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The exterior of each yeast cell consists of a distinct wall and a plasma membrane with a space (the periplasm) in between the two. The cell wall is a dynamic organelle that determines the cell shape and integrity of the organism during growth and cell division. It provides the cell with mechanical strength in order to withstand changes in osmotic pressure imposed by the environment and other stresses. The chemical composition and structural aspects of the S. cerevisiae cell have been known for some time. The plasma membrane of yeast cells (specifically Saccharomyces spp.) forms the barrier between the cytoplasm and the cell wall. The plasma membrane consists principally of lipids and protein in approximately equal proportions. As a result of the large number of functions that it performs, most of the membrane proteins are functional and not structural. It has several distinct roles: a barrier to the free diffusion of solutes, catalyse specific change reactions, store energy as transmembrane ions and solute gradients, regulate the rate of energy dissipation, provide sites to bind specific molecules for catabolic signalling pathways and provide sites of enzyme pathways involved in the biosynthesis of cell components. The periplasm is not a continuous space because of interrupting invaginations in the plasma membrane and irregularities in the inner surface of the cell wall. The periplasm is not an organelle as such. It is the location where a number of important yeast enzymes are located and active.

The final flavour of beer and spirits is the sum of several hundreds of flavour-active compounds produced in every stage of the brewing process. The great majority (not all) of these substances are yeast metabolites. They are produced during wort fermentation and consist of fermentation intermediates or by-products. Higher alcohols (also called fusel oils), esters, vicinal diketones (VDKs), other carbonyls and sulphur compounds (inorganic and organic) are key elements produced by yeast. These compounds determine a beer’s final quality, particularly when it is fresh. Although ethanol, carbon dioxide and glycerol are the major products produced by yeast during wort fermentation, they have little impact on the flavour of the final beer. It is the type and concentration of the excretion products already discussed that primarily determines beer flavour balance. There are many factors that can modify this balance including the yeast strain, malt variety, fermentation temperature, adjunct type and level, fermentation design and geometry, wort pH and clarity, media buffering capacity, wort gravity, etc.

Yeast cells were first microscopically observed by Antonie van Leeuwenhoek in 1680. In the 1850s and 1860s, yeasts were established as being responsible for alcoholic fermentation. Between 1884 and 1894, Emil Fischer studied sugar utilization by yeast including an understanding of enzymatic specificity and the nature of enzyme-substrate complexes. A number of scientists have played important roles in developing ideas regarding the genus Saccharomyces. These scientists include Meyen, Reese, Pasteur, Cagniard-Latour, Schwann, Buechner, Hansen, Winge and Horace Brown. The notable reputation that yeast (particularly Saccharomyces) enjoys as an experimental eukaryote cannot be overlooked. It has been argued that the combination of recombinant DNA technology and classical biochemistry, enzymology and proteins has created a revolution that has presented scientists access to the biology and application of these organisms.

In excess of a thousand unique yeast species have been identified, and many of them have been characterized (to a lesser or greater extent). Ninety percent (and more) of the fermentation ethanol produced globally employs species of the genus Saccharomyces (predominantly S. cerevisiae and S. pastorianus). However, there are a number of non-Saccharomyces yeast species that can produce ethanol (also called nonconventional yeast species). Nonconventional yeasts are a large and barely exploited resource of yeast biodiversity. Many of these nonconventional yeast species exhibit industrially relevant traits such as an ability to utilize complex substrates, nutrient tolerance against stresses and fermentation inhibition. The evolution of most of these yeast species was independent of Saccharomyces spp. Many of them possess novel and unique mechanisms that are not present in Saccharomyces yeasts. Most of them have been characterized as spoilage yeasts because they have been isolated from contaminated foods and beverages. Yeast species that are included in this category are Schizosaccharomyces pombe, Kluyveromyces marxianus, Schwanniomyces occidentalis, Brettanomyces bruxcellensis, Pichia stipitis, Pachysolen tannophilus and Torulaspora delbrueckii.

