BrewingTechniques
Hop Storage:
How to Get - and Keep - Your Hops' Optimum Value

by Mark Garetz
Republished from BrewingTechniques' January/February 1994.

Most brewers are familiar with the fragile nature of hops - they loose their bittering qualities over time, and their essential oils degrade. Accurate information about how and why hops lose their a-acids, and sound preventive measures like good packaging and storage, can enable brewers to keep their hops fresh and their brews in line with expectations. Simple calculations can also help brewers predict the a-acid levels in their hops at any point in time.

Hops have three main ingredients of relevance to brewers: a-acids, b-acids, and essential oils. Brewers normally concern themselves with only two of the three - a-acids and oils.

The a-acids are bitter but dissolve poorly in wort, so they need to be changed into a form that dissolves well. In brewing, this change occurs through boiling; the process is known as isomerization. The resulting isomerized a-acids are more soluble in wort and retain their bitterness. For all intents and purposes, b-acids are not bitter and are not changed into a bitter form during the brewing process. They do form bitter compounds when oxidized during storage (1).

The oils are responsible for the aroma of hops and enter into the beer's flavor profile when added for short boil times, when steeped, or when added fresh in a hop back or as dry hops in the fermentor.

All three of these components undergo changes as hops age.

Hop Harvesting and Processing

Hops are harvested once a year, beginning in mid-August and continuing through early September, depending on the hop variety. The hops are dried, and in the United States baled in 200-lb bales. The bales are made by compressing the hops and then wrapping them in burlap. Some hops are ground and extruded into pellets. Some hops in the UK are compressed into "plugs" that weigh about 1/2 oz. The level of compression in these plugs is much higher than in the bale. (In the UK, these plugs are known as pellets, technically Type 100 pellets.) In Germany, some hops are compressed into 5-kg (11-lb) "bricks" and then vacuum sealed. The level of compression of both of these is about 3-4 times that of U.S. bales.

The hops are then stored in huge warehouses at about -3 degrees C (~26 degrees F) (temperatures differ depending on the broker and outside temperature) and remain there until they are shipped to a brewer or hop supplier. Most small brewers buy enough hops at the start of the hop season to last all year, but they are stored at the hop broker and shipped to the brewery periodically, providing the brewer the convenience of not needing a huge cold storage facility. Also, because most small brewers lack hop analysis equipment, the hop broker can keep tabs on the a-acid and oil contents as they change over time.

Hop Deterioration

Hops start to lose their a-acids and oils as soon as they are harvested. The rate of loss depends on the storage temperature, the amount of air present, and the hop variety. The lower the temperature, the less the hops deteriorate. It has been shown that the rate of loss halves for every 15 degrees C (27 degrees F) drop in temperature (2).

Oxygen is definitely bad for a-acids; their oxidation components are responsible for the "cheesy" aroma detected in old hops (1). Oxidized a-acids lose their bitterness and cannot be isomerized. Because b-acids form bitter compounds when they are oxidized, some believe that this result of oxidation makes up for the loss of b-acids. In fact, it has been argued that cold storage and anaerobic conditions are not necessary for bittering hops, as long as the boil is long enough and open enough to allow the cheesy aroma to escape. But brewers aren't buying the argument (who can blame them).

The variety of the hop also plays a major role in storage. Hops are usually classified as kettle or bittering hops and aroma hops. Kettle hops have a higher a-acid content than aroma hops, and their storage properties are more important. Under identical storage conditions, certain varieties will lose more a-acids than others. Each hop variety contains differing amounts of natural antioxidants, and some varieties' lupulin glands are more permeable to air than others.

One common test for the storageability of the hops is to measure the amount of a- and b-acids lost over a 6-month period at 20 degrees C (68 degrees F). There is a direct relationship between the losses and the hop storage index (HSI). The HSI is a number obtained by spectrophotometric determinations of the a- and b-acids (3,4). If you know the present a-acids content and the HSI or the percentage lost figures for a particular variety, you can estimate the original and future a-acids content. Formulas for predicting a-acid losses are presented later in this article.

