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to do: - https://www.facebook.com/groups/MilkTheFunk/permalink/3292440674117457/ [[File:Brussels brett blend pellicle1.jpg|thumb|Pellicle from The Yeast Bay Brussels Brett Blend; courtesy of Dan Pixley]] A '''pellicle''' is a type of [[Quality_Assurance#Biofilms |biofilm]] which appears on the surface of beer (the classification of pellicles as a type of biofilm has recieved some debate. See [[Pellicle#Scientific_Terminology|Scientific Terminology]]). It consists of an aggregation of cells, proteins, and polymers <ref name="Branda"></ref><ref name="matt"></ref>. The "bubble" formations are caused by trapped CO2 beneath the pellicle film. Pellicles are often formed by ''[[Brettanomyces]]'', ''Acetobacter'' <ref name="Perumpuli">[http://ir.kagoshima-u.ac.jp/bitstream/10232/21479/4/Diss_Perumpuli+Arachchige_Buddhika+Niroshie_RNK999_2014.pdf.pdf Pellicle of thermotolerant Acetobacter pasteurianus strains: Characterization of polysaccharide and induction patterns. Perumpuli Arachchige Buddhika Niroshie. 2014-09-30.]</ref>, and other gram-negative bacteria such as some species of ''Acinetobacter'', ''Escherichia'', ''Burkholderia'', ''Dickeya'', ''Gluconacetobacter'', ''Pseudomonas'', ''Salmonella'', ''Shewanella'', and ''Vibrio''. Gram-positive bacteria ''Bacillus subtillis'' can also form a pellicle <ref name="Armitano">[http://www.ncbi.nlm.nih.gov/pubmed/25756106 Gram-negative bacteria can also form pellicles. Armitano J, Méjean V, Jourlin-Castelli C. Environ Microbiol Rep. 2014 Dec.]</ref>. It can also be formed by ''[[Saccharomyces]]'' in rare occasions (most likely wild species more so than brewer's yeast), and it might be possible for ''[[Lactobacillus]]'' and ''[[Pediococcus]]'' to form a pellicle (according to Matt Humbard; see reference), however other references have not been found as far as we know <ref name="matt">[http://phdinbeer.com/2015/01/30/beer-microbiology-what-is-a-pellicle/ Beer Microbiology – What is a pellicle? A PhD in Beer blog. Dr. Matt Humbard. 01/30/2015. Retrieved 04/26/2015.]</ref>. Pellicles are not formed by and should not be confused with [[mold]].
==Characteristics==
===Introduction===
Pellicles presumably form when the surface of the beer is exposed to even small amounts oxygen <ref>[http://www.themadfermentationist.com/2009/11/brewing-sour-beer-at-home.html Brewing Sour Beer at Home. The Mad Fermentationist Blog. Michael Tonsmeire. 11/06/2009. Retrieved 02/28/2015.]</ref> Why microbes in sour/funky beer form a pellicle when exposed to oxygen is unknown. The most likely [http://phdinbeer.com/2015/01/30/beer-microbiology-what-is-a-pellicle/ hypothesis by Dr. Matt Humbard is that the formation of a pellicle] allows the organism (particularly ''[[Brettanomyces]]'', which prefers aerobic fermentation) to access the small amount of oxygen that is present in the headspace of the fermentation vessel. Another hypothesis, and one that may be less accurate according to Dr. Matt Humbard, is that the pellicle protects the beer from other microorganisms <ref name="matt"></ref>. Yet another hypothesis that is often cited is that the pellicle protects the beer from oxygen itself <ref>[http://jesterkingbrewery.com/pellicle-photos Jester King Blog on Pellicles. Retrieved 09/24/2015.]</ref>; however, however arguments evidence and rationale for this hypothesis are is lacking.
Popular thought is that the formation of a pellicle is not indicative of the quality of the sour beer that is being produced; it is only an indication that oxygen has entered the fermentation vessel and that the microbes are reacting to that exposure. One myth about pellicles is that the sour beer will be ready to package once the pellicle falls out; there is actually no correlation between the maturity of the beer and pellicle formation or dissipation. Some sour beers never form pellicles, and turn out fine as well, so the formation of a pellicle has no correlation with the quality of the beer.
See also;
* [https://www.facebook.com/groups/MilkTheFunk/permalink/2002649679763236/?comment_id=2002789613082576&comment_tracking=%7B%22tn%22%3A%22R%22%7D MTF thread speculating on the function of pellicles in Sherry Flor.]
