The Beginners Brewpot

by Richard Grzelak

May 1994

Introduction to Yeast

It can be effectively argued that were it not for a simple fungus, civilization would not exist. Yeast, a simple plant, is probably more responsible for the domestication of man than any other factor besides fire. Omnipresent, it has an almost magical ability to transform grain, fruit, and other sugars into bread and fermented beverages, encouraging agriculture.

Despite the fact that it has coexisted with mankind throughout history, it was not until fairly recently identified, and its chemistry understood. Yeast was first identified by Van Leeuwenhoek in 1680, but it was Pasteur in the 1850's who first began to identify the process by which it worked.

Prior to the invention of the microscope, brewers knew that some outside agent was responsible for fermentation, but knew virtually nothing about yeast. In Britain, that agent was referred to as God is Good". The Reinheitsgebot, the Bavarian beer purity law of 1516, which defined the ingredients that could be used in the production of beer, did not originally mention yeast.

There is evidence, however, that whatever this mysterious agent was, the ancients knew that something was responsible, and primitive yeast selection is known to have occurred as early as 1440 BC.

Brewers generally divide yeast into two basic categories, top and bottom fermenting. The result of its interaction with malted grains in either case is the production of beer, however, in English, the distinction is between ales and lagers.

Generally speaking, top fermenting yeasts do their best work at higher temperatures, while bottom fermenting varieties do there best work at low temperatures. Bottom fermenting yeasts tend to work slower than top ferments, and the differences between the various substrains tend to be far more subtle than those between the top fermenting yeasts.

It should be noted that both bakers yeast, and ale yeast are classified as the same species, however, they will produce very different results in the same medium. For example, breads made with ale yeast tend to be flatter and heavier, and beers made with bakers yeast tend to be cloudy, effervescent, and somewhat vinous.

Bottom fermenting yeasts are relatively new to brewing, becoming common only in the last 125 years or so. In that short time, they have become so dominant that most people now think of lagers, particularly those of the pilsner-style, when they think about beer. All brewing nations produce some sort of lager.

Top fermented beers (ales, stouts, porters, and Weiss biers, etc.) are experiencing a resurgence in popularity after several decades of decline. In the United States, micro-brewers have probably fueled the growing demand, however major brewers such as Pabst and Miller are now producing top-fermented beverages.

The United States is also responsible for Steam Beer, a unique style, which was developed on the west coast, and basically involves fermenting lager yeast at ale temperatures. The net result is a spritzy, and somewhat fruity beer.

Yeast is available to the brewer either in dry form, or as a pure liquid culture. The primary advantage of the dry form is its convenience, one need activate it only an hour or so before pitching. The disadvantage is that it allows for the increased likelihood of contamination, either by mutant yeasts, or bacteria.

The pure culture gives the brewer greater control of the final product, and most contest winners are now made using pure cultures. The disadvantage is that the yeast must be activated several days prior to brewing, and once activated, has a relatively short useful life.

Yeast may be reused over a period of several batches, but should practically be used no more than three times. If it is used any more than that, the possibility of mutation and infection is greatly increased.

Yeast can affect the final character of the beer greatly, allowing for esters and diacetyl to form in the beer., and differences between the various strains of pure cultures can be profound. Make a large batch of wort, divide it, pitch different strains, and compare the results.