Hops are the flowers (also called seed cones or strobiles) of the female dioecious (meaning that they have separate male and female plants) plant Humulus lupulus , and are used in brewing for flavor as well as for antimicrobial properties. Although bitterness from boiling hops is generally not desired in sour beers, sour and funky brewers can use hops to help regulate lactic acid bacteria and control acid production to desired levels, especially in aged mixed-fermentation or spontaneous fermentation beers. Additionally, it may be argued that the earthy bitterness from aged hops is desired for lambic based styles (see Hops in lambic below). Potentially other mixed fermentation styles can benefit from some degree of bitterness either from aged or fresh hops such as saisons, farmhouse ales, and experimental styles. So while the mantra for sour beer is that "bitterness and sour don't work together", there are certainly exceptions to this rule. Brewers who are interested in rapid acid production using quick/kettle souring techniques such as wort souring may wish to limit or avoid hop use before acidifying so that sufficient acid is produced quickly.
- 1 Hop Composition
- 2 Characterizing Hop Flavor and Aroma
- 3 Antimicrobial Properties
- 4 Hop Derived Compounds In Beer and Biotransformations
- 5 Aged Hops
- 6 Techniques
- 7 See Also
- 8 References
The main compounds of interest to brewers in hops are their bitter acids and oils contained in the yellow-colored lupulin glands. There are at least 250 significant aroma and flavor compounds found in hop acids and oils. Alpha acids account for roughly 2-17% of dried hops by mass, beta acids account for roughly 2-10%, 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 rest of the weight of hops is made up of 40-50% cellulose and lignin, 15% protein, 8-12% water (after drying), 8% minerals, 3-6% polyphenols and tannins, 1-5% lipids and fatty acids, 2% monosaccharides, and 2% pectin .
The primary alpha acids (also called "humulones" and abbreviated as "α-acids") 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 . While alpha acids are mostly 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. 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. Alpha acids are susceptible to oxidation and the alpha acid content of a hop will decrease with storage.
The isomerization of alpha acids into isoalpha acids is mostly dependent on temperature, and not other variables such as sugar concentration, pH, and calcium concentration . Malowicki and Shellhammer determined a calculation that predicts the isomerization rates of alpha acids into isoalpha acids at different temperatures. Beginning at the boiling temperature of 100°C/212°F, which could be considered a rate of 100%, at 96°C/205°F the rate is 72%, and at 90°C/194°F the rate is 43%. This rate continues to drop significantly as the temperature of the wort decreases. At 82°C/180°F isomerization occurs at a rate of 17%. At a temperature of 50°C/122°F, the isomerization rate is at 1%, and finally 0% at 45°C/113°F. This fact has several impacts on brewing processes. For example, when brewing at higher altitudes where the boiling point of wort is less than 100°C/212°F, the isomerization of alpha acids into isoalpha acids will be reduced to whatever the rate is at that lower temperature. "Hop stands" or "whirlpool additions" where hops are left in contact with hot wort that is less than boiling temperature will continue to isomerize alpha acids .
Beta Acids (lupulones) are similar in structure to alpha acids and have the analogous individual beta acids (lupulone, colupulone, adlupulone, prelupulone, and postlupulone ) to individual alpha acids. In their original form, beta acids do not contribute to the flavor of beer because they are not soluble in beer. They are also not able to isomerize during wort boiling. Beta acids do not become soluble in wort or beer unless they are chemically modified by a process such as oxidation , nor are they soluble in beer when dry hopping . Oxidized 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). 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. For example, only recently it has been shown that the amount of hop oils does not correlate to hop aroma intensity when dry hopping, but the composition of hop oils does . While sulfur compounds make up only a very small fraction of the total oils, they have a significant impact on hop flavor .
Hydrocarbons, specifically terpenoids, make up the majority of hop oil. The majority of these terpenoids are myrcene, which characterizes the aroma of hops (although this compound does not carry over well into beer because it is hydrophobic), caryophyllene, and humulene. Most of these compounds are evaporated off by the brewing process, and others are metabolized into different compounds during fermentation . Linalool (citrus, floral, fruity, tropical ) and geraniol (rose-like, musty, floral ) have been identified as the major compounds that contribute to beer flavor in hop varieties such as Cascade .
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 .
Thioesters are derived from an acid and a thiol. These include S-methyl hexanethioate and S-methyl heptanethioate and derivatives of these, which impart cooked cabbage, sulfuric, and soapy flavors, and their low flavor threshold can have an impact on finished beer. Sulfides and polysulfides found in hops includes 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 .
