In vitro starch digestibility, expected glycemic index and some physicochemical properties of starch and flour from common bean (Phaseolus vulgaris L.) varieties grown in Canada

Food Research International 41 (2008) 869–875

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In vitro starch digestibility, expected glycemic index and some physicochemical properties of starch and flour from common bean (Phaseolus vulgaris L.) varieties grown in Canada Hyun-Jung Chung a, Qiang Liu a,*, K. Peter Pauls b, Ming Z. Fan c, Rickey Yada d a

Guelph Food Research Centre, Agriculture and Agri-Food Canada, 93 Stone Road West, Guelph, ON, Canada N1G 5C9 Department of Plant Agriculture, University of Guelph, Guelph, ON, Canada N1G 2W1 c Department of Animal and Poultry Science, University of Guelph, Guelph, ON, Canada N1G 2W1 d Department of Food Science, University of Guelph, Guelph, ON, Canada N1G 2W1 b

a r t i c l e

i n f o

Article history: Received 24 November 2007 Accepted 16 March 2008

Keywords: Beans Flour Starch In vitro starch digestibility Expected glycemic index Physicochemical properties

a b s t r a c t The physicochemical properties including in vitro starch digestibility and expected glycemic index (eGI) of bean flour and isolated bean starch from different cultivars grown in Canada were investigated. The protein content and total starch content of bean flour ranged from 23.1% to 26.6% and 36.8% to 40.3%, respectively. The apparent amylose content of bean starch was between 38.0% and 41.5%. The bean starch granules were round to oval with a smooth surface. All bean starches showed C-type X-ray diffraction pattern with relative crystallinity ranging between 27.7% and 30.3%. Bean starch had only a single transition (14.3–15.3 J/g) in the DSC thermogram, whereas bean flour showed two small separate endothermic transitions corresponding to starch gelatinization (2.1–3.0 J/g) and disruption of the amylose–lipid complex (1.9–2.5 J/g). The bean flour had a significantly lower pasting viscosity due to low starch content and interference of other components. The bean starch exhibited high thermal stability during the 95 °C hold period in the viscogram. The bean flour had a substantial amount of resistant starch, ranging between 32.4% and 36.0%, whereas the bean starch retained a large portion of slowly digestible starch (63.1–65.8%). The eGI, based on the hydrolysis index, ranged from 12.0 to 12.2 and 65.8 to 68.4 for bean flour and isolated bean starch, respectively. Crown Copyright Ó 2008 Published by Elsevier Ltd. All rights reserved.

1. Introduction Beans (Phaseolus vulgaris L.), which are widely grown and consumed in various regions of the world, are excellent sources of proteins (20–25%) and complex carbohydrates (50–60%), and fairly good sources of minerals, vitamins, and polyunsaturated free fatty acids, as are most other legumes (Rehman, Salariya, & Zafar, 2001; Reyes-Moreno & Paredes-Lopez, 1993). Beans contain a large amount of resistant starch and fibers that are resistant to digestion in the small intestine, and pass into the large intestine where they are the substrate for bacterial fermentation producing short chain fatty acids. They prevent blood glucose levels from rising too rapidly after a meal, resulting in reduced glycemic and insulinemic responses, and thus beans have been considered as low glycemic index (GI) foods (Foster-Powell & Brand Miller, 1995; Reyes-Moreno & Paredes-Lopez, 1993). The GI concept is a tool for ranking foods with respect to their blood glucose raising potential (Jenkins et al., 1981). Consumption of low GI foods

* Corresponding author. Tel.: +1 519 780 8030; fax: +1 519 829 2600. E-mail address: [email protected] (Q. Liu).

could contribute to reduced incidence and prevalence of heart disease, cardiovascular disease, diabetes, obesity, and also some forms of cancer (Jenkins, 2007; Rizkalla, Bellisle, & Slama, 2002). The reduced digestibility of legumes including beans has been attributed to their high content of viscous soluble dietary fibers, the presence of various antinutrients, and relatively high amylose content in starch (Tharanathan & Mahadevamma, 2003; Zhou, Hoover, & Liu, 2004). Although many researchers have reported physicochemical properties of bean varieties from different sources, either commercial or newly released (Almeida Costa, Queiroz-Monici, Machado Reis, & Oliveira, 2006; Gujska, Reinhard, & Khan, 1994; Hoover & Sosulski, 1985; Osorio-Diaz et al., 2002; Rehman et al., 2001; Sayago-Ayerdi, Tovar, Osorio-Diaz, Paredes-Lopez, & Bello-Perez, 2005; Su, Lu, & Chang, 1997; Vargas-Torres et al., 2004), there is a dearth of information on in vitro bean starch digestibility including rapidly and slowly digestible starches, and glycemic index of difference cultivars, as well as those of varieties grown under the same environmental conditions. Furthermore, no study has been yet to be conducted to compare physicochemical properties of bean flour and bean starch. In this study, the in vitro starch digestibility, some physicochemical properties (granular morphology,

