6.1 Overview of the IPA Beer Fermentation Process

6.1.1 Technical Route of Dry Hopping

In this experiment, a relatively mature top-fermented Ale beer fermentation process was adopted and combined with dry hopping technology to enhance the hop aroma of beer.

Dry hopping refers to directly adding hops and their products into the whirlpool tank after wort boiling during whirlpool clarification (whirlpool settling), or during fermentation by adding them directly into the whirlpool tank or the fermentation tank.

The technical route of dry hopping is shown in Figure 3.2. In the kettle (boil kettle), hops or hop products are added into the kettle at certain dosages and hop types. “Dry hopping” means adding different types of hops into the wort during the whirlpool clarification process or into the fermenting beer (fermentation broth) at different fermentation stages, so that hop aroma substances can enter the fermentation liquid as much as possible and increase the hop aroma of the finished beer. There are many ways to add hops in a fermentation tank. The most common method is: place hops or hop products into a hop bag (mesh cage), then put the hop bag directly into the fermentation tank, or hang it into the tank. After fermentation is completed, remove the hop bag from the tank. Some foreign breweries also use dedicated dry hopping vessels, which makes dry hopping easier.

Dry hopping is generally done by pouring hops directly into the fermenter, or adding hops into the fermentation liquid through a hop bag suspended or a hop mesh cage.

During brewing, hops and their products are usually added at different stages of boiling to impart a rich hop aroma. However, during wort boiling, volatilization of aroma substances in hops is difficult to avoid, which ultimately causes hop aroma substances to undergo other chemical reactions with changes in boiling intensity, thus affecting the aroma quality of beer. Dry hopping technology is a relatively mature hop addition method that has been popular abroad for many years. Because hops are added during fermentation, the fermentation temperature is controlled at a maximum of about 20℃, thereby avoiding hop aroma substances being destroyed at high boiling temperatures, and further avoiding chemical changes of aroma substances caused by high temperature. This helps ensure the stability of hop aroma substances, retains the original hop aroma, and is beneficial to the aroma of the finished beer [65]. Therefore, using dry hopping technology and adopting the top-fermented Ale beer process can be determined as the fermentation scheme for IPA beer.

Notes / precautions for dry hopping:
The hops added into the fermenter should remain in contact with the fermentation liquid for less than 7 days; the contact time should not be too long, otherwise the beer may develop an unharmonious herbal/medicinal flavor. Put hops into a mesh bag or similar tool to allow a larger contact area with the fermentation liquid, ensuring dissolution of hop aroma substances. Experiments show that the dissolution rate of hop aroma substances is positively correlated with temperature. Since the volatility of hop oils varies by hop variety, different hops require different optimal dissolution temperatures. To allow hops to contact the fermentation liquid more evenly, hops are often crushed and mixed with the beer; CO₂ is introduced into the fermenter to agitate the hops—this operation is also called hop circulation stirring. Alcohol has a significant influence on dissolving hop aroma substances; experiments indicate that the higher the alcohol content, the more hop aroma substances dissolve. The hop addition amount is closely related to the intensity of hop aroma in finished beer. Therefore, based on different requirements for beer mouthfeel, different hops and hop dosages are selected [66]-[67].

6.1.2 Hop Addition During IPA Beer Fermentation

n general beer fermentation processes, the wort after mashing is boiled, and bittering hops or aroma hops are added in batches at different boiling stages. The significance for beer flavor is that α-acids, the typical bitterness contributors in hops, undergo isomerization to form their isomers—iso-α-acids. Iso-α-acids extract better into wort and have higher bitterness value than the precursor α-acids, giving beer a more harmonious bitterness. About 80% of hop oil is lost during boiling, but these lost hop oils are mainly many substances that provide unharmonious aromas. Most of what remains without being lost due to volatilization are key contributors to hop aroma such as linalool, which provide pleasant fruity and floral aroma types [62]-[64].

