THE GROWTH OF TOP-FERMENTATION YEASTS IN BINARY MIXTURES OF VARIOUS NITROGEN NUTRIENTS

The growth of English top-fermentation yeasts in various nitrogen sources, used singly—including ammonium phosphate and a number of amino acids—has previously been investigated and reported upon. But malt wort, the natural medium for yeast growth, contains, not a single nitrogen source, but a mixture of many derived from the breakdown of barley proteins. It is therefore important to know whether growth in such mixtures is simply the average of growth in the individual nitrogen sources or whether the different nitrogen sources interact with each other in such a way as to produce enhanced or diminished growth. The object of the work described in this paper was to ascertain whether the admixture of two different nitrogen sources introduces any new effects unpredictable from the known behaviour of the same nitrogen sources used singly. Nutrient media were prepared containing graded proportions of various selected pairs of nitrogen sources. Four different top-fermentation brewery yeasts were cultured in these media, measurements being made of fermentation, nitrogen assimilation and rate and extent of yeast growth. It was thus possible to observe the influence of the percentage composition of any particular binary mixture of nitrogen nutrients upon the growth and fermentation of a variety of yeasts. Among the mixtures investigated there was generally found a slight enhancement of growth and fermentation over what would be expected from the individual behaviour of the nitrogen sources. In a few cases, however, the observed enhancement was very large and was practically independent of the particular variety of yeast used. No important depressive effects were found in any of the mixtures. It may therefore be concluded that the mere admixture of different nitrogen sources can make a special contribution to yeast growth and fermentation; this contribution is generally relatively small but may in some circumstances becomes very significant. A chemical interpretation of some of the phenomena of yeast growth in mixed nutrients has been put forward.