New England IPA: A Scientific Study
I started writing The New IPA: Scientific Guide to Hop Aroma and Flavor by collecting every academic article I could get my hands on, not only on studies focused on brewing hoppy beer, but every paper I could find on beer in general. Although the book cites over 300 sources, I ended up reading hundreds of additional studies spanning topics such as hop oils, biotransformation, head retention, hop creep, impact of grains and proteins on haze and flavor, different hopping techniques, and the impact of water chemistry and dry-hopping on bitterness. The goal was to piece together years of research often forgotten or generally undiscovered and apply it to brewing modern New England IPAs, which are full of hop flavor, low on hot-side bitterness, and of course, hazy.
Despite numerous well-intentioned attempts from friends reading drafts of the book to litter the book with hazy commercial IPA recipes, I resisted. I want the book and the entirety of its research to inspire new ideas and get brewers to deviate from typical brewing practices and recipes to not only use the science to understand results (both good and bad), but to take advantage of the information to start testing variables like new processes, ingredients, yeast strains, and hot-side and cold-side hopping techniques.
The one recipe I thought would be fun to assemble (which you will find at the end of this story), however, is to combine a collection of academic study results to build a scientifically-inspired hoppy hazy beer using the data of everything from water profile, grist makeup, yeast selection, and hopping techniques to bring together years of work by those much smarter than I into one powerful and punchy hazy IPA! I’m not suggesting this is the new standard recipe for brewing hoppy beer, but it’s a fun exercise to try and put all of the research into action. The book goes into greater depth into each brewing topic, but this article is a glimpse into the book’s contents and some of the research of the core brewing areas and, more importantly (hopefully), enough useful information to inspire brewers to experiment and improve their hoppy beer!
Water
The sulfate-to-chloride ratio is often the marker used when discussing water profiles for hoppy beer. While it’s a good approach for formulating a recipe, the raw amounts of the minerals themselves are just as important. For example, a sulfate-to-chloride ratio of 10:30 and 100:300 are both 1:3 but would likely result in different tasting beers. Generally, I like to stay under 200 ppm of each as to not impart a minerally taste to the beer. The research indicates that beer brewed with water favoring chloride will result in a fuller beer and those favoring sulfate will result in a dryer beer that emphasizes bitterness.1 Rather than relying solely on mineral water adjustments to achieve a full-bodied hop-saturated beer, it’s important to also consider the impact the grist will have on mineral additions. For example, on average, base malt can add around 50–100 ppm of sulfates and approximately 200 ppm of chloride. The malting processes can result in a higher amount of minerals than the raw grain, potentially from the water the maltsters use.2
So, when making a hazy IPA where both fresh hop character is desired as well as a full mouthfeel and restrained bitterness, a 2:3 sulfate-to-chloride ratio would be a good place to start. Adjust with subsequent batches as well as with grist changes. As the percentage of flaked grains increases in a recipe, you could increase the amount of chloride to replace what you’d typically get from malted grains. At Sapwood Cellars, the craft brewery that I and Michael Tonsmeire opened in 2018, we are generally in the range of 150 ppm of chloride to 100 ppm of sulfate, which is a ratio that works well for several beer styles.
