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Hops
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The main compounds of interest to brewers in hops are their bitter acids and oils. Alpha acids account for roughly 2-12% of dried hops by mass, beta acids account for roughly --- % and oils account for roughly 0.5-3%, though the exact percentages will vary depending on factors such as the hop varietal, growing region, harvest time, and growth conditions for the year.
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 acids 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) 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 (---source---). 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"/>. Iso-α-Alpha acids are susceptible to oxidation and the iso-α-alpha acid content of a hop will decrease with storage.
'''Beta Acids''' (lupulones) are similar in structure to iso-α-alpha acids and have the analogous individual beta acids (lupulone, colupulone, adlupulone) to individual alpha acids. Beta acids but they are not able to isomerize and are therefore not soluble in wortunless they are chemically modified by a process such as oxidation. 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 only 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"/>.
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 posotive 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)] attributed the antimicrobial effects the iso-α-acids acting as ionophores to cause leaking of ions across cell membranes</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. The protonated Protonated iso-α-acids are antimicrobially active act against bacteria by crossing into the cell and dissociating (releasing the H+ ion 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 iso-α-acids from the cell and therefore preserve their proton gradients. This resistance to hop antimicrobial activities through iso-α-acids appears to be stable within bacteria with the trait, and the 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"/> [http://www.sciencedirect.comHop 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"/science/article/pii/S0168160503001533 > 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)] <"/ref>.
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