Yeast Strain Selection
Selection of a yeast strain with the required brewing characteristics is vital from both a product quality and economic standpoint. Yeast strains are selected for the flavor characteristics they impart to beers and for other desirable fermentation characteristics. Some of these characteristics are suitable flocculation properties, genetic stability, suitable attenuation of wort carbohydrates (particularly maltotriose), acceptable foam characteristics in the beer produced, and interaction with fining materials for effective filtration. The criteria for yeast selection will vary according to the requirements of the brewing equipment and the beer style, but they are likely to include the following:
Attenuation refers to the percentage of sugars converted to alcohol and carbon dioxide, as measured by specific gravity. Most yeasts ferment carbohydrates—glucose, sucrose, maltose, and fructose. To achieve efficient conversion of sugars to ethanol (good attenuation) requires the yeast to be capable of completely utilizing the maltose and maltotriose.
The yeast strain itself is a major contributor to the flavor and character of the beer. Different strains will vary markedly in the byproducts they produce: esters, higher alcohols, fatty acids, hydrogen sulfide, and dimethyl sulfide. Thus, the choice of yeast strain depends on such things as the oxygen requirements, cropping methods, attenuation limits, fermentation rate, fermentation temperatures, flocculation characteristics, and the flavor profile (e.g., sulfur compounds, esters, fusel alcohols, etc.).
Degeneration of Yeast
Yeast degeneration refers to the gradual deterioration in performance of the brewing yeast. Yeast degeneration has a harmful effect on the course of brewing fermentations. It is characterized by some of the following symptoms: sluggish fermentations, premature cessation of fermentation (resulting in high residual fermentable levels in beer), gradual lengthening of fermentation times, and poor foam or yeast head formation (Piesley et al., 1977). Some brewers have noticed that the flavor of beer becomes increasingly “dry” as a result of yeast degeneration.
The flocculation characteristics of yeast are of great importance. The term “flocculation” refers to the tendency to form clumps of yeast called flocs. The flocs (yeast cells) descend to the bottom in the case of bottom-fermenting yeasts or rise with carbon dioxide bubbles to the surface in the case of top-fermenting yeasts. The degree and type of flocculation varies for different yeasts. For example, highly flocculating yeasts will tend to settle and clump together very quickly and fail to complete fermentation resulting in a beer that is under attenuated and sweet. Highly flocculating yeasts may need to be re-suspended in the beer (“roused”) to complete the fermentation cycle.
Mutation of Yeast
Yeast mutations are a common occurrence in breweries, but their presence may never be detected. Usually, the mutant has no adverse effect since it cannot compete with normal yeast and generally disappears rapidly. In some cases, though, mutant yeast will overcome the normal brewing yeast and may express itself in many different ways. For example, a mutation could affect the fermentation of maltotriose, or there could be a continuous variation in the fermentation rate. Reportedly, lager yeast mutates more rapidly than ale yeast (Maule, 1980).
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