Grist Selection
The grist selection for hoppy beers can impact more than just flavor. For example, certain grains can impact haze levels, play a role in hop compound retention, influence fruity thiol contributions, and affect beer stability. One study by Luis F. Castro and Carolyn F. Ross found that high protein and carbohydrate beer had significantly more intense dry hop flavor but scored lower on the sensory tests for dry hop aroma due to the binding of volatile compounds keeping them in the beer vs. headspace.3 The more viscous base in hazy IPAs can thus increase the retention of otherwise volatile hop compounds. Furthering this concept is recent research of commercial hazy IPAs that found hazy beers, more than other hoppy styles, had much higher polyphenol concentrations and retained more volatile, green, and resinous compounds like myrcene.4 Haze levels and hop bite-astringency are also likely connected and may be altered with the selection of proteins used in the grist. For example, higher usage rates of unmalted grains, like flaked wheat, results in less permanent haze. One lab test conducted by Sofie Depraetere, Filip Delvaux, Stefan Coghe, and Freddy Delvaux found that beer with 40% unmalted wheat had significantly less permanent haze than the beer with 100% malted barley and beers with 20% unmalted wheat.5 I believe this phenomenon is likely due to the higher proteolytic activity in malt (breakdown of proteins during malting), the wheat proteins are degraded, leading essentially to smaller proteins more likely to remain in suspension. It’s possible that the proteins staying in the beer are more likely to interact with hop polyphenols and otherwise volatile less fruity green hop compounds. So, to encourage haze-inducing proteins in beer using high-modified, protein-rich grains like malted wheat will likely lead to hazier beer than brewing with a high percentage of unmalted wheat.
In other words, using a large percent of high-protein malted grains like malted wheat or spelt combined with intense dry hopping rates could result in hazy but also more astringent beer. I speculate that using under-modified grains like Best Chit malt are great because they are a balance between unmalted and malted grains with a small amount of modification balancing the haze and astringency potential coming from malted wheat while also aiding in head retention and mouthfeel. As a bonus, in my experience, chit malt also aids in dense head formation, similar to a shaving cream-like foam.
Interestingly, even base malt choices may influence the fruity flavor desired in hoppy beers. For example, Full Pint 2-row was tested with five other base malts and found by a sensory panel to produce a fruiter beer with flavors of watermelon rind. Further analysis showed that Full Pint has compounds that may be influencing the fruity flavors, including a volatile metabolite called alpha-ionone, a ketone associated with floral, pear, and watermelon rind flavors as well as a small number of monoterpenes, which could potentially boost a beer’s overall fruity monoterpenes levels with most coming from hops.6
Hot-Side Hopping
Traditional kettle aroma in beer is described as spicy, woody, and herbal — not attributes generally sought in fruity-forward hazy IPAs. Although early bittering additions are typically kept to a minimum to highlight hop flavor over bitterness, early bittering hop varieties choices can impart more flavor than you might anticipate. For example, when hops are boiled, newly formed oxygenated sesquiterpenes can be formed leading to more traditional kettle aroma and flavor.7 Hop varieties high in alpha-humulene and beta-caryophyllene (like East Kent Golding, Nelson Sauvin, Northern Brewer, Golding, Pacifica, Vanguard, Progress, and Fuggle) will impart more of the spicy, woody, and herbal character in beer and might want to be avoided for bittering hops in hazy beers. On the other hand, here are some hops that are high in alpha acids but relatively low in α-humulene and β-caryophyllene: WaimeaTM, Loral®, Citra®, Mosaic®, Galaxy®, Bravo, Galena, and Columbus. Consider these hop varieties safe for early bittering additions when fruit flavors are the focus and spicy herbal flavors are not.
Whirlpool Hopping
Late whirlpool hopping is important for achieving hop saturated flavor because the fruity hop compounds are lost in the boil due to evaporation and trub. One paper found that linalool losses can be as high as 80% of the original concentration within just five minutes of boiling.8 In fact, hydrocarbons from hops have been shown to peak almost immediately when added to boiling wort and then start their decline.9 Sensory tests also show beers that are whirlpool hopped scored higher for estery-fruity and fruity-citrus flavors than beers that were only dry hopped.10 In that study, beers were made with Simcoe® hops with additions at either 60 minutes, 20-minute whirlpool (right after the boil, no cooling was done), and dry hop only (for 48 hours).