The oils also deteriorate and oxidize over time. Some people believe that some oxidation of the oils is beneficial to hop aroma. Not enough research has been done in the area of characterizing the oil content loss rates for various varieties, so we are unable to accurately predict oil losses on a variety-by-variety basis at this time. One could make the assumption that the rate of oil loss is directly related to the loss of a-acids and use the a-acid loss formulas to predict the oil losses as well. But again, due to the lack of experimental data to back this method up, it remains only an assumption.

Exposure to light hastens hop deterioration as well. At home, this is not much of an issue because most freezers are dark inside. But in your local homebrew supply store, a display freezer may have fluorescent lights in it. Although this certainly makes the hop display more attractive, the hops would be better served if the light were removed.

Hop Storage

For optimum preservation of hops' valued qualities, they should be stored as cold as possible (30 to -5 degrees F, or -1 to -21 degrees C) and away from air. The compression of hops into bales, pellets, and plugs helps protect all but the surface layers from air. Even so, air penetrates and causes some oxidation. Cold temperatures slow the oxidation process. Because some hop varieties don't store as well as others, at some point in the season hop brokers take all remaining unsold bales of poor-storing hops and turn them into pellets. Not only do pellets keep out a lot of oxygen, their compact form allows them to be easily vacuum packed to further slow the deterioration.

The reason pellets are so prevalent in the home brewing trade is that they deteriorate more slowly than whole hops when stored in less than ideal conditions. Microbrewers like them for two additional reasons: they are easy to remove from the wort if the brewery uses a whirlpool separator, and they take up much less storage space, making it much more practical to keep them cold.

Although compression of whole hops slows the oxidation because it is harder for the oxygen to get at the hops, when the bale is broken up to be portioned into homebrew-sized quantities the compression is lost and air can get at the hops much more easily. Because of the compression, plugs are a good compromise between pellets and whole hops.

Hop Packaging for Sale to Home brewers

Vacuum packing and inert-gas packaging in an oxygen-barrier material are the best packaging methods. The common type of oxygen-barrier packaging is the "boiling bag" which is clear and made from a lamination of two types of plastic. The inner layer is a food-grade polyethylene (the same material that common plastic sandwich bags are made from). Although it does provide a barrier to water, polyethylene is not a proper oxygen barrier; it does make a good heat seal, which is the main reason it is used. The outer layer is made from polyester (also known as mylar or nylon) and is what provides the barrier layer. The next step up in effectiveness is the aluminized mylar bag (also known as the foil bag or pouch), which adds a layer of aluminum that increases the barrier protection more than 10-fold. It also more than doubles the cost, so it is not widely used despite its advantages.

Some suppliers sell hops in simple polyethylene bags, which provide almost no barrier protection. Hops that have been insufficiently protected offer dubious a-acid values and should be approached with skepticism or not used at all.

To tell the bags apart, think about what a typical sandwich bag feels like; it is made of polyethylene. You can smell the hops right through it (this should tell you something). It also has a slightly frosted appearance and lacks the polished look of polyester. Clear barrier bags are noticeably stiffer and thicker. They are also shiny and polished looking and lack the frosted look of polyethylene bags. Foil bags are usually either silver or gold in color.

What To Do When You Get Them Home

First, if the hops are improperly packaged (and you had no choice but to buy them) you need to get them in suitable barrier packaging as soon as possible. If you're going to brew with them soon and are going to use them all up in a few weeks or so, however, don't worry about it - just put them in the freezer and use them up quickly.

If the hops were properly packaged, don't open them until you need to. Store them in the freezer. Once you've opened them, the biggest problem is what to do with the remainder. If they came in a vacuum-sealed or nitrogen-flushed bag, the best thing to do is reseal the bag with a home-quality vacuum sealer, which can cost anywhere from $20 on sale to $100, depending on the seal width and length and the amount of heat it puts out. Even the cheapest sealer (available at many discount outlets) will put out enough heat to seal standard clear barrier bags. Unfortunately, they may not put out enough heat to seal aluminized bags. Look around in kitchen supply departments and hardware stores. Your best bet is to take an old piece of bag with you and see how it seals. Although you can transfer the hops to the bags that come with the sealer, beware that the bags that come with the cheaper models may not be true barrier bags, though they are better than polyethylene. For replacement bags, I recommend the bags that come with the Dazey Micro-Seal system (Dazey Corp., Industrial Airport, Nebraska). They are true oxygen-barrier bags.

