1 This article was downloaded by: n: 03 Dec 2018 Access details: subscription number Publisher: CRC Press Informa Ltd Registered in England and Wales Registered Number: Registered office: 5 Howick Place, London SW1P 1WG, UK Handbook of Brewing Graham G. Stewart, Inge Russell, Anne Anstruther Hops Publication details Trevor R. Roberts, Russell Falconer Published online on: 19 ct 2017 How to cite :- Trevor R. Roberts, Russell Falconer. 19 ct 2017, Hops from: Handbook of Brewing CRC Press Accessed on: 03 Dec PLEASE SCRLL DWN FR DCUMENT Full terms and conditions of use: This Document PDF may be used for research, teaching and private study purposes. Any substantial or systematic reproductions, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The publisher shall not be liable for an loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.
2 Trevor R. Roberts and Russell Falconer CNTENTS Chapter 7 Hops 7.1 Hop Growing History Hop Growing Today The Hop Plant Hop Classification The Plant Hop Cultivation Conditions for Growing Growing Infrastructure Growing Irrigation Plant Protection Harvesting Drying and Packing Certification Hop Varieties Hop Variety Selection Hop Breeding bjectives New Variety Development Gene Mapping and Molecular Markers Hop Chemistry Whole Hops Hop Resins Soft Resins Hard Resins Hop ils General Terpenes xygenated Compounds Sulfur Compounds
3 146 Handbook of Brewing Glycosides Polyphenols General Proanthocyanidins Flavonoids Pectins Hop Waxes and Fats Hop Storage General Hop Variety Kilning and Moisture Hop ils Sulfur Compounds Hop Resin Acids Bittering Value Isomerization The Isomerization Reaction Significance of the Cohumulone Ratio Significance of the Cis:Trans Ratio Late Hopping Bitter Flavor and Foam Role of Iso-α-acids Reduced Iso-α-acids Hop Products Development of Hop Products Benefits of Hop Products Volume Reduction Increased Stability Reduction in Chemical and Heavy Metal Residues Homogeneity Reduced Extract Losses Use of Automated Dosing Systems Improved Efficiency Quality Benefits Classification of Hop Products Nonisomerized Hop Products Pellets Kettle Extracts Isomerized Hop Products Stabilized and Isomerized Pellets Isomerized Kettle Extracts Reduced Isomerized Hop Products Rho-iso-α-acid (Rho) Tetrahydroiso-α-acid ( Tetra ) Hexahydro-iso-α-acid ( Hexa ) Comparison of Reduced Products Relative Foam Stabilization Importance of Cohumulone Ratio ther Possible Hop-Derived Foam Stabilizers
4 Hops ther Products Type 100 Pellets Beta Extract il-enriched Extracts Pure and Fractionated Hop ils and Hop Essences Alpha Extract Antifoam Storage Stability ther Uses New Products Hop Usage Choice of Hop Product Hop Utilization Calculation of Hop Additions Dry Hopping General Factors Affecting Dry Hopping Dry Hopping Methods Cost of Bittering Hop Analysis General Visual, Physical, and lfactory Examination of Hops Hand Evaluation Hop Leaf and Stem Hop Resins Analysis General Hard, Soft, and Total Resins Lead Conductometric Methods Spectrophotometric Methods HPLC Methods Capillary Electrophoresis Near Infra-Red (NIR) Reflectance Mode Analysis Hop ils General Steam Distillation Method (For Total Essential ils) Gas Chromatography (GC Analysis, For Hop il Components) Adsorbent Fiber Technique (Solid Phase Micro-Extraction, or SPME) Polyphenols Flavonoids Analysis of Worts and Beers BU Analysis Hop Resin Acids by HPLC Flavonoids Hop ils References Bibliography...223
5 148 Handbook of Brewing 7.1 HP GRWING History Although beer is first thought to have been brewed in Babylon as long ago as 7000 BC, hops were almost certainly not used in brewing until very much later. 1,2 There is some evidence to suggest that hops were grown in central Europe before 1000 AD, but it is unclear whether these were used in beer or merely for inclusion in early medicines and herbal remedies. Hops were probably first grown for brewing in Germany and the Czech Republic sometime between 1000 and 1200 AD. Their horticulture and use then gradually spread throughout Europe, eventually being imported into England during the fourteenth century. The famous Reinheitsgebot, or Purity Law, in which it was decreed that beer might be brewed only using (malted) barley, hops, and water, was issued by the Duchy of Bavaria in From Europe, hop-growing spread fairly rapidly with the early European settlers to the United States and South Africa (seventeenth century), Australia and New Zealand (early nineteenth century), and also during the 1800s and 1900s into several other countries, many of which no longer grow significant quantities of this perennial crop (Canada, for example). Apart from their obvious flavor benefits, the attraction of hops to early brewers appears to have been related to their preservative qualities, which were particularly relevant before the introduction of refrigeration Hop Growing Today Today, hops are grown successfully in a number of countries in both the Northern and Southern Hemispheres. Location very much depends on achieving the combination of the right growing and climatic conditions but is also influenced by the need for local production in countries situated a long way from the traditional growing areas. As shown in Table 7.1, world hop production is dominated by Germany and the United States, which between them account for more than 70% of the total output. 3 Table 7.1 Principal Hop Producing Countries Average Growing Area and Baled Hops Production % of Total Country Hectares Hops (mt) Area Hops Germany 17,378 34, USA 13,692 30, China 3,315 7, Czech Republic 4,447 5, Poland 1,392 2, Slovenia 1,229 1, England 1,026 1, Australia 441 1, Spain South Africa New Zealand France Ukraine The rest Total 47,299 90,
