Understanding oxidation processes in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 15

Fig. 1.6 A simplistic stairway for radical processes related to antioxidant scavenging of
radicals, and antioxidant regeneration. 1 -O · and 2 -O · are phenoxyl radicals of
antioxidants like flavonoids and -tocopherol (adapted from Becker et al., 2004).
Ascorbate is the most reducing of the antioxidants and may regenerate any of the phenolic
antioxidants depending on spatial location.
practical optimization of protection of lipids in food for long-term storage
(Porter et al., 1989).
The primary event in lipid oxidation is formation of free radicals followed by
appearance of lipid hydroperoxides as primary lipid oxidation products further
leading to secondary lipid oxidation products as shown in Fig. 1.5. The concen-
tration of lipid hydroperoxides may go through a peak value during storage and
again decrease when the rate of their breakdown to yield secondary lipid
oxidation products becomes faster than their formation depending on tempera-
ture conditions (Aragao et al., 2008). In food systems with low molecular
mobility it is, however, possible to detect and quantify the otherwise highly
reactive free radicals as precursors to lipid hydroperoxides providing further
insight in the very early events of oxidation in food and beverages (Andersen
and Skibsted, 2006). In fresh milk powder, the level of free radicals is thus very
low; however, during storage even under the mildly accelerated conditions of
60 ëC, milk powder turns brown after approximately one week (Fig. 1.7). Once
brown discoloration appears, the water activity increases suddenly in the closed
containers indicating crystallization of the initially amorphous lactose in the
milk powder resulting in decreased water binding and with the concomitant
appearance of free radicals, as detected by electron spin resonance (ESR)
spectroscopy (Thomsen et al., 2005). Further changes in the milk powder
include formation of secondary lipid oxidation products with characteristic off-
flavours providing an example of a food for which early radical formation is