M

monomers, oligomers and polymers, are represented in worts and these are extracted during the wort boil. Three classes of simple polyphenols occur (Hough et al., 1982). The first class are simple phenolic derivatives of hydroxybenzoic and hydro-xycinnamic acids. These include among others, /^-hydroxybenzoic, vanillic, syringic, /?-coumaric and ferulic acids. They have no impact on beer colloidal stability but have roles in beer flavour and aroma. The second class are the flavonols, which are principally extracted from hops. These consist of quercitin and kaempferol and their glycosides. These survive into beer but also have no impact on colloidal stability. The third class are the monomeric and oligiomeric flavanols. Two of the common monomeric flavanols are ( + )-catechin and ( —)-epicatechin. Dimers such as pro-delphinidin B3 and Procyanidin B3 and trimeric forms can be detected in malt. The oligomeric flavanoids consist of short chains of polyhydroxy-flavan-3,4-diol-monomers attached to ( + )-catechin or ( —)-epicatechin. The most abundant polymers of this type are the proanthocyanidins (formerly anthocyanogens) so called because they yield anthocyanidins when exposed to acid conditions in the presence of oxygen. These compounds have the ability to cross-link and form large molecular weight polymeric flavanols and also to form complexes with proteins and polypeptides, the complex flavanols.

Changes in the concentrations of polyphenols, derived from both malt and hops during wort production, are complex. The most significant stages are the formation of 'hot break' during the copper boil and 'cold break' after wort cooling together with oxidation which accompanies wort oxygenation (McMurrough & Delcour, 1994). McMurrough et al. (1983) reported that a hopped wort contained 46.2 mgl 1 total flavanols. Of these 39% were simple types, 18% polymeric and the remaining 43%, complexed.

2.4.4 Lipids

Wort lipids may potentially be derived from malt, adjuncts and hops. Up to 4% of the dry weight of barley is lipid and 3.4% in the case of malt. Commonly used adjuncts contain up to 4% lipid although there is some variation. For example, wheat flour contains only 1%, whereas the lipid content of flaked maize is 3.7% (Anness, 1984). Triacylglycerols form the most abundant lipid group in barley and malts. Up to 70% of the total fatty acids are esterified in this form. There is considerable modification to lipid composition and concentration during malting and wort production. In particular, Anness (1984) reported that 30% of the lipid content of barley grains disappeared during germination, an effect ascribed to metabolism of triacylglycerols.

The lipid composition of a typical commercial grist is shown in Table 2.10. Little of this lipid persists into the finished wort; however, that which does is available for yeast nutrition during fermentation and later in the process to be involved in reactions leading to the formation of deleterious flavours. The method of separation of sweet wort from spent grains has the most significant effect on wort lipid content. Anness and Reed (1985) reported that 4.5% of malt lipids were released into wort by a mash press, only 1% with a lauter tun and just 0.3% in the case of a mash tun. Further lipid is lost in the form of trub separated out after the copper boil. Thus, in the same report Anness and Reed (1985) indicated that 91% of wort lipids in the copper were

the brewing process Table 2.10 Lipid analysis of a commercial grist (from Anness, 1984).

Lipid class

Fatty acid (mg g dry wf1)

%

Phospholipids + Glycolipids

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