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Beer Fermentation
Beer Yeast
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 wort gravity, wort constituents, fermentation temperature, degree of wort aeration, fermentation capacity of the yeast, yeast viability, flocculation characteristics, and previous history of the yeast. For example, highly flocculent yeast strains may settle prematurely, requiring the brewer to either overpitch or to mix and aerate by "rousing."
Low pitching rates can result in long lag and reproductive phases. This, in turn, can increase the production of byproducts such as higher alcohols, esters, and diacetyl. Furthermore, since many sugars are not metabolized with low pitching rates, the resulting beer tends to have a residual sweetness (lower attenuation). Insufficient yeast growth can eventually lead to slower and even stuck fermentations. Poor yeast growth can also lead to high sulfur dioxide levels (35).
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. However, once the yeast inoculum is added, the selective effect against microorganisms other than brewing yeast begins. This is due to falling pH, the antimicrobial effect of hop compounds, the developing anaerobic conditions in the wort, and the production of carbon dioxide and ethanol (5).
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