Past and Future of Non-Saccharomyces Yeasts: From Spoilage Microorganisms to Biotechnological Tools for Improving Wine Aroma Complexity

Abstract

It is well established that non-Saccharomyces wine yeasts, considered in the past as undesired or spoilage yeasts, can enhance the analytical composition, and aroma profile of the wine. The contribution of non-Saccharomyces yeasts, including the ability to secret enzymes and produce secondary metabolites, glycerol and ethanol, release of mannoproteins or contributions to color stability, is species- and strain-specific, pointing out the key importance of a clever strain selection. The use of mixed starters of selected non-Saccharomyces yeasts with strains of Saccharomyces cerevisiae represents an alternative to both spontaneous and inoculated wine fermentations, taking advantage of the potential positive role that non-Saccharomyces wine yeast species play in the organoleptic characteristics of wine. In this context mixed starters can meet the growing demand for new and improved wine yeast strains adapted to different types and styles of wine. With the aim of presenting old and new evidences on the potential of non-Saccharomyces yeasts to address this market trend, we mainly review the studies focused on non-Saccharomyces strain selection and design of mixed starters directed to improve primary and secondary aroma of wines. The ability of non-Saccharomyces wine yeasts to produce enzymes and metabolites of oenological relevance is also discussed.

Keywords: aroma complexity; enzymes; mixed starters; non-Saccharomyces yeasts; primary aroma; secondary aroma; secondary metabolites.

FIGURE 1

Use of selected strains of Saccharomyces and non-Saccharomyces yeasts in winemaking. Spontaneous fermentation (a) allows the development of indigenous yeasts from grapes (mainly non-Saccharomyces) and winery (mainly Saccharomyces) leading to wines with a greater aromatic complexity but with less microbiological control. Inoculation with a selected strain of Saccharomyces cerevisiae (c) leads to a greater microbiological control but can reduce the aromatic complexity of wine. The use of mixed cultures of S. cerevisiae and non-Saccharomyces selected strains (b) allows to obtain wines with both greater aromatic complexity and microbiological control of the process. Autochthonous and inoculated selected yeasts are represented in red and blue color, respectively.

FIGURE 2

Release of primary aroma compounds by yeasts. Monoterpenes and volatile thiols occur in grape as odorless precursors which can be released by enzymatic activities of Saccharomyces and non-Saccharomyces yeasts during fermentation. Monoterpene glycosides are mainly glucosides and diglycosides, in which the glucose moiety (G) has been further substituted mainly with arabinose (Ar), apiose (Ap), or rhamnose (R). A two-step enzyme-catalyzed reaction is the main mechanism proposed for the enzymatic hydrolysis of the diglycosides and subsequent release of the free volatile terpene (T) to wine. First a specific glycosidase cleaves the linkage between both sugars and in a second step the released glucoside is hydrolyzed by a β-D-glucosidase, liberating glucose and the corresponding terpene. Volatile thiols are generated from the odourless cysteinylated precursors cysteine-3-mercaptohexan-1-ol (Cys-3MH) and cysteine-4-mercapto-4-methylpentan-2-one (Cys-4MMP) by the action of carbon-sulfur-lyases.