6.1.3 Selection of the IPA Beer Fermentation Process

The specific fermentation process is as follows: after wort boiling, the wort is cooled through a plate heat exchanger. When the wort is cooled to 20℃, it is pumped into a fermentation tank. The yeast screened as suitable for IPA brewing is propagated and then pumped into the fermentation tank under aseptic conditions, waiting for yeast start-up. Fermentation is controlled at 18℃. During this period, dry hopping technology is used to add an appropriate amount of hops and hop products into the fermentation tank. When the apparent extract decreases to around 4°P, the tank is sealed and naturally pressurized to 0.12–0.15 MPa. When diacetyl reaches 0.08 mg/L, cooling begins at a uniform rate; lagering at 0℃ is performed, and sediments at the bottom of the fermentation tank are discharged regularly.

6.2 Experimental Materials and Methods

6.2.1 Main Experimental Equipment and Instruments

100 L mash tun, 100 L boil kettle, 100 L fermentation tank, gas chromatograph, mass spectrometer, constant-temperature water bath, magnetic stirrer.

6.2.2 Main Reagents

Absolute ethanol, 2-nonanol, sodium chloride, linalool; geraniol; α-terpineol; β-citronellol.

6.2.3 Experimental Methods

6.2.3.1 Study on the Influence of Aroma Hop Variety on Hop Flavor in IPA Beer

Mashing control scheme:
Use a grist-to-water ratio of 1:4. Keep the initial mashing-in temperature at about 56℃ for 30 min, raise temperature to 65℃ for 1 h, raise to 72℃ for 10 min, raise to 78℃ to terminate mashing. The mash pH is maintained at 5.3.

Fermentation control scheme:
After heat exchange by a thin plate heat exchanger, when the temperature drops to around 15℃, pump wort into the fermenter. Control fermentation temperature at 20℃. Measure gravity daily until the apparent extract decreases to around 4°P, then strictly seal for anaerobic fermentation and naturally pressurize to about 0.15 MPa. Measure diacetyl daily. When diacetyl drops below 0.08 mg/L, reduce temperature at 0.4℃/h down to 10℃ and hold for 24 h, then reduce at 0.4℃/h down to 5℃, then reduce at 0.2℃/h down to 0℃. During fermentation, periodically discharge waste yeast and other condensates from the tank bottom.

During IPA brewing, dry hopping was applied during main fermentation at 20℃ by adding different aroma hop varieties into a 100 L fermenter to observe the influence of hop variety on beer flavor changes. During the fermentation stage of 100 L IPA trials, Cascade hops, Gansu No.1 aroma hops, Hallertau hops, Saaz hops, and Xinjiang No.1 aroma hops were added in batches. Fermentation liquid without any hops added into the fermenter was used as a blank control and compared with the fermented beers produced using different hop varieties. The contents of hop aroma substances in the fermented beers were tested. Gas chromatography was used to analyze four main aroma components (linalool, α-terpineol, geraniol, β-citronellol) in beers during the lagering stage. Sensory evaluation was carried out on the final beers to select the best hop variety and addition scheme.

6.2.3.2 Study on the Influence of Aroma Hop Dosage on Hop Flavor in IPA Beer

After hop screening for IPA brewing, Gansu No.1 aroma hop was selected. In addition to hop variety, hop dosage has a significant influence on hop aroma. In brewing, process requirements typically add different hop products at different phases in wort boiling or whirlpool clarification, because in boiling wort, hop resins undergo a series of reactions that remove some substances unfavorable to flavor stability, benefiting a harmonious flavor. In this experiment, while keeping the boiling-stage hop addition fixed, different masses of hop products (pellet hops) were added into the fermenter to determine the optimal hop dosage.

