Yeasts

Yeasts (Kingdom Fungi) are well-known endosymbionts of insects, including bark beetles (Vega and Dowd, 2005), and appear to be present throughout most bark beetle life stages (Six, 2003).

From: Bark Beetles , 2015

Yeasts

R. Joseph , A.K. Bachhawat , in Encyclopedia of Food Microbiology (Second Edition), 2014

Classification

Yeasts are unicellular fungi reproducing asexually by budding or fission and sexually by spore formation. Emil Christian Hansen's studies, over a span of 30 years, provided insight into the biological features of yeasts and facilitated their differentiation and their characterization as species.

Currently more than 500 species of yeasts, belonging to around 50 genera, are known. Most yeast species belong to Ascomycotina, a few are basidiomycetes.

Bakers' yeast and the yeasts used in brewing, winemaking, and distilling are strains of Saccharomyces cerevisiae, belonging to the family Saccharomycetaceae in Ascomycotina.

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Yeasts

G.M. Walker , in Encyclopedia of Microbiology (Third Edition), 2009

Yeasts are eukaryotic unicellular microfungi that are widely distributed in the natural environment. Around 1000 yeast species are known, but this represents only a fraction of yeast biodiversity on Earth. The fermentative activities of yeasts have been exploited by humans for millennia in the production of beer, wine, and bread. The most widely exploited and studied yeast species is Saccharomyces cerevisiae, commonly referred to as 'baker's yeast'. This species reproduces asexually by budding and sexually by the conjugation of cells of opposite mating types. Other yeasts reproduce by fission (e.g., Schizosaccharomyces pombe) and by formation of pseudohyphae as in dimorphic yeasts, such as the opportunistic human pathogen Candida albicans. In addition to being widely exploited in the production of foods, beverages, and pharmaceuticals, yeasts play significant roles as model eukaryotic cells in furthering our knowledge in the biological and biomedical sciences. Several yeasts have had their genomes completely sequenced (e.g., S. cerevisiae in 1996; Sch. pombe in 2002), and research is under way to assign a physiological function to sequenced yeast genes. The study of yeasts not only provides insights into how a simple eukaryote works but also leads to understanding of several human diseases and heritable disorders.

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Yeasts

Marianne Perricone , ... Antonio Bevilacqua , in The Microbiological Quality of Food, 2017

Abstract

Yeasts are eukaryotic organisms that are included in a group of organisms called "fungi," which also includes molds and mushrooms. Yeasts can have both positive and negative effects on fermented products consumed by humans and animals. Yeasts are used as starter cultures in cheeses and bread, as well as wine, beer, and other alcoholic fermentation products, but they can also initiate spoilage in foods, such as yoghurt, fruit juice, salads, and mayonnaise. A group of authors have published a list of yeasts that are frequently associated with the spoilage of foods and beverages: to these lists of "dangerous" spoiling yeasts four other yeast species have also been added, which were the members of the "second-division spoiling yeasts." The ability of some yeasts to survive in harsh conditions makes them potent food spoilage organisms responsible for large economic losses of some food products.

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Yeast

I. Russell , in Brewing Materials and Processes, 2016

Yeast Autolysis

Yeast autolysis can occur in any fermentation if the yeast is not very healthy when pitched and/or is subjected to high stress during the fermentation—whether that stress is high temperature, high pressure, high gravity, ethanol, old age, or a number of combinations of stress factors acting together. When yeast autolysis occurs, the entire cell contents spill into the beer, including many enzymes. Proteases can degrade the beer foam proteins, fatty acids from the spill can lead to off-flavors, and autolyzed yeast can lead to hazes in the beer.

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Yeasts

R. Tofalo , G. Suzzi , in Encyclopedia of Food and Health, 2016

Metabolism

Yeasts are heterotrophic organisms in which the energy and carbon metabolism are interconnected and anabolism is coupled to catabolism. Yeasts preferentially metabolize sugars that are converted principally to ethanol and carbon dioxide, but they can utilize different carbon sources, such as amino and organic acids, polyols, alcohols, fatty acids, and other compounds, depending on the species. According to the process utilized to generate energy (respiration and/or fermentation), they can be classified as:

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nonfermentative yeasts with only a respiratory metabolism;

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obligate-fermentative yeasts, only capable of metabolizing glucose through alcoholic fermentation;

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facultative-fermentative yeasts, possessing either a fully respiratory or a fermentative metabolism or even both in a mixed metabolism depending on the growth conditions, the type and concentration of the carbon source, and/or oxygen availability.

