Chapter 13

Beer Fermentation

Pitching Beer Yeast

Strain

The yeast strain itself is a major contributor to the flavor and character of the beer. 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 (sulfur compounds, esters, fusel alcohols, etc.).

Pitching Rates

The brewer's ability to pitch the correct number of yeast cells to initiate fermentation is crucial to consistently producing a product of superior and constant quality. Pitching rates are governed by a number of factors, including yeast strain, fermentation capacity of the yeast, yeast viability, flocculation characteristics, previous history of the yeast, and desired beer flavor characteristics. Other considerations when choosing pitching rates include wort gravity, wort constituents, fermentation temperature, and the degree of wort aeration. For example, highly flocculent yeast strains may settle prematurely, requiring the brewer to either over-pitch or to mix and aerate the yeast by "rousing."

Viability

The pitching rates should be adjusted to account for the number of viable cells rather than the total number of cells. Dead cells can be determined by staining with methylene blue and counting with a hemacytometer.

Methods for Determining Pitching Rates

Traditionally, brewers determined pitching rates on the basis of either the volume or the weight of yeast slurry to a known quantity of wort. Today most breweries determine pitching rates by a number of alternative methods including the use of a counting chamber (hemacytometer), centrifugation of the yeast slurry, electron cell counting, or of in-line biomass sensors.

Weight-Based

A weight-based pitching is preferable to volume-based since volume varies with temperature, carbonation, liquidity of the slurry, and degree of yeast flocculation (31).

Hemacytometer

A better method in estimating cell concentration is to use a hemacytometer, which is a specialized slide with a counting chamber of known volume.

Centrifugation

Another simple method is to centrifuge a sample to determine the ratio of solids in the sample thus the correct amount of yeast to pitch.

Electron Cell Counting

Although electron cell counting is very accurate, it records budding cells, chains of cells, and non-yeast aggregates as single cells (64).

In-Line Biomass Sensors

More recently, commercially available in-line biomass sensors have become available.

Dosing

Yeast can be directly added to the fermenters (batch), mixed with the wort in a starter tank prior to transfer to the fermenter, or continuously metered into the cold, aerated wort stream on the way to the fermenter.

Microbiological Contamination

Uninoculated wort is liable to contamination by many types of mold; bacteria such as Pediococcus, spp. and Lactobacillus spp.; and wild yeasts such as Hansenula, Dekkera, Brettanomyces, Candida, and Pichia. Other Saccharomyces species may be present.

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