for the cooked beans was wide and flat, where the raw beans showed a narrower peak. Among bean varieties, there was no significant difference observed in the DHG. This parameter decreased after cooking because of reduction in amylopectin content and the reduced crystalline structure. Retrogradation of raw and cooked beans was studied after storage of gelatinised flours at 4 °C for 7 days. The cooked beans did not show any thermal transition after storage. This could be due to the loss of crystalline structure of starch after multiple reheating cycles. The thermal transitions representing retrogradation in raw beans are reported in Table 3. The onset of the retrogradation peaks in the raw beans ranged between 43 and 47 °C and the peak conclusion ranged from 68 to 74 °C. These transition temperature ranges were much broader than those obtained for the initial gelatinisation temperatures of raw bean flours. Starch recrystallises in a less ordered manner in stored starch gels than the crystalline structure found in native starch (Chung, Liu, Wang, Yin, & Li, 2010). There was no significant difference (P < 0.05) for beans grown under irrigation, but there were significant differences between locations and bean variety. DHR for raw beans ranged from

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Fig. 1. Scanning electron microscopy (SEM) images of raw and cooked bean flour. Beans grown in Celaya: (A) Raw Black 8025, (B) Cooked Black 8025, (E) Raw Pinto Durango and (F) Cooked Pinto Durango. Beans grown in Ocampo: (C) Raw Black 8025 (D) Cooked Black 8025, (G) Raw Pinto Durango and (H) Cooked Pinto Durango. All images are 550 magnification.

2.06 to 3.21 J/g and was smaller than DHG Retrogradation enthalpy is lower than gelatinisation enthalpy because, after gelatinisation, re-association of the starch molecules during storage forms a weaker matrix or network (Kim et al., 1997). The DHR between bean varieties and water regimes was not significantly different except for Pinto Durango grown under irrigation condition, which had higher DHR. The retrogradation of starch has been often related to high amylose content. However, amylopectin and intermediate materials also play a significant role in starch retrogradation during storage (Kaur et al., 2009). 3.4. Pasting properties The pasting properties of starch are affected by granule swelling, amylose leaching, starch crystallinity, amylose content and

amylopectin chain length distribution (Chung et al., 2008). Pasting profiles of raw and cooked beans are summarised in Table 4. In general, beans grown in both irrigation and rain fed conditions showed low breakdown, high setback and high final viscosities during the heating and cooling cycles. The low breakdown indicates restricted swelling of starch granules and high amylose content, while high setback indicates a higher tendency for retrogradation (Kim et al., 1997). The high final viscosity is related to the ability of the flour to retrograde and form a strong gel. Chung et al. (2008) found that bean flour had a lower peak viscosity and no breakdown, which could be due to dilution of the starch by protein lipid and fibre. Significant differences were observed in peak viscosity and breakdown (P < 0.05) among bean varieties and water regimes. Black 8025 had higher peak viscosity when grown with irrigation however; Pinot Durango had higher peak viscosity

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M. Ovando-Martínez et al. / Food Chemistry 129 (2011) 358–365 Table 2 Thermal properties of raw and cooked beans A.

Raw bean Celaya: Irrigation Black 8025 Pinto Durango Ocampo: Rain fed Black 8025 Pinto Durango

To (°C)

Tp (°C)

74.65 ± 0.58e 67.12 ± 0.44d

81.37 ± 0.10c 77.00 ± 0.19b

77.47 ± 0.66f 75.01 ± 0.71e

Tc–To (°C)

DHG (J/g)

