Hop Extracts

At the end of the zombie apocalypse fear not, we will still have hops! 

I sometimes joke that if all life on planet Earth ceased and aliens came down to investigate our lifeless planet they might wonder what all this green, bitter, sticky resin is doing in drums. What could we possibly have used it for? There is a dearth of literature on hop extracts, on optimal extraction parameters, and even less on their application in beer. Although many brewers have never dabbled in extracts, some predominant craft brewers have been using them since the mid-1990s, and macro brewers have been using them since the 1950s. More recently they have popped up for the homebrew scene in 3- and 10-mL “hop shots,” literally a syringe prefilled with hop extract ready to be heated and shot right into your homebrew kettle. 

History of Hop Extracts

As early as 1880 the value of hop resins as preservative agents was well known. In fact, the preservative value (P.V.) of hops was measured by hop resin content. The hop resins are comprised of alpha and beta acids and their oxidized derivatives. Both alpha and beta acids have preservative qualities, but the isomerized alpha acids are predominantly responsible for beer bitterness. Until the 1970s hops were bailed and sold as dried whole cones, which were subject to oxidation and could lose up to 30% of their alpha acid content in a year, and 50% in two years. 

It is believed that the original extraction applications using solvents aimed at diminishing alpha acid losses, in effect preserving the preservative value and thus bittering content of the hops. In the 1950s solvent-based extractions were conducted on hops. These extractions were done using hexane, methanol, methylene chloride, ethanol, and other chemicals. The extracts produced were of lower purity and quality than today’s commercially available extracts. The favored solvent in the 1950s was methylene chloride (with extraction still ongoing in Poland and former Czechoslovakia until the early 1990s), and then hexane was favored up until the 1990s.  Hexane is the least polar of the solvents — thus it became favored to eliminate some of the more polar compounds that were thought to interfere with purity of the resin. 

In 1957 Miller Brewing Company developed a process to produce light-stable hops (35% reduced hop acids, known as Rho) from solvent extracts. Miller High Life, Champagne of beers, debuted a light stable product in flint bottles in 1961. The hop product was made using a hexane extract that had undergone further processing to make it light-stable, and thus was born a beer in a clear bottle that did not skunk. Miller’s hexane plant was operational until about 1982 and a larger commercial scale extract facility using hexane was in operation in the U.S. in 1991. Today there exists only one operational ethanol hop extract plant owned by Hopsteiner in Germany. At 90% purity, ethanol extracts about 30% of the hop material, which includes more vegetative matter due to its ability to solvate polar molecules. Also ethanolic extracts do not efficiently extract hop oils, and the oils that are extracted volatilize once the ethanol is evaporated. Thus the major issue with solvent-based extracts was the loss of hop varietal character due to the evaporation of the solvent off the extract and thus the evaporation of any solubilized hop oil volatiles. A secondary issue was certainly the safety concerns with the use of highly evaporative and combustible solvents. Nevertheless, solvent-based hop extracts could contain up to 50% alpha acids, significant beta acids, as well as hop oils. The extracts could be canned or put in drums that do not require refrigeration and in comparison to cone hops they offered supreme alpha acid shelf stability. The added cost of the extraction can be compensated for increased utilization, reduced storage and shipping costs, and reduced loss of alpha acids due to aging. 

In the 1950s Japanese and Soviet scientists started investigating the use of liquid carbon dioxide (CO₂) for the production of hop extracts. The extracts produced were of poor quality, thought mainly due to the presence of iron and other salts or minerals in the poor quality steel of the extraction vessels. In 1975 Laws et al. of the Brewing Research Foundation (BRF) introduced liquid CO₂ hop extracts, later commercialized by Carlton and United Breweries. In 1978 Muller, Vitathum, and Huber developed a supercritical CO₂ hop extraction process. A liquid CO₂ extraction facility was commissioned in Australia in 1980 and subsequently plants were constructed in Germany (Supercritical), the U.K., and the U.S. Miller Brewing Company built their pilot CO₂ extraction plant in the 1980s and today there are several facilities in the U.S. and abroad that operate under various conditions from liquid to supercritical pressures. 

Why CO₂? 

