Chapter 5

Yeast and Winemaking

Wine Yeast

The most common yeast associated with winemaking is Saccharomyces cerevisiae which has been favored due to its predictable and vigorous fermentation capabilities, tolerance of relatively high levels of alcohol and sulfur dioxide as well as its ability to thrive in normal wine pH between 2.8 and 4. For these reasons S. cerevisiae is often simply referred to as the wine yeast. Today, a great variety of S. cerevisiae strains are offered by different companies for the production of wines with different sensory profiles. There are some commercial yeast preparations which include a blend of Saccharomyces and non-Saccharomyces species, referred to as mixed cultures. The non-Saccharomyces species include Torulaspora and Candida strains, amongst others, and are selected for their contribution towards wine aromatics and overall quality.

Yeast Strain Selection

Selecting the most appropriate yeast strains based on the must conditions and the type of wine style desired is an important winemaking decision. Winemakers have a wide range of yeast strains to choose from in order to achieve a desired wine style, and selecting the appropriate yeast strain is a very important consideration. There are some key factors to consider when choosing a strain. First the ethanol tolerance of the strain should exceed the projected final ethanol level of the fermentation. Second the nitrogen requirements should match the nutritional conditions of the juice. The temperature tolerance should also be considered if uniform temperature control is an issue. The compatibility of the yeast strain with the malolactic fermentation is also important to consider if malolactic fermentation is also desired. The production of specific aroma compounds is also a consideration but the ability to produce a spectrum of volatile characters is dependent upon the composition of the juice. The aromas produced will vary depending upon the levels of precursors present. For example, a thiol-enhancing yeast strain is very suitable for production of Sauvignon Blanc, whereas an ester-forming yeast strain is more suitable for production of aromatic Chardonnay. When choosing a yeast type and strain, the following properties should be considered.

Acetaldehyde Production

Acetaldehyde is a by-product of alcoholic fermentation and, as in the case of volatile acidity (VA), it can add complexity to wine when produced in small amounts. At low levels acetaldehyde can contribute pleasant fruity aromas to a wine, however, at higher levels the aroma is considered a defect and is reminiscent of rotten-apples.

Ester Production

Yeast strains should be matched to grape varieties to enable the production and release of ester compounds, higher alcohols, and the many other volatile compounds responsible for shaping the aroma and flavor profile in varietals.

Ethanol Tolerance

Yeast strains vary in their ability to tolerate ethanol levels. Some strains can tolerate alcohol levels up to 14 percent or 15 percent alc./vol. while other strains can tolerate up to 18 percent alc./vol.

Fermentation Temperature

Yeast strains vary in their temperature tolerance. Do not stress the yeast by fermenting at the upper or lower end of the recommended range (Section 6.3). Lower temperaturetolerant yeast strains are recommended for white wines and higher temperature-tolerant yeast strains are recommended for red wines.

Fruit Condition

Yeast strain choice can be optimized for unsound or spoiled fruit. Yeast strains that have a short lag phase, are low volatile acidity (VA) producers, and are positive/neutral for competitive factor, will be quick to outcompete microbial spoilers, won't contribute to VA, and won't be sensitive to microbial attack.

Hydrogen Sulfide Production

Hydrogen sulfide is an important contributor to the socalled reductive off-flavor present in some wines.

Malolactic Bacteria Compatibility

Malolactic fermentation (MLF) is an option that some winemakers choose, especially for the more robust wine styles. Malolactic fermentation is the conversion of malic acid to lactic acid leading to a reduction in acidity and the production of aroma and flavor compounds (Chapter 7).

Nutrient Requirements

Yeast strains vary in their need for yeast assimilable nitrogen (YAN) (Sections 5.6, 6.4). The amount of nitrogen a yeast will need is dependent upon its individual needs, the initial sugar level, and the temperature of the fermentation.

Sulfur Dioxide Production

The formation of sulfur dioxide is a strain characteristic with some strains producing prodigious amounts of sulfur dioxide.

Type of Wine

Each cultured yeast strain has been isolated and developed to allow specific grape varieties to express their full, individual organoleptic characteristics for a desired style of wine.

Volatile Acid Production

Volatile acidity (VA), namely acetic acid, are vital components of a wine's bouquet, but only in very small concentrations.

Volatile Thiol Production

Yeast strains differ greatly in their ability to produce flavor-active compounds. Among the positive contributors are volatile thiols, imparting tropical fruity aromas to wine (Section 1.5). Several thiols are significant aroma compounds in white wines.

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