BrewingTechniques

Factors Affecting Hop Production, Quality, and Brewer Preference

by Alfred Haunold and Gail B. Nickerson

Republished from BrewingTechniques' May/June 1993.

Declining yields of traditional noble aroma hops and the demands of modern production methods are leading hop producers to seek new varieties that retain the qualities of these popular hops while increasing their yield. Meanwhile, high-alpha bittering hops are increasingly being used solely for their bittering contributions. This review of the qualities of commercially available hops sheds light on the hop production scene and puts modern brewers' hop use in a new perspective.

Remarkable similarities and sometimes conspicuous differences distinguish hop varieties developed in major hop-producing countries during the past 50 years. Modern hops have been developed to satisfy both the demands of producers for improved yields and disease resistance and the demands of brewers for improved soft-resin content, aroma, and storage stability.

The trade generally recognizes two major groups: bitter or "kettle hops," and aroma hops. At least 50 major varieties of hops are grown around the world. All contain soft resins (alpha and beta acids) that contribute bitterness and components that contribute aroma, but the actual amounts and relative proportions of these compounds differ significantly from variety to variety.

Washington, Oregon, and Idaho, the three states in the United States where hops are grown commercially, produced marketable quantities of hops from 15 varieties on 42,266 acres in 1992 (Table I). Many, particularly the newer varieties, produced outstanding yields, while others lagged far behind in production. Some had high bittering potential with high levels of alpha acids, and others were relatively low in bittering value. Among the 15 varieties listed in Table I, 6 are valued primarily for their aroma potential, 5 for their alpha-acids potential, 3 for a combination of alpha-acids and aroma potential, and 1 (Cluster) for a combination of positive traits suitable for a variety of general brewing applications.

CHOOSING AMONG VARIETIES

The question of why brewers prefer certain hop varieties has been discussed for many years. Most brewers agree that a certain level of hop bitterness is essential to their beer. Some insist that it makes little difference where this bitterness comes from, while others distinguish between the "harsh" and "mild" bitterness associated with specific hop varieties and demand pleasant aroma, which they hope ends up - at least partially - in the finished product. The type of hop bitterness ("harsh" or "mild") may be associated with the relative proportions of alpha-acid analogs such as humulone and cohumulone (1). However, as major U.S. brewers reduced their hopping rates, taste perceptions associated with bittering quality became less important.

Many traditional brewers continue to prefer traditional, mildly bitter European noble aroma hops, such as Saazer, Tettnanger, Hallertauer Mittelfrueh, and perhaps Goldings and Fuggle, for their pleasant aroma characteristics and despite their relatively low alpha-acids content. Unfortunately, nearly all old aroma hop varieties produce low yields, which makes them unattractive for hop growers unless substantial price incentives are offered. In addition, many are susceptible to diseases such as downy mildew (incited by Pseudoperonospora humuli), powdery mildew (Sphaerotheca humuli Burr.), and Verticillium wilt (Verticillium albo-atrum and V. dahliae), which makes them difficult and expensive to grow. Thus, the acreage devoted to growing most traditional noble aroma hops and the supply of these hops to the market has either declined in recent years or remained steady while demand has increased. Hop breeders have attempted to improve the yield potential of these hops by either selecting higher yielding clones by mass selection (2) or by breeding new varieties similar in quality to the older, traditional varieties but with improved yield potential and disease resistance (3-6).

FACTORS AFFECTING HOP PRODUCTION

Why are the yields of many old aroma hops so low? All hop varieties originally started as seedlings from a cross or from a mutation following a cross. Over the years, yields of old varieties may have declined because of the introduction of off-types by careless planting or because of a lack of adequate resistance to diseases such as downy mildew, Verticillium wilt, or viruses. Selection and propagation of vigorous plants (mass selection) within an existing variety may lead to a more uniform new "variety" with higher yield potential; technically, however, such plants are only clones of the original variety obtained by vegetative propagation.

Hybrid vigor, a phenomenon often observed in the seedling progenies of many crops, is probably at least partly responsible for the higher yield potential of the new hop varieties listed in Table I. After many years and repeated cycles of vegetative propagation, however, hybrid vigor gradually declines. Most traditional European noble aroma hops were selected over 100 years ago, and it is reasonable and expected that their vigor and yield have declined over the years.

These old varieties were also developed when hops were hand-picked in the field without cutting the vines. Cutting the vines at harvest weakens the plant for the following season because it disrupts the translocation of food reserves to the perennial rootstock. Older hop varieties selected under a no-cutting regime are probably more adversely affected by modern machine harvesting techniques than are new varieties that were selected under those conditions.

Geographic area and season: Geographic area, the season, and specific microclimates affect the performance of a hop variety from the standpoints of both agronomics and quality. Relatively few hop varieties are widely adapted to and produce acceptable yields in a range of environments.

