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Traditional food-processing technology and high-protein food production

E. O. Mensah and S. Sefa-Dedeh

Abstract

Studies were undertaken on a high-protein gari-like dry product, developed previously, prepared from corn (Zea mays) and cowpea (Vigna unguiculata) flours and dried by the technique traditionally used for roasting gari, a commonly eaten food made from cassava. The product's water absorption and swelling characteristics were lower than those of gari; but its particle size, crispness, colour, and flavour were rated acceptable when compared to gari.

Editor's note

It is reported that as many as 80 million people in West Africa eat gari, a fermented food prepared from cassava. As its protein content is very low, efforts have been made to develop cereal-and-legume mixtures with gari-like characteristics. This paper is a follow-up to one published in the Bulletin in 1984 [18] describing the process for preparing a product of maize and cowpea meals with an appearance similar to that of gari and comparable organoleptic properties but with greatly improved nutritional value.

One important characteristic of the research reported is that the formulations are based on maize and cowpeas, which are widely grown and consumed in West Africa. The authors quite correctly reflect that most of the protein enrichment of traditional foods has been done using legumes such as soy beans that may not always be locally grown advantageously. Their research applies traditional technology to the production of high-protein food products that have high potential for acceptance by the prospective consumers. It is important that the research attempts to adapt the new products to the taste of the consumers rather than seeking to change their food habits. Another important feature of the work is that it employs technologies normally used by women.

The researchers have sidestepped the possibility of using cassava as a component of their formulations. As indicated, cassava products are widely consumed in the countries for which the new-formulations are intended. It would appear important, therefore, to develop products based on cassava but with a higher content of protein. The authors have indicated that the level of maize in the mixture increases the gari-like characteristics of the product, and attribute this to the relatively high content of fibre in maize. One wonders how the inclusion of cassava would affect the quality of the products and their acceptability to the consumers. The results of the organoleptic test given in table 9 show that the general acceptability of the maize-cowpea mixtures (abropa) to consumers is as good as that of gari. This is somewhat surprising given the differences in the physical and functional characteristics of gari and the cereal-legume mixture.

Introduction

Supplementation with legumes is one way of improving the protein quality of cereal diets. Traditional cereal foods play an important role in the African diet [1], and they are processed by women using simple, traditional methods. Attempts have been made to improve the protein quality of many such foods, including ogi in Nigeria, corn dough in Ghana, kisra in Sudan, and many others [2-5]. Tortillas, native Mexican corn cakes, have been enriched by several workers [6; 7].

Most of the protein enrichment of traditional foods has used soy beans as the source of protein. Another approach is to develop high-protein foods with physical and organoleptic characteristics similar to those of existing foods but based on readily available commodities and technology. In attempting to find solutions to the prevailing problems of protein-calorie malnutrition, the importance of locally available raw materials and technology cannot be overemphasized.

Women play a critical role in producing. marketing. and processing foods, as well as in selecting the diet for the family. The methods by which they produce indigenous foods can contribute significantly to improving the nutritional well-being of the population. especially when the result is a product with high acceptability, good storage characteristics, and low cost.

One such food, gari, made from cassava, is widely eaten in West Africa, especially in Ghana and Nigeria. It is produced by women, using simple methods. It has high acceptability and desirable functional and storage characteristics, but is very low in protein. However, the simple process used to make gari can be adapted to other commodities.

This paper reports further data from studies on a high-protein cereal-legume product with gari-like characteristics, described previously [8], which we have called abropa, meaning literally "good corn."

Gari

The steps used to produce gari granules from cassava include grating, removal of water, fermenting, sieving, roasting, and grinding. Various modifications may be introduced by the processors to give the characteristic quality they desire (see FIG. 1. Summary of gari processing). Some of the methods are summarized in table 1.

TABLE 1. Summary of steps used by gari processors surveyed

None of the women we surveyed grated the cassava manually, as mechanized grating increases output and efficiency and ensures a relatively uniform particle size.

Draining water from the grated cassava in baskets is not common in most parts of Ghana; however, it is done in Cape Coast. Nine of 15 respondents drained the gratings to reduce the fermentation time. They allowed the meal to ferment for one to two days instead of the more usual two to three days.

