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Hops
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The primary '''alpha acids''' (humulones) in hops are humulone, cohumulone, and adhumulone. The ratio of these individual acids to each other can vary based on hop variety much like total iso-α-acid percent, though generally the primary acids are humulone and cohumulone. Cohumulone has been identified by some researchers as a source of a more harsh bitterness, although similar research contradicts this statement <ref>[http://www.scielo.br/scielo.php?pid=S0100-40422000000100019&script=sci_arttext&tlng=es Fundamentals of beer and hop chemistry. Denis De Keukeleire. 1999.]</ref>. While alpha acids are insoluble in wort, the isomerized alpha acids (also called isohumulones) which are formed during boiling are soluble. Isomerization leads to roughly a 70%/30% split between ''cis'' and ''trans'' iso-α-acids respectively, with ''cis'' iso-α-acids being more stable over time and more bitter<ref name="Schönberger and Kostelecky, 2012"> [http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.2011.tb00471.x/abstract Schönberger and Kostelecky, 2012]</ref>. Alpha acids themselves do not taste bitter, but isomerized alpha acids (iso-α-acids/isohumulones) contribute to the bitterness of beer and have antimicrobial properties. Isocohumulone is often cited as being more harshly bitter than the other iso-α-acids, but studies of taste perception of individual iso-α-acids have not agreed with this. However isocohumolone is slightly more soluble than the other acids and therefore a hop with a higher cohumulone composition may result in a beer with higher iso-α-acid for hops of equal iso-α-acid percent and use in brewing but different iso-α-acid breakdown<ref name="Schönberger and Kostelecky, 2012"/>. Alpha acids are susceptible to oxidation and the alpha acid content of a hop will decrease with storage.
'''Beta Acids''' (lupulones) are similar in structure to alpha acids and have the analogous individual beta acids (lupulone, colupulone, adlupulone, prelupulone, and postlupulone <ref name="Dušek_2014">[http://pubs.acs.org/doi/abs/10.1021/jf501852r Qualitative Determination of β‑Acids and Their Transformation Products in Beer and Hop Using HR/AM-LC-MS/MS. Martin Dušek, Jana Olšovská, Karel Krofta, Marie Jurková, and Alexandr Mikyška. 2014.]</ref>) to individual alpha acids. In their original form, beta acids do contribute to the flavor of beer. They are also not able to isomerize and are therefore not soluble in wort unless they are chemically modified by a process such as oxidation <ref name="Algazzali_2014" />. Oxidatized Oxidized beta acids are soluble and can contribute to bitterness in beer. Oxidized beta acids are discussed more under aged hops.
===Oils===
Hop oil contains a small percentage (~1%) of sulfur related compounds (thiols, sulfides, polysulfides, thioesters, thiopenes, and terpene derivatives). Although these levels are low, the flavor thresholds for these compounds also tends to be very low. Hydrogen sulfide can be released from these compounds during fermentation. Hops that have been treated with sulfur to prevent mildew growth (an older process that is generally no longer used) can result in increased sulfur compound such as sulfuric terpenes, and lend a garlic-like aroma in beer. Few sulfur compounds survive boiling, however late hopping and dry hopping preserves more sulfur compounds which can survive into the beer. Fermentation generally volatilizes sulfur compounds, and some volatilize almost completely during fermentation <ref name="Peppard_1981">[http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1981.tb04054.x/abstract VOLATILE ORGANOSULPHUR COMPOUNDS IN HOPS AND HOP OILS: A REVIEW. T.L. Peppard. 1981.]</ref>.
'''[https://en.wikipedia.org/wiki/Thioester Thioesters]''' are derived from an acid and a thiol. These include S-methyl hexanethioate and S-methyl heptanethioate and derivatives of these, which impart cabbagycooked cabbage, sulfurysulfuric, and soapy flavors, and their low flavor threshold can have an impact on finished beer. '''Sulfides''' and '''polysulfides''' found in hops includes [[Dimethyl _Sulfide|dimethyl sulfide (DMS)]], dimethyl disulfide (DMSD), dimethyl trisulfide (DMST; cooked vegetable, onion). DMTS has been found in wide ranges in hops, from a few ppm to 1450 ppm, and has a very low flavor threshold (1 ppb). These compounds are volatilized during brewing and fermentation, and are generally only found in beers that are dry hopped <ref name="Peppard_1981" />.