So, incorporating late whirlpool hops is an important step to getting hop flavor but factors such as whirlpool temperatures can also play a role as late hopping at 185 °F (85 °C) retained slightly more measured linalool compared to warmer at 203 °F (95 °C) or cooler at 167 °F (75 °C). Sensory differences between the different temperatures showed that beers late-hopped at 203 °F (95 °C) scored highest for citrusy, spicy, and ester descriptors. The 185 °F (85 °C) late hop temperature scored higher for floral and herbal descriptors. The 167 °F (75 °C) hop addition scored the lowest in nearly every category except for the sylvan (woody) characteristic.11 Lowering the temperature of the whirlpool will also lower the isomerization of alpha-acids to iso-alpha-acids, potentially allowing for larger hop additions for flavor vs. bitterness.
Another variable to consider in late-hopping is the hop variety used. Especially looking towards the oxygen fraction of a hop variety as an indicator of its predicted aroma intensity as low oxygen fraction = less intensity, high oxygen fraction = more intensity.12 So, hops high in compounds like geraniol and linalool (part of the oxygen fraction) as well as high in total oil are likely to have a greater impact on hop flavor intensity. Examples of such varieties are Brewers Gold, Centennial, Bravo, Citra®, EkuanotTM, Olympic, Simcoe®, Mosaic®, and CTZ.
Dry-Hopping
Optimal dry-hop duration is a common topic debated among brewers, but some research shows extraction of dry-hops is done relatively quickly. For example, Peter Wolf found that when testing for linalool and myrcene, dry-hopping for a week didn’t result in additional extraction of the oils when compared to just a couple of days of dry-hopping.13 Of course there are way more hop compounds than just linalool and myrcene, but these are often used as markers to determine hoppinesses in lab tests. You can even speed up the extraction of dry hops to just hours if the hops are agitated or recirculated as one test found full extraction of linalool in just four hours when agitated. Even when dry hopping cold, you can get quick extraction. Willi Mitter and Sandro Cocuzza found that unagitated, dry-hopping at 39.4 °F (4 °C) resulted in slightly faster extraction of linalool compared to dry-hopping at 68 °F (20 °C). Day 2 linalool concentrations in both trials were approximately 80% of day 14 concentrations.14
Dry-hop amounts can also play into overall hop flavor. For example, Scott Lafontaine and Thomas Shellhammer found that dry-hopping with Cascade hops beers scored the highest for citrus marks when dosed at 4 g/L, but at higher hopping rates, the citrus aroma scores were suppressed compared to the herbal and tea descriptors. The conclusion (for Cascade hops, anyway) is that dry-hopping at a rate of 4–6 g/L (approximately 4.5 oz./130 g in a 5-gallon/19-L batch) may be the best way to maintain the citrus quality of the hop and over dry-hopping may alter the perception of a hop variety.15
Dry-hopping in multiple stages and with fewer hops may also help to get more overall extraction from dry hops, as one paper by Ray Marriot shows that linalool extraction dropped 20% when dry-hopping in larger amounts compared to smaller.16 For homebrewers, this would mean a drop of about 20% extraction when dry-hopping around 5 gallons (19 L) of beer at a rate of 400 grams vs. 50 grams (14 vs. 1.8 oz.) at once. Try experimenting with low dry-hop dosages (45–90 grams/1.6–3.2 oz. at a time for homebrewers), and dosing two to three different times throughout the fermentation to increase extraction rates.
