Jourml of’ Food Engineering 33 (1997) 337-34(, 0 1997 Elsevier Science Limited. All rights reserved

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ELSEVIER

The Effects of Soaking and Cooking Time on the Cooking Properties of ltvo Cowpea Varieties

K. A. Taiwo,O C. Akanbi” & 0. 0. Ajibola’

“Technology Planning and Development Unit, Obafemi Awolowo University, Ile-Ife, Nigeria “Department of Food Science and Technology, Obafemi Awolowo University, Ile-Ife, Nigeria

“Department of Agricultural Engineering, Obafemi Awolowo University, Ile-Ife, Nigeria

(Received 26 March 1995; accepted 2 August 1996)

ABSTRACT

The effects of soaking and cooking time on water absorption capacity cookability, percent split, and texture of two varieties of cowpea, IITA-2246 and IFE-BPC, were investigated. Soaking prior to cooking had an effect only on the cookability, while the amount of water absorbed and the penetration depth (u measure of texture) of a penetrometer head in the seeds increased with cooking time. The properties were found to differ notably for the two varieties with IITA- 2244 having a softer texture than IFE-BPC. Splitting in cooked beans increased with cooking time with maximum of 92% split in IITA-2246 and 63% split for IFE-BPC. Higher temperature soaking reduced the cooking time of both varieties. 0 1997 Elsevier Science Limited. All rights reserved

INTRODUCTION

Grain legumes are generally cheap and are good sources of dietary protein for a large part of the world’s population (Khokhar & Chanhan, 1986). They are, how- ever, consumed less because of prolonged cooking time for certain products (Vindiola et al., 1986). Hardness in dried beans must be overcome by cooking in order to render them palatable, digestible and to inactivate antinutritional factors. Excessive cooking, however, can result in a decreased nutritive value.

Soaking and cooking of beans are separate processes which may or may not be performed simultaneously. In some processes the beans are soaked until they are saturated with absorbed water, whilst heat is introduced to induce softening (Hincks et al., 1989). Appreciable research efforts have been devoted to optimising soaking and cooking treatments of legumes. Sefa-Dedeh et al. (1978) reported that the

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338 K. A. ZZwo et al.

hardness of cooked cowpeas decreased with soaking time while soaking significantly increased the softening of blackbeans upon subsequent cooking (Silva et al., 1981) and reduced the cooking time of faba beans (Singh et al., 1988).

Methods developed to decrease the cooking time of dry beans have largely depended on soaking prior to cooking. (Silva et al., 1981; Drake & Muehlbauer, 1985). The effects of soaking beans at room temperature for various durations on the cookability of Ghanaian cowpeas have been reported (Sefa-Dedeh et al., 1978).

The aims of this study were to:

study the water absorption characteristics of the two cowpea varieties when soaked at different temperatures and for varying lengths of time. Study the effect of different soaking conditions on the cooking rate, texture and percent split of the two cowpea varieties when cooked for different lengths of time.

MATERIALS AND METHODS

The two varieties of cowpea beans, IFE-BPC and IITA-2246, used in this study were obtained from the National Seed Service, Ibadan, Nigeria. They have brown seed coats, but the seed coat of IFE-BPC is smooth while that of IITA-2246 is wrinkled. The beans were dry cleaned manually and sorted for whole beans.

The cowpea beans were sundried on the farm to moisture contents of 11.2% and 10.6% for IFE-BPC and IITA-2246 respectively. The freshly harvested beans were bagged in thick polythene bags and kept in a freezer until needed. Bean samples were allowed to equilibrate to room temperature before use.

Samples of 10 g seeds were soaked in 50 ml distilled water in a round bottom flask and heated in a thermostatically controlled water bath (WF 455). The temperatures used in this study were room temperature (27+2”C), 45, 60 and 75°C and soaking times were 0.25, 0.5, 0.75, 1, 2, 3, 4, 5 and 6 h. At these soaking times, beans were removed from the water bath, drained for 2 min, blotted dry to remove surface water and then weighed. The increase in the weight of beans was noted and taken to be water uptake. No corrections were made for the leaching of solids into the soak water. Experiments were replicated twice.

