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→Saccharomyces cerevisiae var. diastaticus
''Saccharomyces cerevisiae'' var. ''diastaticus'' is a variant of ''S. cerevisiae'' 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. 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. However, all ''diastaticus'' strains tested in one study produced phenols (there have been a couple of strains found that do not produce phenols, such as the Sacch Trois strain), with most being above flavor threshold. 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 contaminations 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>.
This variant 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. cereivisae'' 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 Krogarus. Milk The Funk Facebook thread post on the significance of STA2 and STA3 genes in diastaticus 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 ''diastaticus'' 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 diastaticus strains that do not ferment dextrins. March 2018.]</ref>. Krogarus et al. (2019) discovered that a region of genes just upstream of the ''STA1'' gene (promoter) called ''1162 bp'' 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 region is needed for the ''STA1'' gene to become effective. 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 Krogarus 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 ''1162 bp'' promoter could be because humans later started selecting for strains that didn't dry the beer out too much <ref name="krogarus_2019">[https://wwwlink.biorxivspringer.orgcom/contentarticle/10.11011007/654681v1 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 Krogarus 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 Krogarus].
When beer containing this yeast is packaged too early, it will continue to slowly ferment dextrins and cause over-carbonation. When pitching a proper cell count or pitching rate of a ''diastaticus'' yeast strain into the wort, some strains will fully ferment as quickly or nearly as quickly as any other brewers yeast, while other strains may take as long as 16 days to fully ferment a simple 12.4°P (1.050 SG) wort. A highly dextrinous wort may take longer to fully ferment. The problem of slow fermentation in already packaged beer is usually only a concern when ''diastaticus'' is introduced as a very small cell count, for example as an accidental contamination <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1982499288444942/?comment_id=1983013578393513&comment_tracking=%7B%22tn%22%3A%22R1%22%7D Caroline Whalen Taggart. Milk The Facebook post on how quickly diastaticus ferments. 02/09/2018.]</ref>. The enzyme produced by these strains is heat stable and can continue to work on starches and dextrins even after the yeast is killed by heat pasteurization <ref>[https://www.mbaa.com/publications/tq/tqPastIssues/1983/Abstracts/tq83ab19.htm Factors That Control the Utilization Of Wort Carbohydrates by Yeast. G. G. Stewart, I. Russell, and A. M. Sills. MBAA Technical Quarterly, Vol. 20, No. 1, 1983.] </ref>. Almost all strains of ''S. cerevisiae'' var. ''diastaticus'' are able to ulilize all of the sugars found in wort (e.g. glucose, fructose, sucrose, maltose, maltotriose), although one strain tested in a recent study could not efficiently ferment starch, maltose, (80%) or maltotriose (45%) <ref name="Meier-Dörnberg_2018" />. ''S. cerevisiae'' var ''diastaticus'' can grow at 37°C and can also remain viable at refrigeration temperatures <ref>[http://www.ebc2017.com/inhalt/uploads/P095_Begrow.pdf Wade Begrow. "Recent notable microbiological contaminations of craft beer in the United States". Presentation poster at EBC 2017. Retrieved 11/19/2017.]</ref>. Some strains of ''diastaticus'' are as flocculant as typical ale strains, while others are less flocculant <ref name="Meier-Dörnberg_2018" />. One strain of ''diastaticus'', the Belle Saison strain from Lallemand, is killer neutral, which means that killer wine strains will not kill it (it is not verified if Belle Saison is the same as WY3711, although it is suspected to be). Other strains of ''diastaticus'' may or may not be killer neutral (more data is needed; see [[Saccharomyces#Killer_Wine_Yeast|killer wine strains]] for more information on this topic).