Abstract
Proline is the predominant amino acid in grape juice, but it is poorly assimilated by wine yeast under the anaerobic conditions typical of most fermentations. Exploiting the abundance of this naturally occurring nitrogen source to overcome the need for nitrogen supplementation and/or the risk of stuck or sluggish fermentations would be most beneficial. This study describes the isolation and evaluation of a novel wine yeast isolate, Q7, obtained through ethyl methanesulfonate (EMS) mutagenesis. The utilisation of proline by the EMS isolate was markedly higher than by the QA23 wild type strain, with approximately 700 and 300 mg/L more consumed under aerobic and self-anaerobic fermentation conditions, respectively, in the presence of preferred nitrogen sources. Higher intracellular proline contents in the wild type strain implied a lesser rate of proline catabolism or incorporation by this strain, but with higher cell viability after freezing treatment. The expression of key genes (PUT1, PUT2, PUT3, PUT4, GAP1 and URE2) involved in proline degradation, transport and repression were compared between the parent strain and the isolate, revealing key differences. The application of these strains for efficient conduct for nitrogen-limited fermentations is a possibility.
Highlights
Amino acids, ammonium, peptides and proteins represent the main forms of nitrogen present in grape must
The nitrogen-containing compounds utilised by yeast during wine fermentation are referred to as yeast assimilable nitrogen (YAN), with amino acids and ammonium contributing between 60% and 90% of YAN [2]
This study describes the use of ethyl methanesulfonate (EMS) random mutagenesis to develop novel wine yeast with an improved capacity to utilize proline
Summary
Ammonium, peptides and proteins represent the main forms of nitrogen present in grape must. The incidence of stuck or sluggish fermentations due to nitrogen deficiencies is of great concern to winemakers around the world To avoid such problems, winemakers often employ supplementation strategies, e.g., the addition of diammonium phosphate prior to or during fermentation or the use of YAN-rich rehydration additives. Winemakers often employ supplementation strategies, e.g., the addition of diammonium phosphate prior to or during fermentation or the use of YAN-rich rehydration additives This practice can influence yeast metabolism and impact wine quality; for example due to the accumulation of ethyl carbamate, a known carcinogen [11,12] or changes to wine aroma profile [13]. Such a strain was sought as a means of overcoming nutrient-related fermentation problems, thereby improving fermentation reliability and overall wine quality
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