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 . The volatile thiols 3-sulfanyl-4-methylpentan-1-ol (3S4MP; grapefruit ), and 3-sulfanyl-4-methylpentyl acetate (3S4MPA; passionfruit, grapefruit ) 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/4MMP) .
Characterizing Hop Flavor and Aroma
Hops provide a wide array of aromas and flavors to beer. These flavors and aromas are variety and crop dependent. Hop farmers often provide their own hop flavor and aroma descriptors independently of each other, but attempts to standardize these descriptors have been made as far back as 1756, and as recently as 1978. More recently, Drexler et al. (2017) worked with a perfumer to establish 12 major categories of hop flavor descriptors. Each major category contains more specific descriptors. These descriptors can be measured on a 0-10 scale, and a spider graph can be drawn to represent them. Drexler et al. (2017) proposed that even though expensive gas chromatography is available for hops which measures specific compounds, sensory analysis is still the best way to quantify how different varieties of hops actually smell and taste in beer .
The proposed categories, example hop variety, and the specific descriptors by Drexler et al. (2017) are as follows :
- Floral (ex: Ella): Elderflower, Chamomile Blossom, Lily of the Valley, Jasmine, Apple Blossom, Rose, Geranium, Carnation, Lilac, Lavender
- Citrus (ex: Mandarina Bavaria): Grapefruit, Orange, Lime, Lemon, Bergamot, Lemon Grass, Ginger, Tangerine
- Sweet Fruits (ex: Mosaic®): Banana, Watermelon, Honeydew Melon, Peach, Apricot, Passion Fruit, Lychee, Dried Fruit Plum, Pineapple, Cherry, Kiwi, Mango, Guava
- Green Fruits (ex: Hallertau Blanc): Pear, Quince, Apple, Gooseberry, White Wine Grapes
- Red Berries (ex: Monroe): Cassis (Black Currant), Red Currant, Blueberries, Raspberries, Blackberries, Strawberries, Wild Strawberries, Cranberries
- Cream Caramel (ex: Triskel): Butter, Chocolate, Yoghurt, Honey, Cream, Caramel, Toffee, Coffee, Vanilla, Tonka
- Woody Aromatic (ex: Relax): Tobacco, Cognac, Barrique, Leather, Woodruff, Incense, Myrrh, Resin, Earthy, Cedar, Pine
- Menthol (ex: Polaris): Mint, Lemon Balm, Camphor, Menthol, Wine Yeast
- Herbal (ex: Columbus): Lovage, Thuja, Basil, Parsley, Tarragon, Dill, Fennel, Thyme, Rosemary, Marjoram, Green Tea, Black Tea, Mate Tea, Sage
- Spicy (ex: Saazer): Pepper, Chili, Curry, Juniper, Aniseed, Nutmeg, Liqorice, Clove, Ginger Bread, Fennel Seeds
- Grassy-Hay (ex: Herkules): Green-Grassy, Fresh Cut Grass, Hay, Tomato Leaves, Green Peppers, Nettle
- Vegetal (ex: Summit®): Celery Stock, Celery Root, Leek, Onion, Artichoke, Garlic, Wild Garlic
- MBAA Podcast interviews Georg Drexler about his study.
- Experimental Brewing Podcast talks about the Drexler et al. study (starts at ~36 minutes in).
- YCH hop variety database.
- Brulosophy's "The Hop Chronicles", an attempt to characterize hop flavor and aroma.
- Hopsteiner presentation on specific blends of hops that match the flavor profile of another hop.
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 . 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 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. Hop resistant bacteria cultured in the absence of hop acids can lose their resistance if grown in an environment without antibacterial hop compounds 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.
(in progress) Another antimicrobial mechanism resulting from oxidative stress has been attributed to both iso-α-acids and humulinic acids. Humulinic acids are either not bitter tasting or much less bitter than iso-α-acids but are similar in structure to and are formed from the degradation of iso-α-acids. This oxidative stress-driven antimicrobial activity is due to potential for oxidation-reduction (redox) reactions within bacterial cells between Mn2+ ions and these specific hop acids. The redox potential is due to different conditions inside (higher pH, higher Mn2+) and outside (lower pH, lower Mn2+) of the bacterial cell. Iso-α-acids or humulinic acids passing into the cell, form complexes with Mn2+ and transfer electrons out of the cell. By targeted molecular modifications Schurr et al. (2015) determined that the Mn oxidative stress-driven antimicrobial effect of iso-α-acids was more important than antimicrobial effect of the ionophore proton transfer discussed above in the overall antimicrobial activity of hops.
