|Bioconversion of Organic Residues for Rural Communities (UNU, 1979)|
|Nutritional evaluation in humans|
Food Products Derived from Animals
Domestic animals such as ruminants, swine, and poultry will probably be fed more and more products containing materials from bioconversion processes. It is not expected, however, that this practice will change the quality of the protein derived from the animals. Changes in the chemical composition of animal tissues may occur, as well as deposition in the tissues of heavy metals, insecticide and herbicide residues, and other additives. If the levels of these substances become too high, the animals will show a decrease in overall performance, which, in turn, should lead to elimination of the product from the feed. If the animals gain weight and show good feed-conversion efficiency and overall performance, it is a good indicator that food products made from such animals will be of high enough quality to be used for human feeding. This, however, does not imply that quality control evaluation should not be carried out on food products obtained from animals fed bioconversion products or biomass (6).
TABLE 1. Protein Intake for Nitrogen Equilibrium Using the Conventional and Short-Term NBI (g/kg) in Adult Human Subjects
|50/50 beef/soy||0.59||0 57|
Source: Bressani et al. (8).
Conventional Fermented Foods
Fermented foods have been consumed for a long time by populations living in various parts of the world. Although there are several kinds, only three will be discussed in terms of protein quality. These are: (a) foods such as tempeh, a fermented food based on soybeans, a high-oil, high-protein seed; (b) fermented foods based on cereal grains, mainly rice, and (c) fermented foods based on starchy foods such as cassava.
The impact of the fermentation process on the protein quality of the end-product will be considered first. To predict its protein quality, it is essential to know the value of the starting material and the value of the biomass itself. In the case of tempeh, soybean protein is deficient in sulphur amino acids and rich in lysine. The biomass produced on it also contains protein deficient in sulphur amino acids and rich in lysine. Therefore, the protein quality of tempeh will be equivalent to the average of the protein content in the soybean and in the fermented biomass, depending on the amount of protein supplied by each source.
There are no data available on the protein quality of biomass produced on cereal grains. Cereal-grain protein, however, is deficient in lysine, while microbial protein is a rich source of this amino acid. Table 2 shows the nutritional impact of small amounts of yeast added to maize, wheat, and rice. In each case, there is a significant increase in protein quality, suggesting that production of biomass on cereal grains for human feeding would be beneficial to the consumer, assuming that the product would be acceptable organoleptically (9 - 11).
TABLE 2. Supplementary Effect of Small Amounts of Torula Yeast Added to Various Cereal Grains
|Cereal grain||Amount of
Sources. Bressani and Marenco (9); Jarquin et al (10); Elias et al. (11).
There have been few nutritional studies on the protein value of biomass grown on starchy foods. Therefore, to predict its possible use, the analogy of supplementing cassava with beans will be used. SCP and legume foods are also deficient in sulphur amino acids and rich in lysine. The results in Figure 4 show that body weight in rats is maintained when cassava is supplemented with 30 per cent of beans, providing 7.5 9 of protein. However, when bean protein is supplemented with methionine, the body-weight gain of rats fed the bean-cassava diet is maintained with only 15 per cent of beans, providing about 4.5 9 of protein. These results imply that, in order to increase protein content in starchy foods by biomass production, the protein content should be higher than 8 per cent to maintain body weight in experimental animals.
Algae have been used as food for centuries. They form a part of the diet of the people living around Lake Chad in Africa, and were eaten by the Aztecs in Mexico. Among the several thousands of green and blue algae known, the following have been found adequate for large- or small-scale cultivation: the green algae, Chlorella vulgaris, Scenedesanus acutus, Coelastrum proboscideum, and the blue-green algae, Spirulina maxima. The following discussion of algae is based on information from other laboratories, as we have not gone beyond chemical and animal studies with Microcystis sp. (12).
TABLE 3. Some Observations Made on Human Subjects Fed Algae Protein (10 to 500 g/day)
Poor appearance of food
Poor digestibility of nutrients
Nausea at high levels of intake
Urine and blood analyses normal
After additional processing:
More acceptable organoleptically
Gastro-intestinal problems persisted
Not many results have been reported recently on nutritional evaluation trials using algae grown on different types of biomass for human subjects. This is probably not due to a lack of interest in manufacturing such products, but rather to the initial results obtained in 1963 - 68 that showed a variety of adverse effects in subjects fed algae or other SCPs. As shown in Table 3, clinical trials with algae were particularly discouraging (13 - 17). Most materials containing algal protein showed low digestibility for most nutrients, and caused gastrointestinal discomfort. Unacceptable smell, taste, and disagreeable flavour produced nausea. However, it became evident that further processing by alcohol extraction improved the product significantly.
In more recent reports, materials produced and processed by improved technologies have yielded products that offer more promise. Some of these results are shown in Tables 4 and 5, indicating better protein digestibility and biological value, while urine and blood analyses are no different from those observed after feeding casein, a universally acceptable protein. However, it is costly to create acceptable foods from algae. These results suggest that the production of such materials for rural communities would be better directed towards animal feed, where they will be more beneficial to man in the long run.