0963-9969/$ - see front matter Crown Copyright Ó 2008 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.foodres.2008.03.013

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crystalline structure, molecular structure, swelling factor, amylose leaching, thermal properties and pasting properties), and expected glycemic index of both flour and isolated bean starch from common bean varieties grown in Canada were investigated. 2. Materials and methods 2.1. Materials Three bean cultivars (Majesty, dark red kidney bean; Red Kanner, light red kidney bean; AC Nautica, navy bean) from the 2005 growing season in Elora, ON, Canada were provided by the Department of Plant Agriculture, University of Guelph, Canada. Bean seeds were ground to flour using a cyclone mill (A10 analytical mill, Tekmar Co., Cincinnati, OH), and passed through a screen with 125 lm openings. Bean starch was isolated from milled bean flour according to the method described by Otto, Baik, and Czuchajowska (1997) as modified by Chung et al. (2007). 2.2. Chemical composition Moisture content of bean flour and isolated starch was determined by AACC standard method (AACC, 2000). Total starch content was measured by AACC method 76.13 B (AACC, 2000). Protein content (nitrogen  6.25) was determined using a protein analyzer (ThermoQuest CE Instrument, NA 2100, ThermoQuest Italia S.P.A., Ann Arbor, MI) with four standards (atropine, DLmethionine, acetanilide and nicotinamide). Apparent amylose content was determined following the method of Williams, Kuzina, and Hlynka (1970). Starch (20 mg, db) was dissolved in 10 ml of 0.5 N KOH solution in a test tube. The mixture was vigorously mixed and then heated in a boiling water bath for 10 min. The test tubes were then cooled to ambient temperature, and the mixture diluted with water to 100 ml in a volumetric flask. The diluted solution (1.0 ml) was mixed with 1 ml of 0.5 N HCl and 0.5 ml iodine solution (2% KI and 0.2% I2 w/v) and then adjusted to a final volume of 50 ml. After the contents were allowed to stand for 15 min at ambient temperature, the absorbance was measured at 640 nm. 2.3. Granular morphology Polarized light microscopy of bean starch was performed with a binocular microscope (DME, Leica Canada, Mississauga, ON) equipped with a digital camera with real time viewing (Micropublisher 5.0, Q-Imaging, Burnaby, Canada). Granule size was determined using software (QCapture Pro 5.1, Q-Imaging, Burnaby, Canada) at 400 magnification. Granule morphology was also examined with a Hitachi S-4500 field emission scanning electron microscope (Hitachi Ltd., Tokyo, Japan) equipped with Quartz PCI digital image acquisition software (Quartz Imaging Corp., Vancouver, BC, Canada). The starch samples were sprayed on a metal plate previously covered with double-sided adhesive, coated with gold: palladium (60:40) using a Polaron SC500 sputter coater (Quorum Technologies, East Sussex, UK), and examined at 5.0 kV accelerating voltage and 1000 magnification. 2.4. X-ray diffraction X-ray diffractograms of bean starch were obtained from a Rigaku RPT 300 PC X-ray diffractometer (Rigaku-Denki Co., Tokyo, Japan) at 40 kV and 100 mA with 3–35° of the scanning range and 2.0°/min of scan speed. The crystallinity of starch was calculated following the method of Nara and Komiya (1983) using the Origin 6.0 software (Microcal Inc., Northampton, MA).