Batch size can also impact dry-hopping extraction. One study (from Schnaitter et al.) determined a 5-gallon (19-L) vessel had substantially higher concentrations of hop compounds extracted from dry-hopping than the two larger industrial- scale batches with the same dry-hop rates. Sensory testing revealed that the reduced batch size resulted in higher intensities in smell and taste as well, but the hop characteristics were defined as less fruity than the larger batches and more of a raw hop and herbal character in the smaller batch.17
Biotransformation
Hops contain volatile flavors detectable in their original state, but also contain many non-volatile glycosides that can also contribute to the aroma, but first must be released through enzymatic hydrolysis (β-glucosidase).18 One way to encourage β-glucosidase activity to unlock flavors from hops is by purchasing commercially-available enzymes with proven glycosidase activity or pitching yeast that has been tested to produce β-glucosidase during fermentation (more on this later). A study from Takoi et al. that investigated glycosidically-bound flavor potential in beers made with 42 hop varieties found when using a commercial glucosidase enzyme to release the monoterpene alcohols that Amarillo® showed the most glycosidically-bound geraniol potential.19 The results suggest to me that if a commercial enzyme is used, or a known β-glucosidase-producing yeast, then Amarillo® would be one of the best hop choices to try and boost flavoring potential. Combining Amarillo® with β-glucosidase could potentially shift the aroma and flavor of Amarillo® more toward lime/citrus and away from floral. Of the geraniol-dominant (rose-like) hops tested, Bravo, Chinook, and Mosaic® showed the most glycosidically-bound geraniol potential, which also makes them excellent choices when a known amount of β-glucosidase is present during fermentation.19
Multiple studies have examined the β-glucosidase activity of hundreds of different yeast strains and the highest activity has continually found to reside in non-Saccharomyces yeast strains. Studies have also shown a handful of wine yeast strains that will produce the enzyme during fermentation (like QA23, 58W3, 71B-122).20
One method of biotransformation that has been detectable in some ale strains like Lallemand dried West Coast ale yeast is the transformation of fruity terpenoid alcohols during primary fermentation.21 Specifically, geraniol was found to convert into citronellol (lemon-lime) and linalool was converted into terpineol (lilac) during ale fermentations. One study by Andrew King and J. Richard Dickinson found that most of these terpene alcohols were converted within the first few days and after that, the decreases occurred at much lower and steadier rates,22 suggesting these compounds must be in the wort at the start of fermentation. Regarding dry hopping and biotransformation, late stage fermentation dry hopping did not reflect this same monoterpene alcohol biotransformation, according to a study by researchers at Sapporo Breweries.23 In that study, the later the hops were added during dry hopping, the less chance of biotransformation of geraniol to β-citronellol. Some hops varieties have been tested to have more free geraniol than geraniol precursors (bound), which means the free geraniol is more readily available for biotransformation early during fermentation. Examples of hops high in free geraniol are MotuekaTM, Bravo, Cascade, Chinook, Citra®, Mosaic®, and Sorachi Ace. Conversely, hops like Comet, Hallertau Blanc, Polaris, Amarillo®, Summit, and Vic SecretTM were found to have more bound geraniol and would result in less biotransformation to β-citronellol during primary fermentation.
Hop Thiols
Hop thiols are sulfur compounds found in hops at very low concentrations but have low taste thresholds, which means these thiols (mainly 4MMP (boxtree), 3MH (grapefruit), and 3MHA (passionfruit), which is converted from 3MH), can have a flavor impact in hoppy beer even in low amounts. Like with monoterpene alcohols, which can be in a free or bound state, thiols in hops are also in either free or bound states, meaning the bound thiols need to be unlocked by an enzyme (called β-lyase) to impact the flavor of the beer. Hops high in bound thiol precursors should be added during the late-hopping phase to help cleave these precursors during fermentation. On the other hand, hops that are rich in free thiols can be added as the dry-hop, where enzyme activity from yeast isn’t required to release the aromatic thiols.
Citra® was determined to be one of the most versatile hops, meaning it works great both as a late-hop and a dry-hop because it contains similar amounts of both free and bound thiols. Three other varieties have a good percentage of both free and bound concentrations of 4MMP are Apollo, Eureka, and Simcoe®. Cascade, Hallertau, Hallertau Perle, Saaz, Citra®, and Calypso are great choices for late hot-side hopping (to push 3MH levels) and potentially 3MHA levels (during fermentation).24 You can purchase commercial products that may contain the β-lyase enzyme and release bound thiols like Rapidase Revlation Aroma (I would start experimenting with a dosage rate of 0.25–0.5 grams per 5 gallons/19 L of beer).
