Relationships Between Selected Properties of Black Gram Seeds and Their Composition

Abstract

Seeds of different black gram varieties were evaluated for their mass, density, hydration capacity, hydration index, cooking time, and instrumental textural properties (hardness, cohesiveness, and chewiness). Starch was also isolated from these varieties and evaluated for yield, amylose content, solubility, water binding capacity, and turbidity. The pasting (pasting temperature, peak viscosity, breakdown viscosity and set back) and gelatinization (T _o , T _p , T _c , ΔH _gel, and PHI) properties of starches were measured using Rapid Visco Analyzer and Differential Scanning Calorimeter, respectively. Physico-chemical, cooking, and textural properties of seeds of different black gram varieties were related to physico-chemical, gelatinization, and pasting properties of their starches using Pearson correlation. Selected properties of black gram seeds were significantly correlated to their starches. The hardness and chewiness of cooked seeds decreased with increase in amylose content of starch (p < 0.05). The physical parameters (seed mass, seed volume, seed density), hydration capacity and cooking time of black gram seeds were positively related to the starch yield (p < 0.01). Fiber content of black gram seeds was positively correlated to the water binding capacity (WBC) of starch (r = 0.610). Seed mass and hydration capacity was positively correlated to turbidity of starch (p < 0.05). Density and hydration index of seeds was related to the pasting properties of their starches. The varieties with higher seed density and hydration index had starches with low peak viscosity and breakdown viscosity and vice-versa (p < 0.01). Hydration capacity and hydration index showed positive correlation with T _c (p < 0.01). Selected instrumental textural parameters had positive correlation with ΔH _gel (p < 0.01).

Keywords:

• Black gram seeds
• Instrumental TPA
• Cooking time
• Starch
• Physico-chemical
• Gelatinization
• Pasting

Introduction

Legumes are second only to the cereals as a source of human and animal food. India is the largest producer of legumes in the world (13,013,700 MT), the total world production being 54,171,307 MT.1 Legumes are rich source of proteins (20–24%), carbohydrates (50–60%), fiber, fat (2–3%), B-complex vitamins (B₁ and niacin) and minerals.2 Legumes have been considered as the most significant food source for people of low incomes.3 Starch is the major carbohydrate of legume seeds.4 Starch is considered of commercial importance due to its high industrial demand as an ingredient for a variety of processed foods.5

Black gram (Phaseolus mungo) is an important legume crop throughout a large part of tropics. It contains on average 10.9% moisture, 24% protein, 1.4% fat, 0.9% fiber and 59.6% carbohydrate.6 Microstructure of legume seeds influences water absorption characteristics and texture of soaked and cooked legumes. Textural changes during soaking are dependent on the water absorption pattern.7 Grain hardness is very important factor in determining grain quality.8 Cooking of legumes include gelatinization of cell starch and the consequent deformation of the granules. Cooking time is a highly significant aspect of cooking quality. Cooking time is affected by the starch itself, the permeability of the seed coat, and the internal structure of the seed endosperm material, all of which would be affected by soaking in water.9 Starches from different sources vary in their qualitative and quantitative make-up as well as in some physico-chemical properties. DSC monitors changes in the physical properties of starches, offering a thermodynamic approach to the study of starch gelatinization.10 The physical properties of starch granules are determined by the fine structure of the polysaccharides and the percent distribution of amylose and amylopectin.11 There is lack of information on seed-starch property relationships among legumes. Therefore, the present investigation was undertaken to establish relationships between physico-chemical, cooking, and instrumental textural properties of seeds of different black gram varieties with the physico-chemical, gelatinization and pasting properties of starches isolated from these varieties.

Materials and Methods

Materials

Representative samples of 13 improved black gram varieties viz. UG-414, UG-218, UG-909, UG-1017, UG-1093, UG-841, UG-916, UG-902, UG-562, UG-920, UG-1008, UL-338, and KU-3 from 2002 harvest were obtained from the Regional Research Center, Gurdaspur, Punjab.

