by Gregory K. Lewis
Introduction
Historically the management of hop additions to beer was, for most brewers, a relatively simple process. Whole hops were added to the boiling wort and the spent hops provided a filter bed for the wort as it was passed on to fermentation. Some English brewers also added whole hops to finished beer in order to impart a dry hop flavor to cask-conditioned ales. Brewers could vary the amount of bitterness and the intensity and quality of hop aroma by varying the varieties of hops used, the amount used and the time of addition during the wort boiling process.
Over the past one hundred years or so this situation has changed dramatically and, due to a number of factors, the whole issue of hop addition to beer has become much more complex. The major factors contributing to this process have been:
- a need to reduce the costs associated with hop usage. This includes purchasing hops with higher alpha acids contents, using lower volume products to reduce shipping and handling charges and increasing utilization rates through using different products or modifying the brewing process itself.
- a need to provide a higher level of consistency both in the brewing process and in the final beer. It is important with a number of large brewing plants to provide a consistent product from site to site over a long time period.
More recent considerations are:
- a need to demonstrate publicly an awareness of environmental and public health issues. From this has developed a trend towards hops and hop products with reduced health risks (pesticide residues, solvent residues, nitrate residues, etc.) plus a demand for less packaging materials which can be costly to dispose of or recycle.
- a need to produce specialty beers to attract and hold a particular group of consumers. This has to be done within the current brewing infrastructure of large plants with little ability to vary brewing methods on a day to day or even week to week basis. The role of specialty beers to fill particular market niches is however becoming increasingly important in an evermore segmented marketplace.
Many of the considerations above are as important for micro breweries as they are for the large scale brewers.
Although a relatively small factor in the production cost of beer, hops are an extremely important contributor to the flavor and aroma of the product. It is, therefore, not surprising that such a large effort has been devoted to hops in respect of product consistency, health concerns and specialty beers.
Work in these areas has been done by the brewing industry itself, hop trading operations and career brewing and hop chemists. It has resulted in a tremendous array of hop products which seems to be growing larger each year. The purpose of this paper is to attempt to classify this diverse group of products by considering their preparation, their perceived use in the brewing process and their relative advantages and disadvantages in terms of the current and future developments in brewing defined earlier.
Beginning with a look at whole, raw or leaf hops the review will continue through progressively more refined and "purpose-specific" products. Occasionally mention will be made of specific areas in which developments now occurring may result in new and interesting products in the future.
Major Sources of Hop Bitterness and Aroma
Leaf hops
Consideration of this product will help to set a basis for comparison and comment on the advantages or disadvantages of the other products .
Leaf hops are normally traded in normal density "grower" bales or in bales which have been recompressed in one form or another to reduce shipping volumes.
In composition and use terms, leaf hops cannot be considered to be an efficient brewing material. The useful soft resins and oils comprise less than 15% of the hops. Some consider the proteins and tannins of the hop to be important in brewing although most of these in beer come from the malt.
Even if as much as one-half of the hop proteins and tannins are important this only raises the level of useful compounds in the hop to about 24%. The remainder of the proteins and tannins (10%) plus plant vegetable matter and constituents (56%) and water (10%) total to about three-quarters of the product and this is not utilized in brewing. With low utilization of resins, boiling-off of the volatile essential oils and less than 50% recovery of the proteins and tannins actually less than 10% of the total weight of leaf hops are recoverable in beer.
If one considers the removal of wort in spent hops - even after pressing - it can be said that leaf hops are probably the most inefficient brewing material of all.
To this must be added the fact that the product is relatively inconsistent in quality and specifications, unstable in storage, subject to a range of contaminants and bulky and expensive to ship, handle and dispose of. Taking all this into account it is not surprising that alternatives to leaf hops have long been sought. On the other hand a number of the world's major brewers continue to use leaf hops, regarding the use of a "wholesome, natural" product as outweighing all the considerable disadvantages.
Hop Powder Pellets and Hop Pellets
Although hop extracts were developed and in use prior to hop pellets, the latter are closer to whole hops in composition terms and are reviewed first. There are four main types of product under this heading:
- whole hop pellets, made from whole (unmilled) hops
- hop powder pellets, made from milled hops and presented at different levels of enrichment.
- stabilized hop powder pellets
- isomerized hop powder pellets
Whole hop pellets are prepared by freeing hops from foreign matter and then pelleting them without milling. In the past they were used by brewers with traditional hop backs who could not use powder pellets. They are not a significant product in modern beers but still see some use in Eastern Europe. Another form, the "half-ounce" plugs, were designed for use in dry hopping. These products reduced the volume of leaf hops and allowed for packaging in a vacuum but they did not greatly impact on utilization of hop materials in brewing.