The alpha-acids content of the German variety Perle, for example, is 3-4% higher in Oregon than in Bavaria where the hop was developed. The U.S.-bred Nugget regularly has alpha-acids levels of 11-15% by weight in Oregon and Washington but only 10-12% in Germany, despite good cone production in both locations. Cascade, selected in Oregon, generally contains more alpha than beta acids when grown in Oregon but slightly more beta than alpha acids when grown in the Yakima Valley of Washington. Although the total level of alpha acids may fluctuate somewhat between seasons and locations, the alpha-acids composition of most varieties, as indicated by the relative proportion of major alpha-acids components such as humulone and cohumulone, changes relatively little, regardless of area of production or season (7).

The aroma and particularly the total quantities of major hop aroma components, such as humulene, caryophyllene, farnesene, and others in the essential oil, are affected by climate, but the relative proportions of these components vary relatively little, regardless of the area of production or the season. An aroma hop will always have its typical characteristics, regardless of where or when it was grown. The virus-free Saazer hop grown in northern Idaho is a typical Saazer, with yield potential and alpha-acids content enhanced by its virus-free status and favorable growing conditions. Although most of the nearly 300 compounds in hop essential oils are lost during brewing, some are found as transformation products in the finished beer in proportions related to the variety and hopping rate used by the brewer (8,9).

Seeded vs. Seedless hops: Most hops around the world are propagated by cuttings, are grown in the absence of male plants, and therefore are seedless. Growers would love to plant 2-4 male plants per acre, which would increase yields by as much as 25% because pollination stimulates the growth of the cone (Figure 1) and adds weight in the form of seeds. The seed coat, or perianth, is maternal tissue, and it also develops resin glands identical to those found on the bracteoles of the cones. Thus, the total soft-resin production of seeded hops on a per-acre basis is somewhat higher than that of seedless hops because of both the larger cone size and additional lupulin glands on the seed coat. The relative alpha-acids content expressed as a percentage of weight, however, is slightly lower in seeded than in seedless hops because of the dilutive effect of larger cone size and higher yields (10).

hop cones (34k)
Figure I. Effect of pollination on hop cone size. Nonpollinated seedless cone (left); semiseeded cone (center); fully seeded cone (right). Arrows point to enlarged bracteoles which carry a seed at the base. All three cones are from the same plant.

Oregon is one of only a few major hop-growing areas around the world whose customers still accept semiseeded hops; yields of seedless Fuggle and Tettnanger, two old European aroma hops grown commercially in Oregon, are too low for commercial production. Increasingly, however, the trade demands reduced seed content for Oregon hops below the traditional 6-8% currently allowed. This demand poses no problem for triploid varieties such as Willamette, Mt. Hood, and Liberty, which are genetically sterile and produce few if any seeds. Nearly sterile triploid males, which stimulate growth to produce larger cones without producing unwanted seeds, have been used in recent years to stimulate yields of diploid hops without affecting cone quality (10). Because of the chromosomal imbalance of triploids at meiosis (11), fertilization is impaired and only an occasional seed develops.

It is technically difficult to demonstrate why seeded whole hops could pose a problem for the brewer who practices traditional kettle brewing. The seeds are not crushed, and little if any seed fat is leached from intact hop seeds. Barley malt or grain adjuncts contribute far greater quantities of seed fats to the brew. Experienced taste panels, however, seem to be able to detect differences among test brews made with seeded or seedless hops. These differences, however, are not consistently in favor of seedless hops, although many traditional brewers are biased in that direction.

Seeded hops may, however, pose a problem for hop processors. When hops are ground to make powders, pellets, or extracts, seed fats leach out at various processing stages and ultimately can end up in the beer. To overcome this problem, processors developed elaborate deseeding techniques, and most processed hop products today contain little hop seed fats. The added expense of deseeding, passed on to the consumer, may be avoided by growing seedless hops. Recent sales contracts for high-alpha and aroma hops in Oregon emphasize reduced seed content.

THE CHEMISTRY BEHIND BITTERNESS AND AROMA

Alpha-acids composition: Hop varieties differ significantly in relative proportions of humulone, cohumulone, and adhumulone, the three major alpha-acid analogs. Humulone and cohumulone levels vary inversely, and the level of adhumulone varies relatively little and generally accounts for only 8-15% of the alpha acids present. Traditional European aroma hops have low levels of cohumulone (17-24% of their alpha acids), whereas Cluster hops have a cohumulone content ranging from 38% to 42%, the highest among hops used for brewing. As much as 90% of the alpha-acids content of native North American hops occurs as cohumulone (12), and it is not known whether such hops have ever been used for brewing. Nugget, a new American high-alpha hop, has about 25% cohumulone in its alpha-acids fraction, which approaches the levels found in European-type aroma hops (13). Nugget, however, is valued primarily as a bittering hop because of its high alpha-acids content and the good storage stability of its soft resins.

