Nitrogen metabolism of saccharomyces and non saccharomyces wine strainsphenotypic and genomic characterisation

Laburpena

S. cerevisiae is the most important yeast species for winemaking. In our study, different approaches were applied to evaluate the nitrogen requirement of a set of commercial S. cerevisiae strains. The selected high and low nitrogen demanding strains were sequenced and genomic differences were compared. Allele swapping of GCN1 gene revealed that the polymorphism of this gene is not the main reason for the nitrogen requirement differences in our strains. The cryotolerante Saccharomyces species such as S. uvarum, S. eubayanus and S. kudriavzevii are becoming more and more popular for wine making. However, the nitrogen requirements of these species is unclear. We studied the fermentation behaviour of Saccharomyces non-cerevisiae yeast species under different nitrogen concentrations and temperatures. The selected cryotolerant Saccharomyces species, particularly S. eubayanus showed lower nitrogen requirement than our control S. cerevisiae strain especially when the fermentations were carried out at low temperature. Although the cryotolerant yeast species possess many beneficial characteristics for fermentations at low temperature, they have lower tolerance to ethanol and sometimes are not able to finish the fermentation when used as pure culture. In our study, hybrids between cryotolerant species and S. cerevisiae were constructed. The fermentation capacity of the hybrid strains was tested under four conditions by combining two temperatures, 12°C and 28°C, and two nitrogen concentrations, 60 mg/L and 300 mg/L. The hybrid strains conferred better fermentation rates under low-temperature or low-nitrogen conditions and also produced larger amounts of acetate esters and higher alcohols, which increase aroma intensity and complexity in wine. The utilisation of non-Saccharomyces yeast strains in wine fermentation diversifies the characteristics of wine and improves wine quality. However, their nitrogen requirements and nitrogen source preferences are not well known. Our study explored the nitrogen utilisation profile of three popular non-Saccharomyces species: Torulaspora delbrueckii, Metschnikowia pulcherrima and Metschnikowia fructicola. The nitrogen source preferences to support growth and fermentation as well as the uptake order of different nitrogen sources during wine fermentation were investigated. While T. delbrueckii and S. cerevisiae strains shared the same nitrogen source preferences, Metschnikowia sp. displayed a lower capacity to efficiently use the preferred nitrogen compounds, but were able to assimilate a wider range of amino acids. The other important question is how nitrogen sources are metabolised and redistributed after entering the cell. A quantitative analysis using 15N-labelled NH4Cl, arginine, and glutamine, and 13C-labelled leucine and valine revealed the specificities of the nitrogen metabolism pattern of two non-Saccharomyces species, Torulaspora delbrueckii and Metschnikowia pulcherrima.