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Yeast

An overview

A. Speers1,  and J. Forbes2     1The International Centre for Brewing & Distilling, Heriot-Watt University, Edinburgh, Scotland, UK     2Dalhousie University, Halifax, NS, Canada

Abstract

This chapter discusses the biological structure of yeast and how this structure impacts the brewing process and the ultimate product in terms of flavour, colour and quality. Both ale and lager strains of yeast are used in the brewing process along with, increasingly, 'brets'. There are subtle differences in each and their respective roles in the brewing process are discussed. Finally, the mechanism of flocculation is examined in some detail.

Keywords

Ale yeast; Cytoplasm; Fermentation; Flocculation; Lager yeast; Plasma membrane; Yeast

1.1. Yeast species/strains used in brewing and distilling

The formal classification of brewing yeasts over the past 50 years has changed enough that many brewing scientists (and most brewers!) avoid using the current genus and species to identify their yeast and simply label them as either ale or lager strains. These yeasts are used to produce most beers - that is, either 'ales' or 'lagers'. Ale is normally made with Saccharomyces cerevisiae that rises to the top of the fermenter at the cessation of fermentation while lager is made with S. carlsbergensis, which settles to the bottom of the tank towards the end of the fermentation. In the past, Barnett, Payne and Yarrow (1983) stated that both types of yeast should be characterized as variants of S. cerevisiae. However, the strains differ in their DNA profiles, ability to ferment melibiose, (ale strains lack melibiase activity) and their maximum growth temperature (lager strains do not grow above 34 °C (Webb, 1977)) and for these reasons, Stewart (1990) has argued that the two types of yeast should be classified as separate species. Additionally, the increasing importance of a third species, Brettanomyces, has been recognized following the massive growth of the craft brewing movement in the United States. 'Brets', as they are termed in the industry, are used in various stages in the production of lambic-type beers. They are considered a spoilage yeast in lager and ale fermentations as they produce volatile phenolic flavours and acetic acid due to their ability to produce off flavours by the production of volatile phenols (Libkind et al., 2011), their ability to produce acetic acid (Wijsman, van Dijken, van Kleeff, & Scheffers, 1984) and their ability to over attenuate products below 1 °Plato (Kumara & Verachtert, 1991). Those involved with the wine industry have spent significant amounts of time and money learning to isolate and characterize Brettanomyces spp. to develop better methods of early detection and eradication (Conterno, Joseph, Arvik, Henick-Kling, & Bisson, 2006; Dias et al., 2003; Oevelen, Spaepen, Timmermans, & Verachtert, 1977). Despite the large amount of negative attention Brettanomyces receives, this interesting microbe has been shown to contribute favourable organoleptic qualities to a number of products and to be of use in several industrial applications. Belgian lambic beer producers have promoted the unique organoleptic characteristics of Brettanomyces species in concert with other microbes for hundreds of years to produce a beer that is crisp, acidic and refreshing (De Keersmaecker, 1996; Oevelen et al., 1977). However, in comparison to ale and lager yeast less is known about Brettanomyces species employed in brewing. Since the early 2000s the advances in molecular biology have added to our understanding of the lager yeasts (Libkind et al., 2011; Walther, Hesselbart, & Wendland, 2014). It appears that a newly discovered and sequenced species, S. eubayanus, and S. cerevisiae have combined to form the hybrid lager yeast genome. It is hypothesized that materials containing S. eubayanus strains were imported from Patagonia to Europe where hybridization events have occurred to form the S. carlsbergensis progeny, but more recent studies suggest that the origin of the S. eubayanus strain may be Asia (Bing, Han, Liu, Wang, & Bai, 2014). Two types of lager yeast are in common use in the brewing industry. The first, Group I, the so-called Saaz type (i.e. 'Unterhefe No. 1' isolated by the Carlsberg brewery in 1883) is principally a triplod strain with an almost complete copy of the Saccharomyces cerevisiae genome and slightly more than a diploid copy of S. eubayanus genome (Walther et al., 2014). These same researchers noted that the Group II lager (i.e. the Froberg type, Weihenstephan WS34/70) has a tetraplod with roughly two copies of chromosomes from S. cerevisiae and two from S. eubayanus. It has been suggested that the low fermentation temperatures (e.g. as low as 5 °C) that Group I lagers were exposed to may have driven the difference between Group I and II lager yeasts (Walther et al., 2014).

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). 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).
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 ß1-6-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). 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...