The main nitrogen substrates used by yeasts during growth in food are inorganic ammonium compounds and free amino acids. Decarboxylation of amino acids leads to the production of amines, whereas metabolism of arginine and citrulline gives urea, precursor of the potential carcinogen ethyl carbamate. Whereas yeasts produce organic acids, with succinic and acetic acids being the main acids produced during sugar fermentation, some yeast species can use acetic and lactic acids and weak monocarboxylic acids under starvation conditions. Consumption of organic acids by yeasts may thus create conditions favorable for certain spoilage microorganisms. In cheese, lactate catabolism by some species such as Debaryomyces hansenii, Kluyveromyces lactis, and Kluyveromyces marxianus induces an increase of pH at the cheese surface, having a key role during ripening.

Sulfates are utilized to produce volatile sulfur compounds such as hydrogen sulfide, sulfur dioxide, and at lesser amounts mercaptans, thioesters, and other organic sulfites. Yeasts also show proteolytic and lipolytic activities depending on the species and strain.

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Yeast

A. Speers , J. Forbes , in Brewing Microbiology, 2015

1.2 Yeast cell structure

Yeast is the most important part of the brewing fermentation process. Yeast converts sugar to alcohol, carbon dioxide and other compounds that influence the flavour and aroma of beer. Brewer's yeast is a eukaryote and belongs to the kingdom Fungi. By some scientific classifications, all beer-brewing strains of yeast are placed in the genus Saccharomyces (sugar fungus) and species cerevisiae (Walker, 1998). However, the brewing industry uses a classification which divides yeast into two types: ale yeast (S. cerevisiae) and lager yeast (S. carlsbergensis ). The distinction is kept so as to separate yeasts used to make ales from those used to make lagers ( Briggs, Boulton, Brooks, & Stevens, 2004).

Most of the organisms in the kingdom Fungi are multicellular; however, yeast is a single-cell organism. A single yeast cell measures about 5–10   μm in diameter and is usually spherical, cylindrical or oval in shape (Boulton & Quain, 2001, pp. 5–360). Yeast occurs in single, pairs, chains and clusters (Stewart & Russell, 1998). Figure 1.1 is a simplified diagram of yeast cell structure. The cell wall is a barrier that is mostly composed of carbohydrates surrounding the cell (Boulton & Quain, 2001). It is a rigid structure which is 250   nm thick and constitutes approximately 25% of the dry weight of the cell (Stewart & Russell, 1998). There are three cross-linked layers comprising the cell wall (Figure 1.2). The inner layer is a chitin (a long-chain polymer of an N-acetylglucosamine) layer, composed mostly of glucans; the outer layer is mostly mannoproteins while the intermediate layer is a mixture of both the inner and outer layer (White & Zainasheff, 2010).

Figure 1.1. Main features of a typical yeast cell (Stewart & Russell, 1998).

Figure 1.2. Molecular organization of the cell wall of S. cerevisiae. GPI-CWP are GPI-dependent cell wall proteins, Pir-CWP are pir proteins on the cell wall and β1-6-Glc are glucan molecules, which are highly branched. Therefore, they are water soluble, which tethers GPI-CWPS to the cell wall (Kils, Mol, Hellingwerf, & Brul, 2002).

To reproduce asexually, a yeast cell clones itself, thereby creating a new daughter cell. Cell separation is achieved when the layers of the cell wall separate, leaving the bud scar on the mother cell and the birth scar on the daughter cell (Stewart & Russell, 1998). The bud scar is composed mainly of chitin. The average ale yeast cell will not bud more than 30 times over its lifetime while lager yeast will bud only 20 times before they are unable to bud further (Wyeast Laboratories, 2009).