- See also Lactobacillus hop tolerance.
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. 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. 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. 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. 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.
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 .
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 ), 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). The terpenoid citronellol (citrus and floral ) can be esterified by yeast fermentation into citronellyl acetate (fresh, rosy, fruity odor reminiscent of geranium oil ). 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 .
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 by 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 . Other yeast species can also convert monoterpenes. For example, a strain of Kluyveromyces lactis was found to reduce geraniol to citronellol. This strain and a strain of Torulaspora delbrueckii produced linalool from both geraniol and nerol, and could also form geraniol from nerol . Many species of Debaryomyces, Kluyveromyces, and Pichia were found to transform geraniol into linalool, and nerol into linalool and alpha-terpineol .
Sulfur based compounds known as thiols have also been shown to be produced by yeast fermentation from hop derived precursors (suspected to be S-glutathione). So far, science has found that these include the volatile thiols 3-sulfanyl-4-methylpentan-1-ol (3S4MP; grapefruit) and 3-sulfanyl-4-methylpentyl acetate (3S4MPA; passionfruit, grapefruit). These thiols were found in beers dry hopped separately with Amarillo, Hallertau Blanc, and Mosaic hop varieties. The amounts of these two thiols were higher than expected based on the content of these thiols in the hops alone .
In general, different yeast strains have a large impact on how hops are perceived in the final beer, including both perceived bitterness and flavors. For example, POF+ (phenolic positive) strains of Saccharomyces cerevisiae tends to mask the hop derived aromas in dry hopped beers . A beer hopped with the Tradition hop variety produced fruit flavors when fermented with Abbaye ale yeast, and woody/spicy flavors when fermented with US-05. When the beer was brewed with Citra hops, with US-05 the beer had sweet fruits/citrus flavors and more bitterness, but when fermented with the Abbaye ale strain the beer had a more one dimensional sweet fruit/floral flavor and less bitterness .
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 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 . Beta-glucosidase enzyme can also be added to beer to enhance the breakdown of glycosides and 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 . 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 .
Much of the work on hop derived glycosides has been done using hop oils, and might not apply to whole cones or pellet hops. Sharp et al. (2017) found that when using pure beta-glucosidase extract on beer hopped with whole leaf hops that the amount of increased monoterpenes such as linalool, terpineol, citronellol, nerol, and geraniol is small and insignificant. The fatty alcohol 1-octanol (waxy, green, citrus, orange, aldehydic, fruity ) was the only measured flavor compound that was increased significantly . The alcohol 1-octanol can be esterified into octyl acetate, which is a classically "citrusy" aroma, so perhaps certain yeasts can create this ester during mid-fermentation hopping .
See Glycosides for more information on glycosides.
See Aging and Storage.
Aging hops leads to oxidation of acids and oils. Generally, brewers seek to avoid this to preserve the aromatic and bittering properties of their hops by freezing them and storing them in vacuum sealed packaging (oxygen exposure is by far the larger factor for hop degradation, followed by ambient room temperatures, which is significant because hops are often not stored in vacuum sealed packaging). However, some beer styles, including lambic and historical styles, make extensive use of aged hops. Aged hops still retain some antimicrobial properties and can be used for microbial inhibition. In addition to their antimicrobial activity, aged hops contribute important flavor and aroma compounds and precursors to beer. These flavor descriptors often include herbal, tea-like, Earth-like, and a more dull bitterness. Low amounts of Isovaleric Acid might also contribute to the complexity of a beer that has been brewed with aged hops (although the presence of isovaleric acid in aged hops is considered temporary, and will eventually age out of hops that are aged). Historically, some brewers had issues keeping mildew from growing on aged hops that are aged in higher humidity areas (sulfur was used to combat mildew, which often gave the beer a sulfur, rotten egg aroma) .
In lambic brewing, the term aged hops refers to hops (usually Noble varieties such as Tettnang, Saaz, Target, and Hallertau) which have been aged for 3-5 years in non-refrigerated conditions, and in burlap sacks or some other oxygen permeable bag . It should be noted that the term "aged hops" can also refer to any sort of hop aging (especially in scientific literature), including short-term hop aging (1-6 months, for example) at refrigerated or non-refrigerated temperatures, and in oxygen-rich or vacuum sealed packaging. Much of the information below references hops that have been aged in warm conditions for shorter time periods than what hops are aged for in lambic brewing. The additional aging of hops that are used in lambic brewing or similar beers might have different effects than what has been studied in hops that are aged for shorter periods of time.
