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Post-harvest food conversation

Improvement of the nutritional quality of food legumes
Ligno-cellulosic residues: a suggestion for freeing cellulose

 

Improvement of the nutritional quality of food legumes

Ricardo Bressani

Head, Division of Agricultural and Food Science, Institute of Nutrition of Central America and Panama (INCAP), Guatemala City, Guatemala

Luiz G. Elias
Chief of the Program of Basic Foods, Division of Agricultural and Food Sciences, INCAP

INTRODUCTION

Due to the urgency of providing supplementary protein to a large number of the world's people research on increasing the productivity of legume foods is being undertaken on almost every continent. Agricultural productivity alone, however, is not enough unless it takes into account adequate or improved functional properties as well as nutritional quality. The present report attempts to summarize some of the nutritional characteristics of Phaseolus vulgaris and other food legumes of importance in the diets of Latin American populations. The nutritional properties of these foods are analyzed from the standpoint of the influence of environmental conditions, cultural practices, and genetic factors on protein quantity and quality. An analysis is also made of their supplementary impacts on diets consisting of cereal grains and starchy foods, particularly in terms of improved protein quality of these foods. Finally, the paper describes needed areas of research to maximize the nutritional role of legumes in human dietary systems. The final objective is to stimulate needed interaction between scientists in the agronomic and genetic fields and nutritionists and food technologists in order to demonstrate the true meaning of crop and food improvement for the benefit of human populations.

LIMITING NUTRITIONAL FACTORS IN PHASEOLUS VULGARIS

The factors limiting the more efficient use of nutrients in beans are relatively well-known. These are:
1. antiphysiological factors such as protease inhibitors and lectins;
2. deficiency of sulfur amino acids;
3. the presence of tannins;
4. low digestibility of the protein.

In order to be able to establish nutritional standards for bean improvement, the role of these four factors will be discussed first.

The Role of the Antiphysiological Factors

The presence of such compounds in beans has been clearly shown. It is also well established that cooking carried out under controlled conditions destroys the antiphysiological activity of such substances, as shown in Fig. 1. In this particular case, a higher drum velocity implies shorter retention time of material on the drum surface, at a fixed temperature of 94C. Animals fed raw beans will survive two to four days; however, as cooking time increases, trypsin inhibitor activity decreases and better animal performance is obtained in terms of weight gain and protein utilization.1 On the other hand, excessive cooking results in poorer performance, as shown in Fig. 2, because it either destroys or decreases biological availability of amino acids, in particular Iysine.2

On the basis of these results, it may be concluded that appropriate cooking enhances the nutritive value of beans, and as a consequence, antiphysiological factors do not interfere with nutritional value.

Deficiency of Sulfur Amino Acids

Calculations of chemical score confirmed by actual feeding tests have shown that the protein of beans is limiting in sulfur amino-acid content for experimental animals. In all cases there is a significant increase in net protein value with methionine supplementation. However, it does not occur to the same extent in all samples. This may be due to differences in levels of sulfur amino acids in the protein of the various samples, or to differences in levels of other amino acids, such as tryptophan, or to other factors, such as tannins.3 In addition to sulfur amino-acid deficiency, chemical scores suggest that tryptophan is the second limiting amino acid. Levels of tryptophan reported for 10 black bean cultivars ranged from a low value of 41 to a high of 62 mg/g N4 which represents 66 to 100 per cent of the WHO/FAD 1973 pattern.5 To show the importance of tryptophan, growth performance of animals fed common beans2 and pigeon peas (Cajanus)6 with and without methionine supplementation are shown in Table 1, as this legume grain is even more deficient in tryptophan than beans are.

FIG. 1. Effect of Heat in the Inactivation of Trypsin Inhibitor Activity in Phaseolus vulgaris (Black Coat)

FIG. 2. Effect of Excessive Cooking Time on the Protein Quality of Beans

FIG. 3. Relationship between Protein and Total S. Amino Acids and NPR in Nine Phaseolus vulgaris Cultivars

TABLE 1. Effect of Methionine and Tryptophan Supplementation in Cooked Phaseolus vulgaris and Cajanus cajan

Amino-acid addition Phaseolus vulgaris * Cajanus cajan * * *
Av. wt. gain, g PER Feed efficiency** Av. wt. gain, g PER
None 45 1.20 6.60 48 1.82
+Methionine 124 2.27 3.50 45 1.76
+Tryptophan       58 1.81
+Methionine and tryptophan 129 2.42 3.31 118 2.65

* From Reference 2
** Dietary intake, g/g wt. gain
*** From Reference 6

Some additional improvement in the nutritional quality of common beans is induced by addition of tryptophan. The results with pigeon peas indicate no response to methionine addition unless tryptophan is also added. This information is of value particularly for an understanding of the limiting nutrients of protein in food consumption systems where beans are consumed with cereal grains or starchy foods. An additional example, in which 10 cultivars were fed to rats, is shown in Fig. 3.3 The lower part of the figure illustrates the negative relationship between protein quality and protein content in beans. In the upper part of the figure, a negative relationship is shown between protein in beans and methionine and tryptophan. These results indicate the limitation of these two amino acids in relation to the protein quality of beans and they suggest how these foods should be utilized in food systems.

The Role of Tannins

Tannins are common components in plants. Legume foods are no exception, and the ones most frequently consumed are characterized by differences in the colour of the seed coat, which appears to be related to tannin content. Although at present their effect on the protein quality of legume foods is not well known, some evidence suggests that they can interfere with protein digestibility and amino-acid availability when fed to animals.

