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Yeast Factors
The first step to achieving a good fermentation is to pitch enough yeast. The yeast can be grown via yeast starters or it can be harvested from previous fermentations. When yeast is harvested from a previous fermentation, it should be taken from the primary yeast cake and preferably from the upper layer of the cake or from the secondary. This yeast will have the optimum characteristics for re-pitching. In either case, you should target pitching at least 1/3 cup (75 ml) of yeast slurry to a typical 5 gallon batch of ale or 2/3 cup of slurry for lagers. For stronger beers, OG > 1.050, more yeast should be pitched to ensure optimum fermentations. For very strong beers like doppelbocks and barleywines, at least 1 cup of slurry should be pitched.
The yeast that is obtained from a healthy starter or recently from a prior fermentation will have good vitality and adapt readily to the new wort. With good levels of aeration and nutrients, the yeast will quickly multiply to the numbers necessary for an exemplary fermentation.
Wort Factors
There are two considerations that are needed to ensure that the wort has been properly prepared to support a good fermentation. The first is oxygen supplied via aeration. The role of oxygen in yeast growth will be discussed further in the Adaptation Phase section later in this chapter.
The second consideration is the level of amino acid nutrients in the wort, specifically referred to as Free Amino Nitrogen or FAN. Malted barley normally supplies all of the FAN and nutrients that the yeast need to grow and adapt to the fermentation environment. However, if the recipe incorporates large amounts of adjuncts (e.g. corn, rice, unmalted wheat, unmalted barley), or refined sugars, then the wort may not have the minimum levels of nutrients necessary for the yeast to build strong cells. It is always advisable to add some yeast nutrient powder to worts that are made exclusively from light extracts because these extracts are typically thinned with corn sugar.
In addition, brewers should be aware that in a wort that contains a high percentage of refined sugar (~50%), the yeast will sometimes lose the ability to secrete the enzymes that allow them to ferment maltose. They will adapt themselves right out of a job!
Temperature Factors
The third factor for a good fermentation is temperature. Yeast are greatly affected by temperature; too cold and they go dormant, too hot (more than 10°F above the nominal range) and they indulge in an orgy of fermentation that often cannot be cleaned up by conditioning. High temperatures encourage the production of fusel alcohols - heavier alcohols that can have harsh solvent-like flavors. Many of these fusels esterify during secondary fermentation, but in large amounts these esters can dominate the beer's flavor. Excessively banana-tasting beers are one example of high esters due to high temperature fermentation.
High temperatures can also lead to excessive levels of diacetyl. A common mistake that homebrewers make is pitching the yeast when the wort has not been chilled enough, and is still relatively warm. If the wort is, e.g. 90F, when the yeast is pitched and slowly cools to room temperature during primary fermentation, more diacetyl will be produced in the early stages than the yeast can reabsorb during the secondary stage. Furthermore, primary fermentation is an exothermic process. The internal temperature of the fermentor can be as much as 10F above ambient conditions, just due to yeast activity. Thus it is very important to keep the fermentor in the proper temperature range; so that with a normal vigorous fermentation, the beer turns out as intended, even if it was warmer than the surroundings.
Brewing in the summertime is a definite problem if you don't have a way to keep the fermentor cool. A trick is to immerse (not completely) the fermentors in a spare bathtup during the summer. The water in the tub is slow to warm during the day even though temperatures would be in the 90's, and at night the water would be slow to cool, even when the temperature dropped to 45 F. In this way your are able to moderate his fermentation temperature between 60-70 F, and the beer turned out great. I have used this method myself with wash tubs and had great success. You can periodically add ice to the water to help keep it in the right temperature range for your beer. If you don't have a way to keep the fermentater cool, and the beer temperature gets above 70-75F (21-24C), the beer will probably suffer from off-flavors, such as very fruity, phenolic or solvent-like flavors and aromas. One of the best ways to control the fermentation temperature is to buy a used refridgerator and plug it into an external temperature controller. After sanitation, fermentation tempeature control is the most important factor for producing a good fermenation and a good beer.
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