Non-Saccharomyces yeast nitrogen consumption and metabolite production during wine fermentation

Abstract

Over the last decade, the use of non-Saccharomyces yeasts in the winemaking process has been re-assessed and accepted by winemakers. These yeasts can be used to achieve specific objectives such as lowering the ethanol content, preventing wine spoilage and increasing the production of specific aroma compounds. Since these species are unable to complete alcoholic fermentation, strategies of co- and sequential inoculation of non-Saccharomyces and Saccharomyces cerevisiae have been developed. However, when mixed starter cultures are used, several parameters (e.g. strain yeast, inoculation timing and nutrient competitions) impact the growth of the individual yeasts, the fermentation kinetics and the metabolites/aroma production. In particular, competition for nitrogen compounds could have a major impact, potentially leading to sluggish fermentation when the yeast assimilable nitrogen (YAN) availability is low. Moreover, many aroma compounds produced by the yeasts are directly produced and influenced by nitrogen metabolism such as higher alcohols, acetate esters and ethyl esters which participate in the organoleptic complexity of wine. 

In this context, the first part of this work was to provide an overview of the potentialities of oenological interest of non-Saccharomyces species isolated from grape juices. The fermentations were carried out in enological conditions, at 24°C and the potential of several non-Saccharomyces yeasts to produce hydrolytic enzymes and metabolites contributing to the sensory properties of wines has been reaffirmed. In particular, the use of Starmerella bacilliaris exhibited an increased production of glycerol with a concomitant ethanol decrease. Furthermore, some strains of Hanseniaspora osmophila and Metschnikowia pulcherrima produced esters and thiols, which may have a positive incidence on the sensory quality of wines. 

Then, the nitrogen requirements of non-Saccharomyces yeasts were characterized. The analysis of the complete dataset revealed differences between species and even between strains in their preferred nitrogen sources. For example, S. bacilliaris strains consumed a limited fraction of amino acids during fermentation while exhausting all the available ammonium. Overall, this work enhanced our understanding of yeasts’ nitrogen requirement and metabolism. It also pointed out that an appropriate management of the nitrogen nutrition of yeasts during co- or sequential fermentations to take full advantage of the potentialities of non-Saccharomyces species.