Unrefined palm oil may be added to give the product a yellow colour, which is preferred by some consumers. It is interesting to note that only the women who drained their cassava gratings in the basket added palm oil to the meal.

The normal method for fermenting cassava gratings for gari in Ghana is to pack the grated meal into cotton or jute sacks and weigh it down with heavy stones to press out the water. Fermentation and extraction of water occur simultaneously, the operation taking from one to three days.

All the respondents sieved the fermented meal prior to roasting. One-third of them sieved the gari again after roasting to ensure fine particles of uniform size. Generally, the resulting overtails are ground and mixed with the previously sieved fine fraction.

Each of these operations contributes to the production of the desired quality of gari and affects consumer acceptability. The 15 respondents were asked to indicate the characteristics of gari that they considered important. Crispness was mentioned by 12: other qualities cited, in order of decreasing importance, were taste, particle size. colour, and starch content.

Crispness is a function of moisture content, particle size, and texture. It is achieved by roasting the fermented meal for 30 minutes. Roasting, which involves alternate cooking and drying, causes the starch in gari to gelatinize. Also, the wet fermented meal is dried gradually from a moisture content of about 45% to 5% in the finished product. The method and the duration of roasting, as well as handling, packaging, and storing after processing, can affect the moisture content and therefore crispness.

The taste of gari depends on the degree of fermentation. Long fermentation (generally more than three days) results in an unacceptable sour taste.

Particle size is affected by the efficiency of grating and the method of roasting. When the grating is efficient, particle sizes should be small. If the initial roasting heat is too high, the product becomes lumpy and forms large agglomerates. Undesirably large particle size is avoided by sieving and grinding.

The colour and amount of starch in the product were mentioned by the women as being important, but may not be widely considered significant by the consuming public.

Preparation methods and evaluation of abropa

Corn (Zea mays) was obtained from the Ghana Food Distribution Corporation, Accra. Cowpeas ( Vigna unguiculata var. Westbread) were purchased from a market in Accra. Skim-milk powder was obtained from a food-processing company.

Corn meal was prepared after washing and steeping the corn in water for 16 hours. Cowpea flour was prepared after soaking the seeds in water for five minutes and dehulling them by rubbing and shearing with the hands. The dehulled cotyledons were separated from the seed coat, dried in an oven at 100 °C for 60 minutes, and milled into flour using a laboratory mill (Christy and Norris Ltd., Chalinford, England).

Blends of corn and cowpea flour in the ratios 50:50, ó0:40, and 70:30 were prepared. using the "Legon process" [8], which involves drying of the product by roasting in an earthenware bowl over low heat with intermittent stirring.

To study the effects of various pretreatments on the yield and taste of the product, I00-g samples of flour composed of 70% corn flour and 30% cowpea flour were given the following treatments before dryroasting: 10, 20, and 30 ml of water were added to three samples; and two samples, one without added water and one with 10 ml of water added, were steamed in an exhaust box for 10 minutes.

Moisture was determined using the modified air-oven method of the American Association of Cereal Chemists [9]. Protein was determined using a method of the Association of Official Analytical Chemists [10]. Water-absorption characteristics were determined by mixing I g of sample with 8 ml of water in a graduated centrifuge tube. The mixture was allowed to stand for 30 minutes and then centrifuged at 5,000 rpm for 10 minutes. The volume of supernatant was read, and the amount of water absorbed per gram of sample was calculated.

To determine the effect of adding different amounts of water on the swelling characteristics of the products, 5-g samples were weighed into graduated measuring cylinders. Different amounts of water (10, 12, 14, 16, 18, 20, 25, and 30 ml) were added to each cylinder. After 20 minutes the swelled volume was measured.

Five grams of sample was poured into a 250-ml graduated cylinder. The volume (dry-bulk volume) was measured, 100 ml of water was added, and the mixture was stirred. The volume occupied by the product was measured after 1, 5, 10, 15, 30, 45, and 60 minutes.

