Wild yeasts help create volatile compounds in wine fermentation.

As winemakers strive continually to make better and more complex wines, more attention is being given to the type of yeast used for fermentation. The wild yeasts found in fermenting wine each have different reaction rates and will produce different end products as they ferment the juice sugars to alcohol. Typically, Chardonnay will ferment much more slowly with wild yeasts than with one strain of cultured yeast.

With a slow, lengthy wild yeast fermentation, it is more difficult to predict the outcome, positive or negative, because the winemaker does not know which yeast strains are active in the fermentation process. For example, one of the strains could begin producing bad odors, or all the strains could die off before enough of the sugars have been consumed, leaving a too-sweet wine. In addition, each barrel of fermenting wine will undergo a unique individual fermentation process, resulting in significant variation between barrels.

Scientists in Spain wanted to see how extensively wild yeasts contributed to the volatile composition of wine in inoculated fermentations. To do so, they used Parellada, the finest and most delicate of the traditional Catalan white varieties. When grown in cool mountain microclimates (Upper Penedes), it produces aromatic dry white wines, light and with delicate fruity aromas.

The researchers first sterilized and inoculated Parellada with Saccharomyces cerevisiae strain Na33, a pure inoculated fermentation. They used an inoculated Parellada must, generated from a mixed inoculated fermentation, and the Parellada must that fermented with its wild yeasts--the control fermentation.

From the results obtained in the pure inoculated fermentation, the scientists saw that S. cerevisiae produced appreciable quantities of isoamyl acetate, ethyl hexanoate, ethyl octanoate and ethyl decanoate. However, the wild yeasts also contributed to the synthesis of esters since the total concentration of these substances was higher in the mixed inoculated fermentation than in the pure inoculated fermentation.

2-phenylethyl acetate was only synthesized by wild yeasts when they did not compete with S. cerevisiae. The concentration of total alcohols was similar in the three samples. The important production of isobutanol and 2-phenylethanol in the control fermentation is noteworthy, we're told.

Regarding the acids, the greatest concentration corresponded to the mixed inoculated fermentation. The wild yeasts contributed to the synthesis of these compounds to a significant extent, and S. cerevisiae synthesized appreciable amounts of short-chain fatty acids.

Further information. Carmen Ancin-Azpilicueta, Department of Applied Chemistry, Universidad Publica de Navarra, Campus de Arrosadia s/n, E-31006 Pamplona, Spain; phone: [phone]; fax: [phone]; email: [e-mail].