===Yeast===
Pellicle formation by microbes found in sour beer such as ''[[Brettanomyces]]'' and ''[[Lactobacillus]]'' has not been widely studied. One report by De Roos et al. (2018) reported that in lambic casks that were studied, a pellicle formed at around 6 months into spontaneous fermentation. This pellicle formation was attributed to by ''Brettanomyces'' and oxidative yeasts, and this corresponded with a decline in acetic acid bacteria populations. The researchers hypothesized that the formation of the pellicle was the cause of the reduction in acetic acid bacteria populations <ref name="De_roos_AAB_2018">[https://journals.asm.org/doi/10.1128/AEM.02846-17 Temporal and Spatial Distribution of the Acetic Acid Bacterium Communities throughout the Wooden Casks Used for the Fermentation and Maturation of Lambic Beer Underlines Their Functional Role. ASM Journals. Applied and Environmental Microbiology. Vol. 84, No. 7. DOI: https://doi.org/10.1128/AEM.02846-17.]</ref>.
===Acetobacter===
''Acetobacter spp.'' produce homo and heteropolysaccharides (polysaccharides consisting of one type of sugar or more than one type of sugar, respectively <ref>[http://www.britannica.com/science/heteropolysaccharide Heteropolysaccharide. Encyclopedia Britannica. Retrieved 09/05/2015.]</ref>) that attach to the surface of the cells (capsular polysaccharides - '''CPS'''), as well as polysaccharides that are secreted into the medium in which they live (extracellular polysaccharides, or exopolysaccharides - '''EPS'''). CPS is the mechanism that allows pellicle formation in ''Acetobacter'' as the cells tightly associate to one another via CPS <ref name="Perumpuli"></ref>.
Biofilms are extremely diverse and abundant in nature. Examples of biofilms in the classical sense include dental plaque and the green film produced by algae that covers stones in water streams <ref name="montana">[http://www.biofilm.montana.edu/node/2390 Center For Biofilm Engineering. Montona State University. Retrieved 09/05/2015.]</ref>. Biofilms are encountered in brewing in the form of contaminating microorganisms and poor cleaning/sanitation techniques. Biofilms are a common source of persistent brewhouse infections and can be resistant to the actions of many cleaning and sanitizing agents <ref name="montana"></ref><ref name="MTF_Thread"></ref>. Pellicles in beer do not attach to a solid surface, they appear on the "air-liquid interface" (the surface of the beer) <ref name="MTF_Thread">[https://www.facebook.com/groups/MilkTheFunk/permalink/1131334646894748/?comment_id=1131986723496207&offset=0&total_comments=57&comment_tracking=%7B%22tn%22%3A%22R3%22%7D Conversation about Pellicles on MTF. 08/20/2015.] </ref>. To make matters even more confusing, the two established definitions of a "pellicle" in biology only include the outer boundary of a protozoa cell <ref>[https://books.google.com/books?id=ACBcDjDLuNAC&pg=PA38&lpg=PA38&dq=pellicle+biology&source=bl&ots=58e43A_9h-&sig=ViakT4Ectu4DOlqoSWRCPFJ73iw&hl=en&sa=X&sqi=2&ved=0CGIQ6AEwDGoVChMIp6WkouTAxwIVgpmICh0F1wXO#v=onepage&q=pellicle%20biology&f=false Biology of Protozoa. D.R. Khanna. Discovery Publishing House, Jan 1, 2004. Pg 38.]</ref>, and the protein film that forms on the surface of teeth <ref>[https://en.wikipedia.org/wiki/Dental_pellicle Wikipedia. Dental Pellicle. Retrieved 08/23/2015.]</ref>.