6 Hops The Hop Plant Hop Classification Thought to have originated in Asia (probably China), the hop plant is indigenous to the Northern Hemisphere but is also now grown successfully in parts of the Southern Hemisphere. The classification of the hop plant (Humulus lupulus L.), used in brewing, is shown in Figure The Plant Humulus lupulus is a perennial, climbing plant with three- or five-lobed leaves. It is described as dioecious, meaning that it has separate male and female plants, but it is only the female plants that form the hop cones within which the all-important, yellow lupulin glands develop. Each spring, the hop rootstock produces numerous shoots, which initially grow straight upward. After a short time, they begin to twist in a clockwise direction around any available support (normally a string or wire but, in nature, typically a bush or small tree) while, at the same time, the stems themselves twist to form a gently spiraling helix. The stems, or bines, are hexagonal in cross section, with six rows of hairs growing on the six-ridged angles of the bines; it is these hairs that help the hop plant cling to the supporting string. As the hop grows, lateral shoots appear from buds in the axils of the leaves on the main stem and, eventually, flowers develop on these laterals. It is from these clusters of flowers that the hop cones subsequently form. The hop cones consist of a central strig on which develop petal-like structures called bracts and bracteoles (Figure 7.2). Whereas the bracts appear to have only a protective role, it is at the base of the bracteoles that the sticky, yellow lupulin glands develop. It is within these lupulin glands that the most important constituents for brewing are found, namely the resins and the essential oils. If allowed to grow, any male plants present will flower and pollinate the female flowers, resulting in the eventual development of seeds in the folds at the base of the bracteoles. The male flowers wither and drop off the bine soon after their appearance and are therefore of no use as such in brewing. In most hop-growing areas, any male hops are physically removed from hop fields or hedgerows in order to avoid the production of seeds that are considered by some brewers to be undesirable, owing to the possibility of oxidation of seed fats producing off-flavors in beer. 4 However, in England and in some other regions, it is still common practice for male hops to be planted within hop gardens to fertilize the female flowers, which is known to increase the yield of hops per hectare. rder: Family: Genus: Species: Figure 7.1 The hop plant classification. Humulus Urticales Cannabaceae Cannabis H. japonicus C. sativa H. lupulus H. yunnanensis
7 150 Handbook of Brewing Humulus japonicus is an annual, dioecious plant with lobed leaves originally found in Japan and China. It forms cones with very few lupulin glands and is not used in brewing. However, it is now widely cultivated by horticulturists and, as a strong climber, is often used in gardens as a decorative, leafy screen. Humulus yunnanensis is a little studied species thought to be native to the Yunnan Province of southern China. As far as is known, it is not routinely cultivated and has no use in brewing Hop Cultivation Conditions for Growing Bract Lupulin on bracteole As previously noted, hops are grown fairly widely throughout the world. However, in order to grow hops commercially, the following conditions are extremely important: Deep and fertile soil Warm summer sun and cold winters Adequate supplies of water Absence of pests and diseases or else a means of controlling them Abundance of inexpensive labor or well-developed agricultural technology Adequately changing day length The last point is of particular importance and is the reason why hops are normally only grown between latitudes 35 and 55, where the changing day length is most suitable. The hop plant is referred to as a short-day plant and during its growing cycle responds to different periods of daylight. Below about 13 h of daylight, the plant will not grow and is dormant, but once daylight exceeds around 13 h, it begins to climb and eventually will flower and develop cones. However, flowering will not occur until the plant has developed a variety-specific, minimum number of nodes (distance between the axils on the main bine). By the time that this is achieved, day length in the temperate latitudes has increased beyond 15 to 16 h, under which conditions the plant will continue its vegetative growth but will not flower. Not until the day length reduces to below the critical point are conditions suitable for flowering, and the hop cones then develop and ripen. In countries that are just outside of the normally acceptable latitudes for hop growing (for example, South Africa), hops are however grown successfully with the aid of artificial lights to extend daylight at key points of the year. In more recent years, varieties have been bred to be more tolerant of local South African conditions, addressing to some extent the problems of day length. Strig Figure 7.2 Cross section of a typical hop cone showing the general structure.