For techniques and usage amounts of aged hops, see Aged Hops in Lambic.
Chemistry and Characteristics
During aging, both alpha and beta acids oxidize and degrade with warmer temperatures and more oxygen exposure having a greater impact. The generally accepted theory is that oxygen interacts directly with hop acids. This event is called "autooxidation". An alternative theory to this is that oxygen indirectly oxidizes acids by first oxidizing the hop oils and turning them into pro-oxidants, which then oxidize the hop acids which are mixed in with the oils within the lupulin glands . The oxidation of hop acids corresponds with an increase in the Hop Storage Index (HSI), which is a practical way of measuring the overall freshness of hops. As the oxidation of hop oils rises, the measured HSI number on a lot of hops increases . These oxidized compounds lead to a higher amount of non-alpha-acid bitterness compounds in aged hops and have a remarkable effect on the bitterness of the beer. The bitterness from oxidized hop compounds has been described as more earthy, harsh, and astringent than the sharper, cleaner bitterness from iso-alpha acids .
Storage conditions and variety play a large role in how acid content in hops changes over time. Beta acids are generally more resistant to oxidation than alpha acids. A study by Mikyška and Krofta (2012) found that after 12 months of storage at 20°C in open air, pellet hops lost 64-88% of their alpha acid content and 51-83% of the beta acid content, with the beta acids dropping off more significantly after 6 months (alpha acid content declined steadily throughout the aging period). These amounts varied with different Czech hop varieties (Saaz, Sládek, Premiant, and Agnus), and beta acids degraded slower than alpha acids as seen below  (percentages listed below are how much percent was lost):
|Storage||Oil||Saaz (pellet) ||Sládek (pellet) ||Premiant (pellet) ||Agnus (pellet) ||Saaz (leaf) ||Vital (leaf) ||Pure Beta Acid |
|Open air at 20°C for 12 months|
|Vacuum sealed at 20°C for 12 months|
|Vacuum sealed at 2°C for 12 months|
Oxidized alpha acids (humulinones) are similar in taste perception to iso-α-acids, but have been described as less bitter (an average of about 66% as bitter on a 1 to 1 basis). The quality of the bitterness from oxidized alpha acids has been described in one study as "smoother and less lingering" than iso-alpha acids; this was attributed to humulinones being more polar than iso-alpha acids and therefore do not stick or linger on the tongue as long as iso-alpha acids . 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) . 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 about 25 IBU's, but not in beer with less than about 20 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 spectrophotometer). In beers with less than 20 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 lupulin glands are broken. The exact mechanism by which alpha acids are converted to humulinones is not known . Humulinone content in long-aged hops (1+ years) has not been studied.
Oxidized beta acids produce some compounds that also contribute to the perception of bitterness, specifically hulupones. Unlike humulinones which form relatively quickly from the oxidation of alpha acids, hulupones form at a much slower rate . Also unlike humulinones, they survive boiling and fermentation. While some sensory analysis of beers containing oxidized beta acids describes 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 . 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 .
The bitterness of hulupones has received some debate among researchers. In 1973, a researcher found that hulupones are about 50% as bitter as iso-alpha acids. Briggs et al stated the complete opposite, and that hulupones are twice as bitter as iso-alpha acids. More recent studies using modern analysis techniques found that on a weight for weight basis, hulupones are 35-40% as bitter as iso-alpha acids in one study, and another found that they were 84% (+/- 10%) as bitter as iso-alpha acids (note that this is an average; tasters vary widely in how much bitterness they perceived from different bitter compounds) . While the taste threshold of iso-alpha acids is 5-6 mg/L in light lager, the threshold for hulupones has been measured to be 7-8 mg/L in light lager .
Both humulinones and hulupones have been identified as forming due to the oxidation of hop acids. However, other researchers have reported that both of these bitter compounds formed during the boiling of hops, and another during the storage and aging of beer. In all cases, the amounts of the compounds directly correlated with the amount of hops used .
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 .
