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→Diastatic strains of Saccharomyces cerevisiae
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Diastatic strains of ''Saccharomyces cerevisiae'', historically designated as a variant of ''S. cerevisiae'' (''Saccharomyces cerevisiae'' var. ''diastaticus''), is a group of ''S. cerevisiae'' strains that can ferment certain types of starches and dextrins, and has been identified as a contaminant in breweries and is responsible for a few large recalls. The variant based nomenclature has been called into question (classifying it as a true variant based on one phenotype is not typical in microbiology), and terms like "''STA1+'' strains of ''S. cerevisiae''" or "diastatic ''cerevisiae''" have been proposed as a more scientifically correct designation <ref name="Omega_diastaticus_2020">[https://omegayeast.com/news/improved-functional-assays-and-risk-assessment-for-sta-strains-of-saccharomyces-cerevisiae Improved Functional Assays and Risk Assessment for STA+ Strains of Saccharomyces cerevisiae. Laura T. Burns, Christine D. Sislak, Nathan L. Gibbon, Nicole R. Saylor, Marete R. Seymour, Lance M. Shaner, and Patrick A. Gibney. 2020. Awaiting peer review and publication.]</ref><ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/3308119659216225/?comment_id=3308312722530252&reply_comment_id=3308849332476591 Kristoffer Krogerus. Milk The Funk Facebook group post on diastatic ''cerevisiae'' nomenclature. 02/27/2020.]</ref><ref name="krogerus_diastatic_2020">[https://link.springer.com/article/10.1007/s00253-020-10531-0 A re-evaluation of diastatic Saccharomyces cerevisiae strains and their role in brewing. Kristoffer Krogerus and Brian Gibson. Applied Microbiology and Biotechnology (2020). DOI: https://doi.org/10.1007/s00253-020-10531-0.]</ref>. These strains generally do not produce flavors that are considered unpleasant. For example, acetaldehyde and sulfur dioxide are produced in very low amounts compared to other brewing strains. With the exception of a few strains, such as the "Sacch Trois" strain, most diastatic ''cerevisiae'' strains also produce phenols (POF+), which are considered off-flavors in many beer styles other than Belgian beer styles and German Hefeweizen (there is no genetic link between POF+ and STA1 genes, and the link may just be a coincidence due to human selection). Most strains also produced significant amounts of isoamyl acetate (banana ester) and other fruity esters, making them taste very similar to German wheat strains. A small number of strains also produce above flavor threshold levels of diacetyl <ref name="Meier-Dörnberg_2018">[https://www.ncbi.nlm.nih.gov/pubmed/29518233 Saccharomyces cerevisiae variety diastaticus friend or foe? Spoilage potential and brewing ability of different Saccharomyces cerevisiae variety diastaticus yeast isolates by genetic, phenotypic and physiological characterization. Meier-Dörnberg T, Kory OI, Jacob F, Michel M, Hutzler M. 2018. DOI: 10.1093/femsyr/foy023.]</ref>. This variant is often viewed as a contaminant because of its ability to over-attenuate. A survey of contamination reports in the last ten years at European breweries (50% of which were German breweries, which are obligated by law to report such contaminations) found an increase in reports from 2015, 2016, and 2017. 71% of the contamination incidents originated from the packaging systems (bottling/canning lines). These contaminations were tracked down to the filler environment and/or biofilms in the pipework system of the filler which stemmed from hygienic problems. As such, sometimes contamination can be sporadic with some bottles being contaminated while others are not. The other 29% of the contaminations were tracked down to primary contaminations in the brewhouse, fermentation cellar, and storage cellar <ref name="Meier-Dörnberg">[https://www.mbaa.com/publications/tq/tqPastIssues/2017/Pages/TQ-54-4-1130-01.aspx Incidence of Saccharomyces cerevisiae var. diastaticus in the Beverage Industry: Cases of Contamination, 2008–2017. Tim Meier-Dörnberg, Fritz Jacob, Maximilian Michel, and Mathias Hutzler. 2017. MBAA Technical Quarterly; http://dx.doi.org/10.1094/TQ-54-4-1130-01.]</ref>.