Physico-Chemical Properties of Seeds

The samples were estimated for their ash, fat, protein, and fiber content by employing the standard AOAC methods of analysis.12 The carbohydrate content was calculated by difference. All the results were reported on dry weight basis (db). Seed mass, seed volume, seed density, hydration capacity (increase in weight of seeds after soaking/total number of seeds), and hydration index (hydration capacity/seed mass) were evaluated using the method of Williams et al.9

Cooking Time

Seeds (100 g) of different black gram varieties were placed in beakers fitted with condensers to avoid evaporation losses during boiling. Distilled water was added in the ratio of 1:4 (w/v) to the beakers. Cooking time was determined using the method of Williams et al.9 Degree of cooking was tested by pressing seeds between the forefinger and thumb. Cooking time was reported as time to cook 90% of the seeds.

Instrumental Texture Profile Analysis (TPA)

Instrumental TPA of cooked black gram varieties was performed by using a single cooked grain from each variety, which was placed on the base plate of the Instron Universal Testing Machine (Model 4464, Instron, Buckinghamshire, England) parallel to the plate for testing. The grain was subjected to 80% compression with a cylindrical probe (38 mm diameter) at a crosshead speed of 1mm/s twice in two cycles using a 10 kg load cell. The instrumental textural parameters of hardness, cohesiveness, and chewiness were determined as described by Bourne.13 Eight repeated measurements were performed on each sample.

Starch Isolation

Starch was isolated from different black gram varieties using the method of Singh et al.14 Various black gram varieties (300 g) were steeped in water containing 0.16% sodium hydrogen sulphite for 12 h at 50°C. The steep water was drained off, and grains were ground in a laboratory blender. The ground slurry was screened through nylon cloth (100 mesh). The material left over the nylon cloth was washed thoroughly with distilled water. The filtrate slurry was allowed to stand for 1 h. The supernatant was removed by suction and the settled starch layer was resuspended in distilled water and centrifuged in wide-mouthed cups at 2800 rpm for 5 min. The upper nonwhite layer was scrapped off. The white layer was resuspended in distilled water and recentrifuged for 3–4 times. The starch was then collected and dried in an oven at 40°C for 12 h.

Physico-Chemical Properties of Starches

Amylose content of the isolated starches was determined in triplicate using the method of Williams et al.15 Swelling power and solubility were determined in triplicate using method of Leach et al.16 Turbidity of starches from different black gram varieties, was measured in triplicate as described by Perera and Hoover.17 A 1% aqueous suspension of starch from each black gram variety was heated in a water bath at 90°C for 1 h with constant stirring. The suspension was cooled for 1 h at 30°C. The samples were stored for five days at 4°C in a refrigerator and turbidity was determined every 24 h by measuring absorbance at 640 nm against a water blank with a Shimadzu UV-1601 spectrophotometer (Shimadzu Corporation, Kyoto, Japan). Water binding capacity (WBC) is defined as the percentage uptake of water by starch granules without involvement of any heat treatment. WBC of starches from different black gram varieties was determined using the method described by Yamazaki18 as modified by Medcalf and Gilles.19 A suspension of 5 g starch (dry weight) in 75 mL distilled water was agitated for 1h and centrifuged (3000 × g) for 10 min. The free water was removed from wet starch, drained for 10 min and wet starch was weighed.

Gelatinization Properties of Starches

Gelatinization characteristics of isolated starches were studied by using Differential Scanning Calorimeter-821^e (Mettler Toledo, Switzerland) equipped with a thermal analysis data station. Starch (3.5 mg, dry weight) was loaded into a 40 µL capacity aluminium pan (Mettler, ME-27331) and distilled water was added with the help of Hamilton microsyringe to achieve a starch-water suspension containing 70% water. Samples were hermetically sealed, allowed to stand for 1 h at room temperature and reweighed before heating in the DSC. The DSC analyzer was calibrated using indium, and an empty aluminium pan was used as reference. Sample pans were heated at a rate of 10°C/min from 20 to 100°C. Onset temperature (T _o ), peak temperature (T _p ), conclusion temperature (T _c ), and enthalpy of gelatinization (ΔH _gel) were calculated automatically. Enthalpies were calculated on a dry starch basis. The peak height index (PHI) was calculated as the ratio ΔH _gel/(T _p −T _o ) as described by Krueger et al.20

Pasting Properties of Starches

Rheological properties of isolated black gram starches were studied by using Rapid visco analyzer. The AACC method 61-02 (2000) was followed.21

Statistical Analysis

The data reported in all the tables are an average of triplicate observations. The data were subjected to one-way analysis of variance (ANOVA) and correlation coefficients were calculated using Minitab Statistical Software (MINITAB 2002, Version 13, USA).

