Yeast Strain Selection
Selection of a yeast strain with the required brewing characteristics is vital from both a product quality and economic standpoint. 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:
• Rapid fermentation
• Yeast stress tolerance
• Rate of attenuation at the desired temperature
• Beer flavor
• Good yeast storage characteristics
• Stability against mutation
• Stability against degeneration
A rapid fermentation without excessive yeast growth is important, as the objective is to produce a beer with the maximum attainable ethanol content consistent with the overall flavor balance of the product.
Yeast Stress Tolerance
The yeast strain should be tolerant to alcohol, osmotic shock, and temperature. Another stress point for yeast can be the collection, separation (centrifuging/pressing), and transfer (pumping) throughout the plant.
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 flocculation characteristics need to be matched to the type of fermentation vessel used-a strongly cropping strain will be ideal for skimming from an open fermenter but unsuitable for a cylindroconical fermenter.
Rate of Attenuation
Attenuation refers to the percentage of sugars converted to alcohol and carbon dioxide, as measured by specific gravity. Most yeasts ferment the sugars 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. Brewing yeasts vary significantly in the rate and extent to which they use these sugars.
The selection of the yeast strain itself is perhaps one of the most important contributors to beer flavor. Different strains will vary markedly in the byproducts they produce: esters, higher alcohols, fatty acids, hydrogen sulfide, and dimethyl sulfide. The yeast strain must also be capable of reproducible flavor production.
The storage characteristics of a yeast are very important for maintaining viability during storage between fermentations and rapid attenuation when repitched.
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.
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 (50). Some brewers have noticed that the flavor of beer becomes increasingly "dry" as a result of yeast degeneration.
Click on the following topics for more information on brewer's yeast.