You can also incorporate yeast strains that have been tested to contain the enzyme to release bound thiols, but again, most of the research has been focused on wine yeast. One study showed that the wine strains VIN 13 and VIN 7 released the most 3MH, VIN7 also released the most 4MMP and 3MH, suggesting it may be able to do the same with hops (tested here with grapes).25 In my experience with wine strains in hoppy beers, a little can go a long way. I’d suggest starting with a blend of ale and wine yeast, going as low as 2–5% of the blend in wine yeast to start, in order to prevent a strong phenolic fermentation profile.
New England IPA
(5 gallons/19 L, all-grain)
OG = 1.070 FG = 1.018
IBU = 74 SRM = 4 ABV = 6.9%
Ingredients
10 lbs. (4.5 kg) Great Western Full Pint malt
2.5 lbs. (1.13 kg) malted oats
2.5 lbs. (1.13 kg) Best Malz Chit malt
1.7 lbs. (0.77 kg) white wheat malt
1 oz. (28 g) Columbus hops (mash hop)
8.2 AAU Columbus hops (60 min.) (0.53 oz./15 g at 15.5% alpha acids)
27.6 AAU Amarillo® hops (1st hop stand addition) (3 oz./84 g at 9.2% alpha acids)
31 AAU Bravo hops (2nd hop stand addition) (2 oz./56 g at 15.5% alpha acids)
2 oz. (56 g) Citra® hops (1st dry hop)
2 oz. (56 g) Galaxy® hops (2nd dry hop)
1 oz. (28 g) Citra® hops (2nd dry hop)
1 tsp. Irish moss (15 min.)
0.25 g Rapidase Revelation (fermenter)
SafAle S-04 or Wyeast 1318 (London Ale III) or RVA 132 (Manchester Ale) yeast
0.5 g VIN 7 wine yeast
3⁄4 cup corn sugar (if priming)
Step by Step
Starting with reverse osmosis (RO) water, add calcium chloride and calcium sulfate to achieve 150 ppm chloride and 100 ppm sulfate. Mash in the grains and mash hops to achieve a stable mash temperature at 158 °F (70 °C). Hold at this temperature for 60 minutes, then begin mash-out process. Sparge with enough water to collect 7 gallons (26.5 L) in the kettle. Bring wort to a boil for 60 minutes, adding the second hop addition as the wort comes to a boil and Irish moss with 15 minutes left in the boil.
After the boil is over, chill the wort down to 200 °F (93 °C) and stir in the first hop stand addition. After 10 minutes, check wort temperature. Add the second hop stand addition when wort temperature is 185 °F (85 °C); some additional chilling may be required if wort is still warmer than this. After another 10 minutes begin chilling the wort all the way down to yeast pitch temperature. Aerate the wort and transfer into a sanitized fermenter.
Ferment at 66–68 °F (19–20 °C). On day 1, add the Rapidase Revelation enzymes. On day 4, add the first dry-hop addition. After 3 days, rack the beer into a CO2-purged vessel such as a keg or carboy.
After fermentation is complete (7–10 days) soft crash the beer to 58 °F (14 °C) to encourage the yeast and first hop addition to settle out. After a day or two at 58 °F (14 °C), rack to a CO2-purged keg with the second dry-hop addition, purging the hops and the keg at the same time. Alternatively, you can add the second dry-hop addition to the primary fermenter, but purge the headspace with 10 PSI of CO2 while adding the hops to prevent oxygen exposure. Wait 2–3 days and package the beer as normal or serve out of the keg with the hops if they are placed in a fine mesh bag or a dry hop canister to prevent the keg from clogging while dispensing.
New England IPA
(5 gallons/19 L, partial mash)
OG = 1.070 FG = 1.018
IBU = 74 SRM = 5 ABV = 6.9%
Ingredients
3.3 lbs. (1.5 kg) wheat liquid malt extract
3.5 lbs. (1.6 kg) extra light dried malt extract\
2.5 lbs. (1.13 kg) malted oats
2.5 lbs. (1.13 kg) Best Malz Chit malt1 oz. (28 g) Columbus hops (mash hop)
8.2 AAU Columbus hops (60 min.) (0.53 oz./15 g at 15.5% alpha acids)
27.6 AAU Amarillo® hops (1st hop stand addition) (3 oz./84 g at 9.2% alpha acids)
31 AAU Bravo hops (2nd hop stand addition) (2 oz./56 g at 15.5% alpha acids)
2 oz. (56 g) Citra® hops (1st dry hop)
2 oz. (56 g) Galaxy® hops (2nd dry hop)
1 oz. (28 g) Citra® hops (2nd dry hop)
1 tsp. Irish moss (15 min.)