Hop powder pellets in various forms are used extensively in brewing around the world. Basic preparation consists of removing foreign material, milling in a hammer mill, blending batches of several hop bales together for product consistency, pelleting through a standardized pellet die, cooling and packing in aluminum based foil packs under vacuum (hard pack) or flushed with an inert gas (soft pack).
Enrichment is achieved by milling at -35º C (to reduce stickiness of resin) and sieving to remove the coarser "waste" fraction. By metering a proportion of the "waste" fraction back into the line, standardization of the alpha acids level is assured.
The major advantages of hop powder pellets relate to volume reduction, potential for greater storage stability, standardization and consistency and enhanced utilization.
Compared to extracts they are, however, still bulky and possibly contaminated in one way or another. Users of pellets should also be aware of some potential differences in brewing behaviour between leaf hops and pellets. In leaf hops the resin glands are whole and it takes time for the boiling process to extract the oils and resins from the glands. In powder pellets the glands are ruptured and the contents smeared over the particles with a greater surface area exposed to the wort. This is the reason for the higher level of isomerization and utilization of the alpha acids experienced with powder pellets. The same phenomenon may, however, be less favorable for essential oil utilization. The relatively slow-release of oils from whole glands of leaf hops allows time for oxidation of the major hydrocarbons such as humulene to humulene epoxides, etc. thought to be responsible for good hop aroma in beer. The ruptured glands in powder pellets may lose the vast majority of these hydrocarbons by volatilization before the oxidation products have a chance to form. It is possible to overcome this loss by later additions of pellets but this is wasteful of the alpha acids.
This phenomenon many not be significant in respect of the overall taste profile of many beers but it may help explain why some early powder pellet users had to adjust their hopping practices when moving to pellets or why other brewers even now do not choose to use the product despite its close relationship to leaf hops.
Modified Hop Powder Pellets
Other types of hop powder pellet products are the stabilized or modified pellets in which various additives have been used either to protect the alpha acids from oxidation or to enhance the efficiency of utilization.
In one product up to 4% ascorbic acid is added to milled hops before pelleting. During the heat generating process of pelleting and in subsequent storage the ascorbic acid is oxidized preferentially to the alpha acids.
In a second instance activated bentonite clay is added prior to pelleting at a rate of 10 - 30%. This is thought to aid dissolution and isomerization in the wort. A notable disadvantage, of course, is the increased bulk of the product.
The modified pellet which is most regularly used is the stabilized hop powder pellet in which about 2% magnesium oxide is added prior to pelleting. The heat produced in pelleting converts the alpha acids to their magnesium salts. These salts show a greater propensity to isomerize in wort boiling than the alpha acids themselves. Brewers have obtained increases in utilization of up to 10% (33 - 37% total utilization) using these pellets. This observation led to the most recent development in hop powder pellet evolution - the isomerized powder pellet. In this instance the pellet stabilized with magnesium oxide is heated to 50º C for 14 days after sealing under vacuum in the final package. The brewer, therefore, does not have to rely on wort boiling to isomerize the alpha acids. In this product it is thought that at least 90% of the alpha acids are isomerized. Problems in technical procedures do not currently allow a precise estimate of the degree of isomerization.
Compared to unmodified hop powder pellets isomerized powder pellets show a large increase in utilization from 30 - 35% to 55 - 60%. Even though isomerization is apparently over 90% in the pellet there are still losses of iso-alpha acids in brewing beyond the wort boiling stage as there are in the use of leaf hops and other pellets. Another significant advantage is the reduced need for cold storage of this product.
A notable disadvantage of this product perceived by some brewers is the heating of the hops and its effect on the essential oils. Without question the oil profile of the hop is altered but little data has been found which qualitatively defines these changes. The cooking volatilizes the low boiling point compounds which presumably cannot escape the package and will condense on cooling. During heating in the evacuated package the components will not oxidize but may well isomerize or otherwise degrade. As the isomerized pellets are usually added to the wort later in the boil than standard pellets the aromatic processes, either negative or positive, have less chance to occur than in the case of leaf hops or standard pellets. Considering all this, the safest statement is that the precise effects of isomerized pellets on hop aroma and flavor in beer are unknown. Brewers must explore this themselves with their own taste panels. The contribution of isomerized pellets to hop flavor and aroma is unlikely to be positive but may in fact be neutral or negative. In either case it may be possible to re-create the desired flavor profile by judicious use of aromatic leaf hops at various stages of boiling.