Viscoamylograph curves for 8% slurries of the products were prepared using the Brabender viscoamylograph (Brabender, Duisburg, Germany). The equipment was operated through heating. holding, and cooling cycles using the 500-mg sensitivity cartridge at 75 rpm. Heating and cooling were at the rate of 1.5 °C minute.

A 10-member taste panel evaluated the products with respect to particle size, crispness, colour, flavour, taste, and general acceptability. A nine-point hedonic scale was used. and general acceptability was evaluated by analysis of variance.

Results and discussion

Processing abropa

The treatments given to the raw material. cassava, and the processing conditions are important in producing gari. We employed similar methods to produce a similar food, abropa. from a composite of corn and cowpea flours.

The raw material was pretreated to promote the formation of agglomerates. In developing the process, it was found that the yield of abropa depended on the pretreatment of the raw composite flour: the yield decreased as the amount of water added was increased (table 2). In addition. the sample stuck to the bottom of the earthenware bowl used for roasting, the degree of sticking also increasing with the amount of water added to the flour.

TABLE 2. Yield and taste of abropa after different pretreatments

Based on 100-g samples of composite flour comprising 70 g of corn flour and 30 g of cowpea flour.
a. Expected yield for each treatment was 72 g assuming all the moisture in the raw material was removed during processing.

In addition to low yields, all the resulting products had a raw taste. It was therefore decided to steam the raw material before dry-roasting. which increased the yield. Adding water to the flour before steaming also improved the taste. Because of these improvements in both yield and taste, adding 10% water (v/w) to the composite flour before steaming and dry-roasting became the established method of preparation.

TABLE 3. Moisture and protein content of raw materials and products

a. Ratios indicate proportion of corn flour to cowpea flour.

The product had a relatively low moisture content after roasting (table 3). It should, therefore, store well if appropriately packaged. In general, as the level of corn in the mixture increased, the moisture content of the final product increased. This suggests that the corn meal is able to retain higher levels of moisture than cowpea flour. This may be due to the relatively high fibre content of corn meal (the cowpea flour was prepared from dehulled seeds).

The abropa samples without milk-powder supplementation contained protein ranging from 14% to 19%. The addition of skim-milk powder to the product increased the protein content to 30'U,. This concentration is in contrast to the 1% protein in traditional gari and 7% in corn meal. Mixing the corn and cowpea flours therefore increases the level of protein in the end product.

Abropa may be considered a type-4 protein product according to the classification of Bressani and Elias [11], because the two vegetable proteins are of different quality and quantity and the protein content of the resulting product is higher than that of the lower-quality source (corn) but lower than that of the higher-quality protein source (cowpeas).

Functional properties of abropa

The functional characteristics of a product may be affected by the composition of and the treatment given to its ingredients. Water absorption was affected by the composition of the mixture and temperature (see FIG. 2. Effects of composition and temperature on water absorption. A, B= abropa at 70°C and 29 °C respectively; C, D = abropa with skim-milk powder at 70 °C and 29 °C). As the level of corn increased, so did the amount of water absorbed, with high correlation between the two (R = .99). The cowpea flour and skim-milk powder had a negative effect on the amount of water absorbed. Analysis of variance of the data indicated that the temperature at which the water absorption was measured, the ratio of corn to cowpeas, and the addition of skim-milk protein did significantly affect the amount of water absorbed by the product.

It appears that only corn has a positive effect on water absorption. The cowpea flour was given a heat treatment (100 °C for 1 hour) during processing, which might have affected the nature of the starch and proteins, the two components most likely to affect the water absorption. Heating also could cause denaturing of the protein, which can adversely affect water absorption. It might be advisable to use unheated cowpea flour to improve on the absorption properties of the product.

Figure 3 (see FIG. 3. Effects of quantity of water on the swelling of samples of various compositions at 29 °C A, B. C = abropa at corn/cowpea ratios of 50:50 60:40 and 70:30 respectively SM = skim milk G = gari) shows the effects of different amounts of water on the swelling of 5-g samples of abropa at 29 °C. For all the samples, maximum swelling volume was reached after the addition of 18 ml of water, i.e. when the water content was about 80% The same trend was observed when swelling was measured at 70 °C. To achieve maximum swelled volume, one needs a minimum product-to-water ratio of 1:4. In all the experiments on swelled volume, excess water was used.