89.27 ± 0.77c 89.79 ± 0.36c

14.62 ± 1.33e 22.67 ± 0.68g

3.25 ± 0.60c 3.95 ± 0.64c

81.78 ± 0.20d 81.20 ± 0.19c

86.80 ± 0.58b 87.22 ± 0.44b

9.64 ± 1.16c 12.21 ± 1.04d

2.75 ± 0.53c 2.96 ± 0.36c

99.00 ± 0.22e 99.49 ± 0.52e

103.70 ± 0.45d 103.13 ± 0.87d

9.61 ± 0.57c,b 7.06 ± 0.97ª

1.70 ± 0.26b,a 1.21 ± 0.50ª

99.55 ± 0.83e 99.18 ± 0.34e

104.46 ± 1.24d 102.89 ± 0.37d

8.60 ± 1.20b,a 8.31 ± 0.08b

2.60 ± 1.42c,b,a 3.36 ± 0.30c,b

47.60 ± 0.54a 48.91 ± 0.53b,a

56.45 ± 0.15a 56.66 ± 0.15a

65.72 ± 1.33a 65.89 ± 0.74a

18.13 ± 1.85f,d 16.98 ± 1.22f,e

0.62 ± 0.18a 0.91 ± 0.20a

49.79 ± 0.98c,b 48.50 ± 0.66a

56.41 ± 0.38a 56.81 ± 0.21a

63.11 ± 2.12a 66.32 ± 1.19a

13.32 ± 3.05d,e 17.82 ± 1.77f,d

0.97 ± 0.49a 0.83 ± 0.20a

Amylose–lipid complex of raw bean Celaya: Irrigation Black 8025 94.09 ± 0.23g Pinto Durango 96.07 ± 0.82h Ocampo: Rain fed Black 8025 95.85 ± 1.31h,g Pinto Durango 94.58 ± 0.35g Cooked bean Celaya: Irrigation Black 8025 Pinto Durango Ocampo: Rain fed Black 8025 Pinto Durango

Tc (°C)

Data with the same superscript alphabets in the same column are not significantly different (P < 0.05) by Tukey test. To: onset temperature, Tp: peak temperature, Tc: conclusion temperature, Tc–To: gelatinisation temperature range, DHG: gelatinisation enthalpy A Values are mean ± SEM, n = 3.

Table 3 Retrogradation of raw beans A.

Raw bean Celaya: Irrigation Black 8025 Pinto Durango Ocampo: Rain fed Black 8025 Pinto Durango

To (°C)

Tp (°C)

Tc (°C)

Tc–To (°C)

DHR (J/g)

44.02 ± 0.39b 44.04 ± 0.32b

57.21 ± 0.08a 57.42 ± 0.25a

72.14 ± 0.22b 73.24 ± 0.97c

28.12 ± 0.57b 29.20 ± 0.74b

2.45 ± 0.08a 3.21 ± 0.30b

47.37 ± 0.97c 43.08 ± 0.41a

56.85 ± 1.20a 57.70 ± 0.23a

68.16 ± 1.59a 74.28 ± 1.45c

20.79 ± 1.45a 31.20 ± 1.66c,b

2.06 ± 0.43a 2.46 ± 0.20a

Data with the same superscript alphabets in the same column are not significantly different (P < 0.05) by Tukey test. To: onset temperature, Tp: peak temperature, Tc: conclusion temperature, Tc–To: retrogradation temperature range, DHR: retrogradation enthalpy. A Values are mean ± SEM, n = 3.

Table 4 Pasting properties of raw and cooked beans measured by Rapid Visco Analyzer (RVA)A. Viscosity (RVU)

Raw Bean Celaya: Irrigation Black 8025 Pinto Durango Ocampo: Rain fed Black 8025 Pinto Durango Cooked Bean Celaya: Irrigation Black 8025 Pinto Durango Ocampo: Rain fed Black 8025 Pinto Durango

Peak

Breakdown

Final

Setback

81.63 ± 1.88g 64.34 ± 3.08e

3.38 ± 0.46e 5.13 ± 0.38f

150.38 ± 6.70e 109.42 ± 9.42d

72.13 ± 5.29e 50.21 ± 5.96d

70.46 ± 2.88f 89.96 ± 0.29h

2.63 ± 0.29d 2.04 ± 0.29c,a

146.42 ± 6.75e 157.88 ± 0.30f

78.59 ± 3.58e 69.96 ± 0.29e

3.29 ± 0.04b 3.88 ± 0.45c

1.25 ± 0.25a 1.63 ± 0.05b,a

2.75 ± 0.17b 2.88 ± 0.38b

0.71 ± 0.04b,a 0.63 ± 0.13a

1.88 ± 0.46a 6.50 ± 0.33d

1.50 ± 0.00a 1.30 ± 0.63a

0.84 ± 0.59a 6.17 ± 0.84c

0.46 ± 0.13a 0.96 ± 0.13c

Data with the same superscript alphabets in the same column are not significantly different (P < 0.05) by Tukey test. A Values are mean ± SEM, n = 2.

with rain fed conditions. Raw beans grown under irrigation had higher breakdown compared to raw beans grown under rain fed condition. Between water regimes, the final viscosity was significantly different in Pinto Durango variety while Black 8025 showed

no difference (P < 0.05). The setback was significantly different (P < 0.05) for Pinto Durango and beans grown under irrigation. Pinto Durango grown under irrigation condition exhibited lower peak, final and setback and higher breakdown compared to all other