The only other disadvantage of isomerized pellets is the perception of some brewers that they are a chemically processed product. In Germany, for example, the product could not be used as it falls outside the terms of the "Rheinheitsgebot" or German Beer Purity Law. This is true of all "chemically processed" hop products.
Hop Extracts
Hop extraction in one form or another goes back over 150 years to the early
nineteenth century when extraction in water and ethanol was first attempted (Gardner, 1987). Even today an ethanol extract is available in Europe but by far the predominant extracts are "organic" solvent extracts (hexane) and CO2 extracts (super critical and liquid). CO2 (typically at 60 bar pressure and 5-10º C in extract plants) is in a liquid state and is a relatively mild, non-polar solvent highly specific for hop soft resins and oils. Beyond the critical point (typically at 300 bar pressure and at 60º C in extract plants) CO2 has the properties of both a gas and a liquid and is a much stronger solvent. See the comparisons of various hop extract compositions.
At its simplest, hop extraction involves milling, pelleting and re-milling the hops to spread the lupulin, passing a solvent through a packed column to collect the resin components and finally, removal of the solvent to yield a whole or "pure" resin extract.
The main organic extractants are strong solvents and in addition to virtually all the lupulin components, they extract plant pigments, cuticular waxes, water and water- soluble materials.
Supercritical CO2 is more selective than the organic solvents and extracts less of the tannins and waxes and less water and hence water-soluble components. It does extract some of the plant pigments like chlorophyll but rather less than the organic solvents do. liquid CO2 is the most selective solvent used commercially for hops and hence produces the most pure whole resin and oil extract. It extracts none of the hard resins or tannins, much lower levels of plant waxes, no plant pigments and less water and water soluble materials.
A consequence of this selectivity and the milder solvent properties is that the absolute yield of liquid CO2 extract per unit weight of hops is less than the other solvents. Additionally, however the yield of alpha acids with liquid CO2 (89 - 93%) is lower than that of super critical CO2 (91 - 94%) or the organic solvents (93 - 96%). Following extraction there is the process of solvent removal which for organic solvents involves heating to cause volatilization. Despite this, trace amounts of solvents do remain in the extract. The removal of CO2, however, simply involves a release of pressure to volatilize the CO2.
Solvent extracts are increasingly falling out of favour worldwide due to perceived problems with the residues. CO2 extracts on the other hand are gaining favour as they are seen to be produced with a "natural" solvent.
Within the brewing process the major advantages of extracts relate to reduced bulkiness, improved storage, standardization, consistency, utilization and reduced wort losses. Additionally, reduced contamination with nitrate and pesticide residues, especially with CO2 extracts, is very important in some brewer's minds. The major disadvantage of extracts is the slightly higher cost per bitterness unit compared to whole hops or pellets. This varies from 8 - 10% higher with organic solvent extracts and perhaps as much as 15 - 20% for the CO2 forms.
In brewing the immediate availability of the resins to the wort is favorable for utilization. However, as for pellets, the immediate availability of the essential oils to the boiling wort may be detrimental to flavor.
Comparing the extracts themselves the solvent residues are the main problem with the organic solvent extracts. Further, the heating of the extract to remove solvent markedly modifies the aroma profile.
This modification of the hop aroma profile also applies to some extent to super critical CO2 extracts which are prepared at about 60º C. Supercritical CO2 also extracts less oil than does liquid CO2 but more than the organic solvent extracts. As mentioned, liquid CO2 extracts are the most pure whole resin extracts and moreover the low temperature extraction (5 - 10º C) results in an aroma profile most closely resembling that of the leaf hops from which they were prepared. The only real disadvantage of liquid CO2 extract compared to the others is that of its higher cost resulting from the lower extraction efficiency.
Modified hop extract products
As with pellets, processing of extracts outside the brewery has over recent years developed a useful addition to the hop product line for brewers - isomerized extracts. There are two basic types of isomerized extracts:
- the "normal" form in which the alpha acids have been separated from the whole resin extract, purified, isomerized by various means and presented in the form of a standardized solution.
- a more "novel" form in which the alpha acids are isomerized whilst still in the pure resin extract form - this is known as "isomerized kettle extract (I.K.E.)".
