ROO●, which in turn abstract hydrogen from α-methylenic groups or other molecules, RH, to form hydroperoxides, ROOH, and yield R● groups that react with oxygen, and so on. Due to resonance stabilization of the R● species, the reaction is usually accompanied by shifting in the position of double bonds resulting in the formation of isomeric hydroperoxides that often contain conjugated diene groups.

Lipid oxidation gives rise to formation of a number of breakdown products, some of which are responsible for various off-flavors known as rancidity (Nawar, 1996). Even if only a single type of substrate is involved (e.g., one unsaturated fatty acid), the rate and pathway of its oxidation will depend on many factors that include its molecular structure (i.e., the number and location of double bonds), concentration, type of oxidant, oxygen tension, temperature, surface area, pH, time, physical state, and pro- and antioxidants present (Nawar, 1996).

Numerous antioxidant compounds have been studied, including α-tocopherol, α-tocopherol acetate, ascorbyl palmitate, butylated hydroxytoluene, butylated hydroxyanisole, di-t-butylhydroquinone, green tea catechins, and flavonoids, with mixed results (Lindsay, 1996). Briefly, it appears that the degree of oxidation inhibition apparently attained with antioxidants is affected by the method used to measure it and on the system studied.

Although moisture reduction may discourage or inhibit microorganisms from growing in a food during storage, the moisture that remains may promote some chemical reactions such as nonenzymatic browning and enzymatic reactions. Depending on the system, these reactions are normally slowed down at low a_w values, and, in general, at a_w < BET¹, the rates can be very slow and the product may remain in good condition through extended storage if it is properly formulated, processed, and packaged.

There is one exception, however, with respect to oxidative deterioration of lipids and fat-soluble nutrients. It has been shown that lipid oxidation can be increasingly high at moisture levels below a “critical a_w” (Nelson and Labuza, 1992a, 1992b). This critical a_w value is reached when a reduction in the moisture content is accompanied by a decrease in the oxidation rate up to a minimum. At moisture levels below this point, oxidation may rise again. Thus, there is a line of demarcation for lowering a_w: in the a_w range of 0.2 to 0.3, lipid oxidation is likely to be accelerated, whereas at a_w between 0.3 and 0.6, lipid oxidation and other deteriorative reactions are minimized. There are a number of proposed explanations for this effect that implicate the state of hydration of catalysts (e.g.,