Other '''thiol''' (organic sulfur) based compounds contribute to a pleasant aroma and flavor in beer, such as 4-mercapto-4-methyl-pentan-2-one (4MMP), which is found in high quantities in North American varieties such as Simcoe (highest amount), Summit, Apollo, Topaz, and Cascade hops, as well as varieties from Australia and New Zealand. The character of black currant, muscat-like aroma in beer brewed with these hops has been attributed to 4MMP. It is thought that 4MMP is only found in North American, Australian, and New Zealand hops and not European hops because European hops are often treated with copper ions, which has been shown to decrease the amount of 4MMP in hops. Interestingly, beers brewed with these hops showed a 33% increase in 4MMP after fermentation; it is thought that the precursor cysteine conjugate is responsible for the increase in 4MMP during fermentation <ref name="Kishimoto_2008" />. The volatile thiols 3-sulfanyl-4-methylpentan-1-ol (3S4MP; grapefruit <ref name="Cibaka_2016">[https://www.uclouvain.be/cps/ucl/doc/inbr/documents/JAFC_2016_64_8572_8582.pdf 3‑Sulfanyl-4-methylpentan-1-ol in Dry-Hopped Beers: First Evidence of Glutathione S‑Conjugates in Hop (Humulus lupulus L.). Marie-Lucie Kankolongo Cibaka, Laura Decourriere, Celso-JoséLorenzo-Alonso, Etienne Bodart, Raphael Robiette, and Sonia Collin. 2016.]</ref>), and 3-sulfanyl-4-methylpentyl acetate (3S4MPA; passionfruit, grapefruit <ref name="Cibaka_2016" />) have been identified in Nelson Sauvin hops as the compounds that give these hops their "wine-like, Sauvignon Blanc" character. Similar thiols have been described as the major contributors to the aroma of Sauvignon Blanc wines themselves: 3-sulfanylhexan-1-ol (3SH) and 4-methyl-4-sulfanyl-pentan-2-one (4MSP) <ref name="Kiyoshi_2009">[http://pubs.acs.org/doi/pdf/10.1021/jf8034622 Identification and Characteristics of New Volatile Thiols Derived from the Hop (Humulus luplus L.) Cultivar Nelson Sauvin. Kiyoshi Takoi, Marie Degueil, Svitlana Shinkaruk, Cécile Thibon, Katsuaki Maeda, Kazutoshi Ito, Bernard Bennetau, Denis Dubourdieu and Takatoshi Tominaga. 2009.]</ref>.
Hops are known to have antimicrobial properties against gram positive bacteria. This includes bacteria which can be present in beer both as spoilage organisms and as intentionally added in sour and mixed fermentation beer such as ''[[Lactobacillus]]'' and ''[[Pediococcus]]''. Certain other bacteria found in beer such as ''Acetobacteraciae'' are gram negative and are not susceptible to the antimicrobial properties of hops. Certain Gram positive bacteria are known to be more resistant to the antimicrobial effects of hops. Multiple mechanisms have been proposed to explain why hops are antimicrobially active.
One mechanism of the antimicrobial activity of hops is due to the role of alpha acids and similar hop acids (such beta acids and iso-α-acids) as ionophores, or compounds which can transport ions across cell membranes<ref name="Fernandez and Simpson, 1993"> [http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2672.1993.tb02782.x/full Fernandez and Simpson (1993)] </ref> <ref name="Sakamoto and Konings, 2003"> [http://www.sciencedirect.com/science/article/pii/S0168160503001533 Sakamoto and Konings (2003)] </ref>. The protonated iso-α-acid (the form of the acid with an associated H+ ion, an H+ ion is a proton) is the antimicrobially active form. This means that for a beer with a given iso-α-acid concentration, the antimicrobial effects will be stronger at lower pH values because a greater percentage of the acid will be protonated. Protonated iso-α-acids act against bacteria by crossing into the cell and dissociating (releasing H+ ions from the iso-α-acid), therefore disrupting the cellular proton gradient which is necessary for cells to function, before binding an equal charge in metal ions and crossing back out of the cell. Cells with a resistance to hop bitter acids are better able to eject undissociated disassociated iso-α-acids from the cell and therefore preserve their proton gradients. The mechanism to expel iso-α-acids appears to be specific toward this type of compound rather than by a more general antimicrobial resistance mechanism such as multi-drug resistant bacteria possess<ref name="Sakamoto and Konings, 2003"/>. Hop resistant bacteria cultured in the absence of hop acids can lose their resistance if grown in an environment without antibacterial hop compounds<ref name="Fernandez and Simpson, 1993"/> and some hop resistant microbes need to be acclimated to hop acids by growth in sub-limiting levels of antibacterial acids before they are able to resist higher levels<ref name="Sakamoto and Konings, 2003"/>.