Surprisingly, users of high-alpha hops have been little concerned with alpha-acids composition. There is no clear consensus regarding the relative bittering potential of the major alpha-acid analogs, which differ not only in chemical structure but also in physical characteristics (1,14). Low cohumulone content, however, seems to be favorable for head retention (1).

Composition of hop essential oils: Among the nearly 300 components of hop essential oils, relatively few appear to have any significance for beer flavor (Figure 2 and Table II). Myrcene, the largest component, generally accounts for 40-70% of the total oil content, depending on variety, but is readily oxidized and lost after harvest. Myrcene is generally present in lower quantities in aroma hops than in high-alpha hops. Humulene, generally present in large quantities in older traditional aroma hops (18-25% of the oil), undergoes transformation reactions to form epoxides and other oxidation products that contribute to beer aroma (8,9). The sesquiterpenes cadinene, selinene, and muurolene and their various oxidation products, often referred to as posthumulenes, may contribute to hoppy beer flavor in ways not yet fully understood.

Perhaps because of the relatively low hopping rate of many major commercial American beers, hoppy flavor is less obvious in many major brands than in microbrewery products. When high-alpha hops or hop extracts are used to obtain the basic bitterness, hoppy flavor may be further reduced because fewer hops are required to achieve the final bitterness.

INTERCHANGEABILITY

If a brewer is primarily concerned with bitterness, and particularly if the brewer uses high-alpha hops and the soft-resin extracts made from them, the varietal origin of the bitterness is of no concern. Several major brewers buy hops primarily on the basis of alpha content, regardless of varietal origin or alpha-acids composition, and they achieve a consistent level of quality in their beer. Others carefully select specific hop varieties grown in certain geographic areas with additional considerations given to growing season, farm management, and hop storage.

Alpha-acids content of specific hop varieties does make a difference, but if bittering is the primary objective, the variety with the highest alpha-acids content is probably the most cost-effective one, regardless of alpha-acid composition. A brewer might select bitter hops and use them interchangeably without fear of affecting the quality of the product. Such a brewer might also use a combination of bitterness plus aroma obtained from either a single variety or from varietal blends.

Traditional aroma hop users, however, focus their attention on low-alpha aroma hops. Within the aroma group, some varieties are more easily interchangeable than others. Tettnanger, Spalter, and Saazer, for example, are considered to be in the "Saazer Formenkreis" ("Saazer group of varieties") (15). These hops can be used interchangeably, although some traditional brewmasters hesitate to do so. Cascade, on the other hand, contains some aroma components not found in the same proportions in other aroma hops (Table II). Although Cascade was initially evaluated as a supplement for Fuggle, it soon became evident that its use had to be selective to avoid detectable changes in the flavor profile of established brands of beer. One major brewer and several microbrewers, however, established a successful specialty niche by using Cascade as a primary flavoring hop. Willamette, a triploid aroma hop developed from Fuggle, can readily be substituted for Fuggle despite its slightly higher alpha-acids content, which may even be an added bonus for the brewer (3).

Production of the traditional Hallertauer Mittelfrueh has declined significantly in Germany because of Verticillium wilt and low yields (4,16). The new German aroma hops Hallertauer Tradition and Spalter Select are now being evaluated by traditional aroma hop users who have found them to be comparable to traditional aroma hops in brewing performance (17). The oil chromatogram of the new American aroma hop Mt. Hood (5) is virtually indistinguishable from that of Hallertauer Mittelfrueh, and its slightly higher soft-resin content and composition resembles that of both Hallertauer Mittelfrueh and Hersbrucker, two traditional German aroma hops that have been used interchangeably in recent years, particularly whe1n Hallertauer Mittelfrueh was in short supply.

Because major users of aroma hops are likely to be blend brewers, it probably makes little difference if Saazer or Spalter hops are replaced by Tettnanger as long as the relative proportion in the blend remains unchanged. Experienced taste panels might notice some differences that may also be associated with minor variations in soft resins and their oxidation products, but the consumer is unlikely to detect any differences. A prime example is the folpet fungicide scare of 1986-1987, when large quantities of Saazer hops could not be imported to the United States because of fungicide residues for which there was no tolerance. Apparently, Saazer hops were successfully replaced in commercial brewery blends by Tettnanger and other aroma hops with similar brewing characteristics, and consumers detected no taste or flavor differences in the beer.