Whole resin extracts are sometimes referred to as "kettle extracts" as they are added to boiling wort in the kettle. Separation of the alpha acids from the kettle extract leaves behind the so-called "base extract" containing the oils, beta acids, other resin materials plus the impurities. The separated alpha acids are isomerized into "isomerized extract". In the newest range of products, where alpha acids are isomerized in the kettle extract, the name "isomerized kettle extract" or I.K.E. logically applies.
There are two or three patented process by which alpha acids are fractionated from kettle extract and subsequently isomerized. These methods commonly involve the production of potassium or magnesium salts of the alpha acids which are then isomerized by heating. The isomerized extracts are invariably water based, standardized to 20 - 30% isomerized alpha acids and are ideal for addition to beer on a post fermentation basis.
In the I.K.E. products the alpha acids are isomerized by heating with an alkali metal carbonate under controlled conditions. If potassium carbonate is used the product contains the potassium salt of the iso-alpha acids and is known as P.I.K.E. In another product magnesium is used in the isomerization. Adjustment of pH then liberates the iso-alpha acids in the free radical form.
From a brewing standpoint the isomerized hop extracts (fractionated alpha acids extracts) are well established. The ideal use for these is where high gravity brewing is carried out and a high level of bitterness in the pre-blended beer is required. An economic analysis of the cost of bitterness based on the specific brewery characteristics can determine the optimum use rate for isomerized extract relative to bitterness provided in other forms. The base beer is prepared and the isomerized extract is used to supplement and adjust the bitterness at the time of blending. One major brewer uses an isomerized extract exclusively for bitterness, but a base extract is used in the kettle to prevent bumping in the boil and to impart non-isohumulone bitterness and hop aroma. The advantages of the isomerized extract relates to its simplicity of use and flexibility in bitterness adjustment which it gives the brewer. Its disadvantages relate to its cost plus the fact that the brewer must consider other products for an aroma source. It is also a chemically processed product.
Some early users of isomerized extracts experienced problems with gushing in beer and this problem is often quoted in respect of this type of product. Subsequent work has shown beyond doubt that the gushing phenomenon was associated with impurities used in the manufacture and use of the product and not directly with the product itself. Once higher grade reagents were used the problem disappeared. This is unfortunately less well known.
The only isomerized kettle extract used to any real extent at present is the potassium carbonate prepared Isomerized Kettle Extracts (PIKE) product. It is gaining more users worldwide as its benefits become more well known. It combines the advantages of an isomerized product with those of an extract and, when prepared from liquid CO2 extract, has aromatic properties resembling the hop variety used in its manufacture. The user can choose to have the oils separated during the isomerization process and then re-incorporated into the extract. Alternatively the oils may be left in during isomerization or removed prior to it and not added back. This provides a much greater degree of control over the oils than is possible with pre-isomerized pellets. Known disadvantages so far are that, at least for a while, minimum notice and quantities are needed for certain varieties and that it too may be considered a chemically processed product.
The free-acid form of isomerized kettle extract contains nothing which hops do not normally contribute to beer. As such, this form of I.K.E. may in fact be consistent with the terms of the Rheinheitsgebot.
Other Sources of Bitterness and Aroma
With the discussion of isomerized (alpha acids fraction) extracts, we have begun to fractionate the whole resin extract down into its component parts. Various other fractions may be isolated from whole resins and a number of these have important and interesting roles in brewing. This section considers fractionated sources of hop bitterness and the next, fractionated sources of hop aroma and flavor.
Reduced iso-humulones and hydroisohumulones
It has long been known that iso-alpha acids in the presence of sunlight undergo transformations to forms which produce a "skunky" or "light struck" flavor in beer. This is why beer is packaged in light proof bottles. Research on the chemical transformation of iso-alpha acids (isohumulones) revealed that upon reduction to dihydro (rho) isohumulones the capacity to produce skunkiness is lost and the potential to produce light resistant beer using reduced isohumulones was recognized.
Hydrogenation of the dihydroisohumulones produces the tetrahydroisohumulones and further reduction generates the hexahydroisohumulones. These two forms of isohumulone are also light resistant.
Brewing experience with the different forms of reduced isohumulones show that they differ in their relative bitterness and in their capacity to contribute foam stability. See a summary of the effects relative to standard isohumulones together with the other details of these products.
Using various chemical procedures it is also possible to prepare the hydroisohumulones from the beta acids (lupulones) of the hop resins.