8 Hops Growing Infrastructure Hop bines are normally supported on strings (typically coir or polypropylene) or on fine wires, which are fixed between ground-anchored pegs and strong overhead wirework supported on tall wooden or concrete poles. In most countries, the height of the wirework varies from 4 to 7 m and is laid out in rows between 1.6 and 3.0 m apart, with hop plants spaced at 1.5 to 2.0 m intervals along each row. The choice of spacing between rows and plants depends on practical issues such as tractor access, type of irrigation, and light access for the growing hops. Several different string pattern systems are in use, with up to four, steeply angled strings per plant. Plant spacing, wirework height, and stringing arrangements can all significantly influence yields. In more recent years, specially bred dwarf varieties, with shorter internodes between laterals, have been developed. These dwarf varieties are grown as a dense hedge on a semipermanent netting to a height of 2 to 3 m. Infrastructure setup costs are significantly lower than traditional tall varieties and, although operating costs are also substantially lower, these savings are largely offset by lower yields Growing Following wintering below ground, in early spring, the rootstock produces numerous shoots, which may be initially removed along with any dead plant material from the previous year. By late spring, stringing is completed and, once shoots grow to around 50 to 80 cm, up to four shoots are manually trained clockwise around individual strings while unwanted shoots are removed. As the bines grow up the strings, the lower foliage (up to 1 m) is often removed, usually by use of chemical defoliants (or in Australia and New Zealand by grazing sheep), in order to discourage the spread of any pests and diseases and later to aid the harvesting process. Soil inputs such as nitrogen, phosphates, potash, magnesium, straw, and dung are often added in order to restore essential nutrients and promote vigorous growth. However, as some of these applications can adversely affect levels of the all-important α-acids (see Section 7.2) as well as promoting the development of disease, the grower must exercise great care when deciding on amounts and times of addition Irrigation In many temperate or maritime climates, irrigation is unnecessary but is increasingly being introduced to improve consistency and maximize yield. In other areas, such as the Yakima Valley in Washington State, or Xinjiang in China, irrigation is essential, owing to insufficient rainfall. Three types of irrigation systems are commonly used: verhead using spray guns that drench the entire plant. Rill systems in which water is run along ditches adjacent to the hop plants. Trickle small bore piped systems, which can either drip feed water at ground level or from overhead mounted pipework. Every one of these systems has its own advantages and disadvantages. Nowadays, trickle irrigation methods are considered to be the most efficient, offering the opportunity to include chemical additions with the irrigation water (a practice known as fertigation) as well as minimizing water wastage an important consideration in areas of water shortage or high water costs Plant Protection Although expensive, the application of agrochemicals in order to suppress pests and diseases is necessary in most Northern Hemisphere growing areas. Chemical applications are made throughout
9 152 Handbook of Brewing Table 7.2 Common Pests and Diseases of Hops Pests Damson hop aphid (Phorodon humuli) Red spider mite (Tetranychus urticae) Diseases Powdery mildew (Sphaerotheca humuli) Downy mildew (Pseudoperonospora humuli) Verticillium wilt (Verticillium albo-atrum) Virus diseases (esp. Hop mosaic and hop latent viruses) the growing season either as a preemptive measure or in response to the first signs of problems. However, in order to meet statutory minimum residue levels (MRLs) on hop cones at harvest, agrochemical applications are ceased at predetermined periods before harvest. Agrochemicals are either surface active, requiring good foliar contact and applied by using sprays, or systemic and therefore taken up into the root system via soil drenches or trickle irrigation. When using agrochemicals, the hop grower has also to consider health and safety issues, effects of spray drift on neighboring areas, and possible contamination of groundwater due to runoff. The most common pests and diseases are shown in Table 7.2. thers can occur but to a much lesser extent and generally in response to specifically favorable conditions. In most Southern Hemisphere growing regions and in a few isolated, newer hop-growing areas in the Northern Hemisphere, certain pests and diseases are rare or even absent, resulting in very low chemical applications. This added benefit presents the fortunate growers with unique selling and pricing opportunities, particularly at times when green and organic products are much in demand Harvesting Comments Can cause defoliation and significant cone damage; treated by foliar and systemic chemicals as well as natural predator programmes Silvery discoloration of leaves and cones resulting in total loss of crop in severe cases; can be controlled by chemical spraying; thrives in hot weather Comments Fungal disease causing white pustules on leaves and severe cone damage; spreads rapidly, but can be controlled by early spraying before disease establishes a hold Germinating fungal spores on leaves cause black discoloration and severely reduced growth resulting in poor yields; can be treated by both spray and systemic chemicals Fungal disease which can quickly devastate a hop field; no known chemical control available; good hygiene essential together with removal and destruction of infected plants Can cause significant reduction in yields and α-acid levels; no known chemical control; heavy contamination requires replacement of infected plants with virus-free rootstock In late summer, when the hop cones have ripened, the hops are harvested. The bines are severed approximately 1 m from the ground and then cut or pulled from the overhead wires, either manually or by machine. The bines are then transported to local, static picking machines. There they are hung upside down on a moving rail to better expose the cones hanging from the lateral growths and passed through picking machines where a series of revolving, metal wire picking fingers strip the hop cones and leaves from the bines. The cones are then separated from leaves, bine fragments, and so forth on a series of vibrating belts, sieves, and air classifying devices, with the cleaned hop cones being transported to the drier while the unwanted material is cut into small pieces for disposal or spreading back onto the land if there is no fear of spreading disease. In China, where the hop plant density per acre is higher and the wirework much lower (approximately 2 m), hop cones were traditionally picked by hand. However, owing to labor shortages, hop bines are now cut in the field and transported to picking machines for bine stripping as with traditional, taller varieties. This change has resulted in a reduction of approximately 15% in yield compared to hand picking.