The water swelling index followed the same trend as the water absorption: it increased with increasing levels of corn. The addition of skim-milk powder did not affect the swelling index as it affected water absorption; however. the temperature of incubation and the ratio of corn to cowpeas did (p <= .01). This suggests a relationship between water absorption and swelling index.

The swelled volume of the abropa samples was compared to that of gari. Gari has excellent swelling properties, swelling to about four times its initial dry bulk volume after 15 minutes of soaking in excess water at 29 °C. Under the same conditions, abropa swelled to a little over two times its initial dry bulk volume (see FIG. 4. Effects of length of soaking time on the swelling of samples of various compositions al 29 °C. A, B, C = abropa at corn/cowpea ratios of 50:50 60:40 and 70:30 respectively. SM = skim milk. G = gari). It increased with soaking time and the level of corn. The skim-milk powder tended to have a depressing effect on the swelled volume. A similar trend was measured at 70 °C.

Cooked-paste characteristics

The cooked-paste characteristics of these blends are important to their use. Brabender viscoamylograms on 8% slurries of abropa showed no change in viscosity when the samples were heated to 95 °C'. held at 95 °C for 30 minutes, and cooled to 50 °C. This is in contrast to the behaviour of gari under the same conditions. The gari samples showed a peak viscosity of 150 Brabender units. The inability of the abropa sample to gelatinize after heating is probably due to the heat treatment given to the raw material (cowpea seeds) and the abropa (steaming and roasting). This could cause total or partial gelatinization of the starch in the composite flour. This has implications for the use of the product. A higher concentration of flour will be required to cause gelatinization.

Sensory evaluation

Five characteristics of the products were rated on a nine-point scale (from 1 = most acceptable to 9 = unacceptable) to indicate their acceptability. The samples all were rated acceptable (mean score between 1 and 5) for all characteristics. although the mean scores for the abropa samples were poorer than those for the gari samples. For each attribute, however, one or more of the abropa samples were found more acceptable than the gari. This was confirmed by the high general acceptability of all the abropa samples.

TABLE 4. Mean acceptability scores for gari and abropa samples

a. Ratios indicate proportion of corn flour to cowpea flour: -SM = without skim milk, and +SM = with skim milk.

The data on the panel scores was subjected to analysis of variance (ANOVA) for each quality attribute using orthogonal polynomials.

The differences in scores on particle-size acceptability between gari and abropa as well as between abropa with and without skim milk were not significant (p>.05); the differences between samples with different ratios of corn to cowpeas were significant (p <= .05). This suggests that the proportion of corn affected particle size to the extent that it influenced the product's acceptability. It may be possible to adjust the method and degree of grinding to provide ¿I corn flour that will blend better with cowpea flour. In this study, the flours were prepared separately and mixed, but it may be possible to co-mill the flours to reduce the number of operations in the process and mix the ingredients better.

Similar analyses were carried out on the raw panel scores for crispness, colour, flavour. taste, and general acceptability. In all these comparisons, there were no significant differences in acceptability scores between gari and abropa with or without skim-milk powder. The level of corn in the product, however, consistently significantly affected acceptability scores for all attributes. This suggests the importance of the ratio of corn to cowpeas on the consumer acceptability of abropa.

The panelists' mean scores on the taste of abropa showed a high correlation with its general acceptability (R=.99). This confirms the observation of gari processors that taste is an important attribute.

Conclusions

The high-protein cereal-legume product abropa. made from cowpeas and corn, had good water absorption and swelling properties. However. it differed significantly from gari, the traditional food whose characteristics were being simulated. Nevertheless abropa was highly acceptable and has a higher protein content than gari. The ratio of corn to cowpeas used in the mixture affected water absorption and swelling properties, and general acceptability. It appears that the heat treatment given to the cowpea flour during preparation destroyed its ability to contribute to the water-absorption and swelling properties of the abropa. The product is easy to produce using traditional methods, and it can be an important vehicle for combating protein malnutrition in areas where cereals and legumes are grown.

Acknowledgements

This work was partially supported by funds from the United Nations University.

References

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