Cooking tests were carried out on three sets of samples; unsoaked beans, beans soaked at room temperature for (1 h) and beans soaked at 45°C for (1 h) before cooking. The beans were cooked in 50 ml distilled water in tin coated cans. The cans were placed in an aluminum pot (with a tight fitting cover) containing water and heated at atmospheric pressure for the following periods: 30, 45, 60, 75, 90, 105 and 120 min after an initial heating time of about 5 min. The samples were then drained and weighed, and the increase in weight was recorded as water uptake. Adequacy of cooking (cookability) was determined by mouthfeel. Experiments were run in dupli- cates.

The extent of splitting in the cooked beans was obtained by separating beans into two categories;

beans with little or no split longer than l/5 of the small circumference of the beans, beans with significant split, i.e., all beans with split greater than those in (1).

Cooking properties of cowpea 330

These splits were evaluated as recommended by Van Buren et al., 1986). The beans with significant split were counted and expressed as a percentage of the total number of seeds in the sample.

The texture of the cooked beans was determined using Precision Universal Pen- etrometer (Stanhope-Seta Ltd., Surrey, England). Twenty individual seeds from each pretreatment/cooking condition were used for penetration depth (10th mm/m) and the average measure recorded for each sample as a measure of cookability.

RESULTS AND DISCUSSION

Data on the amount of water absorbed during soaking by the two varieties are shown in Figs 1 and 2. In both varieties, temperature influenced the rate of water absorbed during the first hour of soaking. Samples soaked at 45, 60 and 75°C absorbed higher amounts of water than those samples soaked at room temperature. The early increase in the rate of water absorption at elevated temperatures is in agreement with the results of Van Buren et al. (1986) that soaking at elevated temperature increased the rate of water imbibition of kidney beans. Hsu et al. (1983) reported similar higher absorption rate at higher temperature of soaking soybeans. The initial high rate of water absorption has been attributed to the presence of pressurized gases trapped in the interior of the beans (Phlak et al.,

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. / X- x-x $j I

. J

Soaking time (h)

Fig. 1. Effects of soaking temperature and time on water absorbtion of IFE-BPC cowpea seeds (0: Room temperature; o: 45°C; x : 60°C; A: 75°C).

340 K. A. Taiwo et al.

1989) and the filling of capillaries on the surface of the seed coat and the helium (Hsu et al., 1983).

Prolonging soaking time beyond 1 hour did not result in appreciable increase in the rate of water uptake for samples soaked at elevated temperatures. While water absorption for samples soaked at elevated temperatures leveled off, samples soaked at room temperature continued to absorb water slowly with time. This reduced rate of water absorption with prolonged soaking time has been attributed to the filling of free capillary and intermicellar spaces with water and the swelling of hydrocolloids (Phlak et al., 1989). Moreover, after one hour of soaking, the lower the soaking temperature, the higher the amount of water absorbed.

Except for samples soaked at room temperature that increased in weight after 1 h, there was no notable increase in weight gained with soaking time at elevated temperatures. It was also observed in this study that as the beans increased in size due to water uptake, the coat of IITA-2246 seeds changed from wrinkled to smooth.

The importance of these results is that cowpea varieties studied need not be soaked for more than 1 hour at temperatures between 45 and 75°C. In situations or processes where time is an important factor, cowpea seeds of both varieties may be soaked at temperatures between 45 and 75°C instead of the long overnight (or 16 h) soaking frequently reported (Singh et al., 1988; Tuan & Phillips, 1991) and practiced in the industrial production of canned beans. High temperature, short time soaking will also eliminate the possibility of excessive leaching of soluble solids and micro-

0 0.25050 6

Soaking time (h)

Fig. 2. Effects of soaking temperature and soaking time on water absorption of IITA-2246 cowpea seeds (0: Room temperature; o: 45°C; x : 60°C and A: 75°C).

Cooking properties of cowpea 341

bial activities experienced when soaking is undertaken for a long period of time. At the higher temperatures, it was noted that water absorption reached 90% of its maximum value after one hour of soaking (for both varieties) and did not change significantly with further increase in time. From the foregoing, an optimal soaking condition was chosen for both varieties, i.e. soaking at 45°C for 1 h.