10 Hops 153 The introduction of dwarf varieties resulted in the development of mobile field harvesters capable of mechanically stripping cones from bines in the hop garden. In more recent years (particularly in the United States), mobile field harvesters have been developed to pick even the tallest varieties. Although the stripped cones still need to be cleaned in the picking shed, significant savings in harvest labor costs can be achieved. wing to their lower center of gravity, the dwarf mechanical pickers can tolerate some degree of uneven ground; however, those used to pick tall varieties only work well in large, flat hop fields, common in the United States but not typically found in many hop-growing countries in Europe Drying and Packing The freshly picked hop cones have a moisture content of approximately 80% and are dried in oast house kilns to a moisture level of 7% to 12%. Drying is typically achieved by forcing heated air for typically 6 to 8 h through a bed of cones of up to approximately 1 m depth. Normally, the air is heated using either oil or gas burners and can be passed either directly through the cone bed or indirectly via a heat exchanger. Drying temperatures are normally in the range of 60 C to 75 C, but this will vary depending on bed depth and cone size as well as on air speed. Physical damage and degradation of resins and oils can occur if drying is not carefully controlled, and better results are generally obtained if the air-on temperature is kept relatively low. In former times, sulfur candles were often burned underneath the kiln floors, producing sulfur dioxide that had the effect of reducing browning and maintaining a good appearance to the hop cones. However, for quality and environmental reasons, this practice has now been banned in most countries (see also Sections and ). After completion of drying, subsequent cooling, and discharge from the kiln, residual moisture may be unevenly spread within and between the hop cones. It is therefore necessary to condition the hops by holding them in heaps at ambient temperatures for 24 h or, in specially designed conditioning boxes, by employing gentle air flow at 20 C to 24 C for a few hours, during which time moisture is redistributed more evenly throughout the entire mass of material. Following completion of this conditioning period, the hops can then be packed either for immediate transport to brewers or, more commonly, for cold storage prior to sale and/or processing into hop products. Normally, hops are compressed into rectangular bales that are then wrapped in burlap or polypropylene cloths. Bale size and weight typically depend on the country and eventual destination. In Germany, hops are initially gently compressed into farmers bales weighing 50 kg to 60 kg prior to processing, and they are often packed into relatively small, high compression, cylindrical ballots weighing up to about 150 kg, whereas in the United States, a rectangular, approximately 200-lb (90 kg) bale is the standard package either for sale or for transport to the processing plant Certification Most hop-producing countries have their own formal certification procedures for hops (and usually for hop products) designed to ensure a common, minimum standard of product. These regulations should specify: 1. Identification and traceability of harvested hops 2. Rules regarding the blending of hops 3. Minimum requirements for packaging, stipulating rules on sealing and labeling, which identify crop year, variety, name, and place of production 4. Acceptable levels of moisture and extraneous matter ( leaf and stem ) 5. Provision of signed certificates from an authorized source confirming the relevant details
11 154 Handbook of Brewing Hop Varieties The range of hop varieties available to the brewer is both varied and extensive. In more recent years, the choice has been constantly changing as older varieties are phased out while newer varieties with increased disease-resistance, higher yield, and more diverse quality characteristics are introduced. Hop varieties have in the past been grouped into three broad categories aroma, dualpurpose, and high-alpha. However, more recently, this categorization has become blurred as very good aroma varieties with higher levels of α-acids have been introduced while new highalpha varieties with very acceptable and distinctive aroma characteristics have also found favor with brewers, particularly for late kettle and dry hopping. Nowadays, these hops are often referred to as flavor hops, removing the traditional reference to the alpha levels. In Table 7.3, a few examples of varieties from around the world have been loosely grouped together in the three categories mentioned here. Those also referred to as flavor hops are identified by superscripts Hop Variety Selection Traditionally, brewers selected their hop varieties by manual evaluation (see Section 7.5.2), possibly supported by limited analyses such as α-acid and total oil contents. Nowadays, the introduction of gas chromatography (GC) and high performance liquid chromatography (HPLC) analyses provides additional information, which can be included in selection criteria (see Section 7.2). A high proportion of the specific α-acid cohumulone is considered by some to give a more harsh bitterness compared to the other α-acid homologues (see also Section ), and consequently brewers often specify maximum levels. Similarly, ratios of other components, particularly within the oil fraction, can be used to determine both the acceptability of a variety as well as comparing its quality with those of other varieties. However, in reality, the final evaluation of any new variety, either as a replacement for an existing hop or for the production of a new beer, relies on trial brewing and tasting of the resultant beer. Table 7.3 A Few Examples of Aroma, Dual-Purpose, and Alpha Varieties Hop Type Aroma Dual-Purpose Alpha (and Super-Alpha) Country USA Germany England New Zealand a b Excellent Aroma; α-acids Typically 3 7% Mt Hood Cascade Willamette US Fuggle Tettnanger Hersbrucker Huell Melon b Tradition Golding Fuggle Progress Pacifica Motueka Riwaka Dwarf variety Flavor hop often used for late and dry hopping Good Aroma; α-acids Typically 6 11% Cluster Centennial b Amarillo b Perle Brewers Gold Mandarina Bavaria b Hallertau Blanc b Challenger Northdown First Gold a Hallertau Aroma Acceptable Aroma; α-acids Typically 10 20% Chinook b Citra b Simcoe b Apollo Zeus Herkules Magnum Po laris b Target Admiral Pilgrim Green Bullet Pacific Gem Nelson Sauvin b
