Chemistry Reference
In-Depth Information
14.2.3.4.
Fatty Acid Metabolism
FFAs are involved in several types of reactions which vary in impor-
tance with the type of dairy product involved (Figure 14.6). Methyl ketones
are produced from fatty acids by oxidative degradation. The production of
methyl ketones involves oxidation of fatty acids (mainly from C
6:0
to C
12:0
)to
-ketoacids, which are then decarboxylated to corresponding methyl ketones
with one carbon atom less (Hawke, 1966). Methyl ketones are responsible for
the characteristic aroma of blue-veined cheeses (Gripon et al., 1991). How-
ever, they play a limited role in Cheddar cheese flavour. Ultimately, methyl
ketones can be reduced to secondary alcohols, which do not contribute to
cheese aroma. Another reaction in which polyunsaturated and, perhaps,
monounsaturated fatty acids can be involved, is oxidation. The extent of
oxidation in cheese is, however, rather limited, possibly due to a low redox
potential. This, together with the presence of natural antioxidants, could
prevent the initiation of oxidation mechanisms, or create conditions under
which the primary oxidation products are reduced (Adda et al., 1982).
Aliphatic and aromatic esters play an important part in the flavour and,
sometimes, the off-flavour of cheese. This synthesis mainly concerns the
above-mentioned short- or medium-chain fatty acids and an aliphatic (etha-
nol) or aromatic (phenylethanol) alcohol or thiol (methanethiol) may be
involved. Esters generally contribute a fruity flavour to dairy products
which is desirable and characteristic in many cheeses (Parmesan, Parrano)
but undesirable in others (Cheddar).
The mechanism of synthesis of esters in cheese is still largely unknown
(Liu et al., 2004b). It is generally accepted that the enzymes of the cheese
microflora are involved in the formation of cheese esters (Hosono et al., 1974;
Harper et al., 1980; Cristiani and Monnet, 2001; Liu et al., 2004b), but some
authors also suggest that cheese esters are not of enzymatic origin (Forss,
1972; Adda et al., 1982). Most S-methyl thioesters can be formed sponta-
neously in cheese from the reaction of acylCoA with methanethiol (Helinck
et al., 2000). Two ester-producing reactions could be involved in ester forma-
tion in cheese, esterification and alcoholysis. Until recently, ester synthesis in
cheese was regarded as resulting from the esterification of an alcohol and an
acid. The acid or acyl CoA moieties of esters are formed from the action of the
cheese microflora and their enzymes on lactose, lactate, lipids and proteins of
cheese curd (Urbach, 1997a). It has been shown recently that cheese esters
could also be synthesized directly from glycerides and alcohols via an alco-
holysis reaction. Esterases of lactic acid bacteria can catalyse this reaction,
involving the transfer of a fatty acyl group from triglycerides (and, preferably,
mono- and di-glycerides) to an alcohol, without the direct involvement of
water (Liu et al., 2003, 2004a). Alcoholysis could be a more common route for
