COOKING CHARACTERISTICS OF FOUR CULTIVARS OF BAMBARA GROUNDNUTS SEEDS AND STARCH ISOLATE

COOKING CHARACTERISTICS OF FOUR CULTIVARS OF BAMBARA GROUNDNUTS SEEDS AND STARCH

ISOLATE

PHILIPPA CHINYERE OJIMELUKWE

College of Food Processing and Storage Technology

Federal University of Agriculture Utnudike P.M. B. 7267, Urnuahia

Ahia State, Nigeria

Accepted for Publication March 12, 1998

ABSTRACT

The proximate composition and textural changes during the cooking of four bambara groundnut (Voandzeia subterranea) seed cultivars were evaluated. Bambara groundnut seeds contained 18-21 % protein 6-8% fat, 8-10% moisture and 1.2-2.6% ash. Changes in hardness in relation to cooking time was monitored for 2 h. The cooking time for sofrening was shorter for the cream and brown colored cultivars than for the red black colored cultivars. The swelling and viscosity properties of starch isolates prepared form the seeds were studied. There were no varietal digerences (p 50.05) in starch viscosity and swelling properties among cultivars. Estimation of tannin content of bambara groundnut seeds showed that tannin contents of the red and black seed cultivars (0.96% and 1. I %, respectively) were signijicantly higher @ 50.05) than the tannin contents of cream and brown seed cultivars (0.68% and 0.72 %, respectively).

INTRODUCTION

Bamabara groundnut is one of the most important grain legumes in subsahelian West Africa (Vietmeyer 1978; Mbata and Reichmuth 1996). It is a tropical pulse, usually grown for house consumption in Southern part of Nigeria and produced in commercial quantities in Northern Nigeria.

Aspects of chemical composition, botany and seed structure of some bambara groundnut seed cultivars have been studied (Oyenuga 1968; Vaughan 1970; Amuti and Pollard 1974; Salunkhe and Kadam 1989; Kay 1979; Linneman 1987, 1992). Like many other legumes, bambara groundnuts contains some phenolics and antinutritive factors that have negative effects on protein

Journal of Food Biochemistry 23 (1999) 109-1 17. A f f Rights Reserved. "Copyrighi 1999 by Food & Nutriiion Press, I t i t . , Trumbull, Connecticut 109

110 P.C. OJIMELUKWE

metabolism (Reed et al. 1990). However, this crop has several cultivars that have not been fully characterized.

The high satiety value of this legume has popularized its utilization and accentuated the need for better scientific understanding of how to improve its utilization (Vietmeyer 1978; Vietmeyer 1986). For instance, the long cooking time and the hard to cook defect, associated with legumes such as cowpea (Akinyele er al. 1986) are also noticed in this legume.

A breakthrough in processing and utilization of bambara groundnut seeds, requires thorough understanding of the nature and properties of the seed. The present research highlights the starch properties of four most common Nigerian cultivars of bambara groundnut seeds and provides fundamental scientific data needed for processing and better utilization of bambara groundnut seeds.

MATERIALS AND METHODS

Seed cultivars (red, black, cream and brown), were certified by the Botany Department, Abia State University.

Proximate Analysis

The proximate composition of four cultivars of bambara groundnut seeds was determined using standard procedures (AOAC 1975). The crude protein (AOAC 22.038), moisture (AOAC 22.008) and ash content (AOAC 22.010) were determined. Determinations were carried out on triplicate samples.

DETERMINATIONS OF TEXTURAL CHANGES DURING COOKING

Seed samples (50 seeds) from each cultivar which were of similar sizes and have an average weight of about lg each, were boiled in 250 mL distilled water at 98C. Change in weight as cooking progressed was monitored at 30 min intervals initially for the first 1 h and at 15 min intervals for the next 1 h. Seeds were removed from each seed lot (separated by partitioning with metal networks in the same electrothermal heater), drained of water (using perforated metal cooking spoon) and dried to remove surface moisture before weighing. The maximum force required for initial deformation of seeds and change in hardness as cooking progressed were monitored using a hardness tester (Monsato hardness tester, Manesty Machines, Liverpool, England) usually used for evaluating the hardness of small objects. Test for hardness was carried out on triplicate samples (single seeds), selected at random from each seed lot and cooled to ambient temperature. Regression equations were used to transform data obtained from textural studies during cooking.