12 Hops Hop Breeding bjectives The objectives of the plant breeder in developing new varieties of hops can be summarized as follows: Resistance to pests and diseases Increased harvested weight yield of hop cones Higher α-acids leading to increased alpha yields Better storage stability (of α-acids and aroma components) Improved aroma character through higher concentrations of certain hop oil components such as linalool (see Section ) Unique or distinctive aromas and flavors Lower production costs Improved sustainability Increased yields of constituents previously considered unimportant such as β-acids and polyphenols (e.g., xanthohumol) (see Sections and 7.2.5) New Variety Development The introduction of a new variety into commercial production is a lengthy and expensive process, which, in a country where diseases are prominent, can take up to 10 years from selection of parents to plant registration and release. The key stages of traditional hop breeding are shown in Table 7.4. In countries lacking disease problems, it is possible to shorten this process considerably (although not entirely without some risks), and in some instances, availability can be further accelerated by release ahead of organized brewing trials. Some varieties have been developed using laboratory techniques that alter the normal diploid chromosome content of the plant cells, facilitating formation of tetraploid plants that may then be crossed with diploids to produce triploid offspring that are frequently more vigorous and always much less prone to seed production. As the offspring possess a single male component together with a double set of female chromsomes, the female characteristics are more strongly expressed. ne disadvantage of this technique, however, is that the triploids are infertile and cannot be used for further crossbreeding Gene Mapping and Molecular Markers All of the recently released new varieties have been developed using classic hybridization techniques. For reasons of public acceptability, genetic manipulation in the form of gene insertion methodology is not currently employed in hop breeding programs. However, the use of gene mapping and molecular markers is being progressively introduced. Small fragments of DNA have been identified, which correlate with the expression of certain phenotypes, chemotypes, and genotypes in the hop plant. These DNA fragments represent specific regions on the hop chromosome, which has Table 7.4 Typical Stages in the Development of a New Hop Variety Year 1 Selection of parents; pollinate and collect seeds Year 2 Grow seedlings (often under exposure to mildew spores) Years 3 5 Selection on the basis of vigor, disease resistance, cone production, and chemical analysis Year 6 Propagation (via root splitting or meristem culture) of selected plants Years 7 8 Further selection on the basis of yield, disease resistance, and chemical analysis Year 9 Commercial farm and pilot brewing trials Year 10 Further field trials and commercial brewing trials; registration and release to growers
13 156 Handbook of Brewing now been mapped, and can therefore be used as molecular markers for desired characteristics. The application of these techniques allows for traditional crossbreeding of hops to be carried out more rapidly and efficiently. Not only can seedling gender be reliably determined, genetic diversity in breeding material can also be precisely determined, which can be used to select potential parents for crossbreeding. Molecular markers for resistance and quality can be reliably selected irrespective of environmental and climatic conditions. Recently, resistance has been broken in many varieties, but DNA-based markers enable the systematic combination (gene pyramiding) of multiple resistance genes, which are much harder for disease to overcome and hopefully will provide long-term protection. This gene pyramiding cannot be detected by traditional phenotype selection. Current breeding efforts are now focused on the identification of genes or gene sequences together with molecular markers for the synthesis of those hop compounds responsible for key flavors in beer. These breeding programs are very expensive and, consequently, when new breeds are approved as a variety, the breeders often seek to protect the rights of their intellectual property. New varieties must first be tested and evaluated over many years by government agencies in their country of origin. Long-term protection is generally granted based on: Distinctiveness: The plant must demonstrate it possesses a new unique feature compared to existing varieties. Uniformity: All plants must display uniformity with regard to its distinguishing feature. Consistency: Key varietal characteristics must remain consistent over several years Whole Hops 7.2 HP CHEMISTRY Because of the presence of the oil and resin-rich lupulin glands, the overall composition of fresh, dried hop cones shows them to be unlike that of other plant material, though the leafy nature of the hop petals ensures the presence of such ubiquitous substances as proteins and carbohydrates. In an excellent and thoroughly comprehensive review of hop chemistry in 1967, Stevens 6 quotes a typical hop analysis as being: Resins 15% Proteins 15% Monosaccharides 2% Tannins (polyphenols) 4% Pectins 2% Steam volatile oils 0.5% Ash 8% Moisture 10% Cellulose, and so on 43%. No doubt this analysis was for a hop of quite modest resin content by today s standards because some of the modern hop varieties contain nearly twice the amount quoted here. (For a more recent review of hop chemistry, see Verzele. 