2.5. Swelling factor (SF) and amylose leaching (AML) Swelling factor (SF) of bean starch was determined using the method of Tester and Morrison (1990) at the temperature of 60– 90 °C. SF of bean flour (ratio of the volume of swollen flour to the dry flour) was measured according to Subramanian, Hoseney, and Bramel-Cox (1994). Amylose leaching of bean starch and flour when heated to 60–90 °C in excess water was determined according to the method described by Chung et al. (2007). 2.6. Thermal properties Thermal analysis was performed using a differential scanning calorimeter (2920 Modulated DSC, TA Instruments, New Castle, DE, USA) equipped with a refrigerated cooling system. The bean starch and flour (12 mg) were weighed into high-volume pans and distilled water (28 ll) was added (70% moisture content). The pans were hermetically sealed and equilibrated overnight at room temperature. The sample pans were heated from 5 to 180 °C at a heating rate of 10 °C/min. Thermal transitions were characterized by To (onset temperature), Tp (peak temperature), Tc (conclusion temperature), and DH (melting enthalpy). 2.7. Pasting properties Pasting properties of the bean flour (11.9% dsb) and isolated starch (9.2% dsb) were determined using a Rapid Visco-Analyser (RVA-4, Newport Scientific, Warriewood, Australia) following AACC method 76-21 (AACC, 2000). 2.8. Chain length distribution Branch chain length distribution of bean starch was determined by high-performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD) (Dionex, Sunnyvale, CA) following the procedure of Liu, Gu, Donner, Tetlow, and Emes (2007). 2.9. In vitro starch digestibility and expected glycemic index Starch digestibility of both bean flour and starch was determined using AACC method 32–40 (AACC, 2000) with pancreatin from porcine pancreas (cat. no. P-1625, activity 3  USP/g, Sigma Chemical Company, St. Louis, MO, USA), and amyloglucosidase (EC 3.2.1.3, 3300 U/ ml, Megazyme International Ireland Ltd., Bray, Ireland). Rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) were determined as suggested by Chung et al. (2007). Hydrolysis index (HI) was obtained by dividing the area under the hydrolysis curve of the sample by the area obtained for white bread. The expected glycemic index (eGI) was calculated using the equation described by Granfeldt, Björck, Drews, and Tovar (1992): eGI = 8.198 + 0.862HI. 2.10. Statistical analyses All physicochemical analyses were performed at least in duplicate. Statistical analyses were carried out with Duncan’s multiple test (P < 0.05) using statistical software SPSS V. 8.2 (SPSS Institute Inc., Cary, NC). 3. Results and discussion 3.1. Chemical composition The chemical composition of bean flour and starch is presented in Table 1. The protein content of bean flour, which ranged from 23.1%

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Sample

Moisture content (%)

Protein content (%)

Total starch content (%)

Apparent amylose content (%)

Bean flour Majesty Red Kanner AC Nautica

10.3 ± 0.1b 9.7 ± 0.1c 9.1 ± 0.3c

24.0 ± 0.1b 23.1 ± 0.6c 26.6 ± 0.2a

40.3 ± 0.8d 39.8 ± 0.2d 36.8 ± 1.1e

9.7 ± 0.3d 9.4 ± 0.4d 9.5 ± 0.6d

Bean starch Majesty Red Kanner AC Nautica

13.8 ± 0.0a 13.4 ± 0.5a 13.7 ± 0.3a

0.0 ± 0.0d 0.0 ± 0.0d 0.0 ± 0.0d

96.7 ± 0.4b 100.0 ± 0.2a 94.5 ± 1.4c

41.5 ± 0.1a 39.6 ± 1.4b 38.0 ± 0.4c

Values followed by a different superscript in each column are significantly different (P < 0.05).

to 26.6%, was found to be higher in AC Nautica (26.6%) than Majesty (24.0%) and Red Kanner (23.1%). Comparable protein content in pinto (24.5%) and navy (25.4%) beans was reported by Gujska et al. (1994). Total starch content of bean flour ranged from 36.8% to 40.3%, and flour from AC Nautica contained lower starch content than the other bean flours (Table 1). These values were lower than those reported by Rehman et al. (2001) for kidney beans (44.4–47.8%), and higher than those of mucuna bean (27.1%; Siddhuraju & Becker, 2005), and common beans (33.6–36.3%; Vargas-Torres et al., 2004), but was comparable to that of black bean (39.2%) and lima bean (39.9%; Tovar & Melito, 1996). The apparent amylose content in bean flour was in the range of 9.4–9.7%, and no significant difference (P > 0.05) was observed among the cultivars. The AC Nautica cultivar is navy bean, whereas the other cultivars are red kidney beans. The high protein and low total starch content in AC Nautica may be attributed to differences in inherited traits. No protein was detected in bean starch (Table 1). The total starch content ranged from 94.5% to 100% in isolated bean starch. These values were comparable to those reported by Tovar and Melito (1996) for red bean (90.2%) and lima bean (98.5%). Isolation of starches from legumes is generally difficult due to the presence of a fine fiber fraction which exists in the cell wall along with the starch granules (Hoover & Sosulski, 1985). The high total starch content and the lack of detectable protein in bean starch indicated that pure starches were obtained using the isolation method. The amylose content of bean starch was in the range of 38.0–41.5% (Table 1). Comparable amylose contents were reported by Su et al. (1997) and Biliaderis et al. (1979) who found 41.4% and 41.5%, and 36.0% and 35.0% for navy and red kidney bean starches, respectively. 3.2. Starch granule characteristics SEM images and polarized light micrographs of isolated bean starch are shown in Fig. 1. The granule size of bean starch ranged

