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Beer-spoiling Yeasts: Genomics, и Detection, and Control

Chris D. Powell1* and Daniel W. M. Kerruish2 1School of Biosciences, University of Nottingham, Nottingham, UK.

  • 2Diageo PLC, Science and Technology, St James Gate Brewery, Dublin, Ireland.
  • *Correspondence: This email address is being protected from spam bots, you need Javascript enabled to view it

https://doi.org/10.21775/9781910190616.11

Abstract

Beer-spoiling yeasts comprise a diverse group of organisms that can have a variety of impacts on beer production. Invariably, contamination of wort or beer by these yeasts leads to inconsistencies within the process, and quality defects in packaged beer. Beer-spoiling yeasts can be broadly separated into non-fermentative (aerobic) and fermentative yeasts. The former typically exploit process steps associated with raw materials, and areas where oxygen ingress is difficult to prevent, such as unpasteurized cask beers or dispense. Fermentative yeasts are arguably more problematic due to their capacity to compete with production strains during fermentation. Major impacts include altered sugar utilization, flocculation and ethanol production, as well as the formation of phenolic compounds, acidity, estery off-flavours, and haze or turbidity. These effects occur primarily due to differences in the genetic, metabolic and physiological characteristics of the spoilage yeast and the production strain. In this chapter, we describe the characteristics and functionality of beer-spoiling yeasts, as well as methods for their isolation and identification.

Introduction

Yeasts are a group of organisms comprising unicellular fungi that are capable of dividing asexually by budding or fission, or sexually by the process of sporulation. Industrial or ‘domesticated' yeasts typically belong to the Saccharomyces genus, and within the brewing industry two main species are encountered: S. pastorianus and S. cerevisiae (see Chapter 4). These organisms divide asexually and tend not to sporulate, due to their complex genetic make-up and unconventional ploidy. However, other yeasts can be found within brewing processes that fit the description above more completely. Such species are typically referred to as ‘beer- spoiling' or ‘wild' yeasts. Beer-spoiling yeasts are those that have a negative impact on the sensorial qualities of the final product, either by affecting process stages or by changing the character of the beer directly. Within the brewing industry, this definition often incorporates ‘wild yeast', which is a generic term used historically to define any type of yeast not deliberately introduced into the brewery environment. It should be noted that, at the present time, and within certain sectors of the industry, there has been a growing interest in the potential applications of strains that do not belong to the traditional brewing yeast Saccharomyces genus (see Chapter 7). This is primarily due to the novel characteristics that non-Saccharomyces yeasts impart to the final product and has resulted in the phrase ‘wild yeast' (isolated from ‘the wild') taking on a slightly alternative meaning. Within this Chapter we use the term ‘beer-spoiling yeast' to indicate unwanted organisms present within the brewing process or beer.

Beer-spoiling yeasts constitute a broad group of organisms that, although phylogenetically diverse, inevitably comprise individuals that share key physiological properties, which allow them to exploit sections of the brewing chain. Although most beer-spoiling yeast are isolated either directly or indirectly from areas of the brewery associated with fermentation, others may be found as contaminants of raw materials through to final pack (Table 11.1). Irrespective of their preferred environment, for simplicity beer-spoiling yeast are often characterized as Saccharomyces or non-Saccharomyces types. Of these, the Saccharomyces spoilage yeasts are often regarded as being the most hazardous since they are difficult to detect, and compete directly with the culture strain. However, there is also a range of non-Saccharomyces species that are associated with beer spoilage and these include members of the genera Brettanomyces, Candida, Debaryomyces, Hanseniaspora (Kloeckera), Kluyvero- myces, Pichia, Torulaspora, and Zygosaccharomyces. There are inevitably incidences of contamination with other types of yeast, but species and strains belonging to these genera tend to predominate.

 
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