INFLUENCE OF THE NITROGEN-SOURCE ON SACCHAROMYCES-CEREVISIAE ANAEROBIC GROWTH AND PRODUCT FORMATION

To prevent the loss of raw material in ethanol production by anaerobic yeast cultures, glycerol formation has to be reduced, In theory, this may be done by providing the yeast with amino acids, since the de nov o cell synthesis of amino acids from glucose and ammonia gives rise to a surplus of NADH, which has to be reoxidized by the formation of gly cerol, An industrial strain of Saccharomyces cerevisiae was cultivated in batch cultures with different nitrogen sources, i.e., ammonium sal t, glutamic acid, and a mixture of amino acids, with 20 g of glucose p er liter as the carbon and energy source, The effects of the nitrogen source on metabolite formation, growth, and cell composition were meas ured. The glycerol yields obtained with glutamic acid (0.17 mol/mol of glucose) or with the mixture of amino acids (0.10 mol/mol) as a nitro gen source were clearly lon er than those for ammonium-grown cultures (0.21 mol/mol), In addition, the ethanol yield increased for growth on both glutamic acid (by 9%) and the mixture of amino acids (by 14%), G lutamic acid has a large influence on the formation of products; the p roduction of, for example, cr-ketoglutaric acid, succinic acid, and ac etic acid, increased compared with their production with the other nit rogen sources, Cultures grown on amino acids have a higher specific gr owth rate (0.52 h(-1)) than cultures of both ammonium-grown (0.45 h(-1 )) and glutamic acid-grown (0.33 h(-1)) cells, Although the product yi elds differed, similar compositions of the cells were attained, The NA DH produced in the amino acid, RNA, and extracellular metabolite synth eses was calculated together with the corresponding glycerol formation , The lower-range values of the theoretically calculated yields of gly cerol were in good agreement with the experimental yields, which may i ndicate that the regulation of metabolism succeeds in the most efficie nt balancing of the redox potential.