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GRAPEGROWING Table 1. Analytical details of wines made using the fermentation protocols in Figure 2 (page 57). Results are the means of three independent experiments. Alcohol Acetic acid Glycerol Residual sugar (% w/v) (g/L) (g/L) (g/L) S. cerevisiae only 10.3 0.4 4.6 – H. uvarum only 2.4 0.8 8.6 65.6 Co-inoculated 10.10.4 4.5 – Sequential inoculation 9.0 0.7 9.2 1.7 strains of the same species may behave differently in mixed culture and should be tested under fermentation conditions. Yeasts collected from a wild fermentation at a Victorian winery were selected to ferment a chemically defined grape juice. The example given here is Hanseniaspora uvarum, a common isolate from grapes and uninoculated wine fermentations (Figure 2). When grown in isolation, H. uvarum cannot complete the fermentation, but when grown with S. cerevisiae added after three days in a sequential fermentation, the sugar is utilized. Most importantly, the alcohol yield is reduced to 9% (w/v) compared with 10.3% (w/v) when the juice is fermented with S. cerevisiae alone (Table 1). Reductions in the order of 1%–2% of ethanol yield are commonly found in these experiments in our laboratories. Talking yeast All yeasts, regardless of genera, need carbon compounds (sugar) to grow, replicate and metabolise. The carbon from sugar would be channeled into cellular structures and machinery and would thus be unavailable to S. cerevisiae to convert into ethanol. Monitoring the carbon flow in these non-Saccharomyces has not been done in wine fermentation conditions, but with the technology of systems biology and metabolic engineering we now have the tools to understand how flux through the fermentation pathways occurs. An intriguing suggestion may be that mixed-yeast fermentations lead to yeasts "talking," where aromatic compounds act as signaling molecules between different yeasts. Chemical cues such as these mediate cellular behavior and metabolism in Your source for informative technical books. ORDER TODAY! www.PracticalWinery.com and click BOOKSHELF 58 p r acti c al w i ne ry & v i ne yard J U LY 20 13