Beer Yeast and Fermentation

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If You Want to Make Good Beer, You Gotta Use Good Yeast

Homebrew School: Yeast and Fermentation

Yeast is one of the four primary ingredients in beer (the other three are malt, hops, and water). Although yeast is an ingredient that the average beer consumer rarely contemplates, it’s often considered the most important ingredient by brewers. As a matter of fact, yeast can have a greater influence and effect on the finished beer than any other single ingredient.

Yeast is a member of the fungus family. It’s a living single-cell organism and one of the simplest forms of life. Because it has cell-splitting capabilities, it is also self-reproducing Yeast is the one ingredient responsible for carrying out the fermentation process in brewing (see more on fermentation below).

Yeast Genus

Of great importance regarding yeast is that beer styles are categorized and classified by the type of yeast used to ferment them. Brewmasters choose a yeast to ferment their beer according to the style they want to make. Beer yeast is classified into two categories, or species, of the genus Saccharomyces: s. cerevisiae and s. uvarum.

The former is commonly known as ale yeast and can be divided into many sub-strains. It is considered a top-fermenting strain, meaning it floats on the top of the beer during active fermentation. The latter is better known as lager yeast and it can also be divided into many sub-strains. Lager yeast is considered a bottom-fermenting strain, meaning it sinks to the bottom of the fermentation vessel during active fermentation.

Dry Yeast vs. Liquid Yeast

There are many inexpensive and easy-to-use dried beer yeasts in the homebrewing market. Simply re-hydrate the dry yeast in warm water before pouring it into the wort (unfermented beer). Unfortunately, for all their ease-of-use, dry yeasts are fairly generic in the beer they produce Homebrewers who are intent on reproducing their favorite commercial style of beer would be best served using the more style-specific (and slightly more expensive) liquid yeasts that are available in a very wide assortment of beer styles. Liquid yeast cultures are considered “ready-to-pitch”; no re-hydration is necessary.

The Magic of Fermentation

Fermentation is nature’s way of performing magic. Yeast cells consume sugar in liquid form and, in turn, excrete alcohol and carbon dioxide in addition to hundreds of flavor compounds. As part of the growth process, a single cell reproduces by cloning itself; splitting into two separate cells. Multiply this by billions and you have fermentation.

The fermentation cycle –though it’s not obvious to the average homebrewer, has three distinct parts:

  • Respiration- the yeast absorbs as much oxygen in the wort as possible in order to complete the metabolic processes involved in fermentation.
  • Active fermentation- the bulk of the cycle, during which the yeast is consuming and excreting. The yeast cells remain in suspension for maximum contact with the liquid sugars.
  • Flocculation- when the liquefied sugars are depleted and the yeast cells begin to clump together, fall out of suspension and prepare for a state of dormancy.

Yeast Type and Fermentation Temperature

The temperature at which beer ferments can have a great effect on the finished product. The top-fermenting ale yeast strains can complete their gluttonous feast in as little as three days. This quick, warm fermentation has a tendency to give the resulting beer a rich and complex aroma and flavor profile. Ales are said to be fruity and estery, often full of buttery or butterscotchy notes.

Bottom-fermenting lager yeast strains can take up to several weeks to completely ferment a brew, and the long cold fermentation results in a very mellow beer, lacking in the fruity, estery character of most ales. These are examples of the profiles to be expected of the combination of yeast type and fermentation temperature.

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