The reaction of dialdehyde starch with casein and field-bean globulin leads to a blocking of the protein amino groups and to a decrease of free lysine, arginine and histidine. Maximum values are reached at high protein concentrations and great molar reagent excess. At best 80-93% of the amino groups or of the available lysine may be blocked in this way. For 1% casein and I and 5% globulin solutions, the pH optimum of the reaction lies at approximately 8; for 5% casein solutions, it is shifted towards the neutral to weakly acidic range. The value for the proportion of unblocked lysine is higher (approximately 5%) when determined by amino-acid analysis after acid total hydrolysis than when measured by means of the colorimetric method according to Carpenter (20%). The difference is designed as reversibly blocked lysine proportion. There is a linear correlation between the proportion of blocked lysine and the relative nutritional value as determined by means of the test organism Tetrahymena pyriformis. Dependently on protein concentration and reagent excess, gel chromatographically detectable cross-linking products of higher molecular weight are formed by the reaction of dialdehyde starch with casein. In 5% protein solutions, such products with molecular weights of less than or equal to 900 000 are the sole detectable components.

The authors deal with chemical and physico-chemical changes in casein and vegetable globulins after reaction with aldehydes or acylation with carboxylic acid chlorides and anhydrides. The stepwise blocking of the alpha- and epsilon-amino groups leads to modified proteins with lowered isoelectric points and changes in the solubility and precipitability characteristics and in the electrophoretic behaviour. There are relationships between the relative nutritive value (as determined by means of Tetrahymena pyriformis) and the blocking of lysine. On acylation of the protein, the relative nutritive value is influenced by the length of the acyl residue and the kind of modification (N or O-blocking).

HOBbIe KIICJIOTbI% nonacax a p n n b I % 26,8 478 -Bnam-HOCTb % 893 7 75 10,o PaCTBO-PHMOCTb B B o n e % AMUHOKHCJIOTH~I~~ COCTaB 6kiOMaCCbI 14 6ema 113 Mycoderma n p k I B e a e H H n x e : Gno~acca [B r H a 100 r nPoHYHTa1 6 e n o~ %I [B OTHOCHTeJIbHbIX

The microbiological analysis with the test-organism Tetrahymena pyriformis W can be used successfully to measure the relative nutritive value of the biomass and protein isolates from different yeasts for screening the groups of micro-organisms to choose those of the desired quality. In general, this method is rapid, comparatively cheap and gives the possibility to test large numbers of samples simultaneously. According to the literature and our own results, the relative nutritive value, the percentage of absorbed "N", retained by test-organism and used for its growth and multiplication expressed as cell count and given in relation to a standard protein (casein) is generally in a good agreement with results obtained with animals. This microbiological method is promising for investigating the influence of certain technological operations on the quality of protein. The investigation has been performed by the use of Baker's yeast Saccharomyces cerevisiae, of fodder yeast Candida, grown on oil hydrocarbons and on wood hydrolysates, yeasts Hansenula and Mycoderma, grown on a medium containing ethanol as the sole carbon source. The best results were obtained with the yeast grown on the ethanol-containing medium. The biological value of the biomass of yeast Hansenula is 91% and of protein isolate 96% as related to casein as the standard protein.