Tannins inhibit enzymatic activity, thus possibly affecting protein digestion. Furthermore, tannins may react with amino acids, thus decreasing their biological availability to the animal. Representative results suggesting their interference with protein digestibility are shown in Table 2. In this example, cooking beans of three different colours increased digestibility. It is of interest to note that the digestibility of the whole seed, raw or cooked, is negatively correlated with levels of tannins shown in the last line of the table. The tannin content of white beans ranges from 0.34 to 0.42 per cent; from 0.57 to 1.15 per cent in black; and from 0.95 to 1.29 per cent in red beans.3,7

TABLE 2. In vitro Per Cent Protein Digestibility of Three Phaseolus vulgaris Cultivars

Sample Red Black White
Whole seed, raw 36.3 41.1 42.7
Whole seed, cooked 80.0 82.0 86.4
Tannin, % range 0.95 - 1.29 0.57 - 1.15 0.35 - 0.42

TABLE 3. Effect of Cooking Broth on the Protein Digestibility and Protein Quality of Beans

  Cooked beans without broth Cooked beans with broth
PD* % PER** PD* % PER**
White beans 81.3 2.94 81.4 2.10
Red beans 78.7 2.93 70.4 2.10
Black beans 77.9 3.26 75.0 1.69
Casein 91.2 3.80    

* PD = protein digestibility.
** PER = protein efficiency ratio. Beans were supplemented with methionine. Tannins in cooking broth: white: 4.6; red, 21.0; black, 10.0 mcg/mg.

These results have been confirmed by other studies. However, it was found that the tannins are leached into the cooking broth. If these leached tannins are not included with the dried matter, the correlations between digestibility and tannins remain negative but of a lower statistical significance. Tannins, furthermore, reduce the protein quality of the beans, as shown in Table 3 In this example, the digestibility of the protein and the PER of beans of different colours is lower when the cooking broth is left with the cooked beans. These effects are more marked with red and black beans than with white. The concentration of tannins in the cooking broth is shown below the table, being higher for red beans, intermediate for black, and low for white.7

The Role of Protein Digestibility

A further factor in beans limiting biological utilization of the protein is its low digestibility. The reasons for such low digestibility are not well established. However, it may be due to a combination of factors rather than to only one. Some examples from human studies are shown in Table 4. In this case, young adult human subjects fed 0.2, 0.4, and 0.6 9 protein from beans/kg/day show an NPU of 0.54, which is significantly lower than that from soybeans or milk, with values of 0.82 and 0.83, respectively.8 That this is caused by the low digestibility of the protein becomes evident when this value is compared to the index of biological value of 0.81, which is not too different from that obtained from soybeans (0.91) and milk (0.93). Apparent protein digestibility of cooked black beans treated in three different ways and fed to young adult humans was 55 4.1 per cent for whole, 58 4.0 per cent for ground, and 48 3.8 per cent for pureed, strained beans. Obviously, the values are low because of large losses of ingested nitrogen in feces, implying that, of the 23 25 per cent protein content of the beans, only about two-thirds is actually absorbed.9

As mentioned above, the reasons for such low digestibility values are not known. Because protease inhibitors have been eliminated through heating, two other factors may be responsible. One is the presence of tannins, whose possible effects have already been discussed, and the other may be the tertiary structure of some of the proteins in beans not readily susceptible to enzyme activity. In this respect, the GI globulin fraction has been suggested as the protein offering greatest resistance to enzymatic action. This fraction, which is part of the globulins in beans, has been obtained by extraction at a pH of 2.4 and precipitation by dilution with distilled water and centrifugation.

From the results presented, it may be concluded that there are at least four conditions that control the digestibility of the protein in legume grains. These are shown in Fig. 4. The evidence indicates that the protein digestibility in some species of legume, especially when raw, is very low.

TABLE 4. Protein Quality of Phaseolus vulgaris and Soybean Protein in Young Adult Human Subjects

Protein source NR*= a + b** (NI)*** NR= a = b (NA)****
P. vulgaris -62.7 + 0.54 (NI) - 57.4 + 0.81 (NA)
Milk -72.3 + 0.82 (NI) - 57.2 + 0.93 (NA)
Soybean protein -72.2 + 0.83 (NI) - 56.3 + 0.91 (NA)

* NR = nitrogen retained.
** (b in NR = a + b (NI))
*** NI = nitrogen intake.
****NA = nitrogen absorbed.

However, this is not a general characteristic of all species. This low digestibility is caused mainly by the trypsin inhibitors and hemagglutinin compounds. The result of the destruction of their inhibitory activity by heat is an increase in protein digestibility. The extent of improvement depends on the method used to prepare them for cooking, and on the control of heat in terms of temperature, pressure, and time. As discussed earlier, conditions vary with the age and species of legume.

An additional increase in protein digestibility probably results from the destruction of the tertiary structure of certain proteins that resists enzymatic hydrolysis. The presence of this type of protein is probably a common feature of most vegetable proteins. Likewise, cell wall breakdown may result in increased digestibility.

Finally, an additional increase in protein digestibility of a significant magnitude can be obtained by minimizing, controlling, or destroying the effects of protein-complexing substances such as tannins in legumes. Although it is recognized that more evidence should be obtained on all of these factors, particularly on tannins, the evidence available shows that poor storage conditions, a seed-coat colour suggesting the presence of phenolic compounds, and possibly other compounds, may reduce protein digestibility, and, therefore, amino-acid availability, thus decreasing the efficiency of protein utiIization.

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