from 24 to 47 lm in length and from 23 to 32 lm in width. This result is in accordance with that reported by Hoover and Sosulski (1985) for navy bean starch granule (22–49 lm in length and 22–40 lm in width). Most of the granules of bean starch were oval or spherical (Fig. 1). As shown in Fig. 1A, the surfaces of all granules appear smooth with no evidence of any fissures or dents. No significant differences in starch granule shape and size among the cultivars were observed (data not shown). The birefringence of bean starch under polarized light showed two different populations of granules (Fig. 1B). The spherical or round granules exhibited the ‘Maltese cross’, showing a dark cross in the center, whereas the elliptical or oval granules had a different image with two cross lines at the two ends of the ellipse and a dark line in the center. Two different birefringence patterns within one cultivar have not been reported. However, further study is required to understand the different birefringence patterns of bean starch. 3.3. X-ray diffraction The X-ray diffraction pattern of bean starch is shown in Fig. 2. All bean starches showed the characteristic C type pattern of legume starches, which is mixture of the A and the B type starch (Colonna, Buleon, & Mercier, 1981; Hoover & Ratnayake, 2002). The bean starches showed peaks at diffraction angles 2h of 5.6°, 15°, 18°, 20° and 23°. A C-type X-ray pattern for bean starches has been previously reported (Hoover & Sosulski, 1985; Hoover, Li, Hynes, & Senanayake, 1997; Zhou et al., 2004). There was no significant difference in peak positions among the cultivars (Fig. 2). However, the relative crystallinity, which was measured based on the diffraction peak intensity, was slightly higher in Majesty

Majesty-S

Intensity

Table 1 Chemical composition of bean flour and starch

Red Kanner-S

AC Nautica-S

0

5

10

15

20

25

30

35

Diffraction angle (2θ) Fig. 2. X-ray diffraction pattern of bean starch. Label indicates different bean starches.

Fig. 1. Scanning electron micrograph (A, 1000) and polarized light micrograph (B, 400) of bean starch from Majesty cultivar.

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(30.3%) than in Red Kanner (27.7%) or AC Nautica (27.9%) although these starches had similar moisture contents. Comparable relative crystallinity ranging between 27.2% and 33.5% for C-type legume starch has been reported (Sandhu & Lim, 2008; Zhou et al., 2004). 3.4. Swelling factor (SF) and amylose leaching (AML) The extent of SF and AML of bean flour and starch in the temperature range 60–90 °C are presented in Table 2. SF ranged between 3.8 and 9.4, and AML ranged from 0.3% to 5.4% in bean flour (Table 2). Although SF and AML were not significantly different among the bean flours, flour from AC Nautica had a lower SF at 70 °C and AML at 90 °C than the other flours. The lower SF and AML of AC Nautica may be attributed to its higher protein content (Table 1), and larger amount of amylose–lipid complex, which is evidenced by the melting enthalpy of the amylose–lipid complex (Table 3) (Hoover & Ratnayake, 2002; Osorio-Diaz, Tovar, ParedesLopez, Acosta-Gallegos, & Bello-Perez, 2005; Ratnayake, Hoover, Shahidi, Perera, & Jane, 2001; Tester & Morrison, 1990) SF and AML of bean starch increased with temperature. Rapid increases in SF and AML occurred between 70 and 80 °C. This was in agreement with earlier observations on various starches (Hoover & Sosulski, 1985; Tester & Morrison, 1990; Yeh & Li, 1996). Hoover and Sosulski (1985) suggested that the rapid increases in SF and AML at temperatures between 70 and 80 °C was due to melting of the starch crystallites. Tester and Morrison (1990) reported that swelling was characterized by an initial phase of slight swelling, a second phase of rapid swelling and a final stage in which maximum swelling was reached. The second phase of rapid swelling was proposed to result from the loss of birefringence and a large decrease in gelatinization enthalpy, which was attributed to dissociation of crystalline clusters. SF ranged from 5.0 to 24.4, and AML ranged from 1.0% to 24.8% in bean starch as testing temperature increased (Table 2). Among the bean starches, SF (be-