The overall effect of oxidized compounds in aged hops has been shown by Val Peacock, a former scientist at Anheuser-Busch. Peacock stored hops at four different temperatures for 18 months. His data showed that although the alpha acid content in the hops and the iso-alpha acid content in the beers brewed with them decreased the older the hops were stored, the measured IBU of the different beers was about the same. This is because the oxidized acids in hops show up in the same spectrum as iso-alpha acids when using the ASBC method of measuring IBUs with a spectrophotometer . This data is shown below:
|Storage Temperature ||Alpha Acid in Hops||Iso-Alpha Acids in Beer||Beer IBUs|
Hop oils also generally degrade over time, however, their degradation rates are more complex. 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 . In the Hallertauer hops, aging resulted in an increase in a spicy/herbal character , 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 . 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) . Another study found that beta-ionone (classified as a ketone, and characterized as "floral" and "woody" ) increased in beers brewed with hops that were aged for 30 days at 40°C versus beers brewed with aged hops .
A recent study at the Shellhammer lab looked at how trained panelists and consumers perceived a lager beer dry hopped with slightly oxidized Hallertau Mittelfrüh hops (exposed to oxygen once, then stored at 38°C for two weeks) versus highly oxidized (daily exposure to oxygen and stored at 38°C for two weeks). They found that the trained panelists detected more characteristics that are associated with noble hops; e.g. more woody, earthy, and herbal characteristics in the lager beers dry hopped with oxidized hops. They also found the oxidized hopped beers to be more bitter (probably due to oxidized alpha and beta acids). Consumers were not statistically able to tell the difference. The study determined that oxidized hops might serve to provide nuanced increases in noble hop character .
Aging hops while exposed to oxygen develops a cheesy aroma due to isovaleric acid, isobutyric acid, and 2-methylbutyric acid. These acids are produced by the oxidative cleavage of acyl side chains of the hop resins . These cheesy oxidation compounds can be esterified to form fruity tasting compounds.
- "Increasing Bitterness By Dry Hopping", article by Scott Janish on oxidized alpha acids.
- Hulupones - oxidized beta acids.
Polyphenols, including polyphenol flavanoids, also degrade in hops as they age. However, storage conditions have less of an impact on the degradation of polyphenols compared to alpha and beta acids. Mikyška and Krofta (2012) found that regardless of how the hops were stored polyphenols started to decay after about 6 months and after 12 months aged hops lost about 30-40% of polyphenols and 20-30% of flavanoids .
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) . 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 .
Kishimoto et al. found an increase in the thiol 3-methyl-2-butene-1-thiol (MBT) in beers that were brewed with 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). Interestingly, this thiol was higher in beers where the aged hops were added to the boil rather than when they were added after the wort was cooled. The authors were not able to determine whether or not the MBT was derived from yeast fermentation, or from boiling the hops, but aging the hops increased the precursors for MBT . MBT has been described as the thiol that produces the "skunky" aroma in lightstruck beer .
Aged Hop Suppliers
Hops Direct "Choice Debittered/Aged Hops" (Leaf - Cascade).
- Hops Direct "Choice Debittered/Aged Hops" (Pellet - Columbus).
- Freshhops "Lambic Hops" (Leaf - Willamette)".
- Yakima Valley Hops "Lambic / Aged Hops" (Pellet).
- Farmhouse Brewing Supply "Lambic Hop Blend" (Pellet - Blend of varieties that are aged for ~5 years and then pelletized ).
- The Malt Miller (UK).
- Brew Store UK (Leaf - Fuggles).
- Brew Store UK (Leaf - Hallertau).
Kettle and Mash Hopping
Kettle hopping sour beers can be a difficult thing for the new sour beer brewer. The usage of hops generally inhibits most lactic acid bacteria species, however there are many exceptions to this. Lactic acid bacteria can have a range of hop tolerance, with species such as Lactobacillus acetotolerans that tolerated Goose Island's Bourbon County Stout at 60 IBU . Some breweries report that their house lactic acid bacteria can tolerate IBU ranges up to 10-20 IBU. White Labs claims that their L. delbuekii (WLP677) is tolerant of up to 20 IBU, however, most Lactobacillus cultures from yeast labs are not hop tolerant . See the Lactobacillus culture charts and hop tolerance for more information.
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  (malt beverages without hops can still be approved by the FDA instead of the TTB; contact the TTB for guidance ). 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 ). In historical German Berliner Weisse brewing, mash hopping or boiling hops during the decoction were also typical techniques (see 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 raw ale brewing . 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. Another advantage of using at least some hops in the kettle is that various compounds from hops contribute to head retention, and using a small amount of hops in the kettle (and perhaps dry hopping) can greatly assist with head retention in sour beers.