0.25 g Rapidase Revelation (fermenter)
SafAle S-04 or Wyeast 1318 (London Ale III) or RVA 132 (Manchester Ale) yeast
0.5 g VIN 7 wine yeast
3⁄4 cup corn sugar (if priming)
Step by Step
Starting with 7 gallons (25.6 L) RO water, add calcium chloride and calcium sulfate to achieve 150 ppm chloride and 100 ppm sulfate. Heat 2 gallons (7.6 L) of the water to 168 °F (76 °C). Place crushed grains and mash hops in a large grain bag and submerge in the water to achieve a stable mash temperature at 158 °F (70 °C). Hold at this temperature for 60 minutes, then remove the grains and wash with 1.5 gallons (5.7 L) water. Top off to 7 gallons (26.5 L) in the kettle. Bring wort to a boil, then remove from heat and stir in all the malt extract. Once extract is fully dissolved, bring back to a boil and boil for 60 minutes total. Add the second hop addition as the wort comes to a boil and Irish moss with 15 minutes left in the boil.
Follow the remainder of the instructions in the all-grain version of this recipe.
References
1 Comrie, A. A. D. Journal of the Institute of Brewing, 1967 73, 335.
2 Justus, A. (2017). 066: “Sulfate to Chloride Ratio” [Web log post]. Retrieved from http://masterbrewerspodcast.com/066-sulfate-to-chloride-ratio
3 Castro, L. F., & Ross, C. F. (2013). “The Effect of Protein and Carbohydrate Levels on the Chemical and Sensory Properties of Beer.” Journal of the American Society of Brewing Chemists. doi:10.1094/asbcj-2013-0913-01.
4 Maye, J. P., Ph.D. (2018). “Hidden Secrets of The New England IPA a.k.a. Hazy IPA a.k.a Juicy IPA.” Lecture presented at Hopsteiner.
5 Depraetere, S. A., Delvaux, F., Coghe, S., & Delvaux, F. R. (2004). “Wheat Variety and Barley Malt Properties: Influence on Haze Intensity and Foam Stability of Wheat Beer.” Journal of the Institute of Brewing, 110(3), 200-206. doi:10.1002/j.2050-0416.2004.tb00203.x.
6 Dong, L., Hou, Y., Li, F., Piao, Y., Zhang, X., Zhang, X., . . . Zhao, C. (2015). “Characterization of volatile aroma compounds in different brewing barley cultivars.” Journal of the Science of Food and Agriculture, 95(5), 915-921. doi:10.1002/jsfa.6759.
7 Praet, T., & Van Opstaele, F. (2016). “Flavor Activity of Sesquiterpene Oxidation Products, Formed Upon Lab-Scale Boiling of a Hop Essential Oil-Derived Sesquiterpene Hydrocarbon Fraction” (cv. Saaz). Journal of the American Society of Brewing Chemists. doi:10.1094/asbcj-2016-1205-01.
8 Mitter, W., & Steiner, S. (2009). “Annual fluctuations in hop quality – options for adjustment in the brewhouse.” BRAUWELT International, 36-37.
9 Mitter, W., & Steiner, S. (2009). “Annual fluctuations in hop quality – options for adjustment in the brewhouse.” BRAUWELT International, 36-37.
10 Sharp, D., Qian, Y., Shellhammer, G., & Shellhammer, T. (2017). “Contributions of Select Hopping Regimes to the Terpenoid Content and Hop Aroma Profile of Ale and Lager Beers.” Journal of the American Society of Brewing Chemists. doi:10.1094/asbcj-2017-2144-01.
