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Brewers Yeast
Yeast Nutritional Requirements
To grow successfully, yeast requires an adequate supply of nutrients – fermentable carbohydrates, nitrogen sources, vitamins, and minerals – for healthy fermentation. These nutrients are naturally present in malted barley or developed by enzymes during the malting and mashing process.
Carbohydrates
Only low-molecular-weight sugars such as the mono-, di- and oligosaccharides are available for yeast growth. Polysaccharides are not used by the yeast. The sugars are, in order of concentration, maltose, maltotriose, glucose, sucrose, and fructose, which together constitute 75 to 85% of the total extract. The other 15 to 20% consists of nonfermentable products such as dextrins, beta-glucans, pentosans, and oligosaccharides. Regardless of concentration, fermentable carbohydrates are usually assimilated by yeast in the following order: sucrose, glucose, and fructose are consumed most rapidly (24–49 hrs); followed by maltose (70–72 hrs); then maltotriose (after 72 hrs) [23]. Some overlap in assimilation does occur. A majority of the strains leave maltotetraose and dextrins unfermented (57).
Nitrogen
Nitrogen is available for yeast growth in wort as amino acids, peptides, and ammonium salts. Yeast prefers to use ammonium salts, but these are present in wort only in very small amounts (30). Amino acids and peptides are therefore the most important wort constituents. Amino acids, collectively referred to as "free amino nitrogen (FAN)," are the principal nitrogen source in wort and are an essential component of yeast nutrition (41). It is the amino acids that the yeast cells use to synthesize more amino acids and, in turn, to synthesize proteins.
Vitamins
Vitamins such as biotin, panthotenic acid, thiamin, and inositol are essential for enzyme function and yeast growth. Biotin is obtained from malt during mashing and is involved in carboxylation of pyruvic acid, nucleic synthesis, protein synthesis, and synthesis of fatty acids. Biotin deficiencies will result in yeast with high death rates. Panthotenic acid is required by many strains of fermentation yeast and is an essential factor in carbohydrate and lipid metabolism and in cell membrane function. Panthotenic acid deficiencies can lead to the accumulation of hydrogen sulfide. Thiamine is essential in oxo-acid decarboxylation. Inositol is required for cell division; deficiencies will decrease the rate of carbohydrate metabolism.
Minerals
Yeasts are unable to grow unless provided with a source of a number of minerals. These include phosphate, potassium, calcium, magnesium, sulfur, and trace elements. Phosphate is involved in energy conservation, is necessary for rapid yeast growth, and is part of many organic compounds in the yeast cell. Potassium ions are necessary for the uptake of phosphate. Calcium improves the flocculation properties of yeast and should be present in a concentration greater than 50 mg/l (22). Magnesium is required for yeast growth and acts as an enzyme activator. Yeast requires sulfur for the synthesis of methionine and for cycteine, which is incorporated into protein, glutathione, coenzyme A, and thiamin. The elements zinc, copper, and manganese are required in trace amounts.
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