Flavours from hops

Hops play several roles in the production ofbeer, but in particular they are crucial as a source of bitterness (from the hop resins) and aroma (from the essential oils) (Neve 1991).
The chemistry of hop resins is somewhat complex, but of most importance are the a-acids, which can account for between 2% and 15% of the dry weight of the hop, depending on variety and environment. The higher the a-acid content, the greater the bitterness potential. When wort is boiled, the a-acids are isomerised to form iso-a-acids. The latter are much more soluble and bitter than the a-acids. Isomerisation in a boil is not very ef cient, with perhaps no more than 50% of the a-acids being converted to iso-a-acids and less than 25% of the original bittering potential surviving into the beer.
Apart from imparting bitterness to beer, the iso-a-acids also promote foaming by crosslinking the hydrophobic residues on polypeptides with their own hydrophobic side-chains, rendering the foam almost solid-like and able to cling to ('lace') the walls of the drinking glass (Hughes & Simpson 1994). Furthermore they have strong antimi­crobial properties and are able to suppress the growth of many Gram-positive bacteria (Fernandez & Simpson 1995). Beer is not entirely resistant to spoilage but certainly the bitter acids have a strong antimicrobial in uence. Other key factors that render beer extremely inhospitable to microbes are its very low pH (typically in the range 3.8-4.6), lack of oxygen, minimal levels of residual nutrients such as sugar and amino acids, its content of ethanol and perhaps the presence of some other antimicrobial constituents such as polyphenols. No pathogens will grow in beer, even alcohol-free beer. All too familiar food scares such as those due to Listeria, Escherichia coli O-157 and Clostridium botulinum cannot be caused by beer.
Increasingly used nowadays are isomerised resin extracts in which one or more of the side-chains of the iso-a-acids has been reduced, using hydrogen gas in the presence of a palladium catalyst (Hughes & Simpson 1993). This is because one of the side-chains is susceptible to cleavage by light, yielding a radical breakdown product that reacts with traces of sulphidic materials in beer to produce 3-methyl-2-butene-1-thiol (MBT), a compound that affords a reprehensible skunky aroma. If the side-chain is reduced, it no longer produces MBT. For this reason, beers that are likely to be exposed to light in package (e.g. by being sold in green or clear glass bottles) often contain these modi ed bitterness preparations, which have the added advantage of possessing increased foam-stabilising properties. Once again, these products are fully cleared for safe use.
Hops contain between 0.03% and 3% w/w of oil, which comprises a complex mixture of at least 300 compounds contributing to beer aroma (Gardner 1997).