Response of different nitrogen supplementation on Saccharomyces cerevisiae metabolic response and wine aromatic profile

Abstract

The wine yeast Saccharomyces cerevisiae can highly affect wine aromatic profile by producing and/or mediating the release of a whole range of metabolites (such as thiols, esters, and terpenes), which in turn contribute to enhanced aroma and flavor. These metabolites depend on yeast metabolism activated during fermentation which can constitute the ‘’metabolic footprint’’ of the yeast strain that carried out the process. The aim of the present research was to study the metabolism of S. cerevisiae under two different nitrogen supplementation status and to investigate the relative expression of specific genes, that are directly related to the biosynthesis of specific potent odornats such as, terpenes and esters. 

The commercial yeast strains 1X (S.cerevisiae) and 2X (S.cerevisiae x S.bayanus) were inoculated in Moschofilero (Vitis Vinifera L. cv) grape must under two different concentrations of yeast assimilable nitrogen (YAN), a  low at 150 mg/L and a high at 300 mg/L. The produced wines were analyzed for their standard enological parameters, their volatile composition by SPE/GC–MS analysis as well as for their sensory profile. Totally 8 fermentations trials, were realised in triplicate. The fermentation was monitored by measuring the optical density and sugar consumption. Metabolic response was tested through real-time RT-PCR of genes implicated in aroma production of esters and terpenes such as ATF1, ATF2, EEB1, EHT1, IAH1, BGL2, EXG1. Sampling for metabolites and gene expression analysis were taken at the time of inoculation, after 48 hours, when two thirds of the sugars were depleted and at the end of the alcoholic fermentation (< 2g/L rs).

In terms of the volatile characterization of the wines, esters, linalool and nerol appeared to be clearly distinct between the different levels of YAN, which confirms the specialization in volatile compounds production among different nitrogen concentration levels. For instance, linalool was found to be at 0.05 mg/L for low nitrogen concentration, while high nitrogen levels resulted to a concentration of 0.12 mg/L. Real-time-PCR results revealed that, in both cases of nitrogen implementation, the analyzed genes were found to be expressed mainly before the fermentation of the 70% of the sugars. In addition, an overexpression of the BGL2 gene, corresponded well to the linalool concentration found, was observed in case of high nitrogen condition. Also, the EHT1 was expressed five times higher in case of high nitrogen concentration. Finally, correlations between ethyl esters and EEB1, acetate esters and ATF2 (p<0.05) were also found in both cases. 

Our study revealed the impact of different nitrogen implementations on the volatile compounds and the relative expression of specific genes. Metabolic analysis of selected volatile components of the wine aroma in conjunction with transcriptional analyses provide a great approach to orient the fermentation process towards a desirable wine aromatic profile.