The usage of the term "biofilm" has been used to describe the layer of film that covers sherry known as "Flor" <ref>[http://aem.asm.org/content/76/12/4089.full Ethanol-Independent Biofilm Formation by a Flor Wine Yeast Strain of Saccharomyces cerevisiae. Severino Zara, Michael K. Gross, Giacomo Zara, Marilena Budroni and Alan T. Bakalinsky. 2010.]</ref><ref>[http://femsle.oxfordjournals.org/content/femsle/237/2/425.full.pdf FLO11 is essential for flor formation caused by the C-terminal deletion of NRG1 in Saccharomyces cerevisiae. Mari Ishigami, Youji Nakagawa, Masayuki Hayakawa, Yuzuru Iimura. 2004.]</ref>. The word "pellicle" generally isn't used, although it has appeared on occasion when referring to sherry flor in the 1960's <ref>[http://www.ncbi.nlm.nih.gov/pubmed/5311988 On the pellicle formation by “flor” yeasts. Cantarelli C, Martini A. Antonie Van Leeuwenhoek. 1969.]</ref>, and as least once in brewing science from the 1960's <ref name="Macrae_1964">[https://onlinelibrary.wiley.com/doi/abs/10.1002/j.2050-0416.1964.tb02001.x SIGNIFICANCE OF THE USE OF HOPS IN REGARD TO THE BIOLOGICAL STABILITY OF BEER: I. REVIEW AND PRELIMINARY STUDIES. R. M. Macrae. 1964.]</ref>. More recently, there have been studies that define a "pellicle" like we see in fermentation as a type of biofilm that forms on the air-liquid interface of a liquid (see references: <ref name="Armitano"></ref><ref>[http://www.ncbi.nlm.nih.gov/pubmed/26122431 Motility, Chemotaxis and Aerotaxis Contribute to Competitiveness during Bacterial Pellicle Biofilm Development. Hölscher T, Bartels B, Lin YC, Gallegos-Monterrosa R, Price-Whelan A, Kolter R, Dietrich LE, Kovács ÁT. J Mol Biol. 2015 Jun 26.]</ref><ref>[http://www.ncbi.nlm.nih.gov/pubmed/24035282 Modulation of curli assembly and pellicle biofilm formation by chemical and protein chaperones. Andersson EK, Bengtsson C, Evans ML, Chorell E, Sellstedt M, Lindgren AE, Hufnagel DA, Bhattacharya M, Tessier PM, Wittung-Stafshede P, Almqvist F, Chapman MR. Chem Biol. 2013 Oct 24.]</ref><ref name="Branda">[http://www.horadent.com/uploads/media/2005_Branda.pdf Biofilms: the matrix revisited. Steven S. Branda, Ashild Vik, Lisa Friedman and Roberto Kolter. Jan 2005.]</ref><ref>[http://onlinelibrary.wiley.com/doi/10.1046/j.1365-2958.2003.03877.x/full Genes involved in matrix formation in Pseudomonas aeruginosa PA14 biofilms. Lisa Friedman andRoberto Kolter. Dec 2003.]</ref><ref>[http://aem.asm.org/content/77/15/5079.short Increased Transfer of a Multidrug Resistance Plasmid in Escherichia coli Biofilms at the Air-Liquid Interface. Jaroslaw E. Król, Hung Duc Nguyen, Linda M. Rogers, Haluk Beyenal, Stephen M. Krone1, and Eva M. Top. 2011.]</ref><ref>[https://books.google.com/books?id=PO3QrZJnuGgC&pg=PA7&dq=biofilm+air-liquid+interface&hl=en&sa=X&ved=0CDAQ6AEwBGoVChMIjP--39D5xwIVFDWICh285gg_#v=onepage&q=biofilm%20air-liquid%20interface&f=false Bacterial Biofilms. Tony Romeo. Springer Science & Business Media, Feb 26, 2008. Pg 7.]</ref>). Lebleux et al. (2019) defines biofilm as, "Biofilms are complex associations of single- and multiple- species interconnected cells embedded in a hydrated self-produced matrix established at a solid/liquid or liquid/air interfaces." <ref>[https://www.sciencedirect.com/science/article/abs/pii/S0168160519303952 New advances on the Brettanomyces bruxellensis biofilm mode of life. Manon Lebleux, Hany Abdo, Christian Coelho, Louise Basmaciyan, Warren Albertin, Julie Maupeu, Julie Laurent, Chloé Roullier-Gall, Hervé Alexandre, Michèle Guilloux-Benatier, Stéphanie Weidmann, Sandrine Rousseaux. 2019. DOI: https://doi.org/10.1016/j.ijfoodmicro.2019.108464.]</ref> However, however those objecting to the definition of a biofilm as including air-liquid interface aggregates say that the use of the term in a few studies does not warrant a change in the textbook definition <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1142388839122662/ Discussion of this wikipage on MTF. 09/06/2015.]</ref>. The importance of understanding the established terminology of "biofilms" and "pellicles" versus the brewing terminology becomes apparent when trying to research the topic of pellicles in beer. Currently beer pellicles have not been studied scientifically much at all, whereas pellicles of dentistry and microbiology have been studied in depth, as well as biofilms in general. Thus, brewers should take care when reading scientific publications, and understand that brewing terminology does not usually overlap with scientific terminology in regards to pellicles.
==Images==