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A '''pellicle''' is a type of biofilm-like formation which appears on the surface of beer that consists of an aggregation of cells, proteins, and long chain sugars (polysaccharides)polymers <ref name="Branda"></ref>. The "bubble" formations are caused by trapped CO2 beneath the pellicle film. Pellicles are often formed by [[Brettanomyces]], [[Pediococcus]], and [[Lactobacillus]], as well as ''Acetobacter spp'' <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 <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 <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>. Note that "pellicles" in brewing have not been scientifically defined, and there have been objections to defining them as a type of biofilm. See [[Pellicle#Scientific_Terminology|Scientific Terminology]] for more information.
==Characteristics==
The presence of ethanol in concentrations of 1-4% encourages pellicle production in some strains of ''Acetobacter pasteurianus'' (although the presence of ethanol encourages pellicle formation, the amount of ethanol did not make a difference). In some strains of ''A. pasteurianus'', sugar encourages pellicle formation. It has been suggested that CPS production in ''Acetobacter'' is a function of stress tolerance by acting as a barrier around the cell that protects it from acetic acid in the environment. Strains that form a pellicle in the presence of ethanol and/or higher temperatures can fully ferment vinegar whereas those that do not create a pellicle perform poorly in comparison <ref name="Perumpuli"></ref>.
==Handling/Racking==
[[File:Pellicle bottle.JPG|thumb|Pellicle formation in a bottle a few days after packaging; courtesy of Dan Pixley]]
Often a pellicle will form on the surface of the beer inside the bottle shortly after packaging. This is no different than the pellicle forming in the fermentation vessel, and presumably occurs because of the oxygen in the headspace of the bottle. The pellicle will eventually settle out either on it's own during aging, or when the bottle is refrigerated. Other than for aesthetics, there should be no concern if this happens.
==Scientific Terminology==
As with some things in science that are not greatly explored, terminology isn't always agreed upon or fully established, and thus researching such a topic without a lot of prior knowledge can be challenging. Pellicles are a good example of this.
It is sometimes stated that a pellicle is a type of biofilm. However, there have been objections made to defining a pellicle as a type of biofilm. Many microbiology books define a [http://www.biofilm.montana.edu/node/2390 biofilm] as an aggregate of microorganisms where the cells adhere to each other on a solid surface <ref>[https://books.google.com/books?id=-ZDKAQAAQBAJ&pg=PT87&lpg=PT87&dq=brock+microbiology+biofilm&source=bl&ots=PwNZ20KFqq&sig=19lvlfqYj8bE7roiA7g4oJ3Xl0E&hl=en&sa=X&ved=0CCcQ6AEwAWoVChMImZaUmN_AxwIVVFmICh1pQgk0#v=onepage&q=brock%20microbiology%20biofilm&f=false e-Study Guide for Brock Biology of Microorganisms, textbook by Michael T. Madigan.]</ref>. Pellicles do not adhere to a solid surface, and are said to form at the "air-liquid interface" (surface of beer). As such they do not fit the definition of a biofilm in the strictest sense. However, the IUPAC, which is an international federation of National Adhering Organizations that works to standardize nomenclature in chemistry and other fields of science, defines a biofilm as an "aggregate of microorganisms in which cells that are frequently embedded within a self-produced matrix of extracellular polymeric substance (EPS) adhere to each other and/or to a surface <ref>[http://www.degruyter.com/view/j/pac.2012.84.issue-2/pac-rec-10-12-04/pac-rec-10-12-04.xml Pure and Applied Chemistry. Volume 84, Issue 2, Pages 377–410, ISSN (Online) 1365-3075, ISSN (Print) 0033-4545, DOI: 10.1351/PAC-REC-10-12-04, January 2012.]</ref>." The "and/or" part of the definition allows for pellicles to be defined as biofilms since the cells adhere to each other.
Biofilms are extremely diverse 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). They are also not colonies within an adhesive <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>. 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>), 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.
The importance of understanding this terminology becomes apparent when trying to research the topic of pellicles in beer. Currently they have not been studied much at all, whereas pellicles of dentistry and microbiology have been studied in depth, as well as biofilms. Thus, brewers should take care when reading scientific publications.
==Images==