Hop Chemistry Science
I recently heard an advertisement for a famous non-alcoholic beer. The actor insisted this beer was made from only the finest water, yeast and hops. I doubt many of you had trouble spotting that malt is the missing ingredient in that statement. But like the copy writer at the ad agency, many in the general public seem to believe that beer is made only from hops.
Historically speaking, hops are a fairly recent innovation in the brewing world. Although evidence exists of their cultivation as early as 200 AD in Babylon and 700 AD in Germany, they were not widely used in brewing in Bavaria until the 11th century. They didn’t gain wide acceptance until the 15th or 16th century in the rest of Europe. In England they were not highly thought of initially, and their use was banned by King Henry VIII. This was only a short time before brewers began emigrating to the United States, so American brewers have been using hops for about as long as their European counterparts.
It is impossible to think of a beer today that does not include hops, despite what some critics of American pilsner style beers may think. In fact it is a legal requirement in the United States and the UK that beer includes hops in the formulation. The Bavarian purity law, the Rheinheitsgebot, written in 1516, also legislates the use of hops.
Hops provide a bitter flavor to beer, a nice flavor, and a pleasant aroma. They enhance the foam on beer, and the way the foam clings to the side of the glass. They also provide protection against beer spoilage from certain micro-organisms. Over the centuries of their use microorganisms have evolved that are resistant to hops, but most of these are found only in breweries. This was once an important factor in the evolution of the beer style known as India Pale Ale. This beer was designed to survive a long sea voyage and hence was heavily hopped. As brewer’s understanding of microbiology and sanitary brewing practices has grown, hopping levels and beer alcohol content has dramatically reduced in this beer style.
Hop cones used for brewing are the dried seed cases of the plant Humulus lupulus. The hop cones consist of a central string or stalk, and between 20 and 50 petals. At the base of the petals the resin, known as lupulin, is produced as a sticky yellow powder exuded from the surface of the leafy petals. The hop plant is a perennial with separate male and female plants. All commercial hops, used for flavoring beers of all sorts, grow on the female plants and will contain seeds if male plants are allowed to produce pollen near them. To prevent seeds developing in the hops, in most countries the male plants are not permitted to be grown anywhere. In England male hops are permitted, except in Hampshire, and most English hops contain seeds. The plants grow up strings or trellis wires during the summer and the hops are harvested and dried in September. In England, the USA, Canada and Australia, hops are packed into the final package on the farm where they were grown. In European countries, the individual farmers’ hops are blended, re-dried and packed into baled in large lots or processed directly from the farmers’ lots. There is a great deal of variation between hops from the different countries, growing regions within the country and even from farm to farm. Typically a hop cone consists of the following components:
10% water
15% total resins
0.5% essential oil
4% tannins
2% monosaccharides
2% pectins
0.1% amino acid
3% lipids and wax
15% proteins
8% ash
40.4% residual carbohydrate (cellulose, lignin)
Brewers are largely interested in the total resins and the essential oils which represent the brewing value of the hop. Both are contained in the yellow dust that is to be found around the base of each petal on the hop flower. This material, produced in small glands, is called the lupulin and is essentially the only portion of the hop a brewer need be concerned with. The rest of the hop’s leafy matter may perform an important role in the brewery as a separation aid. The leafy material acts as a filter screen which aids in clarifying the wort after it has been boiled. The other components, particularly protein and polyphenols, are soluble in boiling wort, although it should be remembered that greater quantities of protein and polyphenols are derived from malt.
The total resins are further sub divided into hard resins, soft resins and uncharacterized soft resins. Soft resins consist of alpha (a) and beta (b) acids and it is those that the brewer is interested in.
Alpha Acids
Alpha acids consist of more than 50% of the soft resins and are largely thought of as the primary source of bitterness in beer. Not directly, though, as they are insoluble in wort and must first be isomerized by heat to become soluble. Unfortunately it requires around 45 minutes of boiling for 30% of the potential alpha-acid from the hops to isomerize and solubilize. This amount drops dramatically as the boiling time diminishes. A number of factors influence the degree of hop utilization; these will be discussed in an article on wort boiling in the May-June 2002 issue of BYO. The isomerization reaction results in a change in the chemical structure of the alpha-acid molecule.
