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A few species/strains of ''Lactobacillus'' can create all of the amino acids that they need for growth. These species are known as '''prototrophic'''. However, most species/strains can only produce some of the amino acids required for growth and must obtain the other amino acids from their environment. These species are known as '''auxotrophic''' for the amino acids that they cannot produce themselves <ref>[http://www.ncbi.nlm.nih.gov/pubmed/17993552 Phenotypic and genotypic analysis of amino acid auxotrophy in Lactobacillus helveticus CNRZ 32. Christiansen JK, Hughes JE, Welker DL, Rodríguez BT, Steele JL, Broadbent JR. 2007.]</ref>. Auxotrophic ''Lactobacillus'' can break down proteins in their environment in order to consume the amino acids that they cannot make themselves, including foam forming proteins in beer, through a process called '''proteolysis''' <ref name="Todar"></ref>. Proteolysis is the breakdown of various proteins into smaller polypeptides or amino acids through the use of various enzymes <ref>[https://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/26351/ShellhammerThomasH.FoodScience.TextureProteolysisViable.pdf?sequence=1 Texture, proteolysis and viable lactic acid bacteria in commercial Cheddar cheeses treated with high pressure. Cheryl Wick, Uwe Nienaber, Olga Anggraeni, Thomas H Shellhammer and Polly D Courtney. 2002. Retrieved 7/7/2015.]</ref><ref name="Haq_Mukhtar">[http://www.researchgate.net/profile/Hamid_Mukhtar4/publication/230245468_PROTEASE_BIOSYNTHESIS_FROM_LACTOBACILLUS_SPECIES_FERMENTATION_PARAMETERS_AND_KINETICS/links/00b7d52c403b158288000000.pdf Protease Biosynthesis from Lactobacillus Species: Fermentation Parameters and Kinetics. Ikram-Ul_Haq and Hamid Mukhtar. Jan 2007. Retrieved 7/7/2015.] </ref><ref>[https://en.wikipedia.org/wiki/Proteolysis Proteolysis. Wikipedia. Retrieved 7/7/2015.]</ref>. Both homofermentative and heterofermentative species have been observed to have proteolytic activity <ref>[https://books.google.com/books?id=qMreBwAAQBAJ&pg=PA133&lpg=PA133&dq=lactobacillus+homofermentative+proteolytic&source=bl&ots=bjxq9rGdha&sig=pCz4WeKek3zTv5oL6Rui1dUuEqw&hl=en&sa=X&ei=AJqcVeT0O8XkoAS0-oC4CQ&ved=0CF0Q6AEwBw#v=onepage&q=lactobacillus%20homofermentative%20proteolytic&f=false Brewing Microbiology. Fergus Priest. Springer Science & Business Media, Jun 29, 2013. Pg 133.]</ref>. This process is a large part of cheese and yogurt fermentation. ''Lactobacillus'' species that have been identified as breaking down proteins (mostly in cheese or yogurt) include ''Lactobacillus bulgaricus'', ''Lactobacillus rhamnosus'', ''Lactobacillus casei'', ''Lactobacillus paracasei'', ''Lactobacillus helveticus'', ''Lactobacillus delbrueckii'', ''Lactobacillus brevis'', ''Lactobacillus cellobiosus'', ''Lactobacillus fermentum'', and ''Lactobacillus plantarum'' <ref name="Haq_Mukhtar"></ref>. Results of using various species/strains appears to demonstrate that different species/strains are worse for degrading head retention proteins than others. For example, it's been reported that B.H. Meyer says that souring with ''L. delbruekii'' creates better head retention than souring with other species such as ''L. brevis''. <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1105663546128525/?comment_id=1105772812784265&offset=0&total_comments=107&comment_tracking=%7B%22tn%22%3A%22R9%22%7D Conversation with Kristen England on MTF. 7/7/2015.]</ref><ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1096427697052110/?comment_id=1096496983711848&offset=0&total_comments=45&comment_tracking=%7B%22tn%22%3A%22R9%22%7D Conversation with Jace Marti on MTF about L. delbruekii head retention problems. 6/21/2015.]</ref>. Different strains of the same species may also have different levels of proteolytic activity <ref>[http://www.ncbi.nlm.nih.gov/pubmed/8568030 Comparison of proteolytic activities in various lactobacilli. Sasaki M, Bosman BW, Tan PS. 1995.]</ref>.