COOKING AND STARCH PROPERTIES OF BAMBARA BEANS 111

Preparation of Starch Isolates

For the preparation of starch isolates, raw cleaned seeds (1OOOg) were soaked for 24 h in distilled water 24C, containing 0.03% sodium azide. They were dehulled manually and blended in a Waring blender with distilled water in 1 : 10 ratio, weight/volume. The blended slurry was sieved to pass a 65 mesh (212 pm) and 325 mesh (45 pm) sieves, U.S.A. standard testing sieves, respectively. The resulting starch suspension was allowed to sediment and the upper layer was decanted. It was reslurried and resedimented several times until the upper layer was clear of any haze. The starch isolate sediment was spread on clean white polythene bags and dried at ambient temperature and humidity with a torrent of air from a ceiling fan in the laboratory. Samples were placed at a sufficient distance from the fan to ensure that they were not blown away. They were dried to a moisture content of 10%. The ash, protein, and moisture contents of the starch isolates were determined using standard AOAC methods (AOAC 1975).

Swelling Of Bambara Groundnut Starch Isolates

Swelling power of starch isolates was determined by the method of Leach et al. (1959) with slight modifications. Starch isolates (3g) were suspended in 10 mL distilled water in 15-mL centrifuge tubes calibrated at 0.1 mL intervals. The tubes were heated at 95C for 1 h and samples were stirred intermittently (10 min intervals), with a glass rod to keep the granules suspended. The tubes were centrifuge at 5,000 g for 15 min at ambient temperature, after which the supernatant was decanted and the volume increase was noted from the calibrations on the centrifuge tubes. Swelling was calculated as the ratio of the volume of swollen sediment to the volume of the initial sediment. Each experiment was replicated five times.

Viscosity Measurements

The Brabender amylograph was used for the determinations of starch viscosity of 8% starch suspensions in distilled water during a heating cycle of 30-95C. Samples (8g1100 mL water) were heated at 1.5C per rnin up to 95C ad held for 15 min before cooling to 50C. Values obtained were recorded in Brabender units.

Determination of Tannin Content

Whole bambara groundnut seed samples were ground in Cyclotex 1053 sample mill (Tecaltor instruments U.S.A.) after initial cracking in a hammer mill to pass a 44 mesh sieve. Defatting was initially carried out using petroleum spirit (40-60C, boiling point) at 26C. After the removal of petroleum spirit by

112 P.C. OJIMELUKWE

filtration under suction (after 3 h fat extraction), tannin extraction was carried out using 10% acidified methanol (flourholvent ratio = 1:lO) at 26C. Samples were mixed occasionally by swirling and were extracted for 30 min. The assay reagent was 4% vanillin in methanol. Prepared tannin extracts were used for the protein precipitation assay as described by Hagerman and Butler 1978. Absorbance was read at 510 nm. Difference between the absorbance and the blank AA was used to estimated the tannin content.

RESULTS AND DISCUSSIONS

Four bambara groundnut seed cultivars were used in the present study. They were (1) red, smooth near round seeds (2) black smooth round seeds (3) cream smooth round seeds and (4) russet-brown seeds some of which were round while others were cylindrical. All the seeds possessed white elliptical hilium and yellow, round wrinkled pods. The proximate composition and tannin content of bambara groundnut seeds are shown in Table 1 and are similar to previous results from other researchers (Oyenuga 1968). There was no statistically significant difference (p 10.05) in the proximate composition of the different seed cultivars, except for the carbohydrate content of the cream colored cultivar (60%) that was significantly higher than that of the black colored seed cultivar (54%), (p 10.05). The tannin contents of the red and black colored seed cultivars were found to be significantly higher (p 20.05) than the tannin contents of the cream and brown seed cultivars.