7 ) f the aforementioned components, the lupulin glands contain virtually all of the hop resins (in the form of α-acids, β-acids, desoxy-α-acids, and uncharacterized soft resins; see Section ), the essential oil, and most of the hop cone fats and waxes (comprising perhaps 5% of the total and included here under Cellulose, and so on ). Seeded hops will contain a relatively large proportion of fats and nonvolatile fixed oils. High-alpha hops (for example, Apollo or Herkules) will tend to have proportionally more of the resin and oil components, while a low-alpha hop such as Hersbrucker will have relatively more of the mainly petal-derived components such as polyphenols and nitrate. Hence, a brewer who for
14 Hops 157 economic reasons switches from brewing with, say, a 7% α-acids Cluster hop to an 18% α-acids Apollo, will substantially reduce the amount of polyphenols added to the wort, though the balance and nature of the resin and oil components may not be much changed. Such changes can have consequences for example, for beer haze stability that are not easily predictable Hop Resins Soft Resins The resinous fraction of fresh hops contains mostly the α-acids and β-acids, each of which consists of analogous series of closely related homologues. Together with the desoxy-α-acids, they constitute the major portion of the so-called soft resin fraction of hops (see also Section ). The close similarities among these compounds arise as a result of the proposed common pathway to their formation 8 (Figure 7.3). H H H Biosynthesis of alpha-acids and beta-acids H H Beta-acid R Acylphloroglucinol H + 2 Isopentenylpyrophosphate R H? H R 4-Deoxy-alpha-acid? H Alpha-acid 4-Deoxyalpha-acids are precursors to both the alpha- and beta- acids, but it is not clear whether the alpha-acids are formed directly or via the beta-acids. Figure 7.3 Biosynthetic pathway for hop resin acids. H P H P R
15 158 Handbook of Brewing In their native state, the hop resin acids exist within the lupulin glands as fully protonated, nonionized species. In this form, they are virtually insoluble in water, but they dissolve readily into methanol or less polar solvents such as dichloromethane (methylene chloride), hexane, or diethyl ether. At room temperature, a preparation of α-acids will be a pasty, yellow solid, while β-acids, also yellow unless highly purified, will be substantially harder and may become semicrystalline. When slowly heated, both become progressively more mobile, though the temperature has to be raised above about 60 C before the β-acids will flow easily. When suspended in warm demineralized water and titrated with strong alkali (sodium hydroxide [NaH] or potassium hydroxide [KH]), both will begin to dissolve because their alkali metal salts are highly soluble. However, the pk a values for the two types of resin acid are rather different the α-acids having pk a values close to ph 4.8, whereas the β-acids do not reach 50% dissociation until the ph value is around The consequence for practical brewing is that the solubility of α-acids in beer (ph usually in the range 3.8 to 4.5) is significant, and this allows a meaningful proportion of any un-isomerized α-acid in the wort to pass into the beer, whereas the amount of β-acid that can possibly remain is very small. Beers brewed with whole hops, normal pellets, or extracts, and especially those that are late- or dry-hopped, normally contain a few ppm of α-acids, but the amount of β-acids never exceeds 1 ppm and is often undetectable. The same major homologues are found in the resin from all hop varieties, though the proportions differ. There are three major forms of α-acid and three analogous forms of β-acids: for the α-acids, these are cohumulone, humulone, and adhumulone and for the β-acids, colupulone, lupulone, and adlupulone. The relative proportions of the compounds are remarkably consistent among different growths of the same variety, and their determination can be used as a tool to help distinguish one variety from another. Analysis of different hop varieties shows that the proportion of the α-acids that is found to be adhumulone is almost always less (and somewhat more variable) than that of the other two major homologues, of which humulone is predominant in most varieties. Table 7.5 illustrates the wide range of values found 10 : Similarly, adlupulone is almost invariably the lesser component among the three analogues of β-acids; but in this case, the relative content of colupulone as a fraction of the β-acids is always substantially higher than that of cohumulone as a fraction of the α-acids. Howard and Tatchell 11 derived the following relationship, held to be true for all varieties: % Colupulone = [% Cohumulone] Table 7.5 Relative Content of Major α-acids in Different Hop Varieties (Ranking by % Cohumulone) Hop Variety % Cohumulone % Humulone % Adhumulone Brewers Gold Galena Cluster Cascade Perle Nugget Willamette Goldings Fuggles Tettnanger Mittelfrüh Source: Data from Nickerson and Williams. Note: 1982 crop hops, grown in regon, USA. 10
