Chapter 4

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

Carbohydrates are available for yeast growth in wort as 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 non-fermentable products such as dextrins, beta-glucans, pentosans, and oligosaccharides.

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 (33). 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 (45). 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.

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.

Zinc

The most important trace element is zinc, and at least 0.10 to 0.15 mg/l should be present in the wort (33). Zinc assists in protein synthesis in yeast cells and controls their nucleic acid and carbohydrate metabolism. Fermentations are accelerated by adding zinc chloride (0.2-0.3 mg/l) to the wort (45).

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