The development of ß-glucosidase producing wine yeast for the release of flavour compounds
Van Rensburg, P
University of Stellenbosch. Faculty of AgriSciences. Institute for Wine Biotechnology
Gey van Pittius, M H
Cordero Otero, R R
Stidwell, T G
The development of recombinant wine yeast strains that can produce ß-glucosidase. ß-Glucosidase producing wine yeast strains could facilitate the release of aroma components and resveratrol during wine fermentation without the addition of expensive commercial enzyme preparations.
In most white and red grape varieties, a significant amount of potential aroma compounds is bound to two sugar molecules. A considerable proportion of the aroma substances is located in the skin of the berries, therefore the maceration promotes the extraction of aroma compounds. One portion of the aroma compounds found in grapes is in free volatile and odorous form, others are glycosidically bound aroma precursors. These odourless aroma precursors can be transformed during processing (e.g. crushing, fermentation or storage) into flavour active compounds, partly explaining the difference in flavour between wine and grape juice. Juice contains only about 1% free aromas which demonstrates the limited value of tasting grapes in the vineyard to determine ripeness. Chemically the sugar-bound grape volatiles consist of monoterpenes, norisoprenoids, benzene derivatives (e.g. volatile phenols) and aliphates. The bound terpenols are present in grapes mainly as disaccharide glycosides: as 6-O-a-L-arabinofuranosyl-ß-D-glucopyranosides and 6-O-a-L-rhamnopyranosyl-ß-D-glucopyranosides or as ß-D-glucopyranosides of geraniol, nerol and linalool. It has been established that especially the monoterpenes (like linalool, citronellol, nerol and geraniol) in the grape are important for the flowery, fruity flavour characters that wines of Muscat varieties, such as Riesling or Gewürztraminer, often exhibit. Since most monoterpenes are bound, the distinctive character of Muscat variety wines can be increased by acid hydrolysis, accelerated by lower wine pH and elevated temperatures, or by enzymatic hydrolysis. Enhancement of wine aroma by enzymatic hydrolysis of glycosidic precursors from the grape can be carried out by ß-glycosidases. Lately, it is believed that the enzymatic hydrolysis is a sequential process, which requires first a-L-arabinosidase, a-L-rhamnosidase or an apiofuranosidase enzyme to cleave the disaccharide (1-6) linkage (Fig. 1). The monoterpenols are then liberated by the action of ß-glucosidase. Although the grapes and wine yeasts contain ß-glycosidases themselves, they are not effective under winemaking conditions. Traditionally, wines have been produced by natural fermentation by yeasts originating from grapes and wineries. There are great variations in the quantity and distribution of the relevant yeast species which can be mainly related to differences in ecosystems and vintages. For this reason, during the last few years many European winemakers have used pure cultures or mixed pure yeast cultures isolated from their own wine region. In the micro vinification processes some mixed yeast cultures (Hansenula, Candida and Saccharomyces) may have intracellular ß-D-glucosidase activity. A wine yeast that produces sufficient levels of ß-glucosidases, can possibly release aroma components during fermentation. The grape variety aroma of wine could be improved by engineering a wine yeast strain that secretes ß-glucosidase into wine.
Van Rensburg, P, Potgieter, N, Briones-Perez, A L, Ubeda Iranzo, J F, Stidwell, T G, Lambrechts, M G, Cordero Otero, R R and Pretorius, I S. 2002. From wild yeast to recombinant wine yeast: Application of ß-glucosidase activity in wine production. Paper presented at the 22nd International Specialised Symposium on Yeast. Pilanesberg, South Africa.
Van Rensburg, P, Pretorius, I S. 2000. Enzymes in winemaking: Harnessing natural catalysts for efficient biotransformations – a review, South African Journal of Enology and Viticulture, v. 21 Sepcial issue (p. 52-73)