Beer Tap into the Art and Science of Brewing by Charles Bamforth

Author:Charles Bamforth
Language: eng
Format: mobi, epub, pdf
ISBN: 9780195154795
Publisher: IB Dave's Library
Published: 2003-03-27T22:06:27+00:00

The Wicked and Pernicious Weed

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There are three variants of the ␣-acids (cohumulone, humulone, and adhumulone); they differ very slightly in the structure of the so-called side-chain labeled “R,” which attaches to the ring (see fig. 5.1, top right). Received wisdom says that “better” hops have a relatively low proportion of cohumulone.

When wort is boiled in the kettle (see chapter 6), the ␣-acids are re-arranged to form iso-␣-acids in a process referred to as “isomerization” (fig.

5.7). The products are much more soluble than the humulones and are bitterer. At the end of boiling, any unisomerized ␣-acid is lost with the spent hop material, and the iso-␣-acids remain. The process is not particularly efficient, with perhaps no more than 50% of the ␣-acids being converted in the boil and less than 25% of the original bittering potential surviving into the beer. Each iso-␣-acid exists in two forms, cis and trans, which differ in the orientation of the side chains (fig. 5.7). The six iso-␣-acids differ in the quality and intensity of their bitterness (see chapter 3).

Essential Oils

The oil component of hops ranges from just 0.03% to 3% of the weight of a hop. Seedless hops tend to contain more essential oil. The oils are produced in the hop late in ripening, after the majority of the resin has been laid down, which highlights the need for harvesting of the hops at the appropriate time.

The oil is a complex mixture of at least 300 compounds. Nobody can yet claim to have established a clear relationship between the chemical composition of the essential oils and the unique aroma characteristics they deliver. The science is enormously complicated, as a glance at the sorts of compound that contribute to hop aroma shows (fig. 5.8). It is most likely that

“late hop character” (i.e., the aroma associated with lagers from mainland Figure 5.7

Isomerization.

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