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p r a c t i c a l w i n e r y & v i n e ya r d a U G U S t 2 0 1 4 65 W I N E M A K I N G wines. 4 Yeast strains differ greatly in their ability to produce flavor-active com- pounds. Among the positive contributors are volatile thiols, imparting fruity aro- mas to wine. The chemical compounds 4-mercapto- 4-methylpentan-2-one (4MMP), 3-mercap- tohexan-1-ol (3MH) and 3-mercaptohexyl acetate (3MHA) are characterized by the aromas of boxwood, grapefruit and pas- sion fruit, respectively. 4MMP and 3MH are present in grapes in a precursor form, conjugated to cysteine or glutathione. 14 During fermentation, yeast take up these precursors and cleave them to release free volatile thiols into a wine. Yeast strains differ in their ability to release and convert the volatile thiols. 8 Another group of compounds that greatly influence wine aromas are esters. An ester is a bond between an alcohol and a fatty acid. 16 Ethyl esters are comprised of ethanol and medium-chain fatty acids, and acetate esters are comprised of acetyl-CoA and ethanol, or other complex alcohols derived from amino acid metabolism. It has also been suggested that ethyl ester formation by yeast strains is a way to detoxify the must from medium-chain fatty acids that are toxic to the ferment- ing yeast. 4 Various factors influence yeast ester formation during fermentation, such as the amount of substrate in the must, the concentration of the co-substrates (acetyl- CoA and alcohol), must turbidity, must nutrient status, fermentation tempera- ture, availability of oxygen and— most important — the genetic ability of the yeast strain conducting the fermentation (expression of acyl-transferase and ester- ase genes). Winemakers have a wide range of yeasts to choose from in order to achieve a desired wine style, and selecting the appropriate yeast strain is a very important consider- ation. For example, a thiol-enhancing yeast strain is very suitable for production of Sauvignon Blanc, whereas an ester-form- ing yeast strain is more suitable for pro- duction of aromatic Chardonnay. Within the commercial yeast products, specific strains are categorized as POF- negative strains that will not produce volatile phenols above their sensory threshold, even if enzymes containing cinnamoyl esterase are used. Examina- tion of yeast strain product literature or consulting with the proper vendor repre- sentative is recommended. Yeast nutrition The concentration of aroma compounds produced by the yeast Saccharomyces cere- visiae is, along with other factors previ- ously mentioned, a function of nutrient availability. During fermentation, yeast utilize nutrients to produce an array of volatile compounds such as esters, higher alcohols, volatile fatty acids, carbonyls and sulfur compounds. In many viticultural regions the natural nutrient composition of grape juice is considered sub-optimal, and this may contribute to a variety of problems such as stuck or sluggish fermentations and the formation of undesirable off-flavors. 1,7 To alleviate these problems and opti- mize wine aroma, additional yeast nutri- ents are often supplemented to the juice prior to or during alcoholic fermentation. The make-up and quality of the yeast nutrient and timing of addition all play a vital role in its effect on wine aroma, as has been reported in various studies. 5,16 It has been proven that the use of yeast- derived products naturally enriched in survival factors such as sterols (especially ergosterol), fatty acids, vitamins and minerals during rehydration greatly enhance the volatile thiol concentration Your pre-insulated piping solution for glycol installations. COOL-FIT ® Plus Systems! Your pre-insulated piping solution for glycol installations. COOL-FIT ® Plus Systems! www.cool-fit.georgfischer.com 2882 Dow Avenue, Tustin, CA 92780-7258 • Phone (714) 731-8800, Toll Free (800) 854-4090 e-mail: us.ps@georgfischer.com • www.gfpiping.com 2882 Dow Avenue, Tustin, CA 92780-7258 • Phone (714) 731-8800, Toll Free (800) 854-4090 e-mail: us.ps@georgfischer.com • www.gfpiping.com Fine Wine Making with Energy Savings www.cool-fit.georgfischer.com