Understanding and reducing oxidative flavour deterioration in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 136

after-taste, warmed-over after-taste and juiciness after 3±4 chews. The aim of
the study was to substantiate the potential of electronic noses as future quality
control systems in the meat industry, i.e. to investigate whether electronic nose
can replace sensory analysis, TBARS and/or headspace GC-MS methods. The
authors concluded that significant positive correlations were found between the
gas sensor signals and the WOF associated sensory attributes and the levels of
secondary lipid oxidation products and that this confirms that the electronic nose
can be a valuable quality control tool in the meat industry. They also found that
WOF-associated attributes correlated to the development of TBARS, hexanal,
pentanal, pentanol and nonanal.
6.3.7 Fish
The most important flavour change during frozen storage of fatty fish such as
salmon is the formation of train oil, bitterness and metal taste (Refsgaard et al.
1998). Likewise, Milo and Grosch (1995) observed the development of a train
oil flavour during prolonged frozen storage in a lean fish species such as cod. On
the basis of GC-olfactometry, Milo and Grosch (1995) suggested that the
increased concentration of 1-octen-3-one, (Z)-1,5-octadien-3-one, hexanal, (Z)-
3-hexenal, (Z)-4-heptenal, (Z,Z)-2,6-nonadienal and (E,Z)-2,6-nonadienal
observed during storage most likely were important contributors to the train
oil odour in the stored cod sample. In contrast, Refsgaard et al. (1998) suggested
that the formation of volatile oxidation products was not the cause of the most
pronounced sensory changes found during frozen storage of salmon. Rather,
they proposed that compounds of low volatility contributed to the increased
intensity of train oil taste, bitterness and metal taste. Subsequently they
hypothesized that the lipid hydrolysis that occurred parallel to lipid oxidation
and which will give rise to free fatty acid formation contributed significantly to
the sensory deterioration of salmon during frozen storage (Refsgaard et al.
2000). This hypothesis was corroborated by data showing that addition of each
of the unsaturated fatty acids: palmitoleic acid (16:1, n-7), linoleic acid (C18:2,
n-6), eicosapentaenoic acid (EPA; C20:5, n-3) and docosahexaenoic acid (DHA;
C22:6, n-3) to fresh minced salmon changed the sensory perception and
increased the intensity of train oil taste, bitterness, and metal taste. The added
level of each fatty acid (1mg/g salmon meat) was equivalent to the concen-
tration of the fatty acids determined in salmon stored as fillet at ÿ10 ëC for 6
months. The effect of addition of the fatty acids on the intensity of train oil taste,
bitterness and metal taste was in the order: DHA > palmitoleic acid > linoleic
acid > EPA. Taken together these findings suggest that the formation of
unpleasant train oil, bitter and metallic off-flavours may not be solely ascribed to
lipid oxidation, but may be a due to combination of lipid oxidation and lipid
hydrolysis.
In a recent study on frozen stored rainbow trout, formation of volatile
oxidation products and free fatty acids increased in parallel to the formation of
rancid off-flavours (Baron et al. 2009). Moreover, an increased grainy, firm and