yond 70 °C) followed the order: AC Nautica > Red Kanner > Majesty. The higher swelling factor (<70 °C) of navy bean (AC Nautica) compared to kidney bean (Red Kanner andMajesty) was in agreement with earlier observations (Hoover & Sosulski, 1985). SF of bean starch was inversely correlated with amylose content (Table 1), which is in agreement with results reported by Sasaki and Matsuki (1998), and Tester and Morrison (1990). AML of bean starch was in the following order: Red Kanner > Majesty > AC Nautica. AML has been shown to be influenced by the amount of lipid–complexed amylose chains and amylose content (Jayakody, Hoover, Liu, & Weber, 2005; Zhou et al., 2004). The lower AML of starch from AC Nautica may be attributed to its lower amylose content, since bean starch did not exhibit amylose–lipid complex (Table 3). Bean starch showed significantly higher SF and AML than did bean flour, even though the difference in starch content was taken into account. This result could be attributed to the presence of protein, fiber, and lipid in bean flour. These components in bean flour may have played some role in their SF and AML. 3.5. Thermal characteristics The endothermic transitions from bean flour and isolated starch are shown in Fig. 3. Bean starch (Majesty-S) showed only a single transition which reflected gelatinization, whereas bean flour (Majesty-F) exhibited two relatively small separate endothermic transitions which were attributed to starch gelatinization and disruption of amylose–lipid complex (Liu, Donner, Yin, Huang, & Fan, 2006; Tester & Morrison, 1990). Transition temperatures (To, Tp and Tc) and melting enthalpy (DH) of bean flour and starch are presented in Table 3. In bean flour, To, Tp, Tc and DH for gelatinization ranged from 73.1 to 73.2 °C, 81.5 to 82.1 °C, 87.2 to 91.3 °C, and 2.1 to 3.0 J/g, respectively. There were no significant differences (P > 0.05) in To and Tp among the cultivars. However, the Tc of AC Nautica flour

Table 2 Swelling factor (SF) and amylose leaching (AML) of bean flour and starch at various temperatures Sample

Swelling factor 60 °C

Amylose leaching (%) 70 °C

80 °C

Bean flour Majesty Red Kanner AC Nautica

4.9 ± 0.1abc 4.8 ± 0.2abc 3.8 ± 0.1bc

5.9 ± 0.0bb 5.7 ± 0.2bb 4.6 ± 0.1cb

8.3 ± 0.3da 8.9 ± 0.4da 8.6 ± 0.2da

Bean starch Majesty Red Kanner AC Nautica

5.6 ± 0.2ac 5.0 ± 0.5abd 5.0 ± 1.4abc

11.4 ± 1.0ab 11.2 ± 1.2ac 10.9 ± 1.0ab

20.2 ± 0.1ca 21.6 ± 0.9bb 23.3 ± 0.6aa

90 °C

60 °C

70 °C

80 °C

90 °C

8.8 ± 0.2ca 9.4 ± 0.3ca 8.8 ± 0.1ca

0.4 ± 0.2bD 0.3 ± 0.1bD 0.3 ± 0.2bD

1.8 ± 0.0cC 1.6 ± 0.0cC 1.4 ± 0.3cC

4.1 ± 0.2cB 3.8 ± 0.0cB 3.5 ± 0.3cB

5.4 ± 0.0cA 5.4 ± 0.1cA 4.5 ± 0.2dA

20.7 ± 0.6ba 24.1 ± 0.1aa 24.4 ± 0.7aa

1.4 ± 0.0aD 1.5 ± 0.3aD 1.0 ± 0.2aD

9.5 ± 0.3aC 9.5 ± 0.6aC 8.3 ± 0.3bC

18.9 ± 0.0bB 20.4 ± 0.2aB 18.5 ± 0.5bB

22.7 ± 0.6bA 24.8 ± 0.3aA 21.9 ± 0.6bA

Values followed by a different Latin superscript in each column are significantly different; values followed by a different Greek superscript in each row are significantly different (P < 0.05).

Table 3 Thermal properties of bean flour and starch Sample

Gelatinization

Amylose–lipid complex

To (°C)

Tp (°C)

Tc (°C)

Bean flour Majesty Red Kanner AC Nautica

DH (J/g)

73.2 ± 0.2a 73.1 ± 0.2a 73.2 ± 0.2a

81.6 ± 0.1a 81.5 ± 0.3a 82.1 ± 0.6a

87.2 ± 0.1c 87.5 ± 0.3bc 91.3 ± 0.2a

2.1 ± 0.1e 2.1 ± 0.0e 3.0 ± 0.1d

Bean starch Majesty Red Kanner AC Nautica

66.9 ± 0.9b 66.7 ± 0.1b 66.8 ± 0.1b

74.5 ± 0.5b 73.7 ± 0.2b 74.3 ± 0.1b

89.0 ± 1.2b 88.5 ± 0.7bc 91.0 ± 0.5a

14.3 ± 0.4c 14.8 ± 0.0b 15.3 ± 0.1a

Values followed by a different italic superscript in each column are significantly different (P < 0.05). nd = not detected.

