The Timing of Nitrogen Addition Impacts Yeast Genes Expression and the Production of Aroma Compounds During Wine Fermentation

Subpopulation-specific gene expression in Lachancea thermotolerans uncovers distinct metabolic adaptations to wine fermentation

Gene expression is the first step in translating genetic information into quantifiable traits. This study analysed gene expression in 23 strains across six subpopulations of Lachancea thermotolerans, shaped by anthropization, under winemaking conditions to understand the impact of adaptation on transcriptomic profiles and fermentative performance, particularly regarding lactic acid production. Understanding the gene expression differences linked to lactic acid production could allow a more rational address of biological acidification while optimizing yeast-specific nutritional requirements during fermentation. By sequencing mRNA during exponential growth and fermentation in synthetic grape must, we identified unique expression patterns linked to the strains originated from wine-related environments. Global expression analysis revealed that anthropized subpopulations, particularly Europe/Domestic-2 and Europe-Mix, exhibited distinct gene expression profiles related to fermentation processes such as glycolysis and pyruvate metabolism. These processes were differentially expressed, along with other important biological processes during fermentation, such as nitrogen and fatty acid metabolism. This study highlights that anthropization has driven metabolic specialization in L. thermotolerans, enhancing traits like lactic acid production, which is a trait of interest in modern winemaking. Correlation analysis further linked lactic acid dehydrogenase genes with key metabolic pathways, indicating adaptive gene expression regulation. Additionally, differences in other metabolites of oenological interest as glycerol or aroma compounds production are highlighted. Here, we provide insights into the evolutionary processes shaping the transcriptomic diversity of L. thermotolerans, emphasizing the impact of winemaking environments on driving specific metabolic adaptations, including lactic acid production.

Insights into the relative contribution of four precursors to 3-sulfanylhexan-1-ol and 3-sulfanylhexylacetate biogenesis during fermentation

The desirable wine aroma compounds 3-sulfanylhexan-1-ol (3SH) and 3-sulfanylhexyl acetate (3SHA) are released during fermentation from non-volatile precursors present in the grapes. This work explores the relative contribution of four precursors (E-2-hexenal, 3-S-glutathionylhexan-1-ol, 3-S-glutathionylhexanal, and 3-S-cysteinylhexan-1-ol) to 3SH and 3SHA. Through the use of isotopically labelled analogues of these precursors in defined fermentation media, new insights into the role of each precursor have been identified. E-2-Hexenal was shown to contribute negligible amounts of thiols, while 3-S-glutathionylhexan-1-ol was the main precursor of both 3SH and 3SHA. The glutathionylated precursors were both converted to 3SHA more efficiently than 3-S-cysteinylhexan-1-ol. Interestingly, 3-S-glutathionylhexanal generated 3SHA without detectable concentrations of 3SH, suggesting possible differences in the way this precursor is metabolised compared to 3-S-glutathionylhexan-1-ol and 3-S-cysteinylhexan-1-ol. We also provide the first evidence for chemical conversion of 3-S-glutathionylhexan-1-ol to 3-S-(γ-glutamylcysteinyl)-hexan-1-ol in an oenological system.

The aromatic profile of wine distillates from Ugni blanc grape musts is influenced by the nitrogen nutrition (organic vs. inorganic) of Saccharomyces cerevisiae

Although the impact of nitrogen nutrition on the production of fermentative aromas in oenological fermentation is well known today, one may wonder whether the effects studied are the same when winemaking takes place at high turbidities, specifically for the production of wines intended for cognac distillation. To that effect, a fermentation robot was used to analyze 30 different fermentation conditions at two turbidity levels with several factors tested: (i) initial addition of nitrogen either organic (with a mixture of amino acids - MixAA) or inorganic with di-ammonium phosphate (DAP) at different concentrations, (ii) variation of the ratio of inorganic/organic nitrogen (MixAA and DAP) and (iii) addition of different single amino acids (alanine, arginine, aspartic acid and glutamic acid). A metabolomic analysis was carried out on all resulting wines to have a global vision of the impact of nitrogen on more than sixty aromatic molecules of various families. Then, at the end of the alcoholic fermentation, the wines were micro-distilled. A first interesting observation was that the aroma profiles of both wines and distillates were close, indicating that the concentration factor is rather similar for the different aromas studied. Secondly, the fermentation kinetics and aroma results have shown that the nitrogen concentration effect prevailed over the nature of nitrogen. Although the lipid concentration was in excess, an interaction between the assimilable nitrogen and lipid contents was still observed in wines or in micro-distillates. Alanine is involved in the synthesis of acetaldehyde, isobutanol, isoamyl alcohol and isoamyl acetate. Finally, it was demonstrated that modifying the ratio of assimilable nitrogen in musts is not an interesting technological response to improve the aromatic profile of wines and brandies. Indeed, unbalance the physiological ratio of the must by adding a single source of assimilable nitrogen (organic or inorganic) has been shown to deregulate the synthesis of most of the fermentation aromas produced by the yeast. Wine metabolomic analysis confirmed the results that had been observed in micro-distillates but also in the other aromatic families, especially on terpenes. The contribution of solid particles, but also yeast biosynthesis (via sterol management in must) to wine terpenes is discussed. Indeed, the synthesis of terpenes in this oenological context seems to be favored, especially since the concentration of assimilable nitrogen (in addition to the lipid content) favor their accumulation in the medium. A non-negligible vintage effect on the terpene profile was also demonstrated with variations in their distribution depending on the years. Thus, the present study focuses on the metabolism of wine yeasts under different environmental conditions (nitrogen and lipid content) and on the impact of distillation on the fate of flavor compounds. The results highlight once again the complexity of metabolic fluxes and of the impact of nitrogen source (nature and amount) and of lipids. Furthermore, this study demonstrates that beyond the varietal origin of terpenes, the part resulting from the de novo synthesis by the yeast during the fermentation cannot be neglected in the context of cognac winemaking with high levels of turbidity.

