Nitrogen-backboned modeling of wine-making in standard and nitrogen-added fermentations.

Nitrogen has a strong impact on the key bio-mechanisms involved during the grape-must fermentation but also on the synthesis of flavour markers determining the aromatic profile of the wine. This paper first presents a consistent dynamical mass balance model describing the main physiological phenomena implied in standard batch fermentations, i.e. consumption of sugar and nitrogen and synthesis of ethanol. It also includes nitrogen compounds such as hexose transporters. Moreover, a common practice in wine-making is the addition of nitrogen during the fermentation in order to boost and shorten the process duration. A tractable representation of this boost effect has therefore been developed as an extension of the first model. It is apparent that yeast makes a different use of nitrogen depending on the fermentation stage at which the addition is effected, balancing the regrowth of biomass and the synthesis of supplementary hexose transporters. These models have been validated in line with experimental evidence deduced from extensive experimental studies.

Modeling the effects of assimilable nitrogen and temperature on fermentation kinetics in enological conditions.

We propose a dynamic model of alcoholic fermentation in wine-making conditions. In this model, the speed at which CO(2) is released is related to the effects of the main factors involved in fermentation in wine-making conditions: temperature (which can vary within a predefined range) and nitrogen additions (which must not exceed the maximal authorized level). The resulting model consists of ordinary differential equations including numerous parameters that need to be identified and important interactions between explicative variables. These parameters were identified by uncoupling the effects of variables during specific experiments. The results were validated on another series of experiments in different conditions.

Glycerol overproduction by engineered saccharomyces cerevisiae wine yeast strains leads to substantial changes in By-product formation and to a stimulation of fermentation rate in stationary phase

Six commercial wine yeast strains and three nonindustrial strains (two laboratory strains and one haploid strain derived from a wine yeast strain) were engineered to produce large amounts of glycerol with a lower ethanol yield. Overexpression of the GPD1 gene, encoding a glycerol-3-phosphate dehydrogenase, resulted in a 1.5- to 2.5-fold increase in glycerol production and a slight decrease in ethanol formation under conditions simulating wine fermentation. All the strains overexpressing GPD1 produced a larger amount of succinate and acetate, with marked differences in the level of these compounds between industrial and nonindustrial engineered strains. Acetoin and 2,3-butanediol formation was enhanced with significant variation between strains and in relation to the level of glycerol produced. Wine strains overproducing glycerol at moderate levels (12 to 18 g/liter) reduced acetoin almost completely to 2,3-butanediol. A lower biomass concentration was attained by GPD1-overexpressing strains, probably due to high acetaldehyde production during the growth phase. Despite the reduction in cell numbers, complete sugar exhaustion was achieved during fermentation in a sugar-rich medium. Surprisingly, the engineered wine yeast strains exhibited a significant increase in the fermentation rate in the stationary phase, which reduced the time of fermentation.