To (°C)

Tp (°C)

Tc (°C)

DH (J/g)

90.3 ± 0.0b 90.4 ± 0.2b 95.5 ± 0.2a

95.7 ± 0.1b 96.1 ± 0.3b 100.2 ± 0.1a

105.0 ± 0.3b 106.1 ± 0.4a 106.7 ± 0.2a

1.9 ± 0.0b 1.9 ± 0.0b 2.5 ± 0.1a

nd nd nd

nd nd nd

nd nd nd

nd nd nd

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100

Endothermic heat flow

7000 6000 Temperature

Viscosity (cP)

Majesty-F 1.0 mW

4000

AC Nautica-S Red Kanner-S Majesty-S

3000

60

Majesty-F

2000

20

40

60

80

100

120

140

3.6. Pasting characteristics The pasting profiles of bean flour and starch are shown in Fig. 4. The three cultivars of bean flour displayed similar pasting patterns and showed only minor differences with respect to peak viscosity and final viscosity. All bean flours exhibited a lower pasting viscosity and no breakdown during the holding period at 95 °C. This might be due to a lower starch content in the flour and the pres-

Red Kanner-F AC Nautica-F

20 25

0

0

Fig. 3. DSC thermogram of bean flour (F) and starch (S) from Majesty cultivar.

(91.3 °C) was much higher than those of the other cultivars. The gelatinization temperature range (Tc–To) of AC Nautica flour (18.1 °C) was also much higher than that of Majesty (14.0 °C) and Red Kanner flours (14.4 °C). This wide gelatinization temperature range suggests the presence of crystallites of varying stability within the starch crystalline domains (Singh, Sandhu, & Kaur, 2004). The higher Tc of AC Nautica cultivar could be explained by a greater protein content (Table 1) and a higher melting enthalpy of amylose–lipid complex (Table 3). The large amount of protein results in increased protein–starch interactions, which requires more thermal energy for disruption and melting (Liu et al., 2007). The lipid–complexed amylose chains decrease the extent of hydration in the amorphous regions, thereby increasing the amount of thermal energy required for crystallite melting (Jayakody et al., 2005). Flour from AC Nautica (3.0 J/g) showed higher gelatinization melting enthalpy than did Majesty (2.1 J/g) and Red Kanner (2.1 J/g). This indicates that more energy is needed to break the intermolecular bonds in starch granules of flour from AC Nautica for starch gelatinization. The amylose–lipid complex melting peak, which was observed around 90–110 °C, showed a higher melting temperature for AC Nautica than the other cultivars. The melting enthalpy of amylose–lipid complex in AC Nautica (2.5 J/g) was also higher than Majesty (1.9 J/g) and Red Kanner (1.9 J/g). However, the melting temperature range (Tc-To) of amylose–lipid complex was narrower for AC Nautica (11.7 °C) than Majesty (14.7 °C) and Red Kanner (15.7 °C). In bean starch, To, Tp, Tc, and DH for gelatinization were observed to be in the range of 66.7–66.9 °C, 73.7–74.5 °C, 88.5–91.0 °C, and 14.3–15.3 J/g, respectively. Significant differences (P < 0.05) were observed in conclusion temperature (Tc) and gelatinization enthalpy (DH) among the cultivars as with bean flour. Starch from AC Nautica had a higher Tc (91.0 °C) than the other cultivars. The gelatinization temperature range (Tc–To) was also much higher in AC Nautica (24.2 °C) than the other cultivars. The melting enthalpy of bean starch followed the order: AC Nautica > Red Kanner > Majesty. However, when values of gelatinization enthalpy were calculated on the basis of amylopectin content, there was no significant difference (P > 0.05) among the cultivars of bean starch. A higher Tc, wider Tc– To, and greater DH in AC Nautica than the other cultivars among the bean starches was also found in its flour.

