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Inquiring Winemaker TIM P A T T er S o N ongoing Search for Low-yielding yeast extension, some theorized, the yeast was responsible for rampaging alcohol levels in wines from California and elsewhere. The short scientific response to this line T of argument was no, the yeasts aren't rais- ing your alcohol levels, it's all that sugar in the grapes you insist on hyper-ripening. That debate has mostly disappeared from the public stage, replaced with a concern that's almost a mirror image. If the yeasts aren't making more alcohol, are there some strains that can yield less? Might a bit of inefficiency be a virtue? And indeed the quest for low-yielding yeasts has been a major research preoccu- pation in both academic enology depart- ments and commercial yeast research labs. The results so far: a lot of fascinating sci- ence, but no silver bullets. Yeast 101 In fermentation mode, yeast converts one sugar molecule to two molecules of ethanol and two molecules of carbon dioxide. This activity is wired pretty deep into the genome; it's what yeasts do. And herein is the prob- lem with the theories about super-yeast that could procure more ethanol: They would somehow have to create 2.5 molecules of ethanol (or some such number) from the same sugar molecule, thus violating the laws of chemistry and of second-grade math. Modern commercial yeast strains do have the ability to ferment high-Brix juices dry. 72 Wines & Vines JUne 2011 hree or four years ago, the high-octane section of the wine world buzzed with speculation that newer commercial yeast strains somehow yielded more alcohol than old standbys. By Most have been selected for their ability to reliably complete a fermentation, on riper and riper grapes, but that increased ethanol tolerance does not change the conversion ratio: The molecule math stays the same. "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." —anne Julien-ortiz, Lallemand In 2007, wine yeast producer Lalle- mand conducted a study in which the ethanol yields of a total of 113 com- mercial yeast strains were measured. And among those who finished the job to dryness, the widest difference in final alcohol was 0.51%. Additional studies, by the Australian Wine Research Institute and others, also have found the widest gap in commercial yeast strains to be in the 0.3-0.5% range. Linda Bisson and her lab crew at UC Davis have taken the comparison contest a bit further. By including non-commercial Saccharomyces cerevisiae strains—vine- yard isolates, strains that failed to suc- ceed commercially, etc.—they have seen a range as wide as 1.5% among strains able to complete a fermentation. Since that dis- parity could mean the difference between an old-fashioned 13.5% Cabernet and a newfangled 15 percenter, it suggests that figuring out the metabolic differences could pay off. Making less with more Yeast are determined creatures, and if something in their genetic code or environ- ment makes them produce less ethanol, they will produce more of something else instead. Those sugar molecules won't just be frittered away. One of the things yeast can produce more of is yeast. The carbon energy source in the glucose and fructose can be put into ethanol, or into more biomass. Bisson's crew has observed that some of the high- ethanol-producing strains rapidly build up biomass during the early part of a fermen- tation and then switch off reproduction and concentrate on pumping out ethanol until they run out of fuel. Some low-etha- nol producers just keep budding and birth- ing, finding a kind of safety in numbers and domination of the fermentation in relation to other microbes. highlights • Higher alcohol levels around the world have spurred interest in finding or de- veloping yeast strains that produce less ethanol. • Researchers have identified genes that can create more glycerol and less alcohol and have bred yeast strains with those capacities, but the sensory impact is often negative. • For now, the quest for lower ethanol yeast is still a work in progress.