Frequency and control of beer- spoiling yeasts
Incidences of beer-spoiling yeasts in breweries
In comparison to reports ofbacterial contamination, there are relatively few surveys documenting the occurrence of yeast contaminants within breweries.
It is debatable why this is the case; either they are genuinely less problematic, or their presence goes largely unnoticed owing to difficulties in isolating and detecting beer-spoiling yeasts. It is also worth noting that contamination is often not as pronounced as with bacterial species and therefore yeast contaminants may remain unnoticed since only minor changes to fermentation performance or flavour profiles are observed, and these may be erroneously attributed to other sources.
In one major study, van der Aa Ktihle and Jes- persen (1998) analysed 101 cropped yeast samples obtained from 45 lager breweries. These were assessed for yeast contamination by plating samples of yeast collected from fermentation vessels onto selective media. In total, 41 of the samples analysed (representing 24 breweries) were identified as containing beer-spoiling yeasts. In total, 126 beer-spoiling yeasts were isolated and identified to the species level, a frequency that was considerably higher than expected. Saccharomyces species accounted for more than 57% of the infections detected, with Pichia and Candida representing 28% and 15% of the contaminants isolated, respectively. In the same study, the capacity of the isolated yeast strains to grow in de-carbonated beer and wort were also examined. Most strains investigated were capable of growth in both wort (99%) and beer (98%). Further analysis revealed that inoculation of 26 of the isolated strains into bottled lager beer resulted in significantly reduced survival and growth. Eight of these isolates were not able to grow in bottled beer even when incubated for 17 days at 21°C. For those isolates that were able to grow, most demonstrated moderate replication potential and a few showed rapid growth. In a separate study, Pham et al. (2011) analysed yeast contaminants isolated from conditioning tanks and fermentation vessels. Analysis of fermentation samples indicated that 90% of isolates were Saccharomyces species, while 61% of samples from conditioning tanks belonged to the Pichia genus (either P. fermentans or P membranifaciens). This was perhaps surprising, given that Pichia are primarily aerobic yeasts. However, these data highlight the possible effects of oxygen in beer post fermentation; in this instance, it is possible that air was either the direct cause of the contamination, or more likely that it facilitated the growth of the Pichia yeast, which had survived through fermentation at subdetection levels.
Irrespective of the source and nature of yeast contamination, there is a range of consequences. These include having a direct negative impact on product quality and consistency, while also increasing product wastage through disposal of contaminated beer. If a contamination issue is not addressed then this could also lead to complaints from the market, bruised consumer confidence, decreased brand loyalty, and potentially a trade recall.