Results and Discussion

Physico-chemical characteristics of different black gram varieties (db) varied significantly (Table 1). The ash, fat, crude fiber, and protein content of different black gram varieties, ranged between 2.70–3.36, 0.80–1.31, 5.05–5.92, and 21.0–23.0%, respectively. The lowest ash content for UG-1017 and highest for UG-916 variety was observed. UL-338 variety had the lowest while UG-1093 variety showed highest crude fiber content among all black gram varieties. Lowest protein content was found for UG-920 and highest for UG-916. The carbohydrate content (calculated by difference) was lowest for UG-916 (67.4%) and highest for UG-920 (69.5%). Raghuvanshi et al.22 reported average ash, fat, fiber, protein, and carbohydrate contents of 3.58, 1.46, 4.12, 24.8, and 66%, respectively for 10 black gram varieties. Environmental conditions exert significant influences on chemical composition of legumes.23 Seed mass and seed volume also varied significantly among different black gram varieties. KU-3 variety showed highest seed mass and seed volume of 5.2 g/100 seeds and 4.3 cc/100 seeds, respectively. UG-1017 and UG-562 variety showed lowest seed weight. UG-920 and UG-1008 varieties showed lowest seed volume of 3.0 cc/100 seeds. Different black gram varieties showed seed density and hydration capacity in the range of 1.21–1.33 g/cc and 3.5–6.3 g/100 seeds, respectively. UG-562 variety showed the lowest hydration capacity and cooking time. Cooking time for different black gram varieties ranged between 41 and 55 min, with the KU-3 variety having the longest cooking time. Cooking time is a heritable characteristic that differs widely among genotypes. Instrumental texture measurements of the cooked samples indicated that the rate and degree of softening varied among different varieties. Hardness and cohesiveness value of cooked black gram seeds from different varieties ranged from 10.54 to 31.28 N and 0.051 to 0.124, respectively. Varriano-Marston and Omana,24 Sefa-Dedeh et al.,25 and Rockland and Jones26 have reported that separation of cells accompanies the softening of legumes during cooking. Chewiness value of cooked black gram seeds ranged between 0.529 and 4.196 Nmm. UG-909 variety gave the lowest cohesiveness and chewiness values. The overall instrumental textural characteristics of the legume might be attributed to differences in gelatinization pattern of the starch and the susceptibility of the cell constituents, notably the protein matrix, to softening.7

Table 1 Physico-chemical, cooking, and textural properties of seeds of different black gram varieties

