Strain Variation in Brewing Yeast

Sue Bickerdike and Jerry Knapp


Introduction

The purpose of this website is to explore with students the importance of strain variation within a microbial species.

This website introduces the reader to yeast nomenclature/taxonomy and goes on to discuss important characteristics of brewing yeasts. It also introduces us to different types of brewing fermentation vessels and the tall tube fermenters used in our demonstrations.

The demonstration of variation between yeast strains is described in two more pages - one on experimental fermentation and the other on analysis of volatile compounds by GC-MS. These pages are, in turn, linked to other parts of the website giving more details on techniques, definitions etc.

Microbes are small and apparently fairly featureless, most people tend to feel that having identified a microbe and a given it a name it should behave the same as other microbes with the same name. This assumption ignores a wide range of variation within a single species.
Variation within species is important:-

  • Biologically/ecologically (some strains are better fitted to particular environments)
  • medically (some strains are for example more pathogenic than others)
  • commercially (some strains give better yields of commercial products, or better products)

Yeasts are fungi and most are members of the Ascomycota although some are members of the Basidiomycota. Given this it follows that the term “yeast” does not refer to a true taxonomic grouping. Even though they may differ much in their phylogeny, the feature that unifies yeasts is that they have similar morphology. They are predominantly unicellular and generally divide by budding, although some use binary fission. Some yeast species may form hyphae or pseudohyphae in certain conditions. There are many genera and species of yeast but those that concern us here are members of the genus Saccharomyces. This name is derived from the latin for ‘sugar fungus’.

Image of budding yeast taken using a deconvoluting microscope

A photograph showing yeast cells budding, taken using a deconvoluting microscope. The yeast cells are stained using Calcofluor stain.

Microbial taxonomy is in a constant state of flux and the classification of yeasts is no exception.
The classical British brewing yeasts (used to produce ales) and baker’s yeasts have always been considered to belong to the same species - Saccharomyces cerevisiae - although they differ in some properties and applications. The type of strain typically used for brewing German and European lagers was first isolated in the 1880’s at Carlsberg laboratories and called Saccharomyces carlsbergensis. This situation continued for the next 90 years as the strains involved showed differences in some tests and in their 'technical' properties. However, in 1970 it was deemed that Saccharomyces carlsbergensis was so similar to S. uvarum that it was reclassified under that name. In 1990 S. uvarum was lumped in together with all the top-fermenting British ale yeasts, in S. cerevisiae, as the differences between these strains were then thought not sufficient to make them into separate groupings. More recently Saccharomyces cerevisiae var. carlsbergensis has been classified as S. pastorianus this is often written as S. pastorianus/carlsbergensis. Although the differences in reactions in diagnostic tests between the two species (cerevisiae and pastorianus/carlsbergensis) seem small (relating to melibiose utilisation, fructose transport and maximum growth temperature) the differences in terms of brewing technology are considerable. Brewers have always considered them to be different. Undoubtedly the taxonomy of these organisms will change in the future...

A species is a group of organisms that are more similar to each other than to other defined groups of organisms. Classification usually takes account of a wide range of different characteristics including morphology, biochemical characteristics, structure, life style, mode of reproduction and genome. Although members of a species are similar they are not necessarily identical. Therefore there is room for considerable variation within a species.

  • With some characteristics the differences may be absolute - either a strain displays a characteristic or it does not – these differences may depend on the possession, or not, of an active gene.
  • In other cases strains may differ in the degree to which they display a characteristic. One strain may produce a great deal of a metabolite and another only a little. This probably depends on the rate at which some genes are expressed and the amount of an enzyme or the enzyme’s product they produce. These differences may relate to expression of a single gene but may also involve expression of several genes.

The differences noted are not sufficient for the strain to be transferred to another species but are often enough to alter important characteristics of a microbial strain and this may alter the usefulness of the strains in industrial processes. This is certainly true of brewing yeasts.

Brewing is a major industry and the importance of strain variation has been well recognised. Strain selection is important when brewing processes are modified and when new products are being produced, it has been discussed by many authors – see for example an article by Iain Campbell in Microbiology Today.

The strains used in our demonstration are from the National Yeast Culture Collection. They are NCYC 240, NCYC 1026 and NCYC 1087.

Full details can be obtained from NCYC.


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Please Note!

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Acknowledgements

We are pleased to acknowledge the financial support of the Faculty of Biological Sciences Learning and Teaching Enhancement Fund. We are also grateful to Black Sheep and Coors Breweries for permission to take and use photographs of their brewery sites. Thanks are also due to Chris Jones for his photographic expertise, Dan Whalley for his technical advise and assistance, and Owen Jackson for his permission to reproduce his BSc project thesis.