Carbon dioxide is a natural product of fermentation. It is also readily available, inexpensive, nontoxic, nonflammable, and chemically inert under most conditions. CO₂ liquefies under reasonable pressures and once liquefied can be passed through various media, through which it behaves as an organic solvent (Hyatt. J. Org. Chem. 1984, 49, 5097-5101). Carbon dioxide exists as a liquid below 88 °F (31 °C) and 73 atm. Above 88 °F (31 °C), CO₂ cannot exist as a liquid, it exists then as a supercritical fluid that behaves as a gas, but when highly compressed the fluid is more dense than liquid CO₂. In the supercritical phase CO₂ behaves as a solvent with a higher diffusivity, lower viscosity, and lower surface tension than liquid CO₂. In general, the supercritical phase is thought to be a better solvent than liquid CO₂. Liquefied and supercritical CO₂ behave similarly in terms of their ability to act as a covalent solvent. It behaves as a good solvent for hop resins (supercritical) and for aromatics (liquid). However, at higher pressures supercritical CO₂ extractions can negatively impact oil content. The reason for this is that at higher pressure the CO₂ gas contains water vapor as well as oils that leave the separator upon condensation. 

A comparison of the essential oil content and composition indicates that liquid and supercritical CO₂ extracts of the same hops can lead to 50% reduction of oil content in supercritical extracts. The majority of this is composed of hydrocarbons that would generally either volatilize or oxidize in the kettle during boil. However, due to this it is suggested that liquid extracts display more varietal character and are preferable for flavor and aroma applications in beer.

During extraction, extract vessels are filled with pelletized hops. (The hops used must be pelletized because liquid CO₂ cannot penetrate intact lupulin glands, which contain the resins and oils. During pelletization hops are ground into a powder then pressed through a die which ruptures the lupulin glands to expose the resin and oils.) Liquid or supercritical CO₂ is then pumped through a heat exchanger to achieve desired extraction temperature, then run through the vessel at specified flow rates to extract hop resins and hop oils out of the pellets. Once the pressure is reduced, the CO₂ evaporates off of the extract in the heat exchanger, the extract is collected, and the CO₂ is recycled. Suggested extraction pressures and temperatures are displayed in Table 1.

Liquid extracts tend to be yellow to gold in color and higher in oil content due to the softer extraction, while supercritical extracts range from light to dark green due to the presence of chorophyll extracted under harder conditions. During supercritical extraction up to 50% of the volatile hydrocarbon fraction can be lost depending on the temperature, pressure, and flow rate of the CO₂. 

During extraction the most non-polar compounds (the oils and the beta acids) elute first, then the alpha acids, then any subsequent more polar compounds. The maximum solubility of hop acids occurs at 45–46 °F (7–8 °C) due to the presence of moisture in the hop cones. Liquid plants thus operate between 41–50 °F (5–10 °C), up to 68 °F (20 °C) to minimize the uptake of moisture in the extracts. Research conducted by BRF indicates that 350 kilos of liquid CO₂ is required to extract 1 kilo of alpha acids. Supercritical plants require less CO₂ due to the higher extraction efficiency of the supercritical liquid phase and thus shorter run times are required to achieve similar extraction of alpha acids. However supercritical extracts contain more non-desirable components such as waxes and chlorophyll and are thus not as suitable for advanced processing into light-stable or pre-isomerized alpha acid products. The alpha acid content of CO₂ extracts is varietal dependent (30–50% alpha), with low alpha, aromatic varieties yielding lower alpha, so the extraction of lower alpha varieties may not make economic sense. 

Use and Benefits of CO₂ Extracts

Due to the viscosity of traditional hop extracts, they must be heated and poured out or metered out of the storage vessel, or in many craft brewing applications, the entire can punctured and suspended into the kettle. Extracts are typically used for the bittering charge, but as Alec Mull (Vice President of Brewing Operations at Founders Brewing) suggests they can be used throughout the boil. Founders has been using extracts for close to 10 years, but on the hot side only, as to this date trials done on the cold side haven’t proven effective. T90s (pellets that have 90% of whole hop material in them) and other concentrated pellets such as T45, T30 (45% and 30% of original hop material) are still used on the cold side for dry hopping. On the hot side, in addition to hop pellets, Founders uses hop extracts for various charges at 60, 30, 20, 15, and 10 minutes to end of boil. In their “irresponsibly” hopped (those with more than 2 lbs. per barrel on the hot side, or about 1 oz. per gallon/7 g per L) beers they have trialed whirlpool strike charges. Alec suggests if you are looking for a place to start, consider replacing your 60-minute bittering charge at 100% alpha substitution to the alpha in your traditional pellet charge. 