TABLE 1. PROXIMATE COMPOSITION AND TANNIN CONTENT OF BAMBARA

GROUNDNUT WHOLE SEED CULTIVARS. (DRY WEIGHT BASIS)

Color of Protein Fat Moisture Ash Carbohydrate Tannin Whole Seed (%) (%) ( % I (%) (%) Tannic Acid

Equivalent Tg( % )

Red 20+2 73=2 8+ 1 1.5+ 1 56+ 1 1 .Oa

Black 21 +2 8 k 1 9* 1 2.1+1 54*2 l . l a

Cream 19*2 1.16 10-12 2.5+1 60k2 0.7b

Brown 19*2 7+ 1 10* 1 2.5+1 56*2 0.7b

Values followed by same letters are not significantly different ( P l 0 . 0 5 ) from one another.

COOKING AND STARCH PROPERTIES OF BAMBARA BEANS 113

A decrease in hardness occurred during the cooking of all the bambara groundnut seeds. Change in deformation force with cooking time was signifi- cantly different from one another from 0-75 min of cooking (p10.05), see Table 2. There was an initial period of very little water imbibition as indicated by increase in weight despite significant changes in deformation force, during the cooking of bambara groundnut seeds. As expected, the weight of the seeds increased as the cooking time progressed. Change in weight of the seed with cooking time was not significantly different from one another (p 10.05) until after about 30 min of cooking. Varietal differences did not affect the average weight gain of seeds during cooking. Average deformation force was greater for the red and black seed cultivars (5.0 g 9 and was significantly different from the average deformation forces of thexream and brown seed cultivars (4.0 gf). Regression plots of change in force with cooking time for the four cultivars of bambara groundnut seeds use i n the study are given in Fig. 1.

TABLE 2. EFFECT OF COOKING ON THE WEIGHT AND HARDNESS OF BAMBARA

GROUNDNUTS SEEDS

Cooking Time Mean Deformation Mean Weight Force (gf) (go

0 30 60 75 90 105 120

12.3a 1 .Oe 7 . l b 1 .0e 4.0c 1 . a 3.lC 1 . 6 ~ 2 . 1 4 I .8b I .3de 1.9b 1 . le 2 . la

Values followed by the same letters within the same column are not statistically significant different ( ~ 5 0 . 0 5 ) from one another.

The regression equations were as follows:

F, = 11.42 - 0.09T; F,, = 1 1.72 - 0.10T; F,, = 10.02 - 0.09T; F,, = 10.26 - 0.09T

These differences in the ch'mge in hardness as cooking progresses indicate that varietal differences affect the cooking time of these beans (Akinyele er al. 1986). There are subtle differences in the deposition of storage materials and

114 P.C. OJIMELUKWE

nature of the seed coat of beans belonging to different varieties. Water absorption characteristics, reactions of components responsible for hardness, structural changes such as the breakdown of the middle lamella, are all factors that contribute to softening (Rockland et al. 1989; Lolas and Markakis 1975).