A popular technique for 100% Brettanomyces Fermentation is to use a typical IPA recipe. Hops do not inhibit Brettanomyces yeast. Some of the fruity characteristics of Brettanomyces can 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.
Brewers have had positive and interesting results dry hopping sour and funky beer. Often fresh American or New Zealand varieties that compliment fruit flavors are chosen, however, other varieties have been used as well, including English and German hops. Just as in dry hopping normal beers, dry hopping sour/funky should be done after the beer has matured. Dry hopping for around 1-3 days before packaging the beer is adequate for extraction, depending on whether or not the beer is recirculated or agitated (agitation of the beer while on contact with the dry hops attains full extraction in 24 hours) . Hopping rates generally range from 0.5-1 ounces per 5 gallons of beer (1-2 pounds per bbl) to achieve hop-forward flavors, although lesser rates can be used to achieve a more subtle character (see the threads below) .
Dry hopping can contribute to bitterness in beer through oxidized alpha acids and oxidized beta acids. Some alpha acids will also dissolve into the beer, which are estimated as being 10% as bitter as iso-alpha acids. Dry hopping also has a linear impact on the pH of beer regardless of the starting IBU or pH: the pH rises by 0.14 per pound of hop pellets per barrel of beer in a beer that started with a pH of 4.2 (~0.5 ounces per gallon) . This rise in pH might be less in more acidic beers that are dry hopped since pH is a logarithmic scale. Dry hopping can also reduce head retention in beers, although this is variety dependent (one study found that dry hopping with Eureka and Apollo hops increased head retention, while dry hopping with Bravo, Centennial, and Cascade decreased head retention). Extended dry hopping times (after 3 days) can also reduce head retention .
Although the mechanisms are not fully understood, dry hopping inhibits Lactobacillus. This is probably due to non-isomerized alpha acids, other acids, or the small amount of isomerization of alpha acids that happens in beer at room temperature . See the links below.
- MTF thread on using very small amounts of subtle effects.
- Caroline Whalen Taggart's data point on the effects of dry hopping on L. plantarum (GoodBelly). No hops finalized at a pH of 3.53, and the dry hopped version finalized at a pH of 4.35. She used around 4 grams per gallon of 10-15 AA hops .
- The Rare Barrel reports on an anecdote that dry hopping in a less sour beer extracts better hop aroma, and Brettanomyces preserves the character.
- Per Buer's experiment on the effects of dry hopping on Lactobacillus:
Aged Hops in Lambic
Modern lambic traditionally uses aged hops at a moderate rate to help limit and select for microbes and regulate acid production. Modern Lambic brewers cite rates in the range of roughly 450 grams of hops per Hl of finished beer  (~43 min in) (see also the notes pertaining hopping rates on the Cantillon page), with some brewers possibly going above this range. The age of hops used depends on the producer and their preferences/stock. Cantillon uses hops that are roughly 3 years old, while 3 Fonteinen reports using hops that are over 10 years old (~48 minutes in). Lambic brewers either add their hops while still collecting wort, sometime before the wort comes to a boil (also known as "first wort hopping"), or shortly after boil is reached (~48 min in). The hops are then boiled with the wort for essentially the full length of the boil . The resulting lambic beers are often surprisingly bitter, especially when young. Historically, there is some evidence that lambic brewers used a combination of aged hops and fresh hops.
- "Lambic characteristics - FG and IBUs in Geuze" on Hors Categorie blog.
- "Hops in spontaneous fermentation" on Hors Categorie blog.
- MTF discussion on the general benefits of using aged hops in sour beers.
- Spontaneous Fermentation
- Gueuze and Lambic Character
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. 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  (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)
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. Considering the difference in strength in German and Belgian hops, 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. Also in the 1940s Le Petit Journal du Brasseur recommends hopping rates of 400-500 g/Hl, roughly in agreement with modern times, and notes that the lambic of this time was softer than historic lambic.
(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 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 hopping grisette table for some hopping rates, PJB, etc.
James Howat of Black Project Spontaneous Ales uses 0.5 ounces of aged hops per gallon of beer for spontaneously fermented beers brewed using traditional lambic techniques .
- MTF discussion on using aged hops in other styles of beers, including historical references and tips on producing lambic-like character from aged hops and commercial cultures.
Additional Articles on MTF Wiki
- "Hoppy Sour Beers", Dec 2016 BYO article by Michael Tonsmeire.
