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WIN e MAKING Sugar Metabolism of Wine Yeast A: Regular metabolism Grape sugars: Glucose Fructose Ethanol CO2 Glycerol B: Modified metabolism Grape sugars: Glucose Fructose an email from Lallemand researcher Anne Julien-Ortiz: "We know that through GMO strategy it is possible to push the yeast to overpro- duce glycerol by the overexpression of the genes GPD1 and GPD2; in this way, the final ethanol level will decrease a little bit. For example, to have about 1.5%-2% less alcohol, the yeast needs to produce at least 25g/L of glycerol—which is a lot! At the same time, the yeast will overproduce acetate (volatile acidity) as well. Once the genes ald4 and ald6 (responsible of this Glycerol Ethanol CO2 A: Consumption of sugars leads mainly to the production of ethanol and carbon dioxide and to a lesser extent, glycerol. B: Modifying yeast metabolism so more glycerol is synthesized reduces the amount of ethanol produced. Somewhat ironically, the strains that go for biomass make themselves more vulnerable to ethanol toxicity, while the strains focused on ethanol en- joy greater resistance, even with their smaller numbers. Besides making ethanol and carbon di- oxide, yeast also produces small amounts of many other compounds. Strains can be isolated, created through conventional breeding or concocted by GMO tech- niques that yield less ethanol and more of something else, especially glycerol. In order to get less alcohol, you're likely to get way more glycerol than you bargained for. Some of the experimental research has involved crossing commercial and "wild" cerevisiae strains, some breeding with non-cerevisiae strains, some with non-Saccharomyces yeast. A number of these trials have been nominally success- ful, reducing alcohol yields by as much as 1.5%. Trouble is, in order to get that much less alcohol, you're likely to get way more glyc- erol than you bargained for. A little extra glycerol is great for red wine mouthfeel; a lot of it is not so nice. Here's a portion of Lalvin Clos® has been selected by the Universitat Rovira i Virgili (Facultat d'Enologia) from the Priorat D.O.C. area where alcohol and polyphenol levels are usually very high. Lalvin Clos® ferments over a wide temperature range, tolerates high potential alcohols and respects the organoleptic and varietal complexity of the top-of-the-range red wines. www.lallemandwine.us Wines & Vines JUne 2011 73 For prestigious red wines production) are deleted, there is another problem: accumulation of acetoin, which is bad for wine quality (very strong lactic notes, off flavor.) "When you force the yeast to do something else, she will find another way, but one that won't be especially positive for the wine sensory profile. So it's really challenging." In brief, be careful when you mess with Mother Nature. Some examples of the low-ethanol, high-glycerol strat- egy yeasts (all of them created through AUSTRALIAN WINE RESEARCH INSTITUTE