Figure 3(a) and (b) shows the effect of pretreatment and cooking time on the weight of the cowpea varieties, IFE-BPC and IITA-2246, respectively. Soaking the cowpeas either at room temperature or 45°C before cooking did not have a defin- able and appreciable influence on the final weight of the cowpeas. However, increasing the cooking time resulted in higher sample weights. This is similar to the results obtained by Mwandemele and McWhirter (1984) who reported that some varieties of soybeans showed an increase in water uptake with increased cooking time. Furthermore, as cooking time progressed some samples began to split and some parts disintegrated into the cooking water. Since no correction was made for the leached soluble solids from the beans, this may partly account for the decline in

26

r (a)

Cooking time (mins)

Fig. 3. Effects of pretreatments and cooking time on the weight of two cowpea varieties (IFE-BPC: l : Unsoaked samples; o: samples soaked at room temperature; +: samples soaked at 45°C; IITA-2246: x : unsoaked samples; A: samples soaked at room temperature;

n: samples soaked at 45°C).

342 K. A. Taiwo et al.

the rate of weight gain with cooking time. Mwandemele and McWhirter (1984) attributed similar reduction in weight gain of soybean during prolonged cooking to the release of cell-bound water.

The effect of pre-cooking treatments on texture (penetrometer penetration depth) as influenced by cooking time is shown in Fig. 4 for the two varieties. For both varieties and all pre-treatment conditions considered, the depths of penetration increased with cooking time, but the rate of increase was different for the two varieties. There was a small but gradual increase in the depth of penetration of IFE- BPC seeds with cooking between 30 and 90 min. Beyond this period, there was a sharp increase in the depth of penetration (Fig. 4). The rate of increase in the penetration depth for IITA-2246 cowpea samples was relatively high for cooking time up to 90 min, but decreased when samples were cooked for longer times. The general increase in penetration depth with cooking time may be attributed to the gelatinization of starch during cooking. Sefa-Dedeh et al. (1978) reported that gelatinization resulted in the hydrolysis of starch of Ghanaian cowpeas during cook- ing. The hydrolysis of starch is associated with the softening of the beans (Sefa-Dedeh et al., 1979). Quast & Silva (1977), in a study of the texture of lima beans reported that the maximum shear force decreased linearly with cooking time. Comparison of the results of penetration depth for the two varieties (Fig. 4) showed that IITA-2246 beans offered lower resistance to penetration than IFE-BPC beans at the pre-cooking conditions and cooking times considered in the study.

120 -

100 -

S ';

60 -

4 c .o 5 40 -

5 E a

20 -

L/ ’ I I I I I I 0 30 45 60 75 90 105 120

Cooking time (mins)

Fig. 4. Effects of precooking treatments and cooking on the texture of two cowpea varieties (IFE-BPC: l : Unsoaked samples; o: samples soaked at room temperature; x : samples soaked at 45°C; IITA-2246: +: Unsoaked samples; A: Samples soaked at room temperature;

q : samples soaked at 45°C).

Cooking properties of cowpea 343

The extent of splitting of beans during cooking was monitored and the results are shown in Fig. 5 for IFE-BPC and IITA-2246, respectively. For both varieties, split- ting of seeds increased with cooking time for all the pre-cooking treatments studied. There was, however, a positive correlation between texture indices (penetration depth) and splitting tendency. For IFE-BPC, a gradual but slow increase in percent split with increase in cooking time up to 75 min was observed. The rate of splitting of IITA-2246 beans was high within this cooking regime. The maximum percent splits recorded for IFE-BPC were 63 and 92% for IITA-2246. In general, IITA-2246 beans split more readily than IFE-BPC seeds. The effect of soaking was such that, for IFE-BPC samples soaked at 45°C for 1 h prior to cooking, there was a higher percent split compared to the unsoaked samples or samples soaked at room tem-

+’ I I I I I J

30 45 60 75 90 105 120

Cooking time (mins)

Fig. 5. Effects of cooking time on the the extent of splitting of two cowpea varieties IFE- BPC: l : Unsoaked samples; o: samples soaked at room temperature; +: samples soaked at 45°C; IITA-2246: x : unsoaked samples; A: samples soaked at room temperature; o: samples

soaked at 45°C).