16 Hops 159 Hops also contain small amounts of so-called minor α-acids, typically amounting to about 2% to 3% of the total α-acid content. Most abundant of these α-acids are posthumulone, prehumulone, and adprehumulone, 12 the former having the shortest side chain structure of the homologous series and therefore being the most polar and in all probability the best utilized during the brewing process. Evidence exists for the presence of at least four more forms, of presently unknown structure. 13 The ratio of total α-acids to total β-acids is also a varietal characteristic that is quite consistent for mature hops. Hop varieties can therefore be classified by various parameters related to their resin content, of which the most important are the cohumulone ratio and the α:β ratio: Cohumulone ratio (or CoH ratio) = % cohumulone/% total α-acids α:β ratio = % total α-acids/% total β-acids Some typical values are shown in Table 7.6. Not surprisingly, these differences have implications for beer quality produced from different hops, as will be described later. Although it is the α-acids that are the prime source of bittering via their isomerization to iso-α-acids, usually during wort boiling (this Section 7.1.1), the contribution of the β-acids is not necessarily insignificant. These latter β-acids do not undergo any kind of isomerization reaction and are mostly eliminated during the brewing process by precipitation, but a small proportion may be converted into hulupones, which are a much more soluble and substantially bitter derivative. 14 Generally of more significance, hulupones may already be present in the hops or kettle hop products used, and the amount of these compounds that ends up in the beer will therefore vary according to circumstance (see also this Section ). A longestablished rule of thumb that has been used by some brewers to assess the bittering potential of hops states: Bittering potential = α-acids + β-acids/9. Clearly this is an oversimplification, but it is worth bearing in mind that (purely for experimental purposes) a drinkable bitter beer was once prepared by a brewer with the addition in the way of kettle hops of nothing but a crude β-acid extract. It is also known by one of the authors of this chapter that a brewer once managed (albeit inadvertently) to sell beer that was bittered (on a bittering unit [BU] basis) with hop pellets of such venerable age that they contained no measurable α-acids (by HPLC) whatsoever. Table 7.6 Typical Key Values for Hop Resin Acids Content of Some Major Hop Varieties (Listing by α-acids Content) Hop Variety and Primary Growing Region Total % α-acids Total % β-acids CoH Ratio α:β Ratio Hallertau Hersbrucker (Germany) Saaz (Czech Republic) Celeia (Slovenia) Willamette (USA) Perle (Germany) Pride of Ringwood (Australia) Galena (USA) Hallertauer Magnum (Germany) Pacific Gem (NZ) Nugget (USA) Zeus (USA) Summit (USA)
17 160 Handbook of Brewing Although the properties of the different homologues within each hop resin acid series are similar, there are, nonetheless, some noticeable differences. Most importantly, in each case, the co-series homologue is found to be a little more soluble and chemically slightly more reactive. The practical consequence of this subtle distinction is that the utilization of cohumulone is usually significantly and often substantially higher than that of the humulone and adhumulone, which are in all respects extremely similar in their properties (see also Section ). Desoxy-α-acids represent a significant proportion of the soft resin fraction, typically at a level of about 5% of the total. However, they do not seem to play any role in the brewing process, being substantially eliminated in the trub and any remainder almost completely by absorption onto the yeast during fermentation Hard Resins In a fresh hop, the amount of the hard resin fraction (Section ) is quite small and is mostly made up of the yellow/orange-colored prenylflavonoid, xanthohumol (see this Section ). However, during storage, the hard resin fraction increases as the α-acids and β-acids from which they are mostly derived decline. With the exception of xanthohumol, the precise chemical identity of the hard resins is not well understood (and may include dimers and trimers), but generally they are more polar and therefore more water-soluble compounds and will tend to pass more readily into the beer than their precursors Hop ils General Dependent on variety, hops contain typically from about 0.4 to 3.5 ml/100 g of steam volatile, essential oils (with some new varieties showing levels of well over 4.0 ml/100 g). As a general rule, high-alpha, bittering hops contain more oil on a dry weight basis than do lower alpha aroma hops, but this is really no more than a reflection of the greater amount of lupulin present in the former. f much more significance is the spectrum of components, which is primarily genetically determined and varies substantially among varieties. 15 Gas chromatographic analysis of hydrodistilled oils from freshly harvested and dried hops reveals a considerable number of compounds, but the chromatographic patterns are distinctive for each variety and generally enable reliable identifications to be made. 16, Terpenes The major components are hydrocarbon terpenes, of which the most abundant are the monoterpene myrcene and the sesquiterpenes α-humulene and β-caryophyllene (Figure 7.4). Together, these three components alone may account for up to about 80% of the oil. ther terpenes that may be of importance on a quantitative basis are the α- and β-selinenes and β-farnesene, some varieties being almost completely devoid of one or more of these compounds, others having substantial content. Table 7.7 gives an idea of the wide diversity of composition of the essential oil from different hop varieties. Although the terpenes may together comprise well over 90% of the total oil of a fresh hop, their importance as such to the flavor of beer is generally inconsequential because they are all virtually water-insoluble and have relatively high flavor thresholds. For the most part, they are driven off during the wort boil or later flushed out during fermentation. (ccasional exceptions may be the use of hops for dry-hopping and in the case of the brewing of green hop ales where freshly picked, un-dried hops are pitched into the kettle, often late in the boil. In these cases, flavor-active