Proteolytic activity has been shown to decrease as pH falls below 5.0 for some species of ''Lactobacillus'' <ref name="Haq_Mukhtar"></ref>. In order to combat poor head retention in beers that are soured with ''Lactobacillus'', it has been suggested by German brewing scientist, Burghard Hagen Meyer, to [[Sour_Worting#How_to_Pre-Acidify|lower the pH of the wort to 4.5-4.8]] with food grade lactic acid or phosphoric acid before pitching ''Lactobacillus'' <ref name="Gail">[https://sourbrewster.wordpress.com/2012/09/18/berliner-weisse-the-old-time-kettle-souring-technique/ ''Berliner Weisse – the old-time kettle-souring technique.'' Brewing Sour blog, by Gail Ann Williams.September 18, 2012. Retrieved 7/7/2015.]</ref><ref>[http://ingenuitybrew.blogspot.com/2013/06/berliner-weisse-test.html ''Berliner Weisse Test''. Ingenuity Brew Blog. June 4, 2013. Retrieved 7/7/2015.]</ref><ref>[http://www.themadfermentationist.com/2012/06/100-lactobacillus-berliner-weisse.html ''100% Lactobacillus Berliner Weisse.'' The Mad Fermentationist Blog, by Michael Tonsmeire. June 25, 2012. Retrieved 7/7/2015.]</ref>. Although the lowering of the wort's pH inhibits the proteolytic activity of ''Lactobacillus'', it does not stop it completely, so continued exposure to ''Lactobacillus'' over time may still degrade the available head formation proteins. Additionally, ingredients that increase head retention such as unmalted chit, malted wheat, and carafoam have been used to help combat poor head retention in beers soured by ''Lactobacillus'' <ref name="Gail"></ref><ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1105839682777578/?comment_id=1105942362767310&offset=0&total_comments=27&comment_tracking=%7B%22tn%22%3A%22R8%22%7D Conversation with Richard Preiss on MTF. 7/7/2015.]</ref>. Professional brewer [https://www.facebook.com/groups/MilkTheFunk/permalink/1105839682777578/ Kristen England of Bent Brewstillery] tested [http://www.mbaa.com/districts/MidSouth/mash/Documents/2014%20PDF%20Hop%20Product%20Applications%20Presentation.pdf?Mobile=1&Source=%2Fdistricts%2FMidSouth%2Fmash%2F_layouts%2Fmobile%2Fview.aspx%3FList%3D85ff6384-aa05-491f-8ef3-d2e9eea713f2%26View%3Da234a10f-469b-4edd-a8a9-9a96b00750ee%26CurrentPage%3D1 Hexa Iso Hop Extract] by dosing at 4 times the recommended dosage and found that it greatly increased head retention in a Berliner Weisse (3.5% abv, pH 3.1, TA ~1, BU 5), with a minor taste difference. Kristen recommends experimenting with lower dosages to avoid too much flavor impact.
Another method that has been reported to help with head retention when [[Sour Worting]] (kettle souring) is to add a pound of DME per 5 gallons of wort during the heat pasteurization process (after the wort has been soured with ''Lactobacillus''). One could also steep specialty grains such as wheat malt, chit malt, carafoam, or carapils, and add the extract into the kettle during the heat pasteurization or boiling process. This will add back head formation proteins that were lost during the ''Lactobacillus'' fermentation <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1069228273105386/?comment_id=1069266549768225&offset=0&total_comments=24&comment_tracking=%7B%22tn%22%3A%22R8%22%7D Conversation with Gareth Young on MTF. 05/08/2015.]</ref><ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1137179879643558/?comment_id=1137362989625247&offset=0&total_comments=8&comment_tracking=%7B%22tn%22%3A%22R%22%7D Conversation with Paul Finney on MTF in regards to head retention of Berliner Weisse. 08/29/2015.]</ref>.