The tendency of radical formation during storage in the surface part (S) and the inner part (I) of the minced chicken breast patties HP treated at 800 MPa were evaluated by measuring the level of radicals formed during incubation at 55 C in the presence of the spin trap PBN (Fig. 3). The ESR spin-trapping method used detects the amount of radicals formed during 3 h of incubation, thus providing a measure of how oxidative labile the respective system is at the time of sampling. On the first day of storage, similar ordering of samples as for TBARS (Fig. 1) was obtained (i.e. the following decreasing radical level: CS > CI = APS > API (Fig. 3)). Therefore, the different levels of radical concentration obtained at day 1 of storage indicated that HP treatment initiated lipid oxidation to a different extent depending on location and packaging. During further storage (at day 3) the radical levels increased for all patties with two different groups corresponding to control packaging (C) (for both locations) and antioxidant active packaging (AP) (for both locations). This increased ability of forming radicals may be due to the rather unoxidised lipids in the meat patties, which is also reflected in the low and almost similar formation of the secondary lipid oxidation products (Fig. 1). The decrease in the formation of radicals at day 7, and the following increase for the samples, APS,API and CI, in addition to the levelling of the CS sample which is in contrast to the TBARS developments, suggests the efficiency of the antioxidant active packaging. During storage lipid hydroperoxides may not have been converted into secondary oxidation products, and have therefore accumulated in the CI, API, and APS samples. Accordingly, upon incubation at 55 C these hydroperox-
ides decompose into reactive species capable of oxidising the ‘‘fresh’’ lipids resulting in the formation of radicals, and explaining the increased tendency of radical formation, as seen in Fig. 3. Contrary to this, the increasingly oxidised CS sample (Fig. 1) obviously
has less fresh lipids to feed the oxidation cycle, and thereby has less tendency to form radicals as reflected in the levelling observed in Fig. 3. The antioxidant activity of rosemary has been associated with the presence of phenolic compounds, which breaks the free radical chain reactions in the lipid oxidation by hydrogen atom donation (Resurrection & Reynolds, 1990; Wong, Hashimoto, & Shibamoto, 1995). At the end of the storage period (day 25), the tendency for radical formation of APS is similar to CI and API. This maybe is due to the fact that antioxidants are very close to the packaging material where the oxidation is also taking place. However, during the ESR incubation studies, where the packaging material with high levels of antioxidant was removed, the systems had very low (negligible) antioxidant concentrations, even for the surface samples.
There are no studies describing an increased lipid oxidation at the surface of meat, and the few applications of antioxidant active packaging for meat describes investigations of beef and lamb
packed in a modified atmosphere