Modelling the effects of assimilable nitrogen addition on fermentation in oenological conditions

Alcoholic fermentation in oenological conditions is a biological process carried out under significant physiological constraints: deficiency of nitrogen and other nutriments (vitamins, lipids …) and different stresses (pH and osmotic). In literature, few models have been proposed to describe oenological fermentations. They focused on the initial conditions and did not integrate nitrogen addition during the fermentation process which is a widespread practice. In this work, two dynamic models of oenological fermentation are proposed to predict the effects of nitrogen addition at two different timings: at the beginning of the process and during the fermentation experiment. They were validated and compared against existing models showing an accurate fit to experimental data for CO₂ release and CO₂ production rate.

Modelling the physiological status of yeast during wine fermentation enables the prediction of secondary metabolism

Saccharomyces non-cerevisiae yeasts are gaining momentum in wine fermentation due to their potential to reduce ethanol content and achieve attractive aroma profiles. However, the design of the fermentation process for new species requires intensive experimentation. The use of mechanistic models could automate process design, yet to date, most fermentation models have focused on primary metabolism. Therefore, these models do not provide insight into the production of secondary metabolites essential for wine quality, such as aromas. In this work, we formulate a continuous model that accounts for the physiological status of yeast, that is, exponential growth, growth under nitrogen starvation and transition to stationary or decay phases. To do so, we assumed that nitrogen starvation is associated with carbohydrate accumulation and the induction of a set of transcriptional changes associated with the stationary phase. The model accurately described the dynamics of time series data for biomass and primary and secondary metabolites obtained for various yeast species in single culture fermentations. We also used the proposed model to explore different process designs, showing how the addition of nitrogen could affect the aromatic profile of wine. This study underlines the potential of incorporating yeast physiology into batch fermentation modelling and provides a new means of automating process design.

Analysis of volatile compounds production kinetics: A study of the impact of nitrogen addition and temperature during alcoholic fermentation

Among the different compounds present in the must, nitrogen is an essential nutrient for the management of fermentation kinetics, also playing a major role in the synthesis of fermentative aromas. Fermentation temperature is yet another variable that affects fermentation duration and the production of fermentative aromas in wine. The main objective of this study was thus to evaluate the combined effects of nitrogen addition-at the start of the fermentation process or during the stationary phase-at different fermentation temperatures on both fermentation kinetics and aroma synthesis kinetics. To study the impact of these three parameters simultaneously, we used an innovative transdisciplinary approach associating an online GC-MS system with an original modeling approach: a Box-Behnken experimental design combined with response surface modeling and GAM modeling. Our results indicated that all three factors studied had significant effects on fermentation and aroma production kinetics. These parameters did not impact in the same way the different families of volatile compounds. At first, obtained data showed that reduction of ester accumulation in the liquid phase at high temperature was mainly due to important losses by evaporation but also to modifications of yeast metabolic capabilities to synthetize these compounds. In a noticeable way, optimal temperature changed for liquid accumulation of the two classes of esters-23°C for acetate ester and 18°C for ethyl esters-because biological impact of temperature was different for the two chemical families. Moreover, the study of these three factors simultaneously allowed us to show that propanol is not only a marker of the presence of assimilable nitrogen in the medium but above all a marker of cellular activity. Finally, this work enabled us to gain a deeper understanding of yeast metabolism regulation. It also underlines the possibility to refine the organoleptic profile of a wine by targeting the ideal combination of fermentation temperature with initial and added nitrogen concentrations. Such observation was particularly true for isoamyl acetate for which interactions between the three factors were very strong.