40

1000

Temperature (°C)

Temperature (°C)

80

5000

Majesty-S

5

10

15

20

Time (min) Fig. 4. Viscogram of bean flour (F) and starch (S). Label indicates different bean flours and starches.

ence of lipid, protein, and fibers. Similar pasting profiles were observed by Liu et al. (2006) with mung bean and broad bean flours. Differences in pasting profile of bean starch were observed among cultivars (Table 4 and Fig. 4). Pasting temperature of AC Nautica was lower than that of Red Kanner and Majesty. Peak viscosity followed the order: AC Nautica > Red Kanner > Majesty. The breakdown of AC Nautica was much higher than that of Red Kanner and Majesty. The setback and final viscosity followed the order: Red Kanner > AC Nautica > Majesty. In general, pasting properties of starch are influenced by granule swelling, amylose leaching, starch crystallinity, amylose content, and branch chain length distribution of amylopectin (Hoover, Swamidas, Kok, & Vasanthan, 1996; Jane et al., 1999; Tester & Morrison, 1990). Amylopectin contributes to swelling of starch granules and pasting, whereas amylose inhibits the swelling (Tester & Morrison, 1990). The high peak viscosity and breakdown in AC Nautica might be explained by its lower amylose content (higher amylopectin content, Table 1) and higher SF (Table 2). The higher setback and final viscosity of Red Kanner reflect a higher extent of amylose leaching (Table 2). 3.7. Chain length distribution The chain length distribution and the average chain length of bean starch are presented in Table 5. The proportion of DP 6–12, Table 4 Pasting properties of bean starch Sample

Pasting temperature (°C)

Peak viscosity (cP)

Breakdown (cP)

Setback (cP)

Final viscosity (cP)

Majesty Red Kanner AC Nautica

75.2 ± 0.0a 75.2 ± 0.0a 73.9 ± 0.0b

1980 ± 18c 2286 ± 18b 2746 ± 9a

189 ± 35b 145 ± 5c 891 ± 4a

3011 ± 32c 4391 ± 8a 3488 ± 0b

4802 ± 21c 6532 ± 5a 5343 ± 6b

Values followed by a different superscript in each column are significantly different (P < 0.05).

Table 5 Branch chain length distribution and average chain length of bean starch Sample

Average chain length

Majesty Red Kanner AC Nautica

17.8 ± 0.2a 17.5 ± 0.2a 17.7 ± 0.2a

Percent distribution (%) DP 6–12

DP 13–24

DP 25–36

23.7 ± 1.5a 25.1 ± 1.1a 24.3 ± 1.3a

59.7 ± 1.0a 59.3 ± 0.6a 59.8 ± 0.6a

16.6 ± 0.5a 15.6 ± 0.6a 16.0 ± 0.7a

Values followed by a different superscript in each column are significantly different (P < 0.05).

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DP 13–24, and DP 25–36, and average chain length of bean starches ranged from 23.7% to 25.1%, 59.3% to 59.8%, 15.6% to 16.6%, and 17.5 to 17.8, respectively. Only marginal differences in chain length distribution were observed among the cultivars (Table 5). Red Kanner had a relatively higher proportion of DP 6–12 and lower proportions of DP 13–24 and DP 25–36, and average chain length. 3.8. In vitro starch digestibility Rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) contents of bean flour and isolated starch are presented in Table 6, and their hydrolysis curves are shown in Fig. 5. The bean flour showed a fairly low starch hydrolysis extent (below 5%) and a similar hydrolysis extent among the cultivars (Fig. 5), whereas the bean starch exhibited relatively similar hydrolysis patterns until 3 h incubation, after which Majesty showed a lower hydrolysis extent than AC Nautica or Red Kanner (Fig. 5). RDS, rapidly digested in the small intestine, ranged from 0.2% to 1.2% in bean flour. SDS and RS content of bean flour ranged between 3.1–4.2% and 32.4–36.0%, respectively. There was no significant difference (P > 0.05) among the bean flour cultivars in RDS and SDS contents. However, flour from AC Nautica had a lower RS content than did the other cultivars, possibly due to its lower total starch content (Table 1). Although the employed methodology and variety of bean were different, digestible starch content (RDS + SDS) of bean flour (4–5%) used in this experiment was much lower than reported values; 27–38% for commercial bean (OsorioDiaz et al., 2002), 11% for black bean (Sayago-Ayerdi et al., 2005), Table 6 Starch nutritional fractions and expected glycemic index (eGI) of bean flour and starch by in vitro starch digestion Sample

RDS (%)

SDS (%)

RS (%)

HI

eGI

Bean flour Majesty Red Kanner AC Nautica

0.2 ± 0.2b 1.2 ± 0.8b 0.6 ± 0.5b

4.2 ± 0.5c 3.1 ± 1.2c 3.8 ± 1.4c

36.0 ± 0.5a 35.5 ± 0.4a 32.4 ± 0.9b

4.4 ± 0.1d 4.5 ± 0.3d 4.6 ± 0.5d

12.0 ± 0.1d 12.1 ± 0.2d 12.2 ± 0.4d

Bean starch Majesty Red Kanner AC Nautica

12.3 ± 0.8a 12.4 ± 1.2a 11.7 ± 1.0a

63.1 ± 1.5b 65.8 ± 2.0a 65.7 ± 1.5a

21.3 ± 0.8c 21.9 ± 0.9c 17.2 ± 0.9d

66.9 ± 0.3c 69.8 ± 0.3a 68.7 ± 0.2b

65.8 ± 0.3c 68.4 ± 0.2a 67.4 ± 0.2b

Values followed by a different superscript in each column are significantly different (P < 0.05). RDS, rapidly digestible starch; SDS, slowly digestible starch; RS, resistant starch; HI, hydrolysis index; eGI, expected glycemic index.

