Lipid oxidation in milk and dairy products is a chain reaction initiated by
formation of free radicals (Richardson & Korycka-Dahl, 1983). Thanks to intensive
studies on both model systems and actual food, the autocatalytic process, including
the formation of secondary lipid oxidation products from the lipid hydroperoxides
formed initially, is fairly well understood. However, actually predicting the rate at
which the first free radicals leading to spontaneous oxidation are formed in milk from
different cows awaits the development of new analytical methods with higher
specificity and sensitivity (Nicholson, 1993; Barrefors et al. 1995). Such methods
would also be valuable for predicting the stability and shelf life of dried dairy
products, which are determined by oxidative phenomena. Electron spin resonance
(ESR) spectrometry has the potential for detecting the early events in lipid
oxidation, as it is the only spectrometric method that will directly detect the
unpaired electron characteristic of the free radical and it is, moreover, a highly
sensitive method (Brudvig, 1995). ESR spectrometry has recently been shown to
provide quantitative information on the level of free radicals in milk powder that
correlates with the level of secondary oxidation products developed upon
reconstitution and that also correlates with subsequent sensory evaluation (Nielsen
et al. 1997; Stapelfeldt et al. 1997a, b, c).
However, in order to explore further the
potential of this method for raw milk, it was considered valuable to measure the
tendency of milk to form free radicals in relation to its level of α-tocopherol, the most
important lipophilic chain-breaking antioxidant (cf. Kamal-Eldin & Appelqvist,
1996).
Formation of phenylacetic acid and benzaldehyde by degradation of phenylalanine in the presence of lipid hydroperoxides: New routes in the amino acid degradation pathways initiated by lipid oxidation products