	Ash*	Fat*	Protein*	Fiber*	CHO*
Black gram varieties	(%)					Seed mass (g/100 seeds)	Seed volume (cc/100 seeds)	Seed density (g/cc)	Hydration capacity per 100 seeds	Hydration index	Cooking time (min)	Hardness# (N)	Cohesiveness†	Chewiness‡ (Nmm)
UG-414	3.17^c	0.91^ab	22.3^b	5.17^ab	68.5^ab	4.4^bc	3.5^bc	1.26^a	5.3^c	1.26^bc	43.5^b	13.67^b	0.077^ab	1.119^ab
UG-218	3.18^c	1.13^b	22.5^b	5.70^d	67.5^a	4.2^b	3.4^b	1.24^a	5.5^cd	1.31^c	47.0^c	25.16^de	0.124^b	2.474^b
UG-909	3.00^b	0.85^a	22.8^bc	5.20^b	68.2^ab	4.5^c	3.7^c	1.22^a	5.6^cd	1.23^bc	50.5^d	13.86^b	0.051^a	0.529^a
UG-1017	2.70^a	1.31^c	22.2^b	5.55^cd	68.2^ab	3.8^a	3.1^ab	1.23^a	4.1^ab	1.09^ab	43.0^ab	14.70^bc	0.105^ab	1.613^ab
UG-1093	2.93^ab	1.08^ab	22.4^b	5.92^e	67.7^a	4.8^d	3.7^c	1.30^a	6.0^d	1.34^c	52.0^de	25.73^e	0.107^ab	2.598^b
UG-841	3.28^cd	1.27^c	22.3^b	5.70^d	67.5^a	4.6^c	3.6^bc	1.28^a	6.1^d	1.23^bc	51.0^d	31.28^f	0.103^ab	4.196^c
UG-916	3.36^d	1.00^ab	23.0^c	5.27^b	67.4^a	4.3^b	3.5^bc	1.23^a	5.6^cd	1.34^c	47.5^c	22.35^d	0.113^ab	2.647^b
UG-902	3.06^bc	1.25^c	21.3^a	5.22^b	69.2^b	4.3^b	3.4^b	1.26^a	5.2^c	1.07^ab	47.0^c	16.69^c	0.097^ab	1.677^ab
UG-562	3.16^c	1.01^ab	22.5^b	5.48^c	67.9^ab	3.8^a	3.1^ab	1.23^a	3.5^a	0.93^a	41.0^a	22.58^d	0.094^ab	3.717^c
UG-920	3.11^bc	1.26^c	21.0^a	5.16^ab	69.5^b	3.9^a	3.0^a	1.30^a	4.1^ab	1.23^bc	44.5^b	15.27^bc	0.084^ab	0.891^a
UG-1008	3.28^cd	0.86^a	22.4^b	5.74^d	67.8^ab	4.0^ab	3.0^a	1.33^a	4.4^b	1.18^b	45.0^b	10.54^a	0.105^ab	0.990^a
UL-338	3.07^bc	0.80^a	21.9^ab	5.05^a	69.2^b	4.3^b	3.4^b	1.26^a	4.9^bc	1.26^bc	44.0^b	23.18^d	0.084^ab	2.345^ab
KU-3	3.26^cd	0.95^ab	21.6^ab	5.32^bc	68.9^b	5.2^e	4.3^d	1.21^a	6.3^d	1.17^b	55.0^e	23.50^de	0.114^b	1.711^ab

Means superscripted with different alphabets in the same column differ significantly (p < 0.05).

*db.

^#Maximum height of the force peak on the first compression cycle (first bite).

^†Ratio of the positive force areas under the first and second compressions.

^‡Product of gumminess and springiness.

CHO = carbohydrate.

