The Essential Oil of Hops: Hop Aroma and Flavor in Hops and Beer

The female flower of the hop plant (Humulus Lupulus) has long been a friend to beer brewers around the world. Depending on the style of beer, hops can add balance to the finished product through bitterness, flavor, and aroma. It is well documented how bitterness is obtained in the finished product via the isomerization of alpha acids, but investigations into the chemistry and utilization of hop essential oils for beer flavor and aroma have, in general, failed to elucidate the complex processes involved. In this article I begin with some background on hops and their use in brewing beer, and then narrow the focus to examine the essential oil of hops. I first discuss the chemistry and composition of hop oil in hops and beer, and then quickly turn to the practical by discussing how to evaluate and analyze hops for the aromatic quality of hop essential oil. I conclude with some methods to obtain and protect hop flavor and aroma in your finished beer, which I think is the most important take home message of all.

As dedicated small scale brewers, you are more than likely familiar with the wide variety of hop products available for use in brewing. The starting point for all of these is the whole unprocessed hop flower. Hop flowers are harvested once a year, processed, packaged, and held in cold storage until sold. Depending on the level and intensity of processing, many different products can be produced including Type 100 pellets (highly compressed whole cones in half ounce plugs), Type 90 pellets (ground whole cones extruded into pellets), and a wide variety of purified and concentrated extracts and essences. In most cases, there are trade-offs, especially with respect to hop oils, that roughly scale with the intensity of the processing. Each of these products has a slightly different contribution to beer, especially when you consider the wide variety of ways that brewers have come up with to introduce hop character into their product.

By far the most common way that hops are used is to add them to the brewkettle and boil them in the sweet wort. For long boils (30-120 minutes) hops accomplish four major things: they add bitterness via alpha acids being isomerized (rearranged without changing their composition) into more stable and soluble iso-alpha acids, they assist in the production of a good hot break by supplying tannins that combine with unwanted proteins, they add to beer stability due to their antibacterial properties, and they lower the surface tension of the wort so that a vigorous boil can more easily be maintained. Late kettle additions (0-30 minutes before strikeout) are responsible for most of the "hop character" of a beer, i.e. hop flavor and aroma from the essential oils of the hop. Running hot wort through a bed of hops on the way to the chiller (i.e. using a hop back or jack) can introduce even more hop oil to the wort. Dry hopping, i.e. adding hops to the fermenter or serving tank, is another method that accomplishes this although the compounds extracted by cool wort or beer are quite different than those found in a wort after the boil.

The various extract and essences available give the brewer even more choices of when to add hop compounds. Some British brewers have abandoned dryhopping in favor of post-fermentation additions of concentrated hop oil. Pure iso-alpha extracts give equal flexibility in the fine tuning of beer bitterness. To put this all in a nut shell, hop character is affected both by what type of hop product you add to the wort or beer, and by the method and timing of the addition. In part this is due to the vast array of very reactive compounds that make up hop oil.

There are two groups of oils found in most plant materials, differentiated by their volatility. The fixed oils, like liquid fats found in nuts, are relatively nonvolatile and do not contribute much to aroma. The volatile oils, also called the essential oils, are so easily vaporizable that we can deduce their presence or absence by simply using our noses. We are all familiar with perfumes and spices, both of which are loaded with either naturally occurring or synthesized essential oils. In hops, the essential oil makes up only about 0.5-3% (vol/wt) of the whole cone, but the contribution to beer is enormous. Think about it, if a typical beer uses hops at a rate of 0.5 ounces per gallon, and we assume that the oils are 10% utilized, then the hop oils end up being less than 0.001% wt/wt (10 ppm) of the finished beer. And yet what is the first thing that hits the nose in a Cascade dryhopped American pale ale? That's right, a big floral aroma that is impossible to miss. To examine just what it is in the hop essential oil that makes such big impressions let's talk chemistry.