- "Dry Hopping Effect on Bitterness and IBU Testing" by Scott Janish.
- YCH hop variety database.
- Per Buer's Video Demonstration of how dry hopping inhibits Lactobacillus.
- Blog Article on Brett and Glycosides by Cy Wood.
- "How hops prevent infection", by Lars Garshol.
- "Unlocking Hop and Fruit Flavors from Glycosides", HBT article by Dennis L Waldron.
- Beer Legends Hop Varieties - gives vital statistics on hops including acid content, physical cone characteristics, storage and growth details, and oil content.
- TTB Hop Requirements for USA commercial breweries.
- Blog article on hops in spontaneous fermentation by Dave Janssen
- Cohumulone ranges by hop variety.
- "CRISPR Yeast and Hop Compounds" by Dr. Matt Humbard; a critical response of yeast being modified to make two hop compounds.
- "Hops". Wikipedia. Retrieved 06/10/2017.
- The Bitterness Intensity of Oxidized Hop Acids: Humulinones and Hulupones. Victor Alexander Algazzali for the degree of Master of Science in Food Science and Technology presented on August 8, 2014.
- Fundamentals of beer and hop chemistry. Denis De Keukeleire. 1999.
- Schönberger and Kostelecky, 2012
- Isomerization and Degradation Kinetics of Hop (Humulus lupulus) Acids in a Model Wort-Boiling System. Mark G. Malowicki and Thomas H. Shellhammer. 2005.
- A Look at Isomerization Reduction Due to Altitude. John Palmer. MBAA TQ 2017 http://dx.doi.org/10.1094/TQ-54-3-0806-01.
- 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.
- John Paul Maye. EBC 2017 Presentation. 2017.]
- Shellhammer, Vollmer, and Sharp. Oral presentation at the Craft Brewers Conference, 2015.
- "Recent Advances in Controlling Hoppy Aroma in Beer." Daniel C. Sharp. OSU Brewing Science Presentation.
- Comparison of 4-Mercapto-4-methylpentan-2-one Contents in Hop Cultivars from Different Growing Regions. Toru Kishimoto, Minoru Kobayashi, Nana Yako, Ayako Iida and Akira Wanikawa. 2008.
- "Geraniol". Wikipedia. Retrieved 01/09/2017.
- 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.
- VOLATILE ORGANOSULPHUR COMPOUNDS IN HOPS AND HOP OILS: A REVIEW. T.L. Peppard. 1981.
- 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.
- The Language of Hops: How to Assess Hop Flavor in Hops and Beer. Georg Drexler, Elisabeth Wiesen, Mark Zunkel, Sebastian Hinz, Alicia Muñoz Insa, Victor Algazzali, Tim Kostelecky, and Christina Schönberger. 1. Joh. Barth & Sohn GmbH & Co. KG, Nuremberg, Germany. 2. John I. Haas Inc., Yakima, WA, U.S.A. MBAA Technical Quarterly. Vol. 54, no. 1. 2017. Pgs. 34–37. DOI: http://dx.doi.org/10.1094/TQ-54-1-0143-01.
- Fernandez and Simpson (1993)
- Sakamoto and Konings (2003)
- Schurr et al., (2015)
- Behr and Vogel, (2010)
- Simpson and Fernandez, 1992
- 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.
- "(Z)-rose oxide ". Good Scents Company. Retrieved 12/29/2016.
- "There are no flies on Emma Stoye". Emma Stoye. Education in Chemistry website. 06/01/2016. Retrieved 01/10/2017.
- "Citronellyl acetate". Perfumers Apprentice website. Retrieved 01/10/2017.
- Biotransformation of monoterpene alcohols by Saccharomyces cerevisiae, Torulaspora delbrueckii and Kluyveromyces lactis. King A1, Richard Dickinson J. 2000.
- Biotransformation of acyclic monoterpenoids by Debaryomyces sp., Kluyveromyces sp., and Pichia sp. strains of environmental origin. Ponzoni C, Gasparetti C, Goretti M, Turchetti B, Pagnoni UM, Cramarossa MR, Forti L, Buzzini P. 2008.
- "Influence of yeast strain on hop aroma development in dry hopped beers." Christina Schönberger, Elisabeth Wiesen, Benedikt Matsche, Barth Innovations Yves Gosselin, Stephan Meulemans, Fermentis. Presentation slides at 35th Congress EBC.
- "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.
- "Seeded and "Unseeded Hops - a Quality Comparison (presentation slides)." Martin Zarnkow. EBC 2015.
