Beer : Health and Nutrition

82 Chapter Three
Lipids
Barley contains about 3% w/w lipid, most of it congregated in the living tissues (embryo
and aleurone) (Anness & Reed 1985). Very little lipid, however, survives into beer,
making this beverage essentially a fat-free food. This is just as well, from an aesthetic
point of view, because lipids are very bad news for beer foam (Bamforth 1985b).
The other adverse in uence of lipids is through their ability to act as precursors of
stale avours in beer (Drost et al. 1971). The unsaturated fatty acids, such as linoleic
acid, may get a good press for their health-giving properties; however, they can be
oxidised, ultimately to yield carbonyl compounds that afford aged character to beer.
For this reason many brewers try to ensure that as little lipid as possible survives the
brewing process and therefore they are meticulous about eliminating solid material at
all stages, because the insoluble lipid associates with solids.
Flavours from hops
Hops play several roles in the production of beer, 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
α-acids, which can account for between 2% and 15% of the dry weight of the hop,
depending on variety and environment. The higher the α-acid content, the greater the
bitterness potential. When wort is boiled, the α-acids are isomerised to form iso-α-acids.
The latter are much more soluble and bitter than the α-acids. Isomerisation in a boil is
not very ef cient, with perhaps no more than 50% of the α-acids being converted to isoα-acids
and less than 25% of the original bittering potential surviving into the beer.
Apart from imparting bitterness to beer, the iso-α-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-α-acids has been reduced, using hydrogen gas in the presence of a