Understanding oxidation processes in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 17

Fig. 1.8 A temperature/water-activity phase diagram for selection of storage conditions
for a glassy food system based on calorimetric investigations (based on Kurtmann et al.,
2009). The milk powder shown in Fig. 1.7, for which lactose is mainly responsible for the
glassy structure, is in the non-glassy domain for the actual storage conditions of 60 ëC and
accordingly unstable with respect to browning and lipid oxidation.
clearly linked to oxidative deterioration of lipids. Deterioration of proteins in dry
food products like milk powder is similar to lipid oxidation coupled to the
increased water activity occurring as a result of possible glass transitions during
storage (Thomsen et al., 2005). Temperature/water-activity phase diagrams,
such as those shown in Fig. 1.8, which combine the information normally
presented in adsorption isotherms at varying temperatures with the information
available from traditional so-called extended temperature/water-content phase
diagrams, have recently been suggested to be used for selection of storage
temperature for specified relative humidities for prolongation of shelf life for
dried foods like milk powder and freeze-dried probiotic bacteria cultures
(Kurtmann et al., 2009). For foods with higher molecular mobility and for
beverages, radicals need to be stabilized prior to ESR detection using the so-
called spin-trapping technique as has been used for beer to predict beer staling
and for wine to describe oxidative deterioration (Andersen and Skibsted, 2006;
Elias et al., 2009).
Among the non-saponifiable lipids, steroids like cholesterol and carotenoids
are also subject to oxidation in food often coupled with oxidation of the
phospholipids and other of the saponifiable lipids. Carotenoids with their many
conjugated double-bonds are highly coloured and bleach upon oxidation, while
lipid peroxyl radicals abstract one of the allylic hydrogens in cholesterol to yield
7-ketocholesterol as the stable oxidation end product (Smith et al., 1973).
Carotenoid bleaching may be used to monitor progression of oxidation in the
lipid phase of processed dairy products, while 7-ketocholesterol is a valuable
indicator of the total load of oxidative stress accumulated during production and
subsequent storage (Sander et al., 1989). Oxidized cholesterol in food causes
some concerns due to their toxicity (Jacobsen, 1987). Carotenoids seem, how-
ever, to counteract the negative health effects of oxidized cholesterol (Palozza et