ON THE HORIZON

Recent efforts by the U.S. Department of Agriculture (USDA) Hop Research Program at Oregon State University (Corvallis, Oregon) are directed toward developing higher yielding Hallertauer Mittelfrueh-, Tettnanger-, and Saazer-type hops that resemble their parents so closely that their flavor and taste contributions to the finished product are virtually indistinguishable (4). The approach is to start with a regular diploid Hallertauer Mittelfrueh, Tettnanger, or Saazer hop with 20 chromosomes (2n = 2x = 20), to double the chromosome number by colchicine treatment (18) to obtain a tetraploid (2n = 4x = 40), and to cross the tetraploid with a male diploid hop having similar quality characteristics to obtain a triploid with 30 chromosomes (2n = 3x = 30). Two-thirds of the genetic contribution of the triploid seedling comes from the mother. Thus, the probability of finding a higher yielding type with quality characteristics very similar to those of the mother plant is increased.

Two new triploid Hallertauer types - Mt. Hood and Liberty - have been released thus far (5,6). Mt. Hood reportedly is now used by some brewers to supplement certain imported aroma hops. Liberty is in expanded brewing trials.

Triploid Tettnanger and Saazer types are now in various stages of field and laboratory testing. Their higher yield potential should guarantee not only increased availability but also reduced cost relative to imports. All should be suitable for replacing some traditional European aroma hops. Their higher yield potential will also guarantee a reliable supply of such hops in the future.

Traditional conservative brewers will always have an appreciation for the mystique of a Saazer, Spalter, or Tettnanger hop that "must be part of the blend." There is no logical reason, however, why a higher yielding hop cannot be created today that is equal to the ideal in most respects and also available in sufficient quantities at reasonable cost.

REFERENCES

(1) F.L. Rigby, "A Theory on the Hop Flavor of Beer," Am. Soc. Brew. Chem. Proceedings, pp. 36-50 (1972).

(2) V. Rybacek, Hop Production. Phylogenesis of Varieties and Subvarieties. Developments in Crop Science 16 (Elsevier Publishing Company, New York, 1991), pp. 71-78.

(3) A. Haunold, S.T. Likens, C.E. Horner, G.B. Nickerson, and C.E. Zimmermann, "Columbia and Willamette, Two New Aroma Hop Varieties," Brewer's Digest 52, 36-39 (1977).

(4) A. Haunold and G.B. Nickerson, "Development of a Hop with European Aroma Characteristics," J. Am. Soc. Brew. Chem. 45, 146-151 (1987).

(5) A. Haunold and G.B. Nickerson, "Mt. Hood, a New American Noble Aroma Hop," J. Am. Soc. Brew. Chem. 48, 115-118 (1990).

(6) A. Haunold, G.B. Nickerson, U. Gampert, and P.A. Whitney, "Registration of Liberty Hop," Crop Sci. 32, 1071 (1992).

(7) G.B. Nickerson, P.A. Williams, and A. Haunold, "Varietal Differences in the Proportions of Cohumulone, Adhumulone, and Humulone in Hops," J. Am. Soc. Brew. Chem. 44, 91-94 (1986).

(8) V.E. Peacock and M.L. Deinzer, "Chemistry of Hop Aroma in Beer," J. Am. Soc. Brew. Chem. 39, 136-141 (1981).

(9) G.B. Nickerson and E.L. Van Engel, "Hop Aroma Component Profile and the Aroma Unit," J. Am. Soc. Brew. Chem. 50, 77-81 (1992).

(10) A. Haunold and G.B. Nickerson, "Hop Yield Stimulation by Triploid Males under Field Conditions," Crop Sci. 19, 27-31 (1979).

(11) A. Haunold, "Meiotic Chromosome Behavior and Pollen Fertility of a Triploid Hop, Humulus Lupulus L.," Crop Sci. 14, 849-852 (1974).

(12) A. Haunold, G.B. Nickerson, U. Gampert, P.A. Whitney, and R.O. Hampton, "Agronomic and Quality Characteristics of Native North American Hops," J. Am. Soc. Brew. Chem. (in press).

(13) A. Haunold, S.T. Likens, G.B. Nickerson, and S.T. Kenny, "Nugget, a New Hop Cultivar with High Alpha-Acids Potential," J. Am. Soc. Brew. Chem. 42, 62-64 (1984).

(14) K. Wackerbauer and U. Balzer, "Hop Bitter Compounds in Beer, Part I. Changes in the Composition of Bitter Substances during Brewing," Brauwelt International, pp. 144-148 (1992).

(15) H. Kohlmann and A. Kastner, Der Hopfen (Hopfenverlag Wolnzach, Germany, 1975), pp. 34-41.

(16) F. Gmelch and G. Rossbauer, "Hop Varieties in the Federal Republic," Annual Report (German Society for Hop Research, Huell, Germany, 1991), pp. 5-7.

(17) L. Narziss, "The New Hop Varieties," Hopfenrundschau International, pp. 18-25 (1992).

(18) A. Haunold, "Detailed Explanation of the Triploid Hop Breeding Program," Brewer's Digest 63, 40-41 (1988).

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