Today a number of the world's main brewing groups produce beer in clear glass bottles. Some of these are not known to use light stable bitterness but to rely on packaging and distribution to eliminate light struck flavor. Others do use reduced isohumlones and/or hydroisohumulones to produce a completely light resistant beer.
The major problem for a brewer in embarking on this course is that the brewing plant must be free from normal isohumulones and even the yeasts must be purged. As little as 0.25 ppm normal isohumulones can produce a detectable light-struck flavor in beer unprotected from light.
A secondary problem is that the brewer must use a base extract free of isohumulones to provide the hop aroma and flavor required in the beer.
The eye-appeal to the consumer of the product in the bottle unobscured by pigment is thought to be largely responsible for the apparent increasing demand for beer in clear glass bottles.
Hop essential oils and essences
Prior to the development of liquid CO2 hop extraction the only way to separate essential oils from the hop was to distill them following prolonged boiling in water. Whilst steam-distilled oils were useful in characterizing hop varieties in oil profile terms they saw little use as brewing materials. The oil profile of the hop is so modified in solvent extracts that to recoup it was pointless. The same applies, but to a lesser extent, with super critical CO2 extracts. The early whole resin fractions to emerge from a liquid CO2 plant are extremely oil rich and because of the gentle extraction conditions the oils are relatively unchanged from the composition seen in leaf hops. Separation of the oil rich fraction and further gentle fractionation using low temperature vacuum distillation produces a whole oil extract comparable with that of the parent variety of hop. This hop oil may be left in variety specific form or oil from different varieties can be blended into a generic oil which is highly consistent from batch to batch and year to year. Both of these hop oil products are releatively new on the market. They are ideal for post-fermentation addition to provide dry hop aroma to beer.
The whole oil of the hop contains between 250 - 300 different components classified as follows:
- hydrocarbons:;myrcene, humulene, farnesene, caryophyllene, etc. which ;have low boiling points and are largely removed from the beer ;in the boil.
-oxygenated compounds:;terpene and sesquiterpene, alcohols, (e.g. linalool, ;geraniol), aldehydes, ketones, esters, epoxides of ;humulene, caryophyllene, etc. such as humulene
;epoxides I and II.
-sulfur compounds:;such as thioesters, straight chain and cyclic terpenoid
; sulfides.
From within this mixture certain oxygenated compounds are thought to be responsible for the characteristic hop flavor imparted to beer by late hop additions to the kettle. Recently it has been shown to be possible, using carefully controlled column chromatography, to fractionate a whole hop oil (Chapman, 1988). Two important fractions contain those oxygenated compounds which are thought to impart favorable aroma and flavor properties to the beer. These fractions are prepared in water-soluble ethanolic solution and presented to the brewer under the name Late Hop Essences.
Their composition is as follows:
Late Hop Essence - Spicy: contains the terpene and sesquiterpene oxides and ;alcohols. It produces the classic spicy flavor in beer, ;improves mouth feel and enhances perceived bitterness.
Late Hop Essence - Floral: contains the ketone fraction. It imparts a light floral ;note to the beer enhancing the smell rather than the taste.
These products are also designed for post-fermentation addition to supply the characteristic late hop flavor to beer. Apart from the standard essences - spicy and floral - variety specific essences are becoming increasingly available as it is realized that different varieties, even in this form, can have very different effects on the finished beer.
In brewing, the whole oils have to be emulsified or shaken with an ethanol based solvent prior to use. The essences, being in ethanolic solution, are fully miscible with beer. All these aroma products are stable and consistent from batch to batch and year to year. They provide an easy means to adjust hop aroma in terms of either late hop or dry hop character. Importantly they are also produced wholly by physical fractionation processes under gentle conditions. There are no chemically induced transformations in their preparation and the products contain constituents virtually unchanged from those in the leaf hop.
Synthesis and Conclusions
This review has hopefully served to show the range of properties of most of the major hop products in use around the world today. In conclusion it is necessary to discuss how well these products have fulfilled the needs of the brewing industry as described in the introduction to this paper.
In terms of cost, major benefits have, without doubt, accrued to the world's brewers. The pelletizing of hops has made the use of evacuable packaging materials practicable for hops in this reduced bulk form. Extraction and isomerization techniques have provided long term stable storage forms for large quantities of alpha acids. This has helped keep hop prices down.
Products with improved utilization have enabled brewers by and large to off-set most of the processing costs and still enjoy the other benefits brought about by these refined products. In PIKE, for example, the cost of iso-alpha acids in beer is very similar to that from leaf hops. The enhanced utilization of this product covers the processing costs and the brewer obtains the other benefits of using a high quality, high purity extract virtually free of charge.