The plasma membrane is a semipermeable lipid bilayer between the cell wall and the inside of the cell. There are several distinct roles that the plasma membrane carries out such as to provide a barrier to free diffusion of solutes, to catalyse specific exchange reactions, to store energy dissipation, to provide sites for binding specific molecules involved in metabolic signalling pathways and to provide an organized support matrix for the site of enzyme pathways involved in the biosynthesis of other cell components (Hazel & Williams, 1990). The plasma membrane is quite fluid and flexible due to its constituents of lipids, sterols and proteins. Additionally, these constituents allow for the creation of a daughter cell.

The formation of double bonds in fatty acids controls their level of saturation. The saturation level determines the ease and extent of hydrogen bonding that can occur between fatty acids (Briggs et al., 2004). Membrane fluidity is necessary for proper membrane function. Lipid bilayers are by their nature fluid and that fluidity is determined by the extent to which the lipids bind to one another (White & Zainasheff, 2010). By controlling the level of saturation in their lipid membranes, yeast cells are able to maintain proper membrane fluidity at different temperatures, which is important during fermentation. Without proper aeration yeast cells are unable to control membrane fluidity through to the end of fermentation which leads to halted fermentations and off-flavours of the final product (White & Zainasheff, 2010).

The cytoplasm is that portion of the cell enclosed by the plasma membrane and excluding other membrane-bound organelles. It is an aqueous colloidal liquid containing a multitude of metabolites (Briggs et al., 2004). The cytoplasm contains intercellular fluid known as the cytosol. The cytosol contains enzymes involved in anaerobic fermentation that enable the cell to convert glucose into energy immediately after it enters the cell (White & Zainasheff, 2010).

The mitochondrion is an organelle where aerobic respiration occurs. Mitochondria consist of a double membrane that is the location of the conversion of pyruvate (a metabolic compound) and the tricarboxylic acid cycle. The nucleus stores the cell DNA and is delineated by a lipid membrane that envelopes the nucleus and is similar to the plasma membrane. The cell uses mRNA to transfer the information out into the cytoplasm for use in protein synthesis (White & Zainasheff, 2010).

The vacuole is a membrane-bound structure that stores nutrients and is also where the cell breaks down proteins. Brewer's yeast vacuoles are large enough to be seen through light microscopy (White & Zainasheff, 2010). The major site for proteolysis is the cell vacuole. Much of the regulation of both specific and nonspecific proteolysis involves the sequestration of target proteins into vacuoles where they are exposed to proteinases (Briggs et al., 2004). The endoplasmic reticulum is a network of membranes and is usually where the cell manufactures proteins, lipids and carbohydrates for membranes and secretion (White & Zainasheff, 2010). Other microbodies are mainly made up by glycogen bodies and lipid granules (Boulton & Quain, 2001).

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YEASTS

T. Deák , in Encyclopedia of Food Sciences and Nutrition (Second Edition), 2003

Sour Dough

Before baker's yeast was available commercially, part of leavened dough was added as an inoculum to the fresh dough. Acidification normally took place in this old dough, hence the name: sour dough. In recent years, consumption of sour-dough breads has greatly increased. The use of sour dough is necessary for the development of characteristic properties of rye breads. Sour doughs contain both heterofermentative lactic acid bacteria and yeasts, and the mixed population usually comprises several different species of both groups. Unlike baker's yeast consisting overwhelmingly of a single yeast species, S. cerevisiae, even commercial sour-dough starters are not made of defined pure cultures. Lactobacillus brevis and L. sanfrancisco are characteristic lactic acid bacteria in sour dough, whereas Candida milleri and S. exiguus are the predominant yeast species.

