Development and assessment of a recombinant Saccharomyces cerevisiae wine yeast producing two aroma-enhancing β-glucosidases encoded by the Saccharomycopsis fibuligera BGL1 and BGL2 genes
The distinctive varietal flavour of grapes and wine is affected by the absolute and relative concentrations of many compounds, including monoterpene alcohols such as citronellol, geraniol, hotrienol, linalool, nerol and α-terpineol. Monoterpenols in grapes and wine can occur either as free volatile and odorous molecules, or as glycosidically bound, odourless, non-volatile complexes. β-Glucosidases constitute one group of glycosidases that can help to unleash this latent pool of grape-derived volatile aglycons and provide an additional source of wine aroma and flavour compounds. However, yeast (Saccharomyces cerevisiae) and bacteria (Oenococcus oeni) most commonly used to initiate alcoholic and malolactic fermentation during winemaking have only limited ability to liberate the aromatic terpenols as well as other aglycones bound to saccharides. Therefore, the purpose of this study was to clone, integrate and express two β-glucosidase genes (BGL1 and BGL2) from the dimorphic yeast, Saccharomycopsis fibuligera, in a commercially available wine yeast strain, VIN13. Using p-nitrophenyl-β-D-glucopyranoside as a synthetic substrate, enzyme assays and kinetic studies indicated that both these two extracellularly produced β-glucosidases were able to cleave glycosidic bonds efficiently. Subsequently, wine from Chenin blanc, Gewürztraminer and Pinotage grapes was made with the transformed and untransformed S. cerevisiae VIN13 strains, and compared. A series of analyses indicated that wines produced by the β-glucosidase-producing VIN13 strain contained slightly higher levels of terpenols in comparison with the wines produced by the untransformed control VIN13 strain. Surprisingly, the wines produced by the transformed strain also contained increased ester concentrations. Many of these fragrant compounds, when produced in appropriate concentrations, would contribute to the fermentation bouquet of wine. The extent to which this acquired capacity of the transformed wine yeast is of practical significance in large-scale wine production is at present unclear but presents a worthwhile proposition for further investigation.