Article

A Peek into the World of the Single Celled

Fermentation is everything to beer. Without it we’d have sweet barley tea to share with friends or something to put on our pancakes. Fermentation is where the alcohol and CO2 that make beer are produced, but it is also where the chemical reactions occur that produce all the other subtle flavors and aromas that give beer its complexity. And it is where many off-flavors are created that ruin beer.

Fermentation actually consists of three distinct but often overlapping stages: the initial stage, yeast cell- growth stage, and fermentation. The initial stage is often referred to as the lag phase, though the lag phase also overlaps the cell-growth stage. The lag phase technically describes the time immediately after pitching when the yeast adapts to the new conditions of its environment and no activity is seen. The lag phase is the period during which the wort is most susceptible to contamination and spoilage by bacteria. Minimizing lag time, therefore, is of the utmost importance to a clean, healthy fermentation.

The initial stage begins when yeast is pitched into the wort. Because the yeast now finds itself in a new environment, it must adjust and prepare itself for the work of fermentation. The initial stage is characterized first by yeast cell wall preparation and then uptake of oxygen, nitrogen, and sugar.

Cell wall preparation consists of the secretion of enzymes needed to make the cells permeable to the sugars and other compounds that will eventually feed it. The cells need an initial store of energy in the form of glycogen and adequate dissolved oxygen to ensure the permeability of the cell walls. If the pitching yeast has been starved, therefore, the glycogen reserves will have been depleted and a long lag time will ensue. With healthy yeast cells and proper pitching rates, temperature, and aeration, the initial stage should be very short, often just a few hours. When the cell walls have been prepared, nitrogen and sugars can be transported into the cell and rapid growth begins.

This stage of yeast cell growth is often mistakenly referred to as “respiration.” Respiration (cell growth in the presence of air, or aerobic, which yields CO2 and water) and fermentation (cell growth in the absence of air, or anaerobic, which yields CO2 and ethanol) are the two ways in which yeast cells can use sugars to produce energy. However, a law called the Crabtree Effect dictates that respiration does not occur with brewing yeast in wort that has a glucose content of greater than 0.4 percent w/v (weight/volume). Since all wort will have a greater glucose concentration than 0.4 percent, respiration cannot occur in brewing. Instead, yeast cell growth in the presence of air is a highly complex stage wherein yeast use up all of the oxygen in the wort for energy buildup and reproduction. Though the yeast is technically fermenting and giving off small amounts of CO2 and ethanol, the majority of its efforts in the presence of oxygen produce not ethanol but sterols and fatty acids that help build cell walls.

Yeast cells multiply by “budding,” which is a form of cell division, and they require strong cell walls to facilitate budding. This stage also marks the beginning of the acidification of the wort. Rapid yeast growth combined with acidification will begin to inhibit any possible bacterial contamination.

During the cell growth phase wort sugars are brought into the cell and broken down into glucose. Glucose in turn is broken down into pyruvic acid. Thus begins a series of enzymatic reactions that yields metabolic energy and CO2, and begins the formation of a number of important and largely unwelcome by-products such as diacetyl (buttery) and fusel alcohols (medicinal). The production of these by-products is greatly affected by temperature. The lower the temperature, the fewer by-products will be produced. Therefore, with a healthy yeast culture and a rapid initial stage, it is crucial that the temperature during cell growth be maintained at the optimum for yeast performance. With most strains this is 65° to 70° F for ale yeasts and 48° to 54° F for lager yeasts.

Fermentation begins when yeast has used up all of the oxygen in the wort. It is very important, therefore, never to introduce any air into the wort once fermentation has begun. Fermentation makes use of a bio-chemical pathway called the E-M-P (Embden-Meyerhof-Parnas) pathway. The pathway begins with pyruvic acid (the same pyruvic acid created by the breakdown of glucose during the cell-growth phase) that is broken down into acetaldehyde and then to ethanol (alcohol). This process generates CO2.