Hop oils are made up primarily of a hydrocarbon fraction, (containing only H and C), and an oxygenated fraction (H, C and O). There are also small amounts of sulfur containing compounds but we will neglect them in this context. The hydrocarbons typically make up 80-90% of the total oil; the terpenes myrcene and b-pinene, and the sesquiterpenes b-caryophyllene and a-humulene are found in the largest quantities. Two of these, b-caryophyllene and a-humulene, can be easily oxidized in air, thus contributing to the oxygenated fraction of the oil as well. Other oxygenated compounds include alcohols such as linalool and geraniol and esters such as geranyl isobutyrate and methyl dec-4-enoate. Although many brewers think esters are all fermentation by- products, hops can contribute a number of fruity aromas, grapefruit and pineapple for example.

Some of you may have already asked the question, "Why hasn't anyone tried to quantify the 'hoppiness' of a given hop sample like they have its bitterness (i.e. alpha acid %)?" Since hop character is attributed to hop essential oil, one would think that the amount of oil in a hop sample would be a way to estimate its hop aroma content. Unfortunately things are not so simple. The total oil content can be an indicator of the overall quality of a hop sample, especially if you have a good idea what the normal oil content for that hop should be. But because there are many factors that affect essential oil production and preservation, every season there is potential for significant variation in both the total amount of oil and the composition of the oil even for a single variety. In fact from farm to farm there can be substantial differences, not to mention the additional variables introduced by the hop processing and storage facilities. Over the period 1975- 1985 the total oil content of fresh Cascade hop samples at the U.S.D.A. hop lab varied between 0.28 and 1.79 ml/100 g of hops and the percent myrcene, usually the the major component in the oil, ranged from 46% to 82%. Since both the total oil content and the relative amounts of hop oil components can vary widely, it would follow that a simple oil content rating would not reliably provide the information we need to evaluate the aroma quality of our hops. There are however three other methods that have all proven to be quite helpful.

First I will outline the high tech method which gives both quantitative and qualitative information but requires distillation equipment and a capillary gas chromatograph (GC). The GC has helped researchers to identify more than 250 essential oil components. Of these, 22 that have been reported to affect hop aroma in the literature have been divided into groups: Humulene and Caryophyllene Oxidation products, Floral-Estery Compounds, and Citrus-Piney Compounds. These 22 compounds make up the Hop Aroma Component Profile (HACP). Just as bitterness in beer is quantified using the Bittering Unit (BU), Nickerson and Van Engel propose that an Aroma Unit (AU) be adopted which is defined as 1 ppm (Ál/kg of hops or Ál/l of wort or beer) of the sum of the 22 HACP constituents. Thus if the AUs of a brewery's aroma hops changed, it would be a simple matter to calculate the change in the amount of hops needed to achieve the historical AUs of a given beer. How well do AUs correlate with actual 'hoppiness'? A commercial brewery's taste panel found that hop aroma, hop taste, and dry hop aroma correlated very well with AU's.

Using data from an earlier study, I checked to make sure that nothing more simple was being overlooked, since as a homebrewer I didn't happen to have a spare GC laying around my basement. Although there was a correlation between the AU's and the total oil content in fresh hops it was "weak" (r = .449), ranking only about a 3 on a scale of one to ten. The correlation was better in hops aged for six months at room temperature (r = .756) but hopefully none of us are brewing with hops that have been subject to such storage conditions. This confirms our previous hypothesis that total oil content is not necessarily a good indicator of potential hoppiness or aroma quality. There were also no significant relationships between myrcene, humulene, or alpha acid levels and AUs.

For those of us without a complete lab at our disposals, here are a couple of tried and true low tech (no tech!?) methods for evaluating the aroma potential and quality of hops). The first is initiated by grabbing a handful of hops and looking/listening/feeling /tasting/smelling them. Jean De Clerck wrote in reference to evaluating fine aroma hops,

"Aroma is tested by smelling a crushed handful of whole cones. The hops should have a pronounced aromatic smell free from extraneous taints and odors, [list of unpleasant odors]. Smell may also be tested by rubbing the cones between the fingers which splits the lupulin grains. The aroma of the sample should not be sharp, but fine and mellow."
One reminder however, is that although this sort of manual evaluation of hops will give you a good feel for the over-all quality of the hop and its potential dryhop aroma, it provides little quantitative information. Most of what you will smell is the hydrocarbon fraction of the essential oil which is rapidly lost during the boil and/or changed during fermentation and is, in fact, rarely found in beer. If after crushing the hops in your hands you pause for a minute or two to let the most volatile of the hydrocarbons dissipate, you will be better able to evaluate the aroma quality.