11 Inui, T. (n.d.). “Study on the attractive hop aroma for beer.” Speech presented at World Brewing Congress 2012, Portland, OR.
12 Schull, F., Forster, A., & Gahr, A. “Comparison of different dosage criteria when using aroma hops for late hopping.” European Brewing Convention 2017 presentation.
13 Wolfe, P.H. (2012). “A Study of Factors Affecting the Extraction of Flavor When Dry Hopping Beer.” Retrieved from http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/34093/Wolfe_thesis.pdf
14 Mitter, W., Cocuzza, S. (2013). “Dry hopping — A study of various parameters.” Retrieved from https://www.hopsteiner.com/wp-content/uploads/2016/03/3_Dry-Hopping-A-Study-of-Various-Parameters.pdf
15 Lafontaine, S., & Shellhammer, T. (n.d.). “Understanding the Impact Hopping Rate Has on the Aroma Quality and Intensity of Dry Hopped Beers.” Retrieved from http://www.ebc2017.com/inhalt/uploads/TUEL18-LAFONTAINE.pdf
16 Marriott, R., (2017). “Dry Hopping – a new look at techniques, utilization, and economics.” European Brewing Convention 2017 presentation.
17 Schnaitter, M., Kell, A., Kollmannsberger, H., Schüll, F., Gastl, M., & Becker, T. (2016). “Scale-up of Dry Hopping Trials: Importance of Scale for Aroma and Taste Perceptions.” Chemie Ingenieur Technik, 88(12), 1955-1965. doi:10.1002/cite.201600040.
18 Ting, P. L., Kay, S. Ryder, D. “The Occurrence and Nature of Kettle Hop Flavor.” In: Shellhammer, Thomas H. ed., 2009, “HOP FLAVOR AND AROMA Poceedings of the 1st International Brewers Symposium,” Master Brewers Association, 25-26.
19 Takoi, K., Itoga, Y., Koie, K., Takayanagi, J., Kaneko, T., Watanabe, T., . . . Nomura, M. (2017). “Systematic Analysis of Behavior of Hop-Derived Monoterpene Alcohols During Fermentation and New Classification of Geraniol-Rich Flavour Hops.” BrewingScience, 70, 177-186.
20 Rosi, I., Vinella, M., & Domizio, P. (1994). “Characterization of β-glucosidase activity in yeasts of oenological origin.” Journal of Applied Bacteriology, 77(5), 519-527. doi:10.1111/j.1365-2672.1994.tb04396.x
21 Sharp, D., Vollmer, D., Qian, Y., & Shellhammer, T. (2017). “Examination of Glycoside Hydrolysis Methods for the Determination of Terpenyl Glycoside Contents of Different Hop Cultivars.” Journal of the American Society of Brewing Chemists. doi:10.1094/asbcj-2017-2071-01.
22 King, A., Dickinson, R. (2003, March). “Biotransformation of hop aroma terpenoids by ale and lager yeasts.” Retrieved from http://onlinelibrary.wiley.com
23 Takoi, K., Itoga, Y., Takayanagi, J., Watari, J. (2014) “Screening of Geraniol-rich Flavor Hop and Interesting Behavior of beta-Citronellol During Fermentation under Various Hop-Addition Timings.” Journal of the American Society of Brewing Chemists. doi:10.1094/asbcj-2014-0116-01
24 Roland, A., Delpech, S., & Dagan, L. (2017). “A Powerful Analytical Indicator to Drive Varietal Thiols Release in Beers: The ‘Thiol Potency’.” BrewingScience, 70, 170-175.
25 Swiegers, J., Francis, I., Herderich, M., & Pretorius, I. (2006). “Meeting consumer expectations through management in vineyard and winery: The choice of yeast for fermentation offers great potential to adjust the aroma of Sauvignon Blanc wine.” Australian and New Zealand Wine Industry Journal.
