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The Saccharomyces genus is perhaps the most well described of all the yeasts, due to the widespread use of S. cerevisiae for industrial purposes, and the availability of many full genome sequences for analysis. Cells are typically globose or ellipsoidal, and vegetative reproduction is by multilateral budding. Most species are capable of forming pseudohy- phae, although this rarely happens in S. pastorianus (lager) yeasts or in S. cerevisiae (ale) brewing strains. Within the genus, most species are able to replicate sexually, producing characteristic tetraploid asci (Fig. 11.4). However, brewing yeasts belonging to S. cerevisiae or S. pastorianus only do so rarely, due to their genetic complexity and hybrid status. As beer-spoiling yeasts, Saccharomyces strains can be found in various locations within the brewery, but are predominantly associated with the fermentation stages of the process.

From a brewing perspective, the term ‘Saccharomyces beer-spoiling yeast' invariably refers to S. cerevisiae strains. However, most Saccharomyces species are capable of contaminating wort, including strains belonging to S. bayanus, S. kudriavzevii, and S. mikatae. In general, Saccharomyces yeasts display properties that are relatively similar to brewing strains, and are therefore difficult to detect, while representing a very serious threat to the brewing process. It is also important to reiterate that the definition of beer-spoiling yeast includes production strains and variants that have not been directly introduced to the process by the brewer. Consequently, this can include the accidental mixing of different types of brewing yeasts (ale/lager) or the use of an incorrect strain within either category. The mixing of production strains used for ale- and lager-type products can be especially problematic due to their intrinsic differences in fermentation properties. This is apparent when considering their response to fermentation temperature (lager products are generally fermented at colder temperatures), the typical flavour profiles associated with these two styles of beer (ale strains produce more fruity notes), and their flocculation characteristics (lager strains are traditionally classified as bottom- fermenting, while lagers are top-fermenting). However, the precise effects of mixing equivalent ‘types' of production strains are difficult to predict and are largely dependent on strain phenotype. The most likely results include variations in flavour, attenuation rate, flocculation and cropping patterns. However, in reality these may be relatively subtle changes, especially if the level of contamination is low, or if there is a high degree of strain similarity. In the latter instance, differences can sometimes be offset by blending of the product, although this is not desirable or recommended, especially for core brands or premium products.

A similar range of effects can be observed when mutants derived from production strains are used. These can be particularly problematic since they are by nature very difficult to detect, especially if there is a gradual change in the concentration of variants with successive fermentations. Arguably the two most commonly encountered types of mutants are flocculation variants (showing either decreased, or more commonly increased flocculation potential) and respiratory-deficient (RD) mutants with defective mitochondria. Changes to the flocculation characteristics of a culture can have consequences with regard to overall production consistency. For example, an unexpectedly early and heavy crop can result in stuck fermentations or can create issues with regard to mechanically removing the yeast crop. If mitochondrial deficient cells, otherwise known as ‘petite' mutants (due to the production of small colonies on solid agar), are present in significant numbers, this tends to influence beer flavour production. As petite cells are slow to grow and divide, this can lead to a sluggish fermentation with an inappropriate balance of flavours. For example, the presence of petite cells is widely associated with unacceptable levels of diacetyl.

More conspicuous effects can arise when nonproduction Saccharomyces strains are encountered. These often display certain properties that are different and/or undesirable to those found in production yeasts (Jespersen et al., 2000). The most noticeable are those that are capable of producing phenolic off-flavour (POF) compounds (see section ‘Production of phenolic compounds') or that have diastatic activity. Most brewing strains are unable to utilize long-chain sugars (dextrins), often claimed to contribute to mouthfeel in beer. Some S. cerevisiae strains (previously classified separately as S. diastaticus) are amylolytic and possess the STA genes, responsible for glucoamylase production (Tamaki, 1978; Adam et al., 2004). This allows the breakdown of dextrins, leading to superattenuation of wort and resulting in a beer that has an unusually low final gravity and low residual extract. Diastatic yeasts can have more disastrous consequences when associated with unpasteurized bottled beer. The production of abnormally high concentrations of carbon dioxide can increase the risk of exploding bottles. Furthermore, many diastatic strains are also POF+ and the use of residual dextrins for growth can therefore result in phenolic flavour production, as well as haze formation and other off-flavours.

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