FCR = -0 09r + 10 02 FR =-009r c 11 42 F B L = - O i x + l : 72

FRR = -0.09~ + 10 26

~~~ ~

Cooking Time t (min)

Change in Deformation Force with Time of Cooking

FIG. 1. REGRESSION PLOTS OF CHANGE IN FORCE WITH COOKING TIME FOR THE FOUR CULTIVARS OF BAMBARA GROUNDNUT SEEDS

COOKING AND STARCH PROPERTIES OF BAMBARA BEANS 115

The cream and brown colored seed cultivars of bambara groundnut seeds would require less processing time than the red and black seed cultivars, based on the findings of the present study. Because tannins are polyphenolics whose interactions with proteins during cooking render proteins unavailable for digestion, the higher tannin contents of the red and black seed cultivars suggest lower protein availability from these seeds. Swelling characteristics of starch isolates from the four bambara groundnut seed cultivars under study are shown in Table 3. The swelling power values are expressed as volume ratios.

TABLE 3. SWELLING POWER OF STARCH ISOLATES AT DIFFERENT TEMPERATURES

Temperature Swelling Power

("C) Red Black Cream Black Mean Values

65 2.0 f 1.5cd 2.2 f l.Ocd 2.6 f I.Ocd 2.5 f l.Ocd 2.3b 70 4.0 f l.Ocd 5.6 f 2.0cd 6.4 f 2.0cd 6.2 f 1.5cd 5 . l b 75 5.6 f 20abc 6.0 f 1.5bc 10.0 f l.Obc 8.6 f l.Obc 7.6a 80 7.5 f l.Oab 8.2 l.Oab 11.2 f l.Oab 10.6 f 1.Obc 9.4a 85 8.0 f 2.0ab 10.0 f 2.0ab 12.8 f 2.0ab 11.2 f l.Oab 10.5a 90 9.0 f 1.0a 10.5 f 1.0a 14.5 f 2.0a 11.6 f 2.0ab 11.4a

Values carrying the same subscript are not statistically different (p l0.05).

The swelling power at 90C was significantly higher than the swelling power at 65C and at 70C (p 50.05) for starch isolates form the four seed cultivars. At 70C, all starch isolates had started swelling and gelling (Table 3). Starch isolates from the red colored bambara groundnut seed cultivar showed the highest resistance to swelling but had swelling power value of 9.0 which was compara- ble (p 50.05) to the swelling power value of other starch isolates. The swelling power of all the starch isolates at 80C and 85C were not significantly different ( ~ 5 0 . 0 5 ) from their swelling power values at 90C.

Swelling usually involves a weakening of bonds that hold the starch molecules together and depends on the affinity of starch for water. With increase in temperature, there is loss of birefringence and increase in viscosity as the starch isolates granules rupture (Leach el al. 1959). Gelatinization temperature of starch isolates ranges from 72-74C, based on amylograph studies.

Parameters that refleet viscosity properties of starch isolates from bamabara groundnut seeds are given in Table 4. Prepared starch isolates contained 0.5-0.7% protein, ash content of 0.22-0.25% and a moisture content of 10.0-10.3%. Final pasting temperatures indicate loss of at least 98% of the total

116 P.C. OJIMELUKWE

birefringence of the starch granules. The drop in viscosity was calculated as the difference between the peak viscosity and the final viscosity after holding for 15 min at 95C (shear thinning). The change in consistency is evaluated by the difference between the viscosity at 50C and the final viscosity at 95 % . The set back value reflects the viscosity after cooling to 50C.

TABLE 4. COOKING CHARACTERISTICS OF 8% STARCH ISOLATES FROM

FOUR CULTIVARS OF BAMBARA GROUNDNUT SEEDS

Seed Cultivars

Viscosity

Red Black Cream Brown Mean

A. Initial Peak Viscosity at 92C (BU) 750 880 780 860 817.5ab B. Initial Viscosity at 95C (BU) 735 770 740 770 153.8b C. Final Viscosity at 95C (BU) 730 760 680 720 122.5b D. Set Back Value at 50C (BU) 1000 860 900 100 937.5d

Shear Thinning (BU) (A-C) 20 120 100 140 95 .Oc Consistency (BU) (D-C) 270 100 220 210 .oc

Mean values followed by the same letters are not statistically different (p 10.05)

Peak viscosity values of starch isolates form the black and brown seed cultivars were highest. The starch isolates from the black colored seed cultivar had the poorest consistency. The ease of cooking is reflected in the shear thinning values and is lowest for the starch isolates from the red colored cultivars. The overall viscosity data reflects that the starch isolate from the black colored seed cultivar is the least stable to mechanical treatment, while other seed cultivars have more stable starch pastes.

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