(in progress)
===Bacterial Resistance to Hop Compounds===
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 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 citral can be hydrolized hydrolyzed into citronellol (citrus aroma). Citranellol Citronellol can then be esterified by yeast fermentation into citronellyl acetate. 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" />.
Terpenes and terpenoids (monoterpene alcohols) can also be transformed by fermentation. Studies have found that geraniol and nerol can transform into linalool by a strain of ''S. cerevisiae'', as well as nerol and linalool to alpha-terpineol, which can then by further transformed to terpin. Geraniol can also be converted into citronellol. Linalool, nerol, and alpha-terpineol gradually decrease during fermentation and aging (perhaps being transformed into ethers), while nerol and citronellol gradually increase. Geraniol also decreases during fermentation, but not as drastically as linalool. Citronellol might be created from geraniol, but also glycosidic activity (although another study found that glycosidic activity in ''S. cerevisiae'' is not very strong). Likewise, dry hopping preserves linalool and alpha-terpineol, and limits citronellol <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. Studies on hop compounds elude to the possibility of compounds being produced by glycosidic activity of ''S. cerevisiae'', however direct evidence of glucosidic acitivity activity in ''S. cerevisiae'' is lacking. Daenen (2008) reviewed the glycosidic activity of many strains of ''S. cerevisiae'', and found that only a few strains expressed any real glucosidic activity, and none that exhibited exo-beta-glucosidase which would be required to break glycosidic bonds in the beer/wort. Daenen did find that enzymatic activity from some strains of ''Brettanomyces'' can efficiently release these bound compounds and release their flavor and aromatic potential <ref name="Praet_2012" />. Beta-glucosidase enzyme can also be added to beer enhance the breakdown of glycosides in intensify hop derived flavors and aromas. For example, one study showed an increase in citrus, orange, grapefruit, and tropical pineapple in a Cascade dry hopped beer that had beta-glucosidase enzymes added to it <ref>"Optimizing hop aroma in beer dry hopped with Cascade utilizing glycosidic enzymes (presentation slides)." Kaylyn Kirkpatrick from New Belgium Brewing Co. Young Scientist Symposium, Chico, CA 2016.]</ref>. There is also some evidence to support that there is higher glucosidase activity in seeded hops, which are generally not used in the brewing industry <ref>"Seeded and "Unseeded Hops - a Quality Comparison (presentation slides)." Martin Zarnkow. EBC 2015.</ref>. See the [[Glycosides]] page for details.
==Aged Hops==
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'''Oxidized alpha acids''' (humulinones) are similar in taste perception to iso-α-acids, but have been described as less bitter (about 66% as bitter) <ref name="Shellhammer, Vollmer and Sharp, CBC 2015"/><ref name="Maye_2016" />. While the taste threshold of iso-alpha acids is 5-6 mg/L in light lager, the threshold for humulinones has been measured to be 8 mg/L in light lager (note that this is an average; tasters vary widely in how much bitterness they perceived from different bitter compounds) <ref name="Algazzali_2014" />. Humulinone content increases in hops after being pelletized (whole leaf hops have less humulinones). In fresh pellet hops that have a relatively low humulinone content, the humulinones contribute little to the bitterness of the beer when boiled, however when dry hopped they readily dissolve into the beer and have a significant impact on the beer's bitterness. With heavy dry hopping, the humulinones also decrease iso-alpha acid content of beer with more than 20 IBU's, but not in beer with less than 25 IBU. The decrease in iso-alpha acids and perceived bitterness/IBU is partially made up for the bitterness of the humulinones themselves (humulinones are picked up in IBU measurements with a [http://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Kinetics/Reaction_Rates/Experimental_Determination_of_Kinetcs/Spectrophotometry spectrophotometer]). In beers with less than 25 IBU, high dry hopping rates greatly increase the bitterness/IBU due to the bitter humulinones. The rate of humulinone formation is limiting, meaning that humulinone formation occurs rapidly during hop pelletization, and the concentration peaks during this time (researchers found that further exposure to air did not increase humulinone content). Scientists believe that this is because when whole leaf hops are baled, only 20% of lupulin glands are broken, whereas when they are pelletized 100% of the lupuline lupulin glands are broken. The exact mechanism by which alpha acids are converted to humulinones is not known <ref name="Maye_2016" />. Humulinone content in long-aged hops (1+ years) has not been studied.
