Presence of Saccharomyces cerevisiae subsp. diastaticus in industry and nature and spoilage capacity of its vegetative cells and ascospores

Do microbes evade domestication? - Evaluating potential ferality among diastatic Saccharomyces cerevisiae

Certain lineages of the wine, beer and bread yeast Saccharomyces cerevisiae have diastatic activity. They contain the chimeric gene STA1 that codes for an extracellular glucoamylase which enables the strains to degrade starch and dextrins. Beer contaminations by diastatic yeasts can be dangerous because they can cause super-attenuation due to the consumption of otherwise non-fermentable oligosaccharides, gushing and off-flavours. Given that diastatic yeasts can be used for beer fermentation it is important to understand the relationship between production and contaminant strains, their natural reservoirs and entry routes into the brewery. Here, we analyze real cases of contamination in a Portuguese craft brewery over a period of 18 months. By analyzing with whole genome sequencing several contaminants, we show that recurrent contaminations by diastatic yeasts are caused by environmental strains. Moreover, some beer contaminants were closely related to diastatic environmental strains isolated in Botswana. We observed the widespread presence of domestication signatures in diastatic strains. Moreover, the combined phylogeny of STA1 and its ancestor, SGA1, suggested a single STA1 origin, as ancient as the entire lineage of diastatic yeasts. Together, our results suggest that diastatic yeasts isolated in natural settings could be escaping from domestication settings and becoming feral.

Heat resistance acquirement of the spoilage yeast Saccharomyces diastaticus during heat exposure

The main fungal cause of spoilage of carbonated fermented beverages in the brewing industry is the amylolytic budding yeast Saccharomyces cerevisiae subsp. diastaticus (Saccharomyces diastaticus). Heat treatment is used to avoid microbial spoilage of the fermented beverages. Therefore, the spoilage capacity of S. diastaticus may be linked to its relative high heat resistance. Here, we assessed whether S. diastaticus can acquire heat resistance when exposed to heat stress. To this end, ascospores of S. diastaticus strain MB523 were treated at 60°C for 10 min followed by growing the surviving spores on a glucose-containing medium. The resulting vegetative cells were then allowed to sporulate again in sporulation medium. This cycle of heat treatment, vegetative growth, and sporulation was performed eight times in three independent lineages. After these eight cycles, the sporulation rate was similar to the start (∼75%) but the resulting ascospores were more heat resistant. The time needed to kill 90% of the population at 60°C (i.e. the D₆₀-value) increased from 6.5 to 9.0 min (p = 0.005). The vegetative cells also showed a trend to increased heat resistance with an increase in the D₅₂-value from 9.2 to 16.2 min (p = 0.1). In contrast, heat resistance of the vegetative cells that had not been exposed to heat during the eight cycles had been reduced with a D₅₂-value of 4.2 min (p = 0.003). Together, these data show that S. diastaticus MB523 can easily acquire heat resistance by inbreeding while subjected to heat stress. Conversely, heat resistance can be easily lost in the absence of this stress condition, indicative of a trade-off for heat resistance.

Identification of spoilage microflora in draught beer using culture-dependent methods

AIMS

To determine whether the culture-dependent spoilage microflora found in draught beer are influenced by beer style.

METHODS AND RESULTS

Four beer styles - lager, ale, stout, and cask ale - were sampled twice from five different public houses (accounts) in four different locations. The microbiological quality of the dispensed beers was determined by a culture-dependent method ('forcing'), measuring the increase in turbidity after incubation at 30°C. The quality of draught beer varied from 'excellent' to 'poor' with cask beer samples having a higher Quality Index (90%) with keg ale the lowest (67.5%). With PCR amplified DNA (ITS1, ITS4, 16S rRNA primers) and BLAST identification of microflora, 386 colonies from agar plates were identified with 28 different microorganisms from five genera of yeast and six of bacteria. Seven microorganisms were found in all beer styles with Brettanomyces bruxellensis, B. anomalus and Acetobacter fabarum representing 53% of the identified microorganisms. A subsequent, limited study using PALL multiplex PCR GeneDisc technology on forced samples (without selection on plates) suggests that draught beer microflora is qualitatively broader. It is noteworthy that the microflora of spoilt draught beer resembles that involved in the production of Belgian Lambic sour beers.

CONCLUSIONS

Draught beer was of variable quality. Culture-dependent analysis suggests that species of Brettanomyces and Acetobacter are core microflora with some microorganisms being associated with beer style.

SIGNIFICANCE AND IMPACT OF THE STUDY

The microbiological quality of draught beer is important both commercially and to the Consumer. Here, we report the core and diverse microflora found in different styles of draught beer using culture-dependent methods.