beverages Review Saccharomyces species in the Production of Beer Graham G. Stewart The International Centre for Brewing and Distilling, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, Scotland, UK; [email protected] Academic Editor: Edgar Chambers IV Received: 20 October 2016; Accepted: 25 November 2016; Published: 2 December 2016 Abstract: The characteristic flavour and aroma of any beer is, in large part, determined by the yeast strain employed and the wort composition. In addition, properties such as flocculation, wort fermentation ability (including the uptake of wort sugars, amino acids, and peptides), ethanol and osmotic pressure tolerance together with oxygen requirements have a critical impact on fermentation performance. Yeast management between fermentations is also a critical brewing parameter. Brewer’s yeasts are mostly part of the genus Saccharomyces. Ale yeasts belong to the species Saccharomyces cerevisiae and lager yeasts to the species Saccharomyces pastorianus. The latter is an interspecies hybrid between S. cerevisiae and Saccharomyces eubayanus. Brewer’s yeast strains are facultative anaerobes—they are able to grow in the presence or absence of oxygen and this ability supports their property as an important industrial microorganism. This article covers important aspects of Saccharomyces molecular biology, physiology, and metabolism that is involved in wort fermentation and beer production. Keywords: amino acids; ethanol; fermentation; malt; peptides; Saccharomyces species; sugars; wort; yeast 1. Introduction The flavour and aroma of any beer is, in large part, determined by the yeast strain employed together with the wort composition. In addition, yeast properties such as flocculation, fermentation ability (including the uptake of wort sugars, amino acids, small peptides, and ammonium ions), osmotic pressure, ethanol tolerance, and oxygen requirements have a critical impact on fermentation performance. Proprietary strains, belonging to individual breweries, are usually jealously guarded and conserved. However, this is not always the situation. In Germany, for example, most of the beer is produced with only four individual yeast strains and approximately 65% of it is produced with a single strain sourced from the Weihenstephan Brewing School, which is located in the Munich Technical University. No satisfactory definition of yeast exists that encompasses commonly encountered properties such as alcoholic fermentation and growth by budding. The latter is not unusual in yeast and nearly all brewer’s yeast strains multiply by budding (Figure1). Brewer’s yeast cultures predominantly come from the genus Saccharomyces—a minority of non-Saccharomyces yeast cultures employed in brewing will be discussed in Section8. Yeast is cultured in an acidic aqueous sugary solution called wort prepared from barley malt and other cereals such as corn (maize), wheat, rice, sorghum, and also cane and beet sugar. The cells absorb dissolved sugars, simple nitrogenous matter (amino acids, ammonium ions, and small peptides), vitamins, and ions through their plasma membrane. Subsequently, they employ a series of reactions known as metabolic pathways (glycolysis, biosynthesis of cellular constituents, etc.) and use these nutrient materials for growth and fermentation. It is important to emphasise that the primary products of glycolysis are: ethanol, glycerol, and carbon dioxide (Figure2). Beverages 2016, 2, 34; doi:10.3390/beverages2040034 www.mdpi.com/journal/beverages Beverages 2016, 2, 34 2 of 18 Beverages 2016, 2, 34 2 of 18 Beverages 2016, 2, 34 2 of 18 Beverages 2016, 2, 34 2 of 18 Figure 1. An electron micrograph of a budding yeast cell containing bud and birth scars. Figure 1. An electron micrograph of a budding yeast cell containing bud and birth scars. Figure 1. An electron micrograph of a budding yeast cell containing bud and birth scars. D-Glucose Figure 1. An electron micrograph ofD-Glucose a budding yeast cellATP containing bud and birth scars. Glucose 6-phosphate ADPATP Glucose 6-phosphate ADP FructoseD-Glucose 6-phosphate ATP ATP Fructose 6-phosphate ADPATP FructoseGlucose 1, 6-diphosphate 6-phosphate ADP Fructose 1, 6-diphosphate ADP Glyceraldehyde 3-phosphate Fructose 6-phosphate ATP Glyceraldehyde 3-phosphate Dihydroxy-acetone phosphate ADP Fructose 1, 6-diphosphate + Dihydroxy-acetone phosphate NAD Glycerol + Glyceraldehyde 3-phosphate NADHNAD 2 Glycerol 1, 3-Diphosphoglycerate ADPNADH2 Dihydroxy-acetone1, 3-Diphosphoglycerate phosphate 3-Phosphoglycerate ADPATPNAD+ Glycerol 3-Phosphoglycerate ATPNADH2 1,2-Phosphoglycerate 3-Diphosphoglycerate H2O ADP Phosphoenolpyruvate2-Phosphoglycerate H2O 3-Phosphoglycerate ADPATP Phosphoenolpyruvate Pyruvate ADPATP 2-Phosphoglycerate H O CO Pyruvate ATP2 2 Acetaldehyde CO Phosphoenolpyruvate NADH2 2 Acetaldehyde NADADP+ Ethanol NADH2 Pyruvate NADATP+ Ethanol CO2 Acetaldehyde Figure 2. 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