Despite the frequent use of the terms yeast viability and yeast vitality in both the brewing and distilling literature, the application of these terms is often confused. There are a number of analyses to show that yeast is alive; the relationships between a cell’s environment and the methods employed for its cultivation are complex! These methods include the ability of cells to grow on solid or in liquid media and strain-based systems; some are based on changes in the charges and functionality of the cell membrane, and others are dyes that penetrate into live and/or dead cells. The methods for assessing cell viability only provide information on live and dead cells in a whole population. Methods for yeast vitality determination are based on various physiological and metabolic aspects of cells. These methods include: Intracellular ATP content based on the luciferin reaction Determination of mitochondrial membrane potential Acidification power test following the addition of glucose Magnesium release test based on the fact that low molecular weight ions such as magnesium and others are released by yeast following addition of glucose to the medium Determination of the activity of enzymes such as esterase, oxidoreductases or several different redox enzymes

Brewers employ a number of methods to crop their yeast which varies depending on whether one is dealing with traditional ale top-cropping, accepted lager bottom-cropping and cylindroconical fermentation systems (also called a Nathan fermenter) where the yeast (ale or lager) is recovered from the cone (sometimes repitched cone to cone), a non-flocculent culture where the yeast, still in suspension, is cropped with a centrifuge or a portion of the yeast, still in suspension, is blended into the fresh wort of a subsequent fermentation. With the traditional ale top-cropping fermentation system, although there are many variations of this process, a single, dual or multi-strain culture can be employed. Yeast quality is influenced by the manner that yeast is cropped and centrifuges play an important part in this regard. The cell wall structure of S. cerevisiae is the critical parameter involved in yeast flocculation. Details of the cell wall have already been discussed in Chap. 5. However, here it is discussed that the wall consists of an inner layer composed predominantly of β-glucose and chitin and a fibrillar outer layer consisting primarily of α-mannan (highly glycosylated) associated with mannoproteins. Yeast management and handling systems, including culture harvesting, are influential in determining the physiological status of yeast.

The importance of the molecular biology of S. cerevisiae is closely related species is well documented. This yeast was the first microorganism to be domesticated for the production of fermented food and beverages and to be described as a living biochemical agent for biological transformations. In 1996, the complete genome of S. cerevisiae haploid strain became the first eukaryote genome to be fully sequenced. Its 16 chromosomes encode approximately 6000 genes, and approximately 5000 of them are individually non-essential to the yeast cell. This publicly available genome sequence has prepared the way to build the first systematic collection of deletion mutants, which enables high-throughput functional genetic experiments. In 2014, S. cerevisiae became the first eukaryote to be equipped with a functional chromosome. S. cerevisiae has three basic mating types – a, α and a/α. Most laboratory strains are haploid or diploid, whereas industrial yeast strains (brewing, distilling, baking and wine) are predominantly diploid, aneuploid and polyploidy. Polyploid strains are genetically more stable and less susceptible to mutational forces than either haploid or diploid strains, thus enabling such strains to be used by brewers with a high degree of confidence. Cells of polyploid (aneuploid) brewing strains – ale and lager – sporulate poorly, and asci containing four spores rarely develop. Moreover, spore viabilities are low, and the spores that are viable often lack the ability to mate. As a consequence, alternative methods of genetic manipulation such as protoplast (spheroplast) fusion and rare mating that introduces foreign genetic material into the genome have been required to facilitate strain improvement. Unlike the other techniques discussed, genetic engineering (recombinant DNA-rDNA) affords the possibility of introducing additional factors. Also, genetic engineering methods permit the transfer of genetic information between completely unrelated organisms. Consequently, the recipient organism becomes able to produce heterologous proteins or peptides that are not produced by their mated constituents. This provides considerable scope for the transfer of new constituents into industrial yeast strains.

