The main purpose of the fermentation is to allow the yeast to convert the sugars from the wort into alcohol. Other yeast byproducts are also produced as a result of normal cell functions of survival and growth. These include fusel alcohols, esters and aldehydes which will determine the character of the beer. The different flavors are influenced mostly by the yeast strain but also by fermentation parameters (temperature, pressure, pitching rate etc…). From that point of view, trials are key to finding your perfect and unique product! Towards the end of the fermentation, the yeast will start to flocculate and accumulate at the bottom of the fermenter, even for ale strains if the fermentation is performed in a cylindro-conical vessel. This phenomenon will help greatly the clarification process of the beer. The yeast should not sit in the cone for long periods as it will likely deteriorate very quickly due to the stress imposed by the brew (alcohol, pressure etc…). If it cannot be re-used straight away, it should be stored appropriately (see section on yeast storage).
The situation is different for ale strains in open fermenters where the yeast will accumulate at the top of the vessel and should be skimmed off. When skimming yeast, the aim is to remove the yeast head at a point in the fermentation process when it is still sufficiently active and to leave just enough yeast in suspension to complete the fermentation process and form a thin but stable protective cover over the surface of the beer. If the yeast is allowed to remain on the surface of the beer until the end of fermentation autolysis may occur due to overheating. This will result in byproducts from the yeast being released and decreased viability may impact on subsequent fermentations. Additionally, the longer the yeast head is kept at ambient temperature, exposed to the air, the greater the possibility of aerial infection.
Nutrients - The use of nutrients or 'yeast food' can increase fermentation performance without impacting on the flavor profile. Nutrient addition is particularly recommended for high gravity or high adjunct brews to avoid stuck/sluggish fermentations and production of off-flavors. Essential nutrients are FAN (Free Amino Acids), vitamins, minerals, sterols, fatty acids, phosphorus etc… There are several products on the market which differ greatly in their composition. Knowledge of wort composition and nutrient deficiency is useful in order to choose a particular nutrient product otherwise trials are recommended. Zinc addition is known to improve fermentation performance even in non-deficient worts. Addition of zinc-enriched yeast (Servomyces) has been proven more efficient than mineral zinc.
Oxygen - Oxygen is important at the beginning of the fermentation when yeast cells need to reinforce their membranes in order to divide and start fermenting. Lack of oxygen can cause poor growth, poor attenuation, poor flocculation, resulting in cloudy beer and a poor yeast crop which is more likely to loose viability upon storage. Too much oxygen in contrast may encourage the yeast to divide too much at the expense of alcohol production. Too much oxygen is also believed to be damaging to the cells creating oxidative stress which will impact on fermentation performance. The ideal oxygen level to be provided to the yeast will depend largely on the strain and on the wort gravity. For a gravity of 1045 (12°P) 6 to 8ppm is recommended whereas for higher gravity wort such as 1080 (20°P) 16ppm or higher is the norm.
Growth Temperature - Temperature affects the rate of yeast growth. When the temperature is too high, yeast growth will be too vigorous, producing an excessive demand on nutrients and resulting in beer depleted of essential nutrients. This will have an effect on final conditioning. In addition to this, and probably more importantly a higher growth temperature will change the yeasts metabolism producing a different range of by products which can have a major effect on flavor. When the temperature is too low, the fermentation tend to be sluggish, resulting in an increased opportunity for contamination.