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.
USA.
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.
USA.