The next consideration was for products which could provide consistency in beer quality from plant to plant over extended periods. A number of products provide this capacity. Large quantities of pellets or extracts with a similar alpha acids content enable a brewer to be precise in terms of his usage of these products from the start of the brewing process. The standardization of alpha acids in pellets and some extracts applies also to aroma components ensuring consistency in both aspects. Beyond boiling and even beyond fermentation and filtration the isomerized (alpha acids fraction) extracts and the oils and essences provide the means by which to finely adjust bitterness and aroma respectively.
In terms of public health and environmental issues, the contribution of hop developments has also been profound. Beginning with the volume reduction associated with pelleting, the amount of packaging material to be disposed of has been vastly reduced. With extracts and more refined products the packaging material problem has been shrunk to a few boxes, cans and glass, aluminum or polypropylene containers. Perhaps recycling of these back into the processing industry where possible is the next stage?
With the use of extracts the problem of disposal of spent hops has been eliminated, at least as far as brewers are concerned. The vegetative material from processing is now the problem of the hop processing plants which are normally in or near agricultural areas and can arrange to have this returned safely to the land.
Whilst pesticide residue levels at within safe legal limits are enforced at the farm gate these problems are further negated by extraction and subsequent refinement. Nitrate levels as well as residues of polar pesticides are greatly reduced particularly under CO2 extraction. The problem of solvent residues in organic solvent extracts is gradually being overcome as more of the world's brewers move towards the more natural CO2 extracts.
The final consideration of the brewing industry under review concerned the need for flexibility to produce specialty beers and to react quickly to changing market circumstances. It is in this very area that the modern generation of hop products will probably have the most significant and exciting impact.
The use of reduced isohumulones and hydroisohumulones for the production of light resistant beer has been described. This, of course, involves changes in materials and procedures right back to the kettle. Some of the other products provide much more flexibility than that. Using these products a brewers can brew one standard base beer in large quantities and divide it up into a number of large finishing vessels. Using a combination of isomerized (alpha acid fraction) extract, Late Hop Essences and whole hop oils a large number of different beers can be produced varying in bitterness, late hop character and dry hop flavor.
At the Brewing Research Foundation in England the philosophy which led to the development of the Late Hop Essences has recently been successfully applied to malts. Specialty malts are milled, extracted with cold water, filtered, washed and the filtrates and washes subjected to ultra filtration through special membranes. About 98% of the flavor- active and colored material is recovered and packaged for the brewer in a form which can be added directly to green beer. At the BRF this technique has been successfully applied to crystal malt, chocolate malt and roasted barley. The use of different combinations of specialty malt extracts in green beers has enhanced such flavors as those described by the terms malty, burnt, toffee, chocolate and smoky. Combining the use of these malt extracts with isomerized hop extracts, Late Hop Essences and whole hop oil in various combinations means that in theory a very large number of beer types in strong, standard or premium grades can be consistently and controllably produced from a single base beer of high purity, high adjunct and low bitterness.
The conclusion of Atkinson (1992) could not be better phrased:
"Clearly the production of a wide range of beer types from a single green beer is attractive. It offers the advantage of improved control of beer quality attributes, improved efficiency in use of plant, greater flexibility, and the ability to respond to new market requirements much more rapidly and economically. Much of the technology which has been described here is not the technology of tomorrow, it is technology that is available today".
As to the future, there will undoubtedly be further improvements in the quality, form and flexibility of hop products. The absolute dependence of brewers upon hops as a major contributor of aroma and flavor in their product will ensure that brewing scientists and hop chemists will continue to make important advances in this exciting field.
References
| Atkinson, B. | Providing Desirable Qualities of Beer Post-Fermentation. Ferment, Vol. 5, No. 5, p. 357-61 (1992) |
| Chapman, J. | Hop Products Contribution to Beer Flavour. Ferment, Vol. 1, No. 1, p. 22 - 26 (1988) |
| Gardner, D.S.J. | Hop Extracts. In "An Introduction to Brewing Science and Technology", Series II, Vol. 1, Hops, Institute of Brewing (1987) |
| Moir, M. | Developments in Hop Usage. Ferment Vol. 1, No. 3, p. 49 - 56 (1988) |
| Stevens, R. | The Chemistry of Hop Constituents. In "An Introduction to Brewing Science and Technology", Series II, Vol. 1, Hops, Institute of Brewing (1987) |
