Contents - Previous - Next
This is the old United Nations University website. Visit the new site at http://unu.edu
Hunger, Technology and Society
Effect of chemical changes during storage and
processing on the nutritional quality of common beans
Iron fortification of chinese soy sauce
Studies on the use of soybean food in infant feeding in china and the development of formula 5410
International union of nutritional sciences (IUNS) officers and committees
Effect of chemical changes during storage and processing on the nutritional quality of common beans
Division of Agriculture and Food Sciences, Institute of Nutrition of Central America and Panamá (INCAP),
Guatemala, City, Guatemala, C.A.
Storage and processing of foods are technologies that man has used since the beginning of history as a prerequisite for insuring availability of the food supply and, in many cases, as a necessary step before consumption for a variety of reasons, from such aspects as increasing stability, improving flavour, decreasing the possibilities of toxicity, and introducing functionality. All of these results of storage and processing introduce chemical changes that affect the nutritional value of food.
Improper storage and incorrect and/or excessive thermal processing are known to affect nutrient content in foods. Vitamins are more susceptible than minerals, and amino acids are somewhere in between. On the other hand, appropriate storage conditions and well-controlled processing have positive effects in retaining the original nutrient content of the food, and in many cases improving the availability of specific nutrients and the overall quality of the product.
Because the effects of conventional storage and processing on nutritive value of foods are well known and documented, more attention will be given in this paper to those storage and processing technologies commonly used in the preparation of legumes, since they are important sources of nutrients in the diets consumed by people whose principal foods are largely vegetable in origin.
Before beginning this discussion, it should be pointed out that chemistry plays an important role in the food chain from production to consumption, and the nutrients or chemical compounds synthesized by the plant and deposited in the seed interact with each other during storage and processing to give specific nutritional values. This suggests the need for geneticists and production specialists to work closely with food chemists and technologists and with biochemists and nutritionists to insure the maximum nutritive quality and acceptability to the consumer.
Furthermore, food legumes present a variety of problems, from production to consumption, that will be used to point out the effects of storage and processing on nutritive value. Common food legumes, in particular common beans, are harvested in periods of the year when there is rainfall, thus they must be dried for storage. The removal of excess moisture by sun-drying, if not controlled, may initiate a condition known as hardening, which makes the beans hard to cook. This is accentuated during storage at high temperatures and high relative humidity. Whether hard to cook or not, beans are milled in certain areas of the world and in others they are thermally processed whole for consumption, mainly to inactivate antiphysiological substances and improve nutritional value. Other processes are also used, such as germination and fermentation. If not properly stored, processed products may lose significant portions of their nutritional value.
This discussion will thus use common beans as the commodity whose nutritional value is affected by a variety of factors during storage and processing. It will also serve to point out areas of needed research as a means to increase the availability, use, and nutritive value of a food of great nutritional significance to people in developing countries.
Processing for storage as grain. After harvest, common beans usually contain relatively high amounts of moisture that has to be removed before storage. It is a common practice carried out by small farmers in developing countries to expose the grain to solar radiation. The exposure time is more or less well controlled; otherwise the farmers have indicated that the beans will become hard. This was confirmed by experimental studies whose results are shown in Table 1 (1). As solar exposure time increased, there was an expected decrease in moisture content, but at the same time, water absorption and cooking time decreased. Changes in other chemical constituents also took place, the significance of which is not well understood; however, this has important implications in the subsequent operations involved in taking beans to the consumer. These changes probably have an influence in the hard-to-cook condition often developing in common beans and other food legumes subjected to storage (2-4).
The hard-to-cook phenomenon. Storage may have direct and indirect effects on the nutritive value of foods and diets. Poor storage conditions with high temperature and relative humidity will result in staple foods with high moisture levels and a decrease in quality because of the Maillard reaction. Such conditions will also result in the growth of fungi that produce toxic compounds that have adverse effects on animals consuming such foods. Poor storage conditions can also affect nutritive value by favouring insect infestation, which also results in losses of dry matter. Finally, the improper use of chemicals to protect grains can also decrease the nutritive value of the staple food. Various examples will be given for situations that are common in tropical developing countries.
TABLE 1. Physical and Chemical Characteristics of Recently Harvested Common Beans Exposed to Solar Radiation Prior to Storage
* Broken grains
Source: Reference 1.