- Octanol. The Good Scents Company. Retrieved 03/31/2017.
- The effect of hopping regime, cultivar and β-glucosidase activity on monoterpene alcohol concentrations in wort and beer. Daniel C. Sharp, Jan Steensels, Thomas H. Shellhammer. 2017.
- Preiss, Richard. Milk The Funk Facebook group. 03/31/2017.
- Conversation with Ron Smith and Aaron Barker on historical storage of hops. 02/07/2016.
- Dave Janssen. "Hops in spontaneous fermentation". Hors Catégorie Brewing blog. 04/28/2016. Retrieved 04/09/2018.
- "Home for Our Aged Hops". Jester King's blog. Retrieved 11/18/2016.
- Aging of Hops and Their Contribution to Beer Flavor. Lam et al. 1986.
- Humulinone Formation in Hops and Hop Pellets and Its Implications for Dry Hopped Beers. John Paul Maye, Robert Smith, and Jeremy Leker. 2016.
- Assessment of changes in hop resins and polyphenols during long-term storage. Alexandr Mikyška and Karel Krofta. 2012.
- Stability of Hop Beta Acids and Their Decomposition Products during Natural Ageing. K. Krofta, S. Vrabcová, A. Mikyska, M. Jurková, T. Cajka , J. Hajslová. 2013.
- The effect of hop beta acids oxidation products on beer bitterness. Karel Krofta, Světlana VRABCOVÁ, Alexandr Mikyška, and Marie JURKOVÁ. 2013.
- Brewing Science and Practice. Dennis E. Briggs, Chris A. Boulton, Peter A. Brookes, Roger Stevens. 2004.
- 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.
- Dr. Patricia Aron. "Bitterness and the IBU: What’s It All About?" HomebrewCon 2017 Presentation. ~34 mins in. Retrieved 09/05/2017.
- Goiris et al., 2002
- Beta-ionone. Good Scents Company. Retrieved 11/22/2016.
- Odorants comprising hop aroma of beer: hop-derived odorants increased in the beer hopped with aged hops. Toru Kishimoto, Katsunori Kono and Kenkichi Aoki. 2007.
- Aroma Properties of Lager Beer Dry-Hopped with Oxidized Hops. Daniel M. Vollmer, Victor Algazzali, and Thomas H. Shellhammer. 2017.
- 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.
- 3-methyl-2-butene-1-thiol. Aroxa website. Retrieved 11/22/2016.
- Private correspondence with Dustin Carver by Farmhouse Brewing Supply. 03/22/2016.
- MTF thread that reported an MBAA presentation by Brett Porter from Goose Island. 07/30/2016.
- "Commercial Brettanomyces, Lactobacillus, and Pediococcus Descriptions; Commercial Yeast Laboratories." The Mad Fermentationist blog. Michael Tonsmeire. Retrieved 12/12/2016.
- "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.
- MTF thread with John Joyce and Joseph Kearns on TTB vs FDA approval for beer/malt beverages without hops. 12/13/2016.
- "Putting Some Numbers on First Wort and Mash Hop additions." David Curtis. 2014 National Homebrewers Conference presentation slides. Retrieved 12/12/2016.
- "Raw ale". Larsblog. Lars Marius Garshol. 05/06/2015. Retrieved 12/12/2016.
- A Study of Factors Affecting the Extraction of Flavor When Dry Hopping Beer (master thesis). Peter Harold Wolfe. 2012.
- Nate Walter and Dan Pixley. Milk The Funk Facebook group. 05/21/2017.
- Janish, Scott. "Zero Hot-Side Hopped NEIPA | HPLC Testing for Sensory Bitterness". ScottJanish.com. Retrieved 03/09/2017.
- Carloine Whalen Taggart. Milk The Funk Facebook group. 07/06/2017.
- Jean van Roy on Basic Brewing Radio
- D. Janssen personal communication with Jean Van Roy, 9-Nov-2013
- Drie Fonteinen on Belgian Smaak
- Video of Cantillon wort reaching a boil from Bill on Lambic.info
- Conversation with Dave Janssen on MTF. 02/24/2017.
- "Brewing Lambic", section "Hopping". Lambic.info website. Retrieved 02/24/2017.
- Dave Janssen's discussion of hopping in spontaneous fermentation
- Dave Janssen's discussion of hopping grisettes
- Howat, James. Facebook live video stream. 12/23/2016. ~5:30 minutes in.