Yeast is added to the wort to induce alcoholic fermentation. Using an enzyme complex, called zymase, they break down glucose, fructose, sucrose and maltose contained in the wort for ethyl alcohol and carbon dioxide, also creating a small amount of volatile and non-volatile by-products. Special breeds of yeast are used in brewing, the so-called. brewer's yeast, unlike wine and distiller's yeast.
Drożdże pod względem botanicznym należą do grupy grzybów. Dzieli się je na dwie podgrupy: spore-forming and non-spore-forming.
Brewer's yeast belongs to the second subgroup. They are included in the Saccharomyceteae family, to the species Saccharomyces.
Saccharomyces yeast always causes an alcoholic fermentation process. They are round or oval in shape, średnicę 9-12μm, surfaces; ok. 150 μm².
The yeast cell consists mainly of protoplasm , surrounding membrane. Living cell protoplasm consists of cytoplasm and liaroplasm. Mitochondria are located in the cytoplasm, microsomes and other bodies that play specific roles in the life of a cell. In young cells, this plasma fills their entire interior. There is a vacuole in older cells, that is, a concentration of nutritional reserve substances.
Yeasts reproduce by budding. The fermentation process is divided into 3 phases: a phase lasting several hours, the so-called. logarithmic, in which intense reproduction is observed; the growth inhibition phase, as a result of alcohol formation, carbon dioxide, organic acids; the phase of reducing the amount of chemical compounds constituting the yeast growth medium.
The optimal temperature for yeast growth is 25-30 ° C. At temperatures above 40 ° C, yeast growth ceases.
Oxygen is not required for the growth of yeast, if there are a sufficient amount of unsaturated fatty acids in the environment, as hydrogen acceptors. However, it is accepted, with oxygen (02) accelerates the reproduction of yeast.
Succharomyces carlsbergeims bottom-fermenting yeast is mainly used in brewing (according to the new Sacciutromyce nomenclature;; grapes;). Sacchcromyces cereuitdae top-fermenting yeast is also used.
Lager yeast ferment at 5-10 ° C ; at the end of the process they settle to the bottom of the fermenter without forming chains, Top-fermenting yeast ferments at a temperature of 15-25 ° C and during the intensive fermentation phase it gathers on the surface of the fermenting wort.
Sacchuromycez uvarum yeast also has the ability to ferment raffinose, because they contain the enzyme melibase, which Saccharomyces cerevisiae yeast does not possess. The latter can ferment part of the raffinose, only separating the fructose molecule from it.