Structural Complexity of the Nitrogen Source and Influence on Yeast Growth and Fermentation

Abstract

The structural complexity of the nitrogen source strongly affects both biomass and ethanol production by industrial strains of Saccharomyces cerevisiae, during fermentation in media containing glucose or maltose, and supplemented with a nitrogen source varying from a single ammonium salt (ammonium sulfate) to free amino acids (casamino acids) and peptides (peptone). Diauxie was observed at low glucose and maltose concentrations independent of nitrogen supplementation. At high sugar concentrations diauxie was not easily observed, and growth and ethanol production depended on the nature of the nitrogen source. This was different for baking and brewing ale and lager yeast strains. Sugar concentration had a strong effect on the shift from oxido-fermentative to oxidative metabolism. At low sugar concentrations, biomass production was similar under both peptone and casamino acid supplementation. Under casamino acid supplementation, the time for metabolic shift increased with the glucose concentration, together with a decrease in the biomass production. This drastic effect on glucose fermentation resulted in the extinction of the second growth phase, probably due to the loss of cell viability. Ammonium salts always induced poor yeast performance. In general, supplementation with a nitrogen source in the peptide form (peptone) was more positive for yeast metabolism, inducing higher biomass and ethanol production, and preserving yeast viability, in both glucose and maltose media, for baking and brewing ale and lager yeast strains. Determination of amino acid utilization showed that most free and peptide amino acids present, in peptone and casamino acids, were utilized by the yeast, suggesting that the results described in this work were not due to a nutritional status induced by nitrogen limitation.Key words: Amino acids, fermentation, nitrogen metabolism, peptides, Saccharomyces, yeast. -2863(9'8-32Yeast are able to use a wide variety of compounds as a carbon and nitrogen source 8,44,45 . Nitrogen is one of the main elements found in many macromolecules of living organisms, playing a central role in structure and function, and most organisms have elaborate control mechanisms to provide a constant supply of nitrogen 8,21,22,37,45 . Thus, a substantial proportion of cellular activity is concerned with procuring and assimilating nitrogen. It has been observed that ammonia, asparagine, glutamine and glutamate are preferentially used by yeast 21,37 . When these primary nitrogen sources are absent, or present in concentrations low enough to limit growth, other nitrogen sources such as nitrite, nitrate, amides, amino acids and peptides can be used. The utilization of secondary nitrogen sources requires the synthesis of specific-catabolic enzymes and permeases, the expression of which is highly regulated by a process known as nitrogen catabolite repression. The latter is prevented in the presence of a preferred nitrogen source 21,45 . Studies on the effect of nitrogen with only one compound as sole nitrogen and/or carbon source...

Effect of Sugar Catabolite Repression in Correlation with the Structural Complexity of the Nitrogen Source on Yeast Growth and Fermentation

“…Ammonium sulfate always induced lower fermentative performance in all conditions used in this study. These results differ from the previously reported study in media containing glucose and maltose, which showed that peptone, a nitrogen source where amino acids predominate in the peptide form, was more efficient for yeast growth and fermentation 7 . Lower biomass accumulation in the media supplemented with ammonium sulfate was detected in both studies and this resulted from the yeast using the carbohydrate as a carbon and an energy source.…”

“…Improved ethanol production by the baking yeast strain Fiso was observed in the medium supplemented with casamino acids when compared with peptone and ammonium sulfate, with higher biomass accumulation and faster fermentation. The results with galactose media differ from those with glucose and maltose media, where high sugar concentrations were found to induce higher biomass and ethanol production in the media supplemented with peptone 7 .…”

“…Also there is a correlation with the structural complexity of the nitrogen source. In glucose, a repressive carbon source, improved fermentation performance was observed in the medium supplemented with peptone 7 . In galactose, a non-repressive sugar, fermentation performance was different for baking and brewing strains and altered patterns of galactose utilisation, depending on the nitrogen supplement, were observed.…”

“…The sources of nitrogen varied from a single ammonium salt (ammonium sulfate), an acid protein hydrolysate (casamino acids) consisting predominantly of free amino acids to an enzymatic protein hydrolysate consisting predominantly of peptides (peptone) 7,10 . Previous results obtained in our laboratory 7 showed that the structural complexity of the nitrogen source strongly affects yeast metabolism. Biomass accumulation and ethanol production, in addition to their dependence on the nature of the nitrogen supplement, were also affected by sugar concentration.…”

“…Little is known about the range of peptides present, or the order in which they are removed from wort 2,5,13,43 . In previous studies we have shown that in general, the supplementa-tion of the growth media, containing maltose or glucose, with a more complex structural nitrogen source such as peptone induced higher biomass accumulation and ethanol production 7 . In this study we continued to study the effect of the complexity of the nitrogen and carbon sources on the metabolism of the yeast Saccharomyces, except that the sugar was the non catabolite repressing sugar, galactose.…”