The second method is to make a hop tea by boiling or steeping a hop sample in water. This evaporates a lot of the hydrocarbons and gives you a better impression of how a particular sample will contribute in the brewkettle. In order to be as consistent as possible, I recommend always using the same container to make the tea (a 1 L erlenmyer flask is good) and the same water to hop ratio (2 g hops per 600 mL water). Prepare the tea the same way you plan to use the hops in your beer, boiling (or steeping) for a predetermined time. After cooling the tea add water to bring the volume back to your starting point and evaluate the flavor and aroma both for quality and quantity/intensity. Although this method does not employ any fancy instrumentation, it can be very effective, especially if good sampling notes are kept. One final thing to remember with respect to both of the low tech methods: if it smells bad, don't use it!

Hop essential oils, like hop bitter resins, are easily lost during storage due to their susceptibility to oxidation. In storage trials at the U.S.D.A. Hop Research Lab at O.S.U. in Corvallis, OR , oil losses were found to range from 28-90% in six months at room temperature depending on the variety. These losses can be slowed by storing hops in the freezer, preferably in a package that allows no air or water exchange (i.e. "barrier"). The best advice to follow in this area is to buy the best hops you can find and take good care of them, especially if they are aroma hops.

As is mentioned above, the compounds found in hop oil are not all the same as those found in hopped beer. Late kettle hops contribute differently than steeped hops and dry hops as most of you already know. Both the fermentation and finishing processes also affect hop character. Consequently a brewer needs to find the type of hop character s/he desires in a given beer and then be consistent in the method used to brew it. Then once you have achieved the Holy Grail of hoppiness, and the bottle is capped or the keg is sealed, hop character starts to go downhill; unfortunately it is inevitable. I'll tell you how this occurs and some steps to take to prevent it.

Peacock and Deinzer found that the two main ways in which hop oil components in beer can be lost are migration into the packaging material and chemical degradation. They write,

"The major mechanism for the loss of hop aroma compounds from beer appears to be chemical degradation . The most likely pathways for degradation of terpenoid and sesquiterpenoid compounds are reaction with oxygen in the headspace of the bottle and acid hydrolysis."
They go on to say that bottle cap liners tend to absorb many hop compounds, especially hydrocarbons and longer chain ketones and esters. Hop oil alcohols appear to not be absorbed well by the cap liners. Since we can not do anything about acid hydrolysis, this diagnosis points to a rigorous elimination of oxygen from all beer packages, and to using a non-absorptive bottle cap liner. Oxygen scavenger caps (PureSeal) are on the market now but their performance with respect to absorption of hop oil compounds has not yet been tested.

We've covered a lot of ground here and you may be wondering what the take home message is to all of this. I think there are four important points to remember when dealing with the essential oils in hops.

  1. Start with the best hops you can get and store them in the freezer in barrier packaging if possible.
  2. Monitor the aroma quality of your hops using either the Aroma Unit (if you can) or by physical evaluation.
  3. Be very consistent in your brewing process making adjustments based on your evaluations.
  4. Try to eliminate as much oxygen as possible from your packaging process.
Steps 3 and 4 will yield benefits in other areas, so the pay-off is even bigger.

Hop oil research is a rapidly changing field and new findings pop up every year. The consensus is that the marvelous hop aroma and flavor we all enjoy is probably due to a synergistic combination of hop oil compounds and their oxidation and fermentation products, some of which may have not even been discovered yet. All I can say is that even though I may not know exactly what it is I am tasting in my dryhopped IPA, I like it!

References

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