Recipe development should be done using grams of alpha as a metric, not oils. The major advantage of using hop extract over pellets is yield of wort and in a very hoppy beer its use can yield as much as 8% more beer due to reduced beer and wort loss. The sweet spot at Founders is 0.4 lbs. (180 g) pellets per barrel (pre-whirlpool charge); above this rate, increased wort/beer yields merit the added cost of extracts. In their double IPA Alec says that extract use improves wort yield by 1.2–1.7 gallons per pound (10–14 L per kg) of substituted pellet. 

The second advantage is that the vegetal aroma that sometimes gets in the way of the hop expression is diminished and the resultant beer is more distinct, brighter, and cleaner. Alec believes that in addition to the financial benefits, the quality benefits result in the production of better beer.

Vinnie Cilurzo agrees. He says that the use of hop extracts reduces the vegetative quality in his beer and the resultant beer has “cleaner” hop character. At Russian River Brewing, Vinnie has been using extracts since 1998. At the time the strongly held belief among craft brewers was that extracts were just for big beer. Nevertheless, a hop sales rep was able to convince him to test out extracts. The gain in yield of beer and the cleaner hop character allowed the infamous Pliny the Elder beer to “stay true to itself.” Vinnie suggests using Warrior hop extract for the bittering charge as its alpha acid content (15–16%) lends it to a more economical and potentially consistent extract. 

At Russian River they heat the entire can of extract up in hot water, swirl it with hot wort and pour it into the kettle as he prefers not to place the can in the kettle. Vinnie says they always use entire cans, never partial cans. At Founders, they poke holes in the metal can using a commercial can opener, or a pointed hammer, to remove the can lid and place the can inside a metal basket which is then dropped into a hop dosing vessel containing hot wort. Or in case of their automated brewing system, the cans are placed in a hop-dosing vessel where wort is pumped through to add the hop extract. Alec suggests Chinook or Bravo as easy/safe starters for bittering charge replacement, as varietal character seems maintained. 

Extract cans and jugs come in various sizes ranging from 150 g to 200 kg drums or totes. It is measured in “grams of alpha” or GMA for short. An example of recipe conversion of Chinook pellets at 13% alpha acids (AA) to extract: 

Lbs. of hops in charge x 454 (g in a lb.) x (AA%) = required GMA

10 lbs. pellets x 454 g x 0.13 (%AA) = 590 GMA

There are a number of suppliers of hop extracts sold in small hop shots for homebrewers. For instance, Yakima Valley Hops advertises one on their website made up of 60% Columbus or CTZ hops and 40% blended aroma varieties. Typical alpha acid concentration of the hop shots is 50–65%. Their recommended dosing is: 

Volume of extract (mL) = AAU (g) x density of hop extract (g/mL) 

Or 1 mL yields around 10 bittering  units (BUs) in 5 gallons (19 L) of 1.050 wort added 60 minutes before the end of the boil. 

When used as a hop bittering charge replacement on the hot side there are many benefits to using extracts over pellets. The economical bang for your buck in increased yields from reduction of solids, reduction of trub formation, reduction of foam. And on a larger scale the reduced cost in shipping and storage makes extract usage a no brainer. And as we’ve heard from some well-respected craft brewers, the use of hop extracts can also produce cleaner, brighter beer while maintaining hop varietal impact. The CO₂ extraction process also reduces and or eliminates the agrochemical (pesticide) and nitrate content of hops. 

Extracts can also be stored at ambient temperature. In my former life as hop queen at MillerCoors we once sampled a drum that had been in storage in Golden, Colorado for over a decade, at ambient temperature. It had a small layer of oxidation on the top of the drum and the rest was still true to its packaged alpha content. I am positive that at the end of the world as we know it, those drums of gooey green extract will persist. 

References: 
• Gardiner, D.S. “Commercial scale extraction of alpha–acids and hop oils with compressed CO₂” https://doi.org/10.1007/978-94-011-2138-5_4

• Mull, Alec. MBAA presentation “Hot side usage- optimization with extract.”

• MasterBrewers podcast – Episode 121 “Hot Side Hop Usage_ Optimization with Extracts with Alec Mull.”

• Pyman, F.L. “The investigations on the preservative principles of hops.” https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.1927.tb05151.x

• Hyatt, JA. The Journal of Organic Chemistry 1984 49 (26), 5097-5101. DOI: 10.1021/jo00200a016

Written by Dr. Pattie Aron