The utilisation of nitrogenous compounds by commercial non-Saccharomyces yeasts associated with wine

[摘要] ENGLISH SUMMARY: Nitrogen is an essential element for yeast growth and metabolism. During winemaking, the uptake of nitrogen sources ensures the completion of alcoholic fermentation while participating in the production of aroma compounds. Preference for certain nitrogen sources has been determined for Saccharomyces cerevisiae, the main wine yeast, but not in other yeast species, collectively referred to as non-Saccharomyces yeasts. Yet, the latter are becoming increasingly used in industry in combination with S. cerevisiae and the preferences of these yeasts should also be investigated in order to ensure optimal fermentation ability and aroma compound production. This could unravel possible competition between yeast species and consequently potential needs for supplementation to avoid stuck fermentations and production of off-aroma compounds.This study investigated the nitrogen source preferences of a number of commercial non-Saccharomyces yeasts (Pichia kluyveri Viniflora® FootZen™, Torulaspora delbrueckii Biodiva™ TD291, Metschnikowia pulcherrima Flavia® MP346 and Lachancea thermotolerans Viniflora® Concerto™) by conducting pure culture fermentations in a synthetic grape must medium with nitrogen sources present at equal assimilable nitrogen concentrations. Uptake varied between yeast species, however P. kluyveri and M. pulcherrima showed similar uptake trends, as did L. thermotolerans and T. delbrueckii. Nitrogen uptake was also proportional to CO2 release, sugar consumption and yeast growth. L. thermotolerans and T. delbrueckii took up a greater amount of the nitrogen sources than P. kluyveri and M. pulcherrima, as well as at a faster rate. Variations in amino acid preferences between yeast species were observed. The presence of ammonium was shown to affect the % uptake of certain amino acids for the different species as well as the onset of amino acid uptake for T. delbrueckii. Sequential fermentations with grape must-like nitrogen concentrations revealed that L. thermotolerans and T. delbrueckii pose a larger problem regarding competition with S. cerevisiae compared to P. kluyveri and M. pulcherrima. This was reflected in fermentation kinetics of sequential fermentations as well as the levels of aroma compound production. This suggests that sequential fermentations with L. thermotolerans and T. delbrueckii may require higher nitrogen source supplementation than P. kluyveri and M. pulcherrima for S. cerevisiae to complete fermentation as well as increase the concentration of desirable aroma compounds. Some correlations were observed regarding nitrogen source uptake and aroma compounds produced, for example the uptake of threonine and resulting propanol production. The data showed that, after inoculation with S. cerevisiae Lalvin® EC1118, the population of non-Saccharomyces yeasts declined and the cells probably autolyse, thereby releasing nitrogen sources that can be taken up by S. cerevisiae. This study improved our knowledge of non-Saccharomyces yeasts' nitrogen source preferences, and establishes some links between nitrogen source uptake and aroma compound production. Competition between yeasts was also demonstrated and linked with nitrogen uptake and fermentation ability.

[发布机构] Stellenbosch University