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beans. Kaur et al. (2009) reported that the changes in the viscosity during cooking relates to the stability and changes during cooling, and ultimately the consistency of the product when it is consumed. The heat treatment, applied during cooking, resulted in the decrease of viscosity for cooked beans compared to raw beans (Table 4). Black 8025 and Pinto Durango showed significant differences in peak and final viscosities between water regimes, while there were no differences in breakdown and setback between water regimes and bean varieties. The pasting properties of the cooked beans may represent the texture of reheated cooked beans. 3.5. In vitro starch digestibility The starch digestibility and estimated glycemic index of raw and cooked beans are shown in Table 5. The raw beans, which were analysed without gelatinisation, did not contain rapidly digestible starch (RDS). The starch in raw legumes is scarcely digestible because of high type 1 and type 2 resistant starch contents, decreasing the starch digestibility. There were significant differences in slowly digestible starch (SDS) and resistant starch (RS) contents between varieties and water regimes (P < 0.05). SDS and RS ranged from 9.41% to 15.35% and 12.20% to 30.43%, respectively. Bean varieties grown under rain fed conditions exhibited higher SDS contents and lower RS contents than beans grown under irrigation. Variations in the starch fractions were probably related to the physicochemical properties of starch and the entrapment of the starch in the protein and fibre matrix. Chung et al. (2008) reported that the low starch content in AC Nautica flour is a possible cause for the decrease of RS content. The hydrolysis index of raw beans ranged from 3.87% to 12.54% and there were no significant differences observed between varieties and water regimes. The estimated glycemic index (eGI) ranged from 11.53% to 19.01% and was similar to eGI for bean flours (12%) (Chung et al., 2008). The low digestibility of starch in raw bean is also attributed to the presence of proteins, lipids, fibre, a-amylase inhibitors and anti nutritional factors (Singh et al., 2010). The availability of the starch to amylase hydrolysis increases after cooking of beans, resulting in changes of the starch digestibility profiles (Table 5). There was an increase in the RDS and SDS content and decrease in RS after cooking compared to raw beans. Beans grown under rain fed conditions presented higher RDS content compared to beans grown under irrigation. The low RDS content in cooked beans may indicate that 20 min after consumption of Black 8025 and Pinto Durango beans, blood glucose levels may not increase. Beans grown under irrigation showed the highest

SDS content. However, there was no significant difference (P < 0.05) between variety and location. SDS is the most desirable form of dietary starch because potential health benefits result from the slow digestion in the small intestine (Kaur et al., 2009). RS content ranged from 5.80% to 8.14% and was comparable to the values reported for beans (5.48%) (Goñi, Garcia-Alonso, & Saura-Calixto, 1997). Beans grown in rain fed conditions exhibited the highest RS content, but the RS content was not significantly different between water regimes or varieties. The RS content in beans is high compared with the RS content found in cereals due to the high amylose content in beans. Sáyago-Ayerdi, Tovar, Osorio-Díaz, Paredez-López, and Bello-Pérez (2005) found higher RS content in black beans (5.33–6.88%) than in tortillas (2.14–3.78%). Glycemic index (GI) indicates the digestibility of the starch in beans in relation to the digestibility of starch in a reference material (white bread). The hydrolysis index (HI) represents the rate of starch digestion. The HI and eGI are presented in Table 5. The eGI ranged to 35.44–47.44%. These values were higher compared to that reported by Sáyago-Ayerdi et al. (2005) in black beans (27%). The differences found can be attributed to the methodology utilised by these authors, who determined the eGI in beans through a chewing and dialysis test; while in this research the eGI was analysed by an indirect method (Granfeldt et al., 1992). Pinto Durango (irrigation and rain fed conditions) had higher eGI compared to Black 8025. However, compared to spaghetti and rice (68% and 88%) the beans have a lower eGI (Goñi et al., 1997). The HI ranged from 31.60% to 45.53%. Black 8025 grown in both irrigation and rain fed conditions showed lower HI compared to Pinto Durango. Significant differences were observed between varieties of bean grown in the same water regime. However, Pinto Durango showed no differences in the HI between growing locations. The HI determined in this research for each bean variety was higher compared to those reported in black bean (21.39%) (Sáyago-Ayerdi et al., 2005). Low in vitro digestibility of starch from cooked beans is attributed to encapsulation of starch granules by the cell wall (SEM images, Fig. 1), which impedes the swelling and solubilisation of starch. The presence of proteins and dietary fibre affect the digestibility of starch due to their presence on the starch granule surface. This limits the rate of enzymatic hydrolysis by reducing the accessibility of enzymes to the granule surface. In general, legumes produce a low glycemic index, which generates slow and moderate postprandial glucose and insulin response. This characteristic of legumes has a beneficial effect in the management of diabetes, obesity and other diseases (Chung et al., 2008).

Table 5 Starch fractions of raw and cooked beans determined by in vitro starch digestionA.