'''Oxidized beta acids''' produce some compounds that also contribute to perception of bitterness, specifically hulupones. Unlike humulinones which form relatively quickly from oxidation of alpha acids, hulupones form at a much slower rate <ref name="Dušek_2014" />. Also unlike humulinones, they survive boiling and fermentation. While some sensory analysis of beers containing oxidized beta acids describe the resulting bitterness as "harsh and clinging", another analysis by Krafta et al (2013) described the bitterness of oxidized beta acids in beer when added in their pure form at the beginning of the boil as "pleasant and not lingering". The more degradation of beta acids into oxidized beta acids that occurs in hops, the more bitter beers brewed with these hops will be <ref name="krofta_2013" />. Two other compounds other than hulupones have been identified as being produced by the oxidation of beta acids, epoxycohulupone and epoxyhulupone. Their effect on beer flavor is not yet known, however it is thought that hulupones have the greatest impact on beer flavor <ref name="Dušek_2014" />.
Other compounds have been associated with the oxidation of beta acids and are extracted during wort boiling. These are described as giving a long-lasting, lingering bitterness on the palate. They include hydroxytricyclo-lupulone, dehydrotricyklolupulone, and hydroperoxytricyklolupulone <ref>[http://www.sciencedirect.com/science/article/pii/S0308814609001770 Structure determination and sensory evaluation of novel bitter compounds formed from β-acids of hop (Humulus lupulus L.) upon wort boiling. Gesa Haseleu, Daniel Intelmann, Thomas Hofmann. 2009.]</ref>.
[[File:Lam Hop Oils.jpg|thumb|upright=2.5|[http://pubs.acs.org/doi/pdf/10.1021/jf00070a043 Data from "Aging of hops and their contribution to beer flavor" by Kai C. Lam, Robert T. Foster, and Max L. Deinzer.] '''Aged I''': 2 weeks at 90°F; '''Aged II''': 60 additional days at 90°F.]]
====Oils====
Hop oils also generally degrade over time, however their degradation rates are more complex. [http://pubs.acs.org/doi/abs/10.1021/jf00070a043 Lam et al. (1986)] found that aging both cascade and North American grown Hallertauer Mittelfrueh resulted in an increase in grapefruit-like character, although the compound that caused this was not identified. In the case of Cascade the intensity of this flavor correlated with the age of the hops <ref name="Lam et al., 1986"> [http://pubs.acs.org/doi/abs/10.1021/jf00070a043 Aging of Hops and Their Contribution to Beer Flavor. Lam et al. 1986.] </ref>. In the Hallertauer hops, aging resulted in an increase in a spicy/herbal character <ref name="Lam et al., 1986"/>, which is in agreement with reports of oxidized sesquiterpenes (specifically humulenol II, humulene diepoxides, caryophyllene, and to a lesser extent humulene monoepoxides and alpha-humulene) contributing a spicy/herbal flavor to beer <ref name="Goiris et al., 2002">[http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.2002.tb00129.x/abstract Goiris et al., 2002]</ref><ref name="Mikyška_2012" />. Many of the oils followed in the Lam et al. (1986) study which increased during a short accelerated aging period (2 weeks at 90°F) then decreased during extended aging (60 additional days at 90°F). The cascade hops lost more of the fruity/citrusy hop oils (myrecene, linalool, and geranial) than Hallertauer, suggesting that different strains of hops can withstand aging better than others. The concentration of hop oils are affected by the brewing process and fermentation (see the table) <ref name="Lam et al., 1986"/>. Another study found that beta-ionone (classified as a ketone, and characterized as "floral" and "woody" <ref>[http://www.thegoodscentscompany.com/data/rw1006632.html Beta-ionone. Good Scents Company. Retrieved 11/22/2016.]</ref>) increased in beers brewed with hops that were aged for 30 days at 40°C versus beers brewed with aged hops <ref name="kishimoto_2007" />.
Aging hops while exposed to oxygen develops a cheesy aroma due to [[Isovaleric Acid|isovaleric acid]], isobutyric acid, and 2-methylbutyric acid. These acids are produced by the oxidative cleavage of acyl side chains of the hop resins <ref name="Briggs_2004" />. These cheesy oxidation compounds can be esterified to form fruity tasting compounds<ref name="Shellhammer, Vollmer and Sharp, CBC 2015"/>.