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YEASTS

CLETUS P. KURTZMAN , JACK W. FELL , in Biodiversity of Fungi, 2004

DISTRIBUTION

As noted earlier, yeasts commonly are distributed throughout the biosphere ( Phaff et al. 1966). Many yeasts are isolated from soils. Phaff and Starmer (1987), for example, noted that occasional soil populations of 105–106 colony-forming units (cells) per gram of soil suggest the presence of actively growing cells. Some species, including Debaryomyces (Schwanniomyces) occidentalis, Lipomyces species, Schizoblastosporion starkeyihenricii, and certain Cryptococcus species, are isolated exclusively from soils. Population levels of aquatic yeasts are usually highest in fresh waters and decrease in marine waters with increased depth and increased distance from land (Hagler and Ahearn 1987). Populations in the open ocean occur at densities often as low as 10 cells/g of sample but may increase to 103 cells/g in waters associated with plankton blooms, current boundaries, surface slicks, thermoclines, or pollutants (Fell 1976).

Despite these observations, the majority of yeast species are collected from fallen plant materials and other organic matter (Phaff and Starmer 1987). The highest densities of yeasts are usually associated with concentrations of assimilatable sugars and other carbon sources. Leaf surface tissues and plant exudates commonly sustain large numbers of yeasts. di Menna (1959) reported 105–107 viable cells/g of fresh foliage. Flowers and decaying fruits and other plant material also support a wide spectrum of species in high numbers; densities as great as 106 cells/g of tissue were reported from decaying cladodes of some cacti (Phaff and Starmer 1987). Insects represent rich sources of yeasts, especially wood-borers and species of Drosophila. Phaff and Starmer (1987) summarized the yeasts found with different types of insects.

Reference cultures are essential for correct identification of yeast species. The primary collections containing reference strains for yeasts are listed in Table 16.2.

TABLE 16.2. Major Yeast Culture Collections *

Italy: Industrial Yeasts Collection (DBVPG), Dipartimento di Biologia Vegetale, Universita di Perugia
Japan: (i) Institute for Fermentation (IFO); (ii) Japan Collection of Microorganisms (JCM)
The Netherlands: Centraalbureau voor Schimmelcultures (CBS), Yeast Division
Peoples Republic of China: Center for Collection of General Microbiological Cultures (CCGMC), Institute of Microbiology, Academia Sinica
Portugal: Portuguese Yeast Culture Collection (PYCC)
Republic of Germany: Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH (DSMZ)
Russia: Department of Type Cultures of Microorganisms (VKM), Institute of Biochemistry and Physiology of Microorganisms, Russian Academy of Sciences
Slovakia: (i) Slovak Collection of Yeasts (CCY), Institute of Chemistry, Slovak Academy of Sciences; (ii) Research Institute for Viticulture and Enology (RIVE)
United Kingdom: National Collection of Yeast Cultures (NCYC), AFRC Food Research Institute
United States: (i) Agricultural Research Service Culture Collection (NRRL), National Center for Agricultural Utilization Research; (ii) American Type Culture Collection (ATCC); (iii) Culture Collection, Department of Food Science and Technology (FST- UCD), University of California—Davis
*
See Appendix III for contact information.

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Overview of Manufacturing Beer: Ingredients, Processes, and Quality Criteria

Sascha Wunderlich , Werner Back , in Beer in Health and Disease Prevention, 2009

Demands on Yeast for Brewing

Yeast for fermentation should be at peak condition. It has to have a high viability and vitality. Viability is the alive–dead rate of yeast cells. Vitality characterizes physiological condition of alive cells. In breweries different strategies are used to ensure optimal yeast condition. Brewers have to decide whether pure culture yeast or (also) repitched yeast is used. Repitched yeast characterizes yeast that has had prior exposure to fermenting wort (sometimes repeatedly). There are also different methods for yeast propagation before starting fermentation. The resulting yeast is always examined for viability and vitality (Heyse, 2000; Briggs et al., 2004; Back, 2005a).

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Major Ingredients for Dough

Sidi Huang , Diane Miskelly , in Steamed Breads, 2016

3.8.3 Rate of Addition

Consistency of yeast activity is important so that once the usage rate is established, it can be maintained. In China, fresh yeast usage rate is about 1% by flour weight. As the yeast activity decreases, the rate of usage will need to increase. Any increase will be determined by assessing product quality but will be influenced by yeast aging and storage conditions. Less dried yeast is required when used instead of fresh yeast. Dried yeast is used at the rate of 50–100% of the fresh yeast and instant active dried yeast at the rate of 33%.

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