ine is not a natural product, but vinegar is,” said Professor Isak Pretorius (managing director of the Australian Wine Research Institute) at a recent wine microbiology symposium. This reminds us of two elemental facts. First, wine is the complex result of microorganisms interacting with their environment. Second, the control of this process by the winemaker helps assure that wine can be enjoyed on a global scale.

Though the wine and grape matrix is often described as providing a difficult environment for the growth and survival of microorganisms, a specific and very distinct group of microorganisms is essential to the production of highquality wines. The yeast Saccharomyces is one of those organisms. Its ability to proliferate and implant into the matrix changes grape must into wine.

Due to the consistency of performance, Saccharomyces cerevisiae is generally the organism of choice to complete the alcoholic fermentation process. Enological strains of

Saccharomyces spp. have been available in the active dried form for more than 40 years. Great strides have been made from a technological viewpoint.

Due to the natural diversity that has brought us to where we are today, we can separate enological yeast strains into two groups: primary strains and specialty (or technological) strains. Primary strains such as EC-1118™ and BC™ can complete alcoholic fermentation with minimal sensory deviations. Specialty strains such as RC212®, QA23™, and ICV D-254™ have supplemental abilities in addition to their secure fermentation performances, which may include the revelation of varietal aromas, production of fermentation aromas, action on the wine texture via the production of polysaccharides, or the stabilization of color (via polyphenolic-polysaccharide interactions).

Irrespective of strain choice, however, good fermentation practices must be followed. This includes applying appropriate nutritional strategies and controlling the fermentation

temperature (thereby minimizing the production of undesirable yeast metabolites).

Choosing a yeast strain for a successful fermentation includes knowing some basic parameters about your grape must and extrapolating details about the resulting wine (potential alcohol). This initial decision of whether the yeast strain can withstand the alcohol that is to be produced is important, as are the temperature constraints of the yeast strain, its nutritional requirements, lag phase length, competition with other organisms, and rehydration specifics. It is also important to determine whether the yeast strain promotes a healthy malolactic fermentation.

Malolactic fermentation (MLF) is the conversion of L-malic acid to L-lactic acid by lactic acid bacteria, namely Oenococcus oeni. Strains of Lactobacillus plantarum (V22™), however, are also available for this conversion in high pH situations (above pH 3.5, with no volatile acidity increase from grape sugar metabolism).