Composition and nutritive value of single-cell protein (SCP)

Abstract

In recent years there has been an intensification of efforts to make use of microbial fermentation for the production of nutrients and feeds containing protein. Therefore we have investigated some yeasts and a bacterium produced on unconventional nutrient substrates such as methanol, crude oil, n-alkanes and whey, comparing them with yeasts produced by normal techniques, using sulphite liquors, molasses and extract of barley-malt as a carbon source. The crude protein content of the dry matter of all the years was between 39 and 68% and that of the bacterium examined was about 82%. As regards the amounts of essential amino acids in SCP, the high concentration of lysine, with 6.6–8.4 g/16 g N in the yeasts, and 5.9 g/16 g N in the bacterium, deserves special mention. By contrast, the amount of amino acids containing sulphur in the yeasts examined is not satisfactory at 1.5–3.3 g/16 g N, whereas the amount in the bacterium was more suitable at 3.6 g/16 g N. Methionine and cystine are thus the first limiting amino acids. Also, the arginine content in some products might be inadequate for the growth of fowl. Besides the protein, the crude fat fraction has to be considered, firstly because of its considerable order of magnitude, and secondly because of its very variable composition. In an animal experiment, a satisfactory to good biological value of the protein of 61–80%, and an NPU of 55–76%, were found. By adding 0.2% DL-methionine to the dry feed, it was possible to improve considerably these criteria. On the basis of their very high lysine content, these materials have a good to very good value as supplements to our basic feeds for pigs. Metabolic trials with pigs yielded among other things, a protein digestibility of 93% for 2 yeasts, and 84% for the bacterium studied. In fattening trials with pigs, an equally good feeding effect was achieved through equal-protein replacement of fish meal and extracted soya bean meal by yeasts and the bacterium. For the feeding of calves, it is advisable at first, to replace only small amounts of the milk protein in the milk-substitute feed with SCP.