Yeast cells. Color-enhanced scanning electron micrograph (SEM) of cells of brewer’s yeast (Saccharomyces cerevisiae). This fungus consists of single vegetative cells. Small daughter cells can be seen budding off from the larger cell. Once they have reached their full size, the daughter cells will be indistinguishable from the mother cells. S. cerevisiae ferments sugar, producing alcohol and carbon dioxide in the process. It is used in the production of beer, wine and bread. Magnification: x6250 when printed 10 centimeters wide. Copyright © Alpine Events South Africa
Brewer’s yeast is a single celled fungus called Saccharomyces cervisiae (ale) or uvarum (lager), whose purpose is to duplicate and consume a substrate by fermenting this “food” into alcohol, carbon dioxide, and over 600 other compounds. Because they process the wort, they also change the way it tastes and smells. For instance, blueberries that are fermented don’t take exactly like blueberries. The same is true or hops and malt. Yeast absorbs iso-alpha acids from hops and metabolizes malt. Yeast provides the overall final flavor and aroma of beer.
There are two kinds of yeast the brewers use: dry and liquid. Dry yeast comes in several varieties and does not need a starter to pitch. It already contains enough yeast to effectively ferment a 5 gallon batch. However, it should to be hydrated before pitching.
To rehydrate dry yeast:
Liquid yeast comes in several dozen varieties. More specific styles of beers can be made with liquid yeast. To improve fermentation performance, it should be “woken up” by making a starter. A yeast starter is important to make sure yeast is viable and that there are enough cells to pitch. Using a stir plate will significantly improve the effort of creating a starter. With a stir plate, 75 million yeast cells can become 260 million in a 2 liter flask. Without a stir plate that number drops to 160 million.
The standard homebrewing pitching rate of yeast is stated as: .75 million cells per milliliter per degree Plato. There are 18,927 ml in 5 gallons. Wort at 10 Plato (1.040) should have 142 million cells (18927*.75*10=141,952 [move the decimal 3 places = 141,952,000 yeast cells]). A brand new, fresh, and well-handled yeast tube from White Labs contains 100 million cells. By the time you get it, it isn’t 100% fresh any more. It loses about 20% viability per month if it was stored and handled correctly. Let’s say you get your vial one month after it was prepared at the factory. It now contains about 75-80 million cells. That is only enough for 5 gallons of wort at 5˚Plato (1.020). So what? The yeast is going to duplicate and convert the sugars to alcohol and CO2. The problem isn’t the conversion; it is what the final product will taste like. In these conditions, yeast is stressed. They must duplicate by budding more than is desired to make great tasting beer, and in doing so produce more esters and higher alcohols. Also, they will do a poor job of cleaning up byproducts, finish with a higher final gravity, and a possible stuck fermentation. An exception for this is when making Germany Wheat or Belgian beers where the yeast ester profile is very desirable. These yeasts are pitched on the low side to encourage their desired flavor profiles.
Temperature plays a significant roll. Concerning ale yeast, too cold (below 55 F) and the yeast stop fermenting. Below 60, it will be slow and not produce the flavors that enhance our beer. Too hot (over 75 F) and yeast will work quickly, but give of too many esters, byproducts, and higher alcohols. In fact, the prime temperature for yeast growth is 90 F, but that would not make very good beer. The sweet spot for most ale yeast fermentation is 66-71 F. In this range, yeast duplication will be quick but controlled and produce the ethanol and esters that we want in our beer. The best way to ferment most ale yeast is to start at 66 F and let it rise to 68 F over three days. Then slowly rise to 71 F over the next week. This higher temperature encourages the yeast to speed up, absorb diacetyl, and flocculate.
When yeast is first pitched in wort it will consume most of the oxygen in about 30 minutes. Over the next 1-15 hours it will duplicate and adapt to the environment of the wort. This is the lag phase. Once the cells have built up their membranes, the cell walls become permeable and start eating the “food” in the wort. At 4 hours to 4 days, the yeast are in their budding or duplicating phase. This is part of the next phase: fermentation. At this point, the yeast consumes the sugars converting them to alcohol and carbon dioxide and hundreds of other compounds. This will take place over 1-5 days or more. At 3-10 days, the conditioning phase starts. During this time the yeast will slowly consume the remaining sugars, flocculate, reabsorb byproducts, and slow down to minimal activity. Each of these phases don’t just start where the last one stopped. As one phase slows the next one begins and so on.
The conditioning phase is often neglected. Sometimes beers are racked into bottles to early. This can often lead to buttered popcorn flavors (diacetyl), over-carbonated beers, or even beer bombs. These are the more obvious issues, but what about a more delicate issue – flavor.
Many brewers say that fermentation is done 2 days after the final gravity is reached. This usually takes about 10-14. However, there are still some chemical reactions happening that we cannot see in the air lock. I do not have the details, but I do know from experience that beers left in the fermenter for an extra week or two turn out much better tasting. What about autolysis (yeast flocculate and over a period of time die and release undesirable compounds into the beer)? Autolysis is not a concern until more than 5-6 weeks (high gravity beers could cause autolysis a little earlier due to the higher alcohol). Any longer and the beer needs to be racked off the yeast, then it can be conditioned for a much longer time in a secondary fermenter. In surveys that were attempting to better define autolysis, beer that was left in the fermenter was actually preferred over younger beer. I know a homebrewer of 40+ years that will not keg or bottle beer until it has been in the fermenter for at least a month. The only exception I can think of is small beers under 1.040. Their flavor tends to fade over a few weeks, so its best flavor is when it’s young – let’s say 2 months from brewing. As always, there are exceptions.