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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 much like total iso-α-acid percent, though generally the primary acids are -------. 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 beta acids are soluble and can contribute to bitterness in beer. Oxidized beta acids are discussed more under aged hops.
There are three primary classes of '''oils''' in hops: hydrocarbons (~64% of the total oils), oxygenated compounds (~35% of the total oils), and sulfur compounds (≤1% of the total oils)<ref name="Shellhammer, Vollmer and Sharp, CBC 2015"> Shellhammer, Vollmer, and Sharp. Oral presentation at the Craft Brewers Conference, 2015. </ref>. Individual flavor and aroma active oils each have different thresholds, solubilities, and volatilities, and individual oils can have synergistic interactions with each other. The chemistry of hop oil taste perception is therefore very complicated and overall is not well understood. While sulfur compounds make up only a very small fraction of the total oils, they have a significant impact on hop flavor<ref name="Shellhammer, Vollmer and Sharp, CBC 2015"/>.
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" />. 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 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 (hulupones) 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" />. 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 caused by oxidized beta acids, epoxycohulupone and epoxyhulupone, however their effect on beer flavor is not yet known <ref name="Dušek_2014" />.
[[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.]]