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 (See Table II). 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 (See Table III). 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.
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