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 favourable 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 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 (supercritical 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.
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 supercritical 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 supercritical 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 (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 product P.I.K.E. 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.