344 K. A. Taiwo et al.

perature. It is probable that soaking at 45°C initiated some of the structural changes expected to take place during cooking thereby rendering the beans more susceptible to splitting. Unsoaked samples of IITA-2246 had an appreciably reduced percent split compared to those soaked at room temperature or 45°C. These results indicate that for industrial processing of beans where wholesomeness is of importance, it may not be necessary to soak IITA-2246 seeds before cooking. However, for IFE- BPC, soaking should not be at an elevated temperature.

Splitting always started with the cracking of the seed coat which then allowed the cotyledons to open up; although the nature of splitting occurring in both varieties was different. The IITA-2246 seeds split transversely below the helium, without resulting in the separation of the two cotyledons. Splitting was such that each part of the split had a part of both cotyledons held together by a fragment of the seed coat. For the IFE-BPC seeds, splitting occurred at the edges in a longitudinal manner. It was observed that most of the unsplit seeds of IITA-2246 variety when cooked for more than 90 min. were the small sized seeds within the samples. This observation was supported by the sieve analysis result of unprocessed (neither soaked nor cooked) beans. On the average 88.5% of IITA-2246 seeds were retained on sieve size (British Standard) 4.75 mm aperture while the remaining 11.5% did not pass through 2.36 mm sieves. The sieve analysis of IFE-BPC seeds resulted in 95% of the seeds being retained on 4.75 mm aperture sieves while the remaining 5% did not pass through the 2.36 mm sieves. It cannot be said, therefore, that the unsplit IFE-BPC seeds were the small sized portion since the maximum percent split was 63% and the small seeds constitute only 5% of the population. It has been suggested that restricted breakdown in beans may cause a reduced ability of hydro- colloidal swelling due to a higher pressure potential (Sefa-Dedeh et al., 1979). Therefore, it may be said that IFE-BPC seeds have tougher texture than IITA-2246 seeds. It was also observed in IITA-2246 seeds that, when they were cooked for 75 min or longer, some of the seeds interlaced forming clumps. The number of such clumps formed increased with cooking time. This phenomenon was absent in the split seeds of IFE-BPC.

Results of time of cooking (cookability) are presented in Table 1. After 30 min. of cooking, cracked seeds of IITA-2246 were adequately cooked. Cooking, of these seeds beyond this period resulted in overcooking which led to the disintegration of the seeds. This short cooking time and the resultant overcooking may be responsible for the seed clumping observed in IITA-2246 with prolonged cooking. The unsplit IITA-2246 seeds soaked at 45°C for 1 h before cooking were adequately cooked after 45 min. The unsoaked IITA-2246 seeds required a relatively longer cooking time. However, both the split and unsplit seeds were adequately cooked by 75 min.

TABLE 1 Cooking Times (min) of the Cowpea Varieties as Influenced by Soaking Conditions

Soaking conditions IFE-BPC IITA-2246

Split seeds Whole seeds Split seeds whole seeds

Unsoaked 105 120 60 75 Room temp/ 1 h 105 120 75 45”C/l h 90 105 :o” 45

Cooking properties of cowpeu 34.5

Soaking did not have an appreciable effect on the cooking time of IFE-BPC seeds. Samples soaked at 45°C for 1 h before cooking had a shorter cooking time. Soaking at room temperature had no influence on the cooking rate as the soaked and unsoaked samples had the same cooking times. Therefore, cookability was influenced by varietal difference. Mwandemele and McWhirter (1984) also observed a difference in the cookability of different varieties of soybeans which they attri- buted to the difference in their characteristics. Notable among these characteristics was the breakdown of the middle lamella resulting in the separation of the cells. The middle lamella, however, becomes relatively soft with cooking due to depoly- merisation of bivalent cations (Nelson & Hsu, 1985).

CONCLUSIONS

From the results obtained, it is suggested that cowpea seeds of IITA-2246 variety be consumed at domestic level where fragmented seeds are acceptable. The IFE-BPC variety is, however, recommended for industrial use (i.e. in the production of canned cowpeas) where the long cooking time can be reduced by soaking the beans at elevated temperatures prior to cooking.

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