Different black gram varieties had starch yields and amylose contents in ranges of 19–25% and 30.2–34.6%, respectively (Table 2). Sathe et al.27 and Srinivasa Rao28 reported higher starch yields for black gram than observed in the present study. It was observed that varieties with higher seed weight resulted into higher starch yield (r = 0.857). Starch yield from different black gram varieties was positively correlated to seed volume (r = 0.795), hydration capacity (r = 0.742), and cooking time (r = 0.618) (Table 3). UG-562 gave the lowest starch yield, while KU-3 variety gave the highest. Amylose content was observed to be highest for UG-916 and lowest for UG-562 starch. An average amylose content of 26.65% for black gram starches has been reported earlier.27 Amylose content of starches separated from different black gram varieties showed a significant negative correlation with various instrumental texture parameters such as hardness (r = −0.452) and chewiness (r = −0.489). Solubility (amount of solids leached out in the solution during heating of starch suspension) for different black gram starches ranged between 0.148 and 0.173 g/g. When starch granules are heated in excess water, the crystalline structure is disrupted and water molecules become linked by hydrogen bonding to the exposed hydroxyl groups of amylose and amylopectin, which brings about their dissolution.29 UG-916 starch showed the lowest solubility value. Solubility of black gram starches showed a significant negative correlation with seed volume (r = −0.497) and hydration capacity (r = −0.514). Legume starches have been shown to exhibit a single stage restricted swelling and low solubility patterns.30 31 This indicates a more strongly bonded micellar network. WBC of starches from different black gram varieties ranged between 73.5 and 84.5%, lowest for KU-3 and highest for UG-1008 starch. Loose association of amylose and amylopectin molecule in the starches has been reported to affect WBC.32 WBC of starches from different varieties of black gram was positively correlated to fiber content of black gram seeds (r = 0.610). The turbidity values of gelatinized starch suspensions from different black gram starches ranged between 1.56 and 1.70. UG-920 starch paste showed lowest turbidity values during 120 h of storage at 4°C. It was observed that varieties with higher seed weight and hydration capacity had higher turbidity values for their gelatinized starch suspensions. Turbidity development in starches during storage has been reported to be influenced by factors such as granule swelling, leached amylose and amylopectin, amylose and amylopectin chain length, lipid and cross-linking substitution.33 The protein and carbohydrate content of seeds did not show significant correlation with any of the starch properties. The results of Rapid visco-analysis revealed that pasting temperature of starches from different black gram varieties ranged between 75.8 and 80.3°C, lowest for UG-562 and highest for UG-909 starch. Pasting temperature of black gram starches showed negative correlation to fiber content (r = −0.532) and cohesiveness (r = −0.568) of cooked black gram seeds. Peak viscosity and breakdown viscosity of different black gram starches ranged from 422 to 514 and 134 to 212 RVU, respectively. Peak viscosity was negatively correlated to seed density and hydration index (p < 0.01). Peak viscosity and breakdown viscosity were highest for UG-562 and lowest for UL-338 starch. Setback values ranged from 102 to 151 RVU, lowest for KU-3 and highest for UL-338 starch. Setback showed a significant negative correlation with fiber content (r = −0.464) and cooking time (r = −0.439) and a positive correlation with seed density (r = 0.506) and hydration index (r = 0.438) of black gram seeds. In DSC analysis, the onset temperature (T _o ), peak temperature (T _p ), conclusion temperature (T _c ), enthalpies of gelatinization (ΔH _gel), and peak height index (PHI) for black gram starches are shown in Table 2. T _o and T _p was found to be lowest for UG-562 starch while UG-909 starch showed the highest values. T _c was observed to be highest for UG-414 and lowest for UG-1017 starch. T _c showed a significant positive correlation with hydration capacity (r = 0.554) and hydration index (r = 0.534) and was negatively correlated to cohesiveness of cooked black gram seeds. ΔH _gel was observed to be highest for KU-3 starch, whereas UG-414 starch showed the lowest ΔH _gel value. PHI was found to be lowest for UG-920, whereas it was highest for UG-218 starch. Granule shapes, percentage of large and small granules and the presence of phosphate esters have been reported to affect the ΔH _gel values of starches.34 35 ΔH _gel and PHI were significantly correlated to various instrumental texture parameters such as hardness, cohesiveness, and chewiness of cooked seeds (p < 0.01).

Table 2 Physico-chemical, pasting, and gelatinization properties of starches from different black gram varieties

Black gram varieties	Amylose (%)	Yield (%)	Solubility (g/g)	WBC (%)	Turbidity (120 h)	Pasting temperature* (°C)	Peak viscosity# (RVU)	Breakdown viscosity† (RVU)	Set back‡ (RVU)	T o (°C)	T p (°C)	T c (°C)	ΔH gel  (J/g)	PHI
UG-414	32.6^ab	23^ab	0.151^a	77.3^b	1.70^b	79.5^bc	450^bc	156^b	130^bc	70.9^c	75.5^bc	80.4^c	6.7^a	1.5^a
UG-218	31.1^ab	21^ab	0.159^ab	76.6^ab	1.62^ab	77.9^ab	445^b	165^bc	127^b	69.2^b	72.9^ab	78.1^b	8.8^ab	2.4^b
UG-909	32.8^ab	21^ab	0.155^ab	74.2^a	1.70^b	80.3^c	449^bc	149^ab	124^b	71.3^c	76.2^c	80.2^c	7.1^ab	1.4^a
UG-1017	31.1^ab	19^a	0.153^a	76.6^ab	1.60^ab	76.8^ab	494^c	183^c	125^b	67.1^ab	71.9^ab	75.9^a	7.5^ab	1.7^ab
UG-1093	32.9^ab	22^ab	0.156^ab	82.4^c	1.62^ab	76.7^ab	494^c	182^c	106^a	67.4^ab	72.4^ab	77.1^ab	8.2^ab	1.6^ab
UG-841	31.7^ab	22^ab	0.157^ab	76.0^ab	1.62^ab	78.5^b	466^bc	161^bc	108^ab	69.3^b	73.3^ab	78.6^bc	9.2^b	2.3^b
UG-916	34.6^b	21^ab	0.148^a	78.4^b	1.64^ab	78.7^b	459^bc	157^b	134^bc	70.1^b	74.1^b	79.1^bc	8.3^ab	2.1^b
UG-902	33.4^b	22^ab	0.156^ab	78.5^b	1.68^b	79.6^bc	475^bc	167^bc	105^a	71.1^c	74.5^b	78.0^b	7.7^ab	2.3^b
UG-562	30.2^a	19^a	0.165^ab	76.1^ab	1.56^a	75.8^a	514^d	212^d	106^a	66.1^a	71.0^a	76.0^a	8.3^ab	1.7^ab
UG-920	32.5^ab	20^a	0.164^ab	74.0^a	1.56^a	77.4^ab	494^c	184^c	122^ab	67.1^ab	72.2^ab	76.8^ab	7.0^a	1.4^a
UG-1008	33.0^ab	21^ab	0.173^b	84.5^c	1.62^ab	77.4^ab	498^cd	193^cd	119^ab	68.1^ab	72.7^ab	77.7^ab	8.8^ab	1.9^ab
UL-338	30.7^a	23^ab	0.171^b	76.3^ab	1.64^ab	79.3^bc	422^a	134^a	151^c	69.9^b	74.5^b	79.2^bc	7.7^ab	1.7^ab
KU-3	32.0^ab	25^b	0.154^a	73.5^a	1.58^a	76.3^ab	497^cd	199^cd	102^a	67.6^ab	72.6^ab	77.9^b	9.4^b	1.9^ab