The breakdown of glucose into pyruvic acid and then the route of the E-M-P pathway is the most common pathway that fermentation will take under normal conditions. However, there are branches at the individual steps of the pathway that lead to other, minor pathways that the yeast may use. These minor pathways give rise to fermentation by-products such as diacetyl, esters, fatty acids, and fusel alcohols. Likewise, bacteria will make use of wort materials using entirely different pathways that lead to the formation of the compounds that cause beer spoilage. Fermentation ends when the vast majority of sugars are used up and the yeast begins to settle (flocculate) at the bottom of the fermenter.

Keeping By-Products in Check

The formation and reduction of unwanted by-products in beer is directly related to the sound fermentation preparation techniques of pitching rate, aeration, and temperature control.

Diacetyl is characterized by a buttery or butterscotch flavor and aroma and is desirable in small amounts in certain styles of beer, notably English-style ales. It is unacceptable in any amount in lagers. Diacetyl has a very low flavor threshold — that is it can be detected in very small amounts (as low as 0.1 part per million).

Compounds created from pyruvic acid and acetaldehyde during fermentation pass through the yeast cell wall and combine with oxygen to create diacetyl. Diacetyl production generally occurs during the cell-growth stage, therefore, and because there is oxygen available, its production cannot easily be avoided. However, all yeasts have enzymes that can reduce diacetyl to harmless compounds. These enzymes work anaerobically during fermentation. Therefore, given the proper amount of time at the right temperature and barring any new introduction of oxygen, yeast will clean up most of the diacetyl in the beer.

Also, diacetyl formation is inhibited by lower temperatures during the cell-growth phase. This is another reason to keep the temperature at the lower end of the working range at the beginning of fermentation. Some lager brewers begin their fermentations at the low temperature and then raise the temperature toward the end of fermentation to allow the yeast to uptake any diacetyl. This is called a diacetyl rest. Ale brewers often hold their beer at fermentation temperature for at least 24 hours after fermentation is complete to ensure the same diacetyl reduction.

The formation of fusel alcohols, fatty acids, and esters are all encouraged by high fermentation temperatures. Esters are compounds formed from an acid and an alcohol, and have strong fruity aromas. Some esters are appropriate to give complexity to most ales, particularly strong ales. But excessive ester formation can give unpleasant aromas such as banana or pineapple.

Esters are formed from fatty acids that are used during the cell-growth stage, in the presence of oxygen, to help build up cell walls. If the wort is not aerated properly, fatty acids will not be used to build cell walls but instead will become attached to fusel alcohols to form esters. High-gravity beers hold less oxygen than regular-gravity beers, so ester formation is generally much higher and to some extent expected.

To keep esters, fusel alcohols, and fatty acids down, worts must be aerated thoroughly, especially high-gravity worts, and temperature must be kept low.

Preparation: Pitching Rate

Proper preparation of yeast and wort for fermentation offers the greatest amount of fermentation control. Finding a good yeast strain that is appropriate for the style of beer you will be making and is clean, fresh, and healthy is the first step. The most important step in yeast preparation is making certain your pitching rate (amount of yeast you are adding) is high enough. High pitching rates will solve many problems of fermentation and are the single easiest way to improve fermentation.

In commercial breweries there is a standard rule that yeast should be pitched at the rate of one million cells per milliliter of wort per degree Plato (four to 4.5 specific gravity points).
This is an amount that is difficult for homebrewers to achieve without culturing up large amounts of slurry. A smaller homebrewing rate is therefore generally agreed upon. There are three main sources of yeast for homebrewers: dry yeast, pure culture yeast (liquid smack pack or slant), and previous batch slurry or slurry from a local brewpub or microbrewery.