4. Conclusions

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The bean flour had fairly low RDS and SDS contents, a high RS content and a low eGI compared to those reported for bean flours. The isolated bean starch contained a substantially larger amount of SDS and had a relatively higher RS content than those reported for cereal starches and other legume starches. The in vitro starch digestibility of bean flour differed only marginally among cultivars. However, in isolated bean starch, Majesty cultivar showed lower digestibility and eGI than the other cultivars. This cultivar differed from the other cultivars in exhibiting higher crystallinity and amylose content which are causative factors in its low digestibility.

80

Total hydrolysis (%)

26% for bean (Goni, Garcia-Alonso, & Saura-Calixto, 1997), and 33% for mung bean (Liu et al., 2006). The hydrolysis index (HI) of bean flour ranged from 4.4 to 4.6 and expected glycemic index (eGI) ranged from 12.0 to 12.2. Sayago-Ayerdi et al. (2005) reported that the HI and eGI of black common bean obtained by in vitro digestion were 21 and 27, respectively. Siddhuraju and Becker (2005) showed that the HI and eGI of mucuna bean were 12.1 and 46.3, respectively. The HI and eGI of bean flour grown in Canada were much lower compared to those reported, possibly due to differences in botanical variety, the employed methods, and growing conditions, as well as the method employed to produce flour which relates to particle size and starch damage. The poor starch digestibility of bean flour used in the experiment has been suggested to have beneficial effects in the management of diabetes and hyperlipidemia (Jenkins et al., 1988; Siddhuraju & Becker, 2005). RDS, SDS, and RS contents of bean starch ranged from 11.7% to 12.3%, 63.1% to 65.8% and 17.2% to 21.9%, respectively. RDS content of bean starch was substantially lower than corn (24.4%), wheat (40.1%) and rice (32.4%) starches (Zhang, Ao, & Hamaker, 2006). SDS content, which is considered a desirable form of dietary starch, was 63.1% for Majesty, 65.8% for Red Kanner and 65.7% for AC Nautica. These values are much higher than those reported by Zhang et al. (2006) for maize (53.0%), waxy maize (47.6%), wheat (50.0%), rice (43.8%) and potato (15.2%) although these were measured using the Englyst method. Besides, the SDS content of bean starch was higher than the values reported for yellow pea (53.7– 59.0%), lentil (58.3–62.2%), and chickpea (45.7–57.7%), which were determined with the same method (Chung et al., 2007). RS content of AC Nautica (17.2%) was lower than that of Majesty (21.3%) and Red Kanner (21.9%). Hydrolysis indices (HI) of bean starches ranged from 66.9 and 69.8, and expected glycemic indices (eGI) were between 65.8 and 68.4. Starch from Majesty had lower HI and eGI than did the other cultivars. In general, digestibility of native starch was influenced by starch source, granule size, amylose/amylopectin ratio, crystallinity, and amylopectin molecular structure (Chung, Lim, & Lim, 2006; Hoover & Sosulski, 1985; Jane, Wong, & McPherson, 1997; Sandhu & Lim, 2008). The lower RS content of AC Nautica may reflect its lower amylose content (Table 1), and higher SF (Table 2) and peak viscosity (Table 4). A higher crystallinity (Fig. 2) and greater apparent amylose content (Table 1) of starch from Majesty could contribute to its low HI and eGI. From our results, we speculate that flour from native common bean grown in Canada under controlled environmental conditions could be considered as an ideal RS material, whereas the isolated bean starch could be an ideal SDS material.

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Acknowledgements

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Hydrolysis time (hr) Fig. 5. Total starch hydrolysis of bean flour and starch: ( ) Majesty-starch, ) Red Kanner-starch, ( ) AC Nautica-starch, ( ) Majesty( ) Red Kanner-flour, ( ) AC Nautica-flour. flour, (

The authors thank Mrs. Elizabeth Donner for her technical support. Financial support for this study through The Saskatchewan Pulse Crop Development Board and Agriculture and Agri-Food Canada MII program is gratefully acknowledged.

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