''STA1+'' strains of S. cerevisiae can produce extracellular glucoamylase (also called [https://en.wikipedia.org/wiki/Alpha-glucosidase alpha-glucosidase], which is the same enzyme that ''[[Brettanomyces]]'' produces to break down starches and dextrins). This enzyme is released outside of the cell and can break down the α-1,4 linkages of starches and dextrins releasing glucose that is then fermented by the yeast. The capability to produce this enzyme is encoded by the ''STA1'' gene, which is a fusion of two other genes that are present separately in all ''S. cerevisiae'' yeasts, ''FLO11'' and ''SGA1'' (the ''STA2'' and ''STA3'' genes are the same as ''STA1''; they were initially found on different chromosomes and so they received different names, but they are all the same gene <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/2697088176986046/?comment_id=2697419373619593&reply_comment_id=2698451940183003&comment_tracking=%7B%22tn%22%3A%22R%22%7D Kristoffer Krogerus. Milk The Funk Facebook thread post on the significance of STA2 and STA3 genes in diastatic strains. 06/01/2019.]</ref>). Not all strains containing one of these genes produce the glucoamylase enzyme or are as effective as others at metabolizing dextrins <ref>[https://link.springer.com/article/10.1007%2FBF00365634 STA10: A gene involved in the control of starch utilization by Saccharomyces. Julio Polaina, Melanie Y. Wiggs. 1983.]]</ref><ref>[http://onlinelibrary.wiley.com/doi/10.1002/yea.1102/full Structural analysis of glucoamylase encoded by the ''STA1'' gene of Saccharomyces cerevisiae (var. diastaticus). Ana Cristina Adam, Lorena Latorre-Garcia, Julio Polaina. 2004.]</ref>. It has been reported by some microbiologists that most brewing strains that contain the ''STA1'' gene do produce the glucoamylase enzyme <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1935201836508021/?comment_id=1936604203034451&reply_comment_id=1937166892978182&comment_tracking=%7B%22tn%22%3A%22R7%22%7D Richard Preiss. Milk the Funk thread about ''STA1'' gene correlation to glucoamylase production. 12/31/2017.]</ref><ref name="mbaa_diastaticus">[http://masterbrewerspodcast.com/068-diastaticus-part-1 Matthew Peetz of Inland Island and Tobias Fischborn of Lallemand. "Master Brewers Association Podcast" 12/25/2017.]</ref>(~16 mins). A study that surveyed 18 strains of ''S. cerevisiae'' that contain the ''STA1'' gene found that only one was not able to ferment dextrins <ref name="Meier-Dörnberg_2018" />. Richard Preiss has also reported that WLP351 has the ''STA1'' gene, but is not able to ferment dextrins <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1888017211226484/?comment_id=2013050695389801&reply_comment_id=2013355312026006&comment_tracking=%7B%22tn%22%3A%22R9%22%7D Richard Preiss. Milk The Funk Facebook group thread on diastatic strains that do not ferment dextrins. March 2018.]</ref>. Krogerus et al. (2019) discovered that a region of 1162 base pairs just upstream of the ''STA1'' gene called "a promoter gene" is missing in strains that test positive for the ''STA1'' gene but do not test positive for fermenting starches, dextrins, or secreting the enzyme. They were able to demonstrate that this promoter gene region is needed for the ''STA1'' gene to become expressed. They also discovered that ''STA1'' gene is found in the Beer 2 group of yeast (see [[Saccharomyces#History_of_Domestication|History of Domestication]] above), and wild ''S. cerevisiae'' strains do not carry the ''STA1'' gene. Coincidentally, Beer 2 yeast strains lack the genes that the Beer 1 yeast strains do for fermenting maltotriose, yet Beer 2 yeasts ferment maltotriose just fine; it was discovered by Krogerus et al. (2019) that the ''STA1'' gene allows the Beer 2 yeasts to ferment maltotriose (although this exact mechanism is not known yet). It was proposed that the ''STA1'' gene evolved in the Beer 2 yeast strains as a means to take advantage of grain fermentation as an evolutionary advantage, and the existence of strains that are missing the promoter gene could be because humans later started selecting for strains that didn't dry the beer out too much <ref name="krogarus_2019">[https://link.springer.com/article/10.1007/s00253-019-10021-y A deletion in the ''STA1'' promoter determines maltotriose and starch utilization in ''STA1+'' Saccharomyces cerevisiae strains. Kristoffer Krogerus, Frederico Magalhães, Joosu Kuivanen, Brian Gibson. 2019. DOI: https://doi.org/10.1101/654681.]</ref>. For more details on the Krogerus et al. (2019) study, see [http://beer.suregork.com/?p=4068 this Suregork Loves Beer blog post] and [https://www.facebook.com/groups/MilkTheFunk/permalink/2697088176986046/ this MTF thread posted by Kristoffer Krogerus].