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Hops
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Due to the multiple mechanisms for hop antimicrobial activity, multiple resistance mechanisms are necessary for a Gram-positive bacterial cell to successfully be hop-tolerant<ref name="Behr and Vogel, 2010"> [http://aem.asm.org/content/76/1/142.short Behr and Vogel, (2010)] </ref>. Hop resistance of bacteria will vary by species as well as within a species with individual strains. The environment in which strains are cultured and maintained may also influence their hop tolerance. The hop tolerance of lactic acid bacteria strains decreases when they are cultured in hop-free environments and strains cultured in media with increasing concentrations of hop compounds show an increase in hop tolerance<ref name="Sakamoto and Konings, 2003"/>. the stability of hop resistance, or the rate at which it is lost when bacteria are cultured in unhopped wort, varies by strain. It can take up to 1 year for maximum loss of hop resistance, suggesting that in some strains have a relatively stable hop resistance<ref name="Sakamoto and Konings, 2003"/>. Because of this intra-species variability and dependence on how the strains were cultured, it is difficult to give specific advice about the hop-tolerance of a wide range of strains offered from different sources. As a general rule, some common lactic acid bacteria species used in sour beer and found as beer spoilage organisms like ''Lactobacillus brevis'', ''Lactobacillus lindneri'' and ''Pediococcus delbrueckii'' have some resistance to hops<ref name="Sakamoto and Konings, 2003"/>. Brewers seeking to make acidic beers with higher doses of hops may wish to seek out one of these species. Some hop-tolerant species benefit from pre-culturing in media with below-limiting concentrations of compounds before being used in more highly hopped wort or beer<ref name="Simpson and Fernandez, 1992"> [http://onlinelibrary.wiley.com/doi/10.1111/j.1472-765X.1992.tb00636.x/abstract Simpson and Fernandez, 1992]</ref>.
==Hop Derived Compounds In Beer and Biotransformations==
The flavor and aroma compounds found in leaf/pellet hops is different than the hop derived flavor and aroma compounds found in finished beer (other than in the case of dry hopping). The brewing process (particularly boiling), and fermentation greatly affect the composition of flavor and aroma compounds that are found in beer. For example, boiling wort and hops isomerizes non-bitter alpha acids into bitter iso-alpha acids. During boiling of the wort, many compounds found in hops are evaporated, such as many of the various sulfur compounds found in hops. The terpene hydrocarbons which make up most of the hop oil content in hops (myrecene, humulene, and caryophyllene) are completely removed by fermentation. It is believed that these terpene hydrocarbons stick to the yeast cells and fall out of solution during fermentation <ref name="Praet_2012">[http://www.sciencedirect.com/science/article/pii/S1373716311001636 Biotransformations of hop-derived aroma compounds by Saccharomyces cerevisiae upon fermentation. Tatiana Praet, Filip Van Opstaele, Barbara Jaskula-Goiris, Guido Aerts, Luc De Cooman. 2012.]</ref>.
Some carbonyl compounds found in hops (citral, geranial, nerol, citronellal, and methyl ketones) can be used as a food source by yeast during fermentation. ''Cyclic ethers'' such as linalool oxides, karahana ether, hop ether, and rose oxide (aroma of roses <ref>[http://www.thegoodscentscompany.com/data/rw1035651.html "(Z)-rose oxide ". Good Scents Company. Retrieved 12/29/2016.]</ref>), increase after fermentation and have been identified as secondary metabolites produced by yeast during metabolism from hop derived precursors. ''Esters'' found in hops can be converted into ethyl esters by yeast during fermentation; for example geranyl esters found in Cascade hops can be hydrolized into geraniol (flowery), and citronellyl can be hydrolized into citronellol. Yeast strains differ in their ability to convert these compounds. For example, one study found that lager yeast was able to form acetate esters of geraniol and citronellol, but ale yeast was not <ref name="Praet_2012" />.
===Glycosides===
Hops contain glycosides, which are flavor compounds that are bound to a sugar molecule. In their bound form, glycosides are flavorless. Enzymatic activity from some strains of ''Brettanomyces'' can release these bound compounds and release their flavor and aromatic potential. See the [[Glycosides]] page for details.
==Aged Hops==
* [http://www.brewstore.co.uk/specially-aged-fuggles-hops-100-grams Brew Store UK (Leaf - Fuggles).]
* [http://www.brewstore.co.uk/specially-aged-hallertau-hops-100-grams Brew Store UK (Leaf - Hallertau).]
==Techniques==