If you keg your beer or otherwise have carbon dioxide or nitrogen available, you can flush mason jars with the gas, put the hops in, and add a layer of gas and reseal the jar. An alternative container is a PET plastic jar like the ones peanut butter comes in. I advise you to practice with the gas first because it's very easy to blast your hops all over the room, and always use a regulator. If you can't do any of this, put the hops in a mason or PET jar and put them in the freezer - it's better than nothing.

How Long Will Hops Last?

Like most things in brewing, the answer is, "It depends." If you keep them very cold and free from oxygen, hops should last a few years. It's not uncommon for hop brokers to be selling pelletized and vacuum-sealed hops from two or three seasons ago. Commercial breweries continue to use last year's crop well into the current year's harvest. This is not to say that the oils and a-acids will be exactly the same as when you purchased them, but the hops won't be "bad"; hops are not considered "bad" until they get below 50% of their original a-acid value (5), at which point the degree of oxidation will produce a definitely cheesy aroma.

Predicting a-Acid Loss

Based on published research (such as the literature referenced in this article) and the known storage properties of commonly used hops, it is possible to predict the a-acid content of your hops at any given point in time. To do this, you must know the following:
  • hop variety and its associated storage properties
  • reasonably accurate a-acid percentage for the hops when you bought them
  • storage conditions (aerobic or anaerobic)
  • the storage temperature
  • and the number of days from the date at which you knew the a-acids to the time for which you are predicting them. To perform the calculations, you must have access to a scientific calculator, a spreadsheet program with logarithm functions, or logarithm tables (available in many math and scientific reference books).

    As previously mentioned, hop storage properties depend on the variety. The data can be reported in any of three ways, each based on the HSI. The first and least common way is the actual HSI number. This is rarely encountered in the hop trade and is primarily used as an in-house reference at research centers' hop laboratories and large breweries. Hop brokers publish the storage qualities as either "percent alpha remaining" or "percent alpha lost" after 6 months of storage at 20 degrees C (68 degrees F). The calculations presented in this article use the latter method, which I simply call "percent lost." If you have the "percent remaining" figure, simply subtract it from 100 to get the percent lost.

    Many uncertainties surround the accuracy of the a-acids level shown on the packaging at the point of purchase. When were the hops harvested? How were they handled and stored? Fortunately, the packaging itself may tell a large part of the story - high-quality packaging and care in storage reflects care and concern for the product and lends reassurance that the labeled values are reliable.

    The rest of the required data is readily available from your kitchen or brewery, depending on your storage conditions. Make no assumptions about how cold your freezer is. It is a wise and important investment to get a freezer thermometer and measure it.

    To determine the "number of days from the date at which you knew the a-acid percentage," consider the day on which you bought the hops to be Day 1. If you bought the hops 1 month ago, it is now Day 30. If you want to know what they will be 1 month from now, that will be Day 60, and so on.

    Step-by-step Instructions for Predicting a-Acid Losses

    To make the math a bit easier, this article provides some of the values already calculated for you. All you need to do is look them up in a table. For those who want to know all of the gory details, the formulas and procedures used to derive the table values are provided in the accompanying box.

    The first step is to look up the percent lost for the hop variety in Table I. Next we need to determine the rate constant (k) based on the percent lost. This rate is the constant used in the log expression to determine the curve of percent lost vs. time. Find the percent lost on Table II and next to it the value for k. Now find the temperature factor (TF) from Table III, based on your storage temperature. From Table IV, find the storage factor (SF) based on your storage method. Last, determine the number of days from the day on which you knew the a-acid content to the day on which you need to know the new a-acid content (Days). The a-acid percentage when we bought the hops is referred to as A.