====Esters====
During fermentation, it is believed that esters are produced by yeast metabolism from hop compounds such as alpha acids, beta acids, polyphenols, and hydrocarbons because they are not found in unhopped beer or in hops themselves. These esters include ethyl 2-methylpropanoate (citrus, pineapple, sweetness), ethyl 2-methylbutanoate (citrus, apple-like), ethyl 3-methylbutanoate (citrus, sweetness, apple-like), 2-phenylethyl 3-methylbutanoate (floral, minty), and 4-(4-hydroxyphenyl)-2-butanone (citrus, raspberry) <ref>[http://pubs.acs.org/doi/abs/10.1021/jf061342c Comparison of the Odor-Active Compounds in Unhopped Beer and Beers Hopped with Different Hop Varieties. Toru Kishimoto, Akira Wanikawa, Katsunori Kono, and Kazunori Shibata. 2006.]</ref>. [https://researchmap.jp/?action=cv_download_main&upload_id=110987 Kishimoto et al.] found that some beer esters were increased when using unidentified pellet hops (described in the study only as "a bitter variety of 11.5% alpha acid") that were aged for 30 days at 40°C versus using fresh pellet hops that were stored cold (4°C). Specifically, in the beers that used the aged hops, they found a significant increase in citrus esters (ethyl 2-methylbutanoate, ethyl 3-methylbutanoate, and 4-(4-hydroxyphenyl)-2-butanone), and a decrease in "green, hop-pellet-like, and resinous" compounds such as myrcene and (Z)-3-hexen-1-ol in the beers made from aged hops. The beers brewed with aged hops were described as more citrusy, while the beers brewed with fresh hops were described as more "hop pellet-like", resinous, floral, and "green". The authors speculated that since these esters were not present in beers brewed without hops that they were derived from the humulone and lupulone oils in the hops during yeast fermentation <ref name="kishimoto_2007">[https://researchmap.jp/?action=cv_download_main&upload_id=110987 Odorants comprising hop aroma of beer: hop-derived odorants increased in the beer hopped with aged hops. Toru KISHIMOTO Kishimoto, Katsunori Kono , and Kenkichi Aoki. 2007.]</ref>.
====Thiols====
For both mixed fermentation sour beers and kettle sour beers, hops are often not used at all. In the case of kettle sours, sometimes brewers opt to add hops after the wort has been soured (see [[Wort Souring]]). Commercial brewers in the USA must by law use 7.5 pounds of hops for 100 barrels of beer <ref>[https://www.ttb.gov/rulings/2008-3.pdf "Classification of Brewed Products as “Beer” Under the Internal Revenue Code of 1986 and as “Malt Beverages” Under the Federal Alcohol Administration Act". TTB Ruling 2008, Number 2008-3. 07/07/2008. Retrieved 12/12/2016.]</ref> (malt beverages without hops can still be approved by the FDA instead of the TTB; contact the TTB for guidance <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1508230109205198/?match=ZGFuIHBpeGxleQ%3D%3D MTF thread with John Joyce and Joseph Kearns on TTB vs FDA approval for beer/malt beverages without hops. 12/13/2016.]</ref>). Since there is no US regulation for when the hops must be added, mash hopping might be a considered technique for commercial breweries in the US and in other parts of the world where hops are a requirement for beer (mash hopping retains only about 30% of the IBU that a 60 minute boiling addition does <ref>[https://www.homebrewersassociation.org/how-to-brew/resources/conference-seminars/ "Putting Some Numbers on First Wort and Mash Hop additions." David Curtis. 2014 National Homebrewers Conference presentation slides. Retrieved 12/12/2016.]</ref>). In historical German Berliner Weisse brewing, mash hopping or boiling hops during the decoction were also typical techniques (see [[Berliner_Weissbier#Historical|Berliner Weisse historical brewing]]). Another historical technique for adding hops to beer is to add a hop tea (hops boiled in water), for example in historical [http://www.garshol.priv.no/blog/331.html raw ale] brewing <ref>[http://www.garshol.priv.no/blog/331.html "Raw ale". Larsblog. Lars Marius Garshol. 05/06/2015. Retrieved 12/12/2016.]</ref>. For lactic acid cultures that are hop tolerant, hops can be used as a way to inhibit the amount of acid produced by them if the brewer desires this.