Means superscripted with different alphabets in the same column differ significantly (p < 0.05).

*The temperature at the onset of rise in viscosity.

^#The equilibrium point between swelling and polymer leaching.

^†Peak viscosity-Holding strength.

^‡Increase in final viscosity upon cooling.

T _o  = onset temperature of gelatinization; T _p  = peak temperature of gelatinization; T _c  = conclusion temperature of gelatinization; ΔH _gel = enthalpy of gelatinization; PHI = peak height index [ΔH _gel/(T _p  − T _o)].

Table 3 Pearson correlations between properties of seeds and starches of different black gram varieties

	Starch properties
Seed properties	Yield	Amylose	Solubility	WBC	Turbidity	Pasting temperature	Peak viscosity	Breakdown viscosity	Set back	T c	ΔH gel	PHI
Fiber	−0.215	0.187	0.087	0.610b	−0.249	−0.532a	0.411	0.424	−0.464a	−0.456	0.585b	0.360
Seed weight	0.857b	0.105	−0.382	−0.107	0.490a	0.101	−0.161	−0.146	−0.312	0.418	0.371	0.148
Seed volume	0.795b	−0.005	−0.497a	−0.281	0.417	0.093	−0.184	−0.138	−0.273	0.417	0.344	0.137
Seed density	0.520a	0.025	−0.180	0.139	0.254	0.424	−0.640b	−0.632b	0.506a	0.605b	−0.022	0.050
Hydration capacity	0.742b	0.138	−0.514a	−0.031	0.509a	0.314	−0.384	−0.386	−0.162	0.554b	0.370	0.373
Hydration index	0.373	0.033	−0.228	0.141	0.184	0.362	−0.572b	−0.585b	0.438a	0.534a	0.010	0.044
Cooking time	0.618b	0.219	−0.373	−0.059	0.340	0.024	−0.016	−0.045	−0.439a	0.242	0.486a	0.263
Hardness	0.264	−0.452a	−0.137	−0.187	−0.162	−0.218	−0.139	−0.066	−0.210	−0.060	0.632b	0.507a
Cohesiveness	0.059	−0.112	−0.090	0.336	−0.361	−0.568b	0.285	0.381	−0.266	−0.462a	0.754b	0.685b
Chewiness	−0.065	−0.489a	−0.029	−0.007	−0.216	−0.301	0.042	0.096	−0.238	−0.230	0.570b	0.513a

^aStatistically significant at 5% level (p < 0.05).

^bStatistically significant at 1% level (p < 0.01).

Conclusion

In summary, various physico-chemical, cooking, and instrumental textural properties of black gram seeds were observed to correlate with physico-chemical, gelatinization and pasting properties of their starches. The results also revealed that selected properties of legume seeds might be useful in assessing the properties of their starches. Further, there is a need to carry out detailed investigation on properties of other legume seeds and their starches.