Raw bean Celaya: Irrigation Black 8025 Pinto Durango Ocampo: Rain fed Black 8025 Pinto Durango Cooked bean Celaya: Irrigation Black 8025 Pinto Durango Ocampo: Rain fed Black 8025 Pinto Durango

RDS (%)

SDS (%)

RS (%)

0.00 ± 0.00a 0.00 ± 0.00a

11.89 ± 0.38c 9.41 ± 0.67a

23.38 ± 0.38d 30.43 ± 0.67f

0.00 ± 0.00a 0.00 ± 0.00a

15.35 ± 0.62d 10.88 ± 0.44b

12.20 ± 0.62c 25.20 ± 0.44e

0.60 ± 0.35b 0.53 ± 0.19b

22.33 ± 1.31f 24.00 ± 0.72f

3.51 ± 0.78d 1.81 ± 0.23c

19.28 ± 1.50e 22.98 ± 1.34f

HI

6.37 ± 0.29b,a 5.96 ± 0.97a

eGI

13.69 ± 0.25b,a 13.34 ± 0.84a

12.54 ± 2.83c 3.87 ± 1.91a

19.01 ± 2.44c 11.53 ± 1.65a

5.80 ± 0.99a 7.79 ± 0.68b,a

31.60 ± 1.68d 45.53 ± 3.80f

35.44 ± 1.45d 47.44 ± 3.27f

8.14 ± 0.91b,a 7.63 ± 1.27a

39.07 ± 2.33e 45.03 ± 1.09f

41.87 ± 2.01e 47.01 ± 0.94f

Data with the same superscript alphabets in the same column are not significantly different (P < 0.05) by Tukey test. RDS: rapidly digestible starch, SDS: slowly digestible starch, RS: resistant starch, HI: hydrolysis index, eGI: estimated glycemic index was calculated from equation proposed by Granfeldt et al. (1992). (eGI = 0.862 IH + 8.19). A Values are mean ± SEM, n = 3.

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4. Conclusions The physicochemical properties of the beans were significantly affected by the cooking process. Cooking the Black 8025 and Pinto Durango beans caused changes in the chemical composition, RFOs, amylose and amylopectin content, thermal, pasting and digestibility properties of the bean flour. These properties were also affected by both bean variety and water regime. The effect of water regime had significant influence on protein content, TS, and the amylose and amylopectin content of Black and Pinto Durango beans. Although the protein and total starch was reduced when beans were grown under irrigation, the irrigation conditions seemed to have a positive effect on starch digestion. The results observed in raw bean indicate that these legumes are good source of RS, especially bean varieties grown under irrigation. The traditional cooking of Black 8025 and Pinto Durango increased the RDS and SDS content and decreased the RS content. However, the RS content was high compared to cereals, which highlights the nutritional value of these types of beans. It would be important to study the effect of water regime on the physicochemical and structural properties of isolated starch from these two bean varieties to determine if these properties play an important role in the in vitro starch digestibility of beans. Acknowledgements The authors thank to Dr. Horacio Guzmán-Maldonado and Dr. Jorge Acosta-Gallegos for donation of the samples. We gratefully acknowledge the technical assistance of Mary Niehaus and Dr. Clifford Hall for the lipid analysis. One of the author (M.O.M.) also acknowledges the scholarship from CONACYT-Mexico and SIP-IPN. The authors L.A.B.P. and P.O.D. acknowledge the support from EDI-IPN, COFAA-IPN and North Dakota State University Agricultural Experiment Station. References Abdel-Gawad, A. S. (1993). Effect of domestic processing on oligosaccharide content of some dry legume seeds. Food Chemistry, 46, 25–31. Aguilera, Y., Esteban, R. M., Benítez, V., Mollá, E., & Martín-Cabrejas, M. (2009). Starch, functional properties, and microstructural characteristics in chickpea and lentil as affected by thermal processing. Journal of Agricultural and Food Chemistry, 57, 10682–10688. American Association of Cereal Chemists. (2000). Approved methods of the AACC (10th ed.). AACC methods 44-15A, 08-01, 46-30, 76-13 and 76-31. AACC, St. Paul, MN, USA. AOCS.(1998). Official methods of analysis.(16th ed.). AOCS method Ba 3-38. Association of Official Analytical Chemist International: Arlington, VA. Campos-Vega, R., Reynoso-Camacho, R., Pedraza-Aboytes, G., Acosta-Gallegos, J. A., Guzman-Maldonado, S. H., Paredes-López, O., et al. (2009). Chemical composition and in vitro polysaccharide fermentation of different beans (Phaseolus vulgaris L.). Journal of Food Science, 74, 59–65. Carbonaro, M., Vecchini, P., & Carnovale, E. (1993). Protein solubility of raw and cooked beans (Phaseolus vulgaris) role of the basic residues. Journal of Agriculture and Food Chemistry, 41(8), 1169–1175. Castellanos, J. Z., Guzmán-Maldonado, H., Jiménez, A., Mejía, C., Muñoz-Ramos, J. J., Acosta-Gallegos, J., et al. (1997). Hábitos preferenciales de los consumidores de

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