A popular technique for [[100%25_Brettanomyces_Fermentation|100% Brettanomyces Fermentation]] is to use a typical IPA recipe. Hops do not inhibit ''Brettanomyces'' yeast. Some of the fruity characteristics of ''Brettanomyces'' can compliment complement the fruity character of hops such as Citra, Amarillo, and Galaxy. For beers that are fermented with just ''S. cerevisiae'' and ''Brettanomyces'' but not lactic acid bacteria (such as some American farmhouse ales), Old World and noble hops are often used as well as North American and New Zealand/Australian citrusy hops, depending on what flavor and aroma profile the brewer is intending.
===Dry Hopping===
===Historic hopping in lambic and other mixed-fermentation beer===
While modern lambic uses aged hops almost exclusively, it was common for historic lambic to blend both aged and fresh hops<ref name="Hors Catégorie Brewing Hops in Spontaneous Fermentation"> [http://www.horscategoriebrewing.com/2016/04/hops-in-spontaneous-fermentation.html Dave Janssen's discussion of hopping in spontaneous fermentation] </ref>. The exact ratio of fresh to aged hops changed over time and could vary depending on the harvest (poor hop years may have relied more heavily on aged hops while years of good harvests would make more use hops of the recent harvest). In addition to the difference in hop age between modern and historic lambic, hopping rates also differ significantly between modern and aged hops. It is important to note that the quality of these hops are certainly different from modern hops, and that hop origin could have a significant influence on suggested hopping rates <ref name="Hors Catégorie Brewing Hopping Grisette"> [http://www.horscategoriebrewing.com/2016/06/hopping-historical-grisettes.html Dave Janssen's discussion of hopping grisettes] </ref> (see the hopping rate table and notes regarding hop origin conversion factors from historical texts). While hop quality would have improved moving to the modern day while hopping rates were dropping, there is mention in historic lambic literature of lambic in the late 1800s being more bitter than lambic from the mid -1900s (and, subsequently, similar to historic saison in the increased hop presence in a mixed-fermentation beer)<ref name="Hors Catégorie Brewing Hops in Spontaneous Fermentation"/>
Historical documents dealing with Belgian brewing show a steady progression from high doses of fresh hops in lambic to the sort of hop composition and origin that are in use today. In 1851 Lacambre mentions rates for Belgian hops of 760-860 g/Hl and specifically highlights the use of young hops. Belgian brewing scientist Henri Van Laer recommended a hopping rate of 700-800 g/Hl in 1890, roughly in agreement with Lacambre though slightly lower. In the early 1900s, citing information from 1896, ''Le Petit Journal du Brasseur'' mentions a hopping rate of 540 g/Hl using a mix of Belgian and Bavarian hops and a split of 2/3 young, 1/3 old in good years (and 50/50 in bad years). In 1928 ''Le Petit Journal du Brasseur'' recommends a larger proportion of aged hops (2/3 aged, 1/3 fresh) and rates of 600g/Hl of Belgian hops<ref name="Hors Catégorie Brewing Hops in Spontaneous Fermentation"/>. Considering the difference in strength in German and Belgian hops<ref name="Hors Catégorie Brewing Hopping Grisette"/>, this fits with a stable or decreasing hopping rate from that given in the early 1900s. In 1937 exclusive use of aged hops is recommended, though as noted in 1946, year old hops may be preferable to hops that were aged longer in poor conditions<ref name="Hors Catégorie Brewing Hops in Spontaneous Fermentation"/>. Also in the 1940s ''Le Petit Journal du Brasseur'' recommends hopping rates of 400-500 g/Hl, roughly in agrement agreement with modern times, and notes that the lambic of this time was softer than historic lambic<ref name="Hors Catégorie Brewing Hops in Spontaneous Fermentation"/>.
(In Progress)
Lambics aren't the only historic mixed-fermentation beer to make use of aged hops. Though the specific mention of aged hops for saison and bieres beers de garde does not seem to be the norm, aged hops were used at times, such as when more acidity was desired. These hops were also more likely to be used toward the beginning of the brewing season in months like October where the current harvest may have been considered too fresh for proper use. Notes: Give some discussion of hopping saison and bieres de garde. See [http://www.horscategoriebrewing.com/2016/06/hopping-historical-grisettes.html hopping grisette table] for some hopping rates, PJB, etc.
==See Also==