Basically, alpha-acids are a class of compounds known as humulones. They consist of a complex hexagonal molecule with several side chains, with ketone and alcohol groups. Examples include humulone, cohumulone, adhumulone, posthumulone, and prehumulone. Each different humulone differs in the make up of the side chain, for instance, humulone has a side chain of isovalerate attached, while cohumulone has isobutyrate as its side chain. These side chains can become detached during extended storage under poor conditions and result in cheesy flavors associated sometimes with old hops. It has become accepted dogma among brewers to think of each of these humulones to have different bitter characteristics. There are some that swear that the bitterness associated with cohumulone is “harsher” than that from humulone. Other studies have shown no difference in sensory impact when each of the different humulones are compared. Nevertheless, the humulone:cohumulone ratio is now quoted in hop analyses and new varieties are being bred with low cohumulone levels in mind. Historically, the most highly prized hop varieties – including noble hops such as Hallertau, Tettnang and Saaz – also happen to be those that have low cohumulone levels.
The alpha-acid levels in hops begin to tail off immediately after harvesting, and continue to decline in storage. The number quoted to you on a packet of hops purchased from a homebrew store was the alpha-acid content when the hops were tested immediately after harvest, and despite the best intentions of the retailer have been subjected to conditions that cause that level to be lower. High temperature and exposure to air will speed up the losses of alpha-acids. In hop varieties with poor storage characteristics up to 50% of the total harvest alpha-acids may be lost in 6 months stored at 70° F. A good hop will still lose 20% of its total acids under the same storage conditions. Hops should be stored in a fridge or preferably a freezer and air must be excluded from the package. This will more than half the deterioration rate of your hops. Since you have no way of predicting what the hops experienced before you bought them (remember, the inside of a UPS truck can get up to 140° F in the summer in Arizona) it is always better to buy from reputable suppliers.
Additional benefits of alpha-acids are seen from their role in foam formation and head retention. They cross link chemically with certain specific proteins in an extremely complex manner to support foam. If you sip the thick foam on a pint of nitrogen poured Guinness you will notice a distinctly more bitter taste than that found in the beer beneath.
Beta acids
These compounds are not actually bitter, but will turn into bitter compounds when they oxidize during storage. The alpha:beta ratio is considered important in gauging how a hop will provide bitterness to beer as the hops age. The bittering potential from alpha-acids reduces with time but the bittering potential from oxidized beta-acids increases. In a hop with a 2:1 ratio of alpha:beta the bittering potential may remain fairly constant. The oxidation reaction will take place to an even greater extent during kettle boiling., and again the chemistry will be discussed during the wort boiling article. Beta-acids consist of lupulone, colupulone, adlupulone and other substances, and like alpha-acids differ in the structure of the side chains. Again there is a difference of opinion in the brewing world as to the character of bitterness derived from beta-acids compared to that of alpha-acids. In Germany oxidized beta-acid bitterness is preferred while in Japan it is considered too “harsh.”
Uncategorized soft resins
Dr. David Ryder gave a talk at the National Craft Brewers Conference in Milwaukee on April 28th, 2000, entitled “Hopping to Perfection.” In the lecture, Dr. Ryder introduced the idea that perhaps uncategorized soft resins may have some brewing value. In his talk (slightly better attended than my own talk on beer costs and quality), Ryder, who is Vice President of Brewing, Research and Quality Assurance at Miller Brewing Company, announced that Miller researchers have discovered that this fraction contains a portion of hop aroma compounds chemically bound to sugars. The upshot of the research suggested that maybe these compounds find their way into beer and yeast transform them into beer flavor compounds. Until the research is published, however, these compounds remain officially “uncategorized.”
Oils
The total oils, formed in the lupulin glands, represent the general aroma characteristics of the hop. It varies in concentration depending on hop variety and from season to season. It may be as low as 0.5 % or as high as 2%. The oils are soluble in boiling wort but are extremely volatile and are largely lost during the wort boiling phase of brewing. A full boil of an hour to an hour and a half needed to volatalize most of the unfavorable aroma characteristics from the malt, and precipitate enough of the denatured protein and polyphenols, results in the complete loss of any of the aroma components from the hops. Brewers get around this issue by adding a portion of the hop charge to the boiling wort 5–10 minutes from the end of the boil. Alternatively, brewers add hops immediately after boiling but before chilling to attempt to extract the aromas and avoid the losses due to volatilization. The action of yeast fermenting sugar and causing vast amounts of CO2 to rise through the wort has the effect of carrying hop aroma away with it. While this may produce a wonderful hop aroma in your fermentation area, it will cause a decrease of hop aroma in the beer. Remember that intense hop aromas are not always pleasant, so losing some may be a good idea. Hop aroma may also be added to the finished beer by a process known as dry hopping where whole hops are added to the beer in a maturation vessel. Oils dissolve slowly into the beer, probably into the alcohol fraction, while there is no increase in bitterness in the beer.
