Understanding Yeast

Yeast History

"God is Good" is the name which yeast were given in the early days of brewing. This is prior to the time which Louis Pasteur, in the mid 1800's, discovered that, in fact there was actually a single cell microscopic organism responsible for the conversion of fermentable barley malt sugars into alcohol, carbon dioxide, and flavor compounds. No longer was the process an unexplainable phenomenon, but a scientific process which we all can be grateful for.

Yeast and Brewing

Yeast, Saccharomyces cerevisiae, and Saccharomyces uvarum are the genus and species of ale, and lager yeast respectively. These are the primary types of yeast cultures which produce most of the worlds beers. Other types of yeast and bacteria are also utilized in various styles of beer and brewing beer like beverages. Many of these organisms were discovered more by chance, than by design. Beverages including wine, fermented milk products, and mead from honey are some examples of what developed from spontaneous fermentation, which is now understood and managed in a scientific manner.

Yeast cells are round, or ovate and reproduce by multipolar budding. They are approximately 6-8 microns in size. The characteristics which are beneficial to brewing are flavor production, the ability to attenuate the wort, and their flocculating nature, or how well they settle out and clear the beer after fermentation is complete.

As described by Gay-Lussac at the beginning of the nineteenth century, the chemical reaction of fermentation is as follows:

C6H12O6 + Saccharomyces cerevisiae = 2C2H5OH + 2CO2

(sugar plus yeast yields alcohol and carbon dioxide)

Yeast and Their Flavor Affects in Beer

Around the world throughout the centuries, yeast cells, like most living things, have evolved and have adapted to their environment in order to survive.

Due to this mutation and adaptation, subspecies, or variations of yeast have evolved due to the specific climate, and food sources. For this reason we are fortunate to have a wide variety of yeast strains today. These strains can be identified by their fermentation characteristics and selected to produce certain beer styles.

Ale yeast ferment at warmer temperatures than will lager yeast. In turn, Ales typically become fruitier, softer and more robust than lager beers. Lager beers tend to be dry, crisp and steely, the classical example of this is the Pilsner Lager Beer. By selecting specific yeast, one can emphasize maltiness, certain fruity esters, hop character, and a number of other fermentation characteristics desirable for a given beer.

In addition to the Ales and lagers of the world, wheat beers, and lambic style beers utilize other relative yeast strains and bacteria which impart unusual flavors and aromas such as apple, banana, plum, apricot, bubble gum, and even sweaty horse hair. These flavor and aroma compounds are an essential aspect of certain beer styles. Without these rare types of cultures which have evolved through time, the dimension and diversity of beer styles would be greatly limited.

By selecting specific yeast strains and providing a certain environment, the brewer creates the beer style of choice. These parameters include the type of water, whether hard or soft, the variety of malts, whether malted lightly or dark, and the choice of hops, whether bitter or aromatic. Taking these components and controlling the environment, the brewer puts their signature on the beer with the silent regimen of the hard working yeast.

Cultivation and Maintenance of Yeast

Yeast cells are maintained with two basic parameters; feed them so they don't die, and keep them clean, or free of contamination by other organisms. By doing this, along with selective culturing, the identity and character of each specific strain can be maintained. This process requires dedicated microbiologists in an environment free of contamination. There is an ongoing process which involves the storage, reviving, testing, and cataloging data which is collected as the identifying characteristics.

Yeast can be stored or maintained on agar slants of food nutrient to allow continued survival of the organism. Yeast can be dried or freeze dried for prolonged shelf storage, or they can be frozen in certain mediums or held under refrigeration in liquid suspension. All methods have benefits and drawbacks. It is the scientists and brewers job to control the parameters which effect the quality of the yeast and their survival.

Three key instruments in this process, are the autoclave, incubator and microscope. The use of these tools and culture medium conducive to yeast growth, allow's one to take care of their yeast. In addition basic laboratory supplies and glassware are used.

The autoclave is a pressure cooker or sterilizer. This equipment is used to sterilize the growth medium and instruments which come in contact with the yeast. This is how the yeast is kept clean. The sterilization of the medium kills any of the many potentially contaminating bacteria and organisms which abound. The autoclave produces steam heat to 250 degrees fahrenheit at 15 pounds of pressure to produce the sterilizing effect, or to kill anything in it. The growth medium and equipment having been processed in the autoclave, are ready to be introduced into the yeast environment aseptically. Note, that the yeast cannot be autoclaved, or they too would die.

When the proper medium is prepared, and yeast aseptically transferred to the medium, the suitable temperature which is conducive to rapid or exponential yeast growth is necessary. This is achieved in an incubator. An incubator is a controlled environment, which provides the optimum temperature. It minimizes temperature fluctuations and extremes. Temperatures too cold or too warm will inhibit growth or even kill the yeast.

The microscope is beneficial to confirm the presence of the desired organism. Single yeast cells cannot be seen by the naked eye. Magnification by several hundred times, to 10,000 times, by the microscope, can bring you into the microscopic world of the unknown. It can also be used to identify potential contaminating intruders. Depending on the magnifying strength, the microscope can bring to life, vivid morphologies, which help identify one organism from another.

Yeast cells 10,000x magnified

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