Specific method for dry hopping into fermenter:
During main fermentation at 20℃, add 60 g, 120 g, 180 g, and 240 g pellet hops into 100 L fermenters. No pellet hops were added as a blank control. According to QB/T1686-2008, gas chromatography was used to analyze the contents of the four main aroma components (linalool, α-terpineol, geraniol, β-citronellol) in beers during the lagering stage. Sensory evaluation was performed on the final beers to select the best hop dosage scheme.

6.3 Results and Discussion

6.3.1 Selection of Hop Variety for IPA Beer

The contents of linalool, α-terpineol, geraniol, and β-citronellol in lagering-stage beers produced by adding Cascade hops, Gansu No.1 aroma hops, Hallertau hops, Saaz hops, and Xinjiang No.1 aroma hops during main fermentation were measured. The results are shown in Table 2.8:

For IPA beer, compared with other beer types, the distinctive hop aroma is the biggest difference. Whether hop aroma matches consumer preference depends entirely on hop variety selection during IPA brewing. Different hops have significant differences in the content and types of aroma substances, leading to different sensory impressions. According to literature, the main substances causing differences in hop aroma are two aroma components—linalool and geraniol. Therefore, for IPA beers brewed with five hop varieties, the contents of linalool and geraniol were determined, and professional beer industry personnel performed sensory evaluation. With meeting drinking preference as the standard, an optimal hop variety was selected. Based on the measured linalool and geraniol contents in Table 2.8, beers brewed with Cascade and Gansu No.1 had significantly higher linalool than the other three hop varieties, while geraniol content in Gansu No.1 and Xinjiang No.1 was significantly higher than in the other three. After sensory evaluation, beers brewed using Gansu No.1 and Xinjiang No.1 showed more typical and prominent hop aroma and better matched the target IPA flavor compared with the other three hop varieties. Considering price and other factors, Gansu No.1 aroma hop was selected as the hop for IPA brewing.

6.3.2 Selection of Hop Addition Amount for IPA Beer

The contents of linalool, α-terpineol, geraniol, and β-citronellol in lagering-stage beers produced by adding 60 g, 120 g, 180 g, and 240 g of Gansu No.1 pellet hops during main fermentation were measured. The results are shown in Table 2.9:

From Table 2.9, it can be concluded that as hop dosage increases, the contents of the four main aroma substances increase as well, indicating hop dosage is closely related to the final beer mouthfeel and aroma type. Although increasing dosage can create more obvious or typical hop aromas such as fruity and botanical aromas, an appropriate dosage standard should be found for two reasons: (1) saving resources and reducing cost—better hop products with richer aroma substances are more expensive; (2) more hops does not necessarily mean better aroma—excessive hop usage can produce overly strong, irritating hop aroma that makes tasting uncomfortable, while appropriate dosage can give a more harmonious flavor. Therefore, based on sensory evaluation and experimental results, the hop dosage range of 120 g/HL–180 g/HL was considered most suitable for brewing IPA beer. Within this range, IPA beers emit pleasant hop aromas, with relatively rich fruity and floral aromas as typical flavor characteristics compared with other beer types.

6.4 Summary of This Chapter

Based on the studies on the influence of hop variety and aroma hop dosage on IPA beer hop flavor, a hop usage scheme can be summarized: based on linalool and geraniol levels and professional sensory evaluation, on the basis of hop additions during boiling, Gansu No.1 aroma pellet hop was selected as the hop type to be added into the fermenter. On this basis, the relationship between hop dosage and main aroma substances was studied. Through data and professional evaluation, the optimal dosage range for brewing IPA beer was 120 g/HL–180 g/HL. Therefore, at 20℃ fermentation liquid in a 100 L fermenter, 120 g/HL–180 g/HL of Gansu No.1 aroma pellet hops were added to highlight the typical hop flavor of the brewed IPA beer.

References

[1] Cai, Y. (2014). Studies on IPA Beer Brewing. Master’s Thesis, Qilu University of Technology.

Author Profile: Alex Chen, Lead Brewery Process Engineer