Most (not all) genera and species of yeast can ferment sugars to ethanol anaerobically. This is why research on yeast fermentation has (and still does) receive extensive financial support in many countries. The question of ethanol metabolism and related areas is the most important in this context. Studies on glycolysis (particularly with yeast) rival their significance, both scientifically and economically, with the discovery of penicillin and subsequent studies to develop it as the first antibiotic. The elucidation of the glycolytic pathway from glucose into pyruvate and subsequently ethanol (or lactic acid) involves a number of enzyme-catalysed stages. Although studies on the glycolytic pathway began with Pasteur in the mid-nineteenth century and then Buchner with cell-free extracts, during the twentieth century, this research was central for generating major advances in biochemistry together with massive economic applications. Glycogen is a major intracellular carbohydrate in yeast cell together with the disaccharide trehalose. Glycogen forms an energy reserve that can be rapidly mobilized to meet a sudden requirement for glucose, usually early in the fermentation/growth cycles. Trehalose plays a protective role against stresses such as osmotic pressure, nutrient depletion and starvation. It improves cell resistance to high and low temperatures, concentrated wort, elevated ethanol concentrations, etc. The Citric Acid Cycle is also known as the Tricarboxylic Acid (TCA) Cycle and/or the Krebs cycle. It is a series of reactions used by aerobic organisms (including yeast) to generate energy through the oxidation of acetyl-CoA derived from carbohydrates, fats and proteins into carbon dioxide and chemical energy. Also, the cycle provides precursors for certain amino acids as well as the reducing agent NADH, which is used for numerous other biochemical reactions (details in Chap. 7).

There are basically three types of beer: lager, ale and stout. In addition, there are distilling yeast strains. Reasons for differences between ale, lager and distilling yeast strains have intrigued many students of Saccharomyces for a long time! The isolation and characterization of Saccharomyces eubayanus has been discussed. It is regarded to be an early ancestor of lager yeast strains and has the capacity to ferment at cold temperatures (

All organisms have evolved to cope with changes in environmental conditions, ensuring the optimal combination of metabolism, cell proliferation and survival. Brewing and distilling fermentations exert a number of stresses on yeast cultures. During the past 50 years or so, the question of process efficiency and intensification has become a major focus and the stresses imposed, and over time this focus has become intensified. The primary brewing technique (not the only one) that is susceptible to intensification is high-gravity (HG) processing, particularly high-gravity wort fermentation processes. The question has often been asked: “Does the use of high-gravity wort inflict unusual and cruel punishment on yeast?” As well as HG conditions, there are a number of other parameters that persist during brewing and distilling fermentations, which exert stresses upon yeast. These parameters include ethanol, osmotic pressure, temperature, cell surface shear, continuous fermentation compared to batch fermentation, wort ionic balance and some minor wort components. Nevertheless, the conditions that prevail in HG worts are the primary factors that exert stresses on yeast. These factors are discussed in detail.

The structure and function of yeast mitochondria and a plethora of eukaryotes have been the subject of intensive investigations for many decades. The predominant concept is that mitochondria are the “energy powerhouses of the cell”. These studies, combined with the evolutionary origin of mitochondria, have focussed on this organelle as an essential, independent, functional cell component. A mitochondrion (plural mitochondria) is a membrane-enclosed organelle found in most eukaryotic cells including yeast. Mitochondria are readily recognizable in electron micrographs of aerobically grown yeast cells as spherical or rod-shaped structures surrounded by a double membrane. They contain cristae, which are formed by the folding of the inner membrane. A number of spontaneous mutations can occur in brewing and distilling strains. The most frequently identified spontaneous mutation is the respiratory-deficient (RD) or cytoplasmic “petite” mutation. In brewing yeast strains, the RD mutation normally occurs at frequencies between 0.5 and 5.0%. The RD mutation has the following influence on brewer’s and distiller’s strains: effects on the uptake of wort sugars, effects of RD mutants on yeast flocculation and sedimentation characteristics and influence of stress conditions (e.g. centrifugation and heat) on the formation of RD mutants. Damage and subsequent dysfunction of mitochondria is an important factor in a range of human diseases because of their influence on cellular metabolism. Fundamental research on the mitochondrion structure and function of brewer’s (and baker’s) yeast strains has been extrapolated to an understanding of mammalian mitochondrial disorders.