    Now use the following formula:
    future alpha = A*1/e(k*TF*SF*Days)
    where e is the base of the natural logarithm. In Excel and other spreadsheet programs, e(n) is expressed as EXP(n). Let's walk through an example. Let's say we bought some Cascade hops at 6.4% a-acid 1 month ago and we want to brew with it 1 week from today. We're storing it in our home freezer, which is ~10 degrees F, in its original nitrogen-flushed oxygen barrier packaging. Table I shows that Cascade's percent lost value is 50%. Table II reveals that a percent lost of 50% gives the value for k as 0.00385. Table III shows that the corresponding value for TF is 0.228, and Table IV shows that the value for SF is 0.5. We bought the hops 30 days ago, and adding the 7 days from now, the value for Days becomes 37.
    So our formula now looks like this:
    future alpha = 6.4*1/e(0.00385*0.228*0.5*37)
    which gives us 6.3% (rounded), which really isn't all that much different but proves that good storage conditions can really make a difference in a poor-storing hop like Cascade. If we stored it at room temperature in a poly bag the numbers would look like this:
    future alpha = 6.4*1/e(0.00385*1*1*37)
    which equals 5.6% alpha - a much more significant difference. It also shows the effect of poor storage conditions.

    Conclusions

    The hop variety, storage temperature and storage conditions all play a role in determining how fast a-acids are lost from the hops. Of these, temperature is the most important factor over which we have control. Next is hop variety and finally the aerobic or anaerobic storage conditions.

    Figure 1 compares aerobic and nonaerobic storage of Eroica and Galena - two high-alpha varieties with different storage properties. You can see that, all other things being equal, it makes sense to choose a hop that has good storage properties. You can also see the dramatic effects of temperature. Sometimes you have no choice - Cascade is Cascade, and there's nothing you can do about it. But if you're looking for a general-purpose bittering hop, Cluster, Perle, and Galena are better choices than Chinook and Eroica (most of the really poor-storing bittering hops such as Olympic and Comet have faded from the market, and I omit them from the table for that reason).

    Now that you know the effects of proper storage on hops, you are in a better position to pick a hop supplier that will give you a fresher product. Consider their packaging and storage temperature. You can also take advantage of close-out sales on last season's hops if you know the hops have been properly stored.

    Finally, to get better and more consistent results when bittering your beer, you now have a tool that you can use to calculate and predict the a-acid percentages at any given point in time. One last word of advice: Don't buy brown hops - they should always be green.

    Acknowledgements

    The author thanks Gail Nickerson for her review of this manuscript and helpful suggestions along the way, Bob O'Hara for help with the math, and Greg Lewis, Ralph Olson, Ralph Woodall, and Marcie Shull from Hopunion USA.

    References

    (1) J.S. Hough, D.E. Briggs, R. Stevens, and T.W. Young, Malting and Brewing Science, Vol 2 (Chapman and Hall, London, 1982).

    (2) R.N. Skinner, R.P. Hildebrand, and B.J. Clarke, "The Effects of Storage Temperature on the Stability of the Alpha-Acids Content of Baled Hops," J. Inst. Brewing 83, 290-294 (1977).

    (3) S.T. Likens, G.B. Nickerson, and C.E. Zimmerman, "An Index of Deterioration in Hops (Humulus Lupus)," American Society of Brewing Chemists Proceedings (American Society of Brewing Chemists, St. Paul, Minnesota, 1970). 68-74.

    (4) ASBC Methods of Analysis, 8th ed. (American Society of Brewing Chemists, St. Paul, Minnesota, 1992).

    (5) Gail Nickerson, personal communication, 1993.

    (6) C.P. Green, "Kinetics of Hop Storage," J. Inst. Brewing 84, 312-314 (1978).

    (7) A.J. Rehberger and L.H. Bradee, "Hop Oxidative Transforms and Control for Beer Bitterness," MBAA Technical Quarterly 12 (1), 1-8 (1975).

    (8) Hop Variety Specifications (Hopunion USA, Yakima, Washington, April 1993).

    (9) J. Wain, C.D. Baker, and D.R.J. Laws, "Deterioration of Pelleted Hop Powders During Long-Term Storage," J. Inst. Brewing 83, 235-240 (1977).

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