|Bioconversion of Organic Residues for Rural Communities (UNU, 1979)|
|Nutritional evaluation of bioconversion products for farm animals|
Micro-organisms such as yeast, bacteria, fungi, or algae are the single-cell proteins used for most bioconversion of wastes or other substrates to make food or feed. A crucial question is: What is the nutritional value for man or animals of the final product of the bioconversion process? A second important aspect is the toxicological status of the product. This is the subject of papers by Scrimshaw and Shacklady appearing elsewhere in these proceedings, and my presentation is based on the assumption that the materials are acceptable toxicologically. I will consider some points that must be taken into account when evaluating a bioconversion product for animal feeding.
The main reasons for using micro-organisms in the conversion of agricultural residues are: First, to degrade that part of the residue that is not available for absorption by animals or man when the material is fed as such. In most cases this means that the enzymes secreted in the animal or human gastro-intestinal tract cannot, or are insufficiently able to, break down the material into components that can be absorbed. This pertains to cellulosic, hemicellulosic, and ligno-cellulosic components. The second purpose is to upgrade the nutritional quality of the residue by increasing its protein content, or, for monogastric animals and man, raising its content of essential amino acids.
Of the four categories of micro-organisms involved in bioconversion processes (yeasts, bacteria, fungi, and algae), a considerable amount of information is available about the nutritional value of yeasts. Species of yeast have been used for many years as a valuable component of animal feeds, supplying proteins and certain vitamins. In addition, some of the large-scale industrial SCP processes developed over the past ten years use yeasts that utilize hydrocarbons (i.e., paraffins) as an energy source and carbon and hydrogen for growth and synthesis of cell constituents. The results of extensive evaluation programmes show that these yeasts form a highly valuable source of protein for monogastric animals.
The second category of SCP, the bacteria, have, for many centuries, contributed to food supplies for man in an indirect manner: the protein supply of the ruminant is largely dependent on the bacteria and protozoa abundantly present in the fore-stomach of the animal, which forms, in principle, a large in vivo fermentation vessel.
Bacteria will be used in several large units being constructed for industrial protein production where methane or methanol will provide the energy. The data available show that bacterial material produced in this way also forms a highly valuable protein source.
The last two categories of SCP, the fungi and algae, have until now not been used to any extent in animal feeding, and this is why very little is known about the nutritional value of these products. The scarce data in the literature show variable results and indicate that, for monogastric animals, digestibility may be a problem. ILOB experiments with a fungal product showed reasonable results for digestibility and growth performance in pigs, but the results in poultry were unsatisfactory. Because fungi and algae will most likely be the microorganisms of choice for the small-scale bioconversion units considered in this work shop, a thorough look at the nutritional value of the material produced is essential. I would especially stress the necessity of testing nutritional value at an early stage of process development in order to be able to provide some sort of guidance for that development, for example, the choice of the micro-organism or the relevance of including a special treatment, if possible.