There may be up to 300 different compounds in hop oil and much of the chemistry associated with their role in beer flavor is yet to be unraveled. Three classes of compounds exist within the hop oil fraction. Hydrocarbons, oxygenated compounds and sulfur containing compounds. The majority of the compounds in fresh hops are hydrocarbons (75%) and oxygenated compounds account for most of the balance. Sulfur compounds represent around one percent but are potent flavor compounds with low taste thresholds.
Hydrocarbons
Principally hydrocarbons consist of a class of essential oils known chemically as terpenes. Monoterpenes such as myrcene, diterpenes such as dimyrcene, and sesquiterpenes such as farnascene, humulene, caryophyllene, selenine and limonene. Each of these essential oils can be isolated and their individual aroma contributions identified. Some are floral, others spicy, some are simply described as “hoppy”.
Myrcene, humulene, caryophyllene, and farnescene are the four major component of hop oil, accounting for up to 80% of the total essential oils. They are, however, extremely volatile and are only found in large quantities in beer that has been dry hopped. The amount of these constituents, and particularly the ratios between them, can be used as clear varietal indicators. Myrcene particularly is a major oil and is characterized as having an unpleasant, aggressively hoppy aroma, with a harsh grassy character. It is very volatile and hence not found in beer in large quantities unless the beer has been heavily dry hopped. Humulene is the hop aroma that is perhaps the most prized and so hops with a lot of this compound are well thought of, largely because it oxidizes readily and it’s oxidized form is pleasantly hoppy. Saaz contains as much as 45% humulene. Some hop merchants quote the humulene:caryophyllene ratio as an indicator of aroma quality.
Oxygen-containing compounds
This class of compound grows with time as hops are stored and their components oxidize. Poor storage of hops will result in hops that may contain up to 50% of these compounds. Since they may represent powerful flavoring agents that find their way into beer through late kettle additions of hops, some brewers deliberately age their hops to enhance these qualities. They consist of oxidized terpenes, higher alcohols, aldehydes, ketones and esters. Linalool and geraniol are higher alcohols and provide a floral character to beers, while geranyl acetate is an ester and provides a fruity character.
Sulfur containing compounds
These compounds are found in trace amounts but may be potent flavoring agents. Hops in the field are treated with sulfur to control mildew, and in Europe some sulfur may have been added to the warm air in the hop kiln. This causes several highly volatile compounds to be produced including dimethyl sulfide (DMS), responsible for cooked vegetable, onion, garlic flavors, and late kettle additions may slightly increase levels of these compounds.
Tannins or Polyphenols
Hops contribute additional polyphenolic compounds to wort, and these compounds are known to contribute significantly to beer haze. Luckily, boiling will cause them to combine with proteins and precipitate out of solution, avoiding additional haze problems with the finished beer. However, beers that are dry hopped often exhibit hazes that are hard to remove and so it is likely that polyphenols from hops dissolve in beer causing hazes.
Noble Hops
Certain hops are prized for their special characteristics. In Europe these varieties are known as noble hops. The origins of this term are unknown but it is likely that through their long history of use in Europe they became prized because beer made from them was favored by drinkers. There are only four true noble hops Hallertau Mittelfruh, Tettnang Tettnanger, Czech Saaz, and Spalt Splater.
Some consider the English varieties Fuggle and East Kent Golding to be noble. They are characterized through analysis as having an alpha:beta ratio of 1:1, low alpha-acid levels (2–5%) with a low cohumulone content, low myrcene in the hop oil, high humulene in the oil, a ratio of humulene:caryophyllene above three, and poor storability resulting in them being more prone to oxidation. In reality this means that they have a relatively consistent bittering potential as they age, due to beta-acid oxidation, and a flavor that improves as they age during periods of poor storage.
While it would appear that researchers have managed to unravel the exact nature of hop aroma, the sheer number of compounds present, the levels at which they are present, the degree to which they become modified during ageing and storage, and the multitude of ways they can be used by brewers make truly accurate use difficult. Likewise control of the actual bittering potential of hops is difficult to predict. This is why some of the larger brewers use extracts, emulsions or oils added directly to finished beer as a way of controlling the character of a beer’s bitterness, hop aroma and flavor. This is also one of the reasons why brewers can rightly feel justified in insisting that they are also “artists.”