18 Hops 161 Major terpenes found in hop essential oils Myrcene Alpha-humulene Beta-caryophyllene Beta-farnesene amounts of myrcene and perhaps also other terpenes can certainly be transferred to the beer.) Notwithstanding the general unimportance of the terpenes as direct contributors to beer flavor, a relatively high humulene to caryophyllene ratio (the H/C ratio) is nevertheless considered by many to be a necessary hallmark of a good aroma hop, a value in excess of 3.0:1 being said to be necessary for a hop having so-called spicy/herbal/floral noble character. 18,19 The presence of β-farnesene is also generally held to correlate with good beer hop aroma xygenated Compounds H Alpha-selinene Dry-hopping apart, hoppy flavor in beer is usually associated with the late addition of hops to the kettle and is generally considered to be due to the presence in the hop oil albeit in relatively small amounts of oxygenated derivatives of the terpenes These components fall into several classes, including alcohols, ketones, and esters, and much research has been devoted to determining which of the various compounds has significance to beer flavor. Very little hop oil survives into finished beer; therefore, to be of significance to beer flavor, a compound must have a flavor threshold measured in ppb rather than ppm, though there is evidence to suggest that synergistic effects play an important role. 25 Undoubtedly, one component that often plays a major role is linalool, which is found in two stereoisomeric forms. These enantiomers have different organoleptic properties, though owing to its much lower flavor threshold, only R-linalool (D (-)-linalool) is of significance H H Beta-selinene The monoterpene myrcene is invariably the most abundant substance in the oil from fresh hops, but its proportion reduces substantially during storage, mostly due to volatilization and polymerization. Amongst the sequiterpenes, humulene in particular is a precursor to some oxygenated compounds that may positively influence beer flavor. Farnesene and the selinenes are mainly significant as markers for identification of different hop varieties. Figure 7.4 Structures of some terpenes found in hop essential oil. H
19 162 Handbook of Brewing Table 7.7 Typical % Content in Hop Essential il Fraction of Some Terpenoid Compounds as Related to Hop Variety (Ranking by Humulene/ Caryophyllene Ratio) Hop Variety Myrcene α-humulene β-caryophyllene β-farnesene α + β Selinenes H/C Ratio Variety Type Hersbrucker Aroma Saaz Aroma Liberty Aroma Challenger Dual purpose Cluster Dual purpose Willamette Aroma Fuggles Aroma Cascade Aroma Perle Dual purpose Nugget High alpha, good aroma Chinook High alpha, good aroma Galena High alpha Target High alpha Columbus High alpha Pride of ringwood Medium alpha
20 Hops 163 to beer flavor. 26 The amount of linalool in beer seems generally to correlate well with its hoppy character. 27 Some other hop oil (or oil-derived) components that have been suggested to be of possible importance are geraniol, humulenol II, α-terpineol, undecan-2-one, methyl-4-deca-4-enoate, humulene diepoxides, and citronellol Sulfur Compounds Sulfur-containing compounds have also been found in hop oil, and because many of these have very low flavor thresholds, they may on occasion influence the beer flavor, generally in a negative manner Such compounds include dimethyl disulfide (DMDS), dimethyl trisulfide (DMTS), and methanethiol. The formation of many sulfur compounds has been shown to be linked to the spray application of elemental sulfur as an anti-fungal agent during the growing season and leads to the appearance of a range of polysulfides, particularly via purely chemical reaction. 28 The former practice of burning sulfur on the kiln to discourage browning and ensure an attractive, pale green coloration to the dried hops also had implications for the formation of flavor-active sulfur compounds. 30, Glycosides In common with many other plants, hops contain a variety of glycosides, and recent evidence 27,32 suggests that hop flavor may partly derive from hydrolysis of these covalently linked combinations of a wide range of organic substances with a sugar moiety, releasing flavor-active chemicals such as linalool that are better known as components of the hop oil. Relatively little is known about the occurrence of these compounds in hops, but their existence may account for some of the hop character and could explain the observation that adding aroma hops to wort before boiling (so-called first wort hopping) can lead to unexpected, pleasant hoppy notes in the resulting beer Polyphenols General All hops contain a complex and, to some extent, variety and even geographically specific 36 mixture of polyphenolic substances, a proportion of which are found to be in the form of glycosides. 34 This polyphenolic content is typically in quantity sufficient to be of significance to physical beer stability 37,38 and sometimes also to a modest extent the beer flavor as well, by contributing a degree of astringency, bitterness, and body to the beer Indeed, despite the very small quantity of hops used in the brewing process as compared to that of the major ingredient, malt, hops may be responsible for up to about 50% of the polyphenolic content of some heavily hopped beers. The hop polyphenols are found mostly in the hop cone petals and strig, and with the exception of the prenylflavonoids, not in the lupulin. Thus, the brewery that uses low-alpha, aroma hops added to the kettle will tend to add more polyphenolic material to the wort than the brewer who uses the newer high-alpha varieties. bviously, it follows that purchase of aroma hops in the form of concentrated (Type 45) hop pellets (see Section ) will also reduce the added amount of such material. Total elimination may readily be achieved by switching to the use of carbon dioxide (C 2 ) extracts or postfermentation hop bittering Proanthocyanidins A portion of the hop polyphenolic material comprises such water soluble substances as catechin and epicatechin (Figure 7.5), flavan-3-ols that are the monomeric building blocks of