Origins and impacts of beer- spoiling yeasts
The natural environment of yeasts is known to be diverse, with species isolated from a huge range of locations and ecosystems. Interestingly, the natural habitat of S. cerevisiae yeasts (encompassing many industrial, and beer-spoiling strains) has been the subject of much debate, although current consensus is that they are primarily associated with the bark and litter of trees, specifically oak (Sampaio and Gonsalves, 2008; see Chapter 4). It is unclear whether oak trees are genuinely the primary habitat for S. cerevisiae yeasts, or whether this association is due to biased sampling; the more we look for this species of yeast, the more we find. Indeed, although the majority of yeasts exist in habitats associated with plant material, detritus, and soils, they can also be found in water (salt and fresh), on animals and insects, and can be dispersed through air. Most beer-spoiling yeasts are likely to originate from these sources and become introduced to the process opportunistically. Unlike certain species of bacteria (e.g. Pediococcus damnosus), spoilage yeasts are typically not unique to industrial locations, but are associated with raw materials such as hops, priming sugars and adjunct syrups, and casks. It is accepted that specific materials are often not the direct cause of contamination but act as a source of entry into the brewery, leading to contamination across different stages of the brewing chain (Table 11.1). Consequently, brewing equipment, surfaces, water supplies, and pitching yeast can also be considered to be potential sources of infection and should attract particular attention from a hygiene perspective, especially if a repeat contamination is observed.
As mentioned above, beer-spoiling yeasts are often described as being Saccharomyces or non- Saccharomyces. This classification is based primarily on the fact that different methods of detection are generally employed for each group. However, there are also broad differences in terms of spoilage potential; the majority of non-Saccharomyces yeasts do not typically compete with production strains during fermentation and cannot establish themselves within the process. This is primarily because many non-Saccharomyces yeasts depend on oxygen for growth, and their influence is therefore limited due to tight oxygen control within the brewing process. In addition, non-brewing strains are generally inefficient at metabolizing maltose, the predominant sugar associated with wort, and consequently many beer-spoiling yeast strains are at a competitive disadvantage during brewing fermentations. Spoilage by these yeasts is therefore restricted to raw materials and the early stages of fermentation, where simple sugars and some oxygen may be present. Exceptions to this can be found in breweries that employ open fermentation vessels or foeders (barrel fermentations), where oxygen ingress may act to stimulate growth.
Ethanol tolerance and the ability to grow at low pH are central properties that enable yeasts to spoil fermenting wort and beer, and this is where the most serious impacts can be found. During fermentation, use of an inappropriate production strain or the presence of a killer yeast (see section ‘Beer-spoiling yeasts and killer toxins') can have particularly negative effects. Typically, spoilage is through the production of inappropriate levels of esters, higher alcohols and vicinal diketone (VDK), which cause flavour imbalance during fermentation. However, certain species of yeasts can also produce specific off-flavours including organic acids, sulfur-containing compounds, and phenolics. An additional impact is related to the performance of the culture yeast; the majority of beer-spoiling yeast strains do not sediment in the same fashion as production strains, often displaying a weaker flocculation potential (see ‘Beer-spoiling yeasts and flocculation'). Many non-production yeasts also do not interact with finings, since they do not exhibit a strong negative charge. The result of this is to create cloudy beer with associated off-flavour production due to cell lysis.
Packaged beer that has been filtered and pasteurized rarely undergoes spoilage by yeasts. Although strains show variability in heat tolerance, their ability to withstand temperatures associated with pasteurization are such that their survival is a relatively rare occurrence (Tsang and Ingledew, 1982). When spoilage does occur, this is generally due to carry-over of culture yeast or spoilage yeasts that are smaller and less subject to fining. Some yeasts can also be an issue in traditional cask-conditioned beers, giving rise to quality defects. Typically, the result of contamination post-fermentation is that residual sugars are utilized and yeasty or phenolic off-flavours are produced. Growth can also result in the formation of haze and sediments and in some instances the development of pellicles or surface films that act to ensure the proximity of yeast to headspace oxygen.