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The studies described in this paper were undertaken in an attempt to answer the question: what happens to each of the individual components of binary mixtures of nitrogen nutrients when yeast is grown upon them? Does yeast assimilate the nitrogen completely from the more assimilable component before attacking the less assimilable one, or are the two components attacked simultaneously, and if so, at what relative rates? The relevance of this inquiry to brewing practice lies in the fact that wort contains a complex mixture of nitrogen nutrients (ammonia, amino acids, amides and peptides) and hitherto it has been a matter of opinion whether or not yeast exhibits any preferential assimilation among these nutrients. The experimental method involved following the course of nitrogen assimilation at various stages during the growth of yeast 6479 in media containing ammonium phosphate, together with an amino acid, by estimations of the amounts of ammonia nitrogen and total nitrogen removed from the media. It is shown that yeast attacks the components of such nutrient mixtures simultaneously and in a fixed ratio which is partly dependent upon their relative concentrations in the mixtures, but which is mainly a specific characteristic of the particular nutrients used. Yeast exhibits a marked preference for the more assimilable component of a mixture, much greater than that which might be expected on the basis of individual nutrient efficiencies. |
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