Poor storage of common beans will increase the hard-to-cook problem. In the example shown in Figure 1, common beans were stored for 0, 3, and 6 months at 35°C and 85 per cent relative humidity (5). At the end of each storage period, samples were cooked under standard procedures at atmospheric pressure and their hardness was measured by the Instrom texturometer. A standard cooking hardness at g-force of 90 was chosen on the basis of current home practices in Guatemala. As seen in the figure, it took 150 minutes at atmospheric pressure to soften the samples stored for O months. However, the samples at 3 months required 170 minutes, while those stored for 6 months were still uncooked even after 210 minutes. This hard-to-cook condition has significant economic implications, not only because the beans are no longer acceptable to the consumer, but because if they are cooked, the increase in energy cost is very high. Furthermore, the excessive cooking that is often applied to soften the grain will decrease protein quality, as shown in Figure 2 for Phaseolus vulgaris. The loss in PER and weight is the result of lower protein availability as well as a loss of Iysine, which becomes inactivated through the well-known Maillard reaction shown by various investigators (6, 7).
FIG. 1 Limiting Nutritional Factors of Common Beans
FIG. 2 Effect of Excessive Cooking Time on the Protein Quality of Beans
An interaction between storage and processing can also affect the nutritional quality of legume foods. In some instances long cooking time is needed to obtain a maximum possible nutritive value from hard-to-cook beans. Table 2 shows that even after 240 minutes cooking time, digestibility of hard-to-cook beans was significantly lower compared to recently harvested beans, as indicated in the lower part of the Table (8).
TABLE 2. Effect of Storage and Processing on the Digestibility of Hard-to-Cook Beans
*Recently harvested beans
The results shown in Figure 3 clearly indicate three factors to be involved: storage temperature and time, relative humidity, and the moisture content of the seed (3). It can be seen that high moisture in the grain favours hardening as storage time increases. The mechanism of the hard-to-cook condition that develops in food legumes under improper storage conditions is not fully understood. The evidence available suggests that an increase in the bound protein takes place in the seed coat and aleurone layer; however, the cotyledons also lose their capacity to absorb water because of changes in pectins and calcium ions, and very often they develop a grey colour, suggestive of carbohydrate-protein reactions (9, 10)
TABLE 3. Protein Quality of Chickpea and Pigeon Pea Infested by Insects
|Food Legume||PER||% Decrease|
|Chickpea dhal (control)||2.21|||
|Chickpea dhal (infested)||1.83||17.2|
|Pigeon pea (control)||2.04|||
|Pigeon pea (infested!||1.66||18.6|
Insect infestation. Studies carried out in India (11), shown in Table 3, indicate that chickpeas or pigeon peas lost about 18 per cent of their protein quality because of insect infestation. The loss may have been caused by contamination from uric acid, a protein metabolite of insects, as well as by increases in fat acidity and microbial contamination, and perhaps even to losses in grain fractions.
Resistance to insect attack has been reported for common and Faba beans (12, 13). It is possible, therefore, that natural resistance to insect infestation exists, and thus chaser cooperation should be established to identify the nature of this resistance in order to increase it to alleviate storage problems, reduce chemical treatment for insect control, maintain nutritive value, and insure increased efficiency of processing.
FIG. 3. Effect of Moisture and Storage Time on Cooking Time of Common Beans
Most staple foods as well as other foods of vegetable and animal origin are processed for consumption. Although not universally used, in certain regions of the world, too legumes are milled to remove the seed coat before the application of thermal processing. Storage conditions have significant quantitative and qualitative effects on legume milling.
Milling of food legumes is carried out by household and commercial methods involving the same basic principles. There is both a wet and a dry method (14). Both procedures have disadvantages. The wet method usually takes longer, and although yields are good, the cotyledons (dhal) become hard to cook and require longer cooking. The dry method has the disadvantage of high milling losses caused by breakage of cotyledons and Powdering. Furthermore, the loosening of the seed coat in this process is not adequate, and even less so when the drying operation of beans before storage has not been satisfactory. In addition, if the grain is insect-infested, milling yields decrease (11). "Control" chick peas with 2 per cent kernel damage had an 82 per cent dhal yield, while infested peas had 15 per cent kernel damage and a 65 per cent dahl yield. Milling techniques have been developed to maximize the yield of edible fractions, and data have been obtained suggesting that the genetic make-up of the cultivars also plays an important role in yield of edible fractions.