Dry yeast contains a high number of cells per gram and so under perfect conditions would provide an ideal pitching rate. However, cell viability (number of live cells) can be very low in dried yeast due to drying, packaging, and storage conditions. Also, dry yeast are more susceptible to being contaminated with bacteria or wild yeast. Dry yeast should be pitched at the rate of 0.5 grams per liter, or about 10 grams for a five-gallon batch of normal-gravity wort. If you are concerned about the yeast’s viability, pitch more.

Dry yeast should be properly hydrated before pitching. Hydrating dry yeast is basically the same as proofing yeast for baking. Mix the yeast into a pint of 90° to 110º F water and let it stand for 15 minutes. Cool the slurry to pitching temperature, and pitch. Pay close attention to good sanitary techniques using sanitized bowl, water, mixing spoon, and so forth.

Pure culture yeast is widely available in the form of smack packs. If you maintain a yeast bank or prefer to buy yeast slants, they will also be pure cultures and will be pitched at the same rate as smack packs. Smack packs that have been activated and have bulged to their capacity should contain about 2.5 billion cells. This is about 100 times less than the million-cell-per-milliliter-per-degree-Plato formula. Therefore, just the contents of a smack pack do not provide enough cells for a quick, strong fermentation.

For ales, smack packs and slant yeast should be grown into at least a one-pint starter, although a one-quart starter would be better. Lager yeasts should be grown into at least a one-quart starter and into two quarts if possible. For homebrewing purposes these rates, though still about 10 times lower than the commercial pitching rate, will give excellent results.

With the proliferation of brewpubs today, many homebrewers may pop into their local brewery and ask the brewer for a bit of slurry. Alternatively, if you brew often enough, you can re-pitch yeast collected from the primary or preferably secondary fermentations. These slurries will generally be healthy and viable and provide the ideal commercial pitching rate.

If you are re-pitching from your own fermentations, you can collect the yeast from the bottom of the fermenter and store it in a sanitized jar for a week or 10 days without losing much viability. Any longer than that and the risk of yeast autolysis (cell death and self-consumption) and loss of viability are too great and the yeast should be discarded. Liquid slurries should be pitched at the rate of one fluid ounce per gallon. Remember that high-gravity beers will need a higher pitching rate, but severe overpitching of yeast can lead to harsh, yeasty flavors and accelerated yeast autolysis.

Preparation: Aeration

Wort aeration is the second key step to fermentation preparation. As we have seen, oxygen plays an important role throughout the fermentation process. Studies have shown that excessive wort oxygenation is difficult to achieve even with pure oxygen and that worts will reach a maximum oxygen saturation level, after which more O2 just won’t dissolve. So shake away at those carboys as long as your arms hold out.

One good way to aerate is to attach a strip of metal, such as the pocket clip from a ball point pen (make sure it’s clean and sterilized), to the end of the racking hose. The chilled wort then sprays out into the fermenter and picks up lots of air. Be sure to give the fermenter several minutes of good shaking afterward just to be sure. There are also aeration systems on the market that use a small oxygen stone and pure oxygen tanks. A 30-second blast from one of these systems will do the trick handily.

Higher gravity worts cannot hold as much O2 in solution, and therefore they must be particularly well aerated.

And Don’t Forget…

There are a few additional points to remember about fermentation preparation. Yeasts that are cultured from pure cultures should be grown in a wort that is as similar as possible to the wort into which the yeast is being pitched. The more adaptation the yeast needs to undergo at pitching, the longer the lag time and the greater the loss of viability.

Sugar should never be used as a culturing medium because of the changes the yeast will undergo. Proper wort composition relying on a malt base with a low proportion (less than 10 percent) of sugars should be observed.

Temperature control is an important tool to coerce the yeast into doing their work the way you want them to do it. Brew when ambient temperatures permit or employ some system of temperature control. Temperatures that are too high or too low or fluctuate greatly will undermine all proper preparations you have made.

Remember that yeast will do all of the hard work for you. If you treat them right and provide a good home for them to live in, they will reward you with great beer every time.

Issue: December 1997