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.
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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.
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 supercritical 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 products are 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 mouthfeel 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.