Roasting. Roasting is an interesting processing technique because it has the capacity to develop attractive flavours in foods so treated. It also induces important functional properties, attributes that should be compatible with nutritional value. The effect of roasting Vicia faba is shown in Table 4. Length of roasting is important in enhancing PER in these beans. As shown in the table, about 15 minutes of roasting at 200°C was the optimum time for maximum protein quality. Chemical analysis for available Iysine showed the expected decrease in this amino acid, which explains the lower protein quality as roasting time increased. This product, converted into a fine flour, is used as a drink for young children in some developing countries (151.
TABLE 4. Effect of Roasting Time (at 200°C) on Protein Quality of Vicia faba
The roasting process has also been applied to common beans, giving results shown in Table 5 (16). The method used was a bed heat exchange dryer operated at 190 to 200°C for 20 to 30 seconds.
TABLE 5. The Protein Quality of Autoclaved and Dry-Roasted ¡Navy Beans
Units/g x 10-3
( % )
|Dry-roasted 20-25 sec.|
Adapted from Reference 16.
Based on the components measured, roasting resulted in a product as good or better in protein quality as that produced by common wet cooking under pressure. Therefore, if processing conditions are well controlled, there is no danger of reducing the nutritional potential of a particular food.
Heat treatment can also be very useful as a way to pre" serve the cooking characteristics of whole food legumes that become hard to cook upon storage, as indicated earlier. In one study, whole black beans were heat-treated for 2, 5, and 10 minutes at 121°C, and for 10, 20, and 30 minutes under steam (98°C). The materials so treated were then stored at 25°C at 7û per cent relative humidity, and samples were withdrawn at 3, 6, and 9 months to evaluate cooking quality (4, 17). The hardness was measured by a puncture test on soaked-cooked samples. The results with beans treated at 121°C are shown in Figure 4, and indicate that the process retained acceptable cooking characteristics compared to the untreated control, which was harder to cook as length of storage increased.
Cooking at atmospheric pressure and under pressure. Wet cooking is probably the most common method of preparing food for consumption, both at home and industrially. Common beans must be cooked before consumption in order to inactivate antiphysiological factors (18, 19). This is shown in Figure 5. One side of the figure shows the effect of processing in inactivating trypsin inhibitors and haemagglutinins, while on the opposite side, the graph shows the improvement in protein quality. The antiphysiological factors are destroyed in about 90 minutes, which results in an increase in protein quality. However, excessive cooking time causes a progressive decrease in nutritive value because of the loss of Iysine. Wet cooking can be carried out under pressure, which reduces the time of exposure compared to cooking at atmospheric pressure. In both situations, however, excess cooking time may result in lower nutritive value.
Drum drying. Another technique for cooking foods is drum drying. Important processing conditions to inactivate antiphysiological factors and up-grade the nutritional quality of the product include temperature, residence time, both dependent on drum velocity, opening between drums, and solid concentration. It is an attractive technology because the material is cooked and dried in one operation. Results in samples of beans processed by this technique in comparison with autoclave and extrusion cooking are shown in Table 6. As shown, drum drying gene a product higher in protein quality than that resulting from autoclave cooking, particularly for cowpeas (20).
Extrusion cooking. One of the most useful recent techniques in food processing is extrusion cooking. Some water may be added for this process. The results, also shown in Table 6, summarize data comparing pressure cooking, drum drying, and extrusion cooking on the same samples consisting of a mixture of common beans and cowpea and cowpeas alone. Note the increased protein quality in legumes cooker.) by the extrusion method compared to the other methods. At present one can only speculate on the possible reasons for this. This procedure may have caused a greater inactivation of antiphysiological factors and thereby increased the susceptibility of protein to a more complete hydrolysis, or it may have changed the carbohydrate fraction to favour better protein utilization (20).
FIG.4. Effect of Time in Storage let 25°C, 70 Per Cent Relative Humidity) on the Hardness of Cooked (18 Hours Soaking, 20 Minutes Boiling) Untreated and Retort-Treated (15 Psi, 121°C) Black Beans
Contents - Previous - Next