Water activity lipid oxidation

Nelson KA, Labuza TP. 1992. Relationship between water and lipid oxidation rates. In Angelo AJ St (Ed.), Water Activity and Glass Transition Theory, pp. 93-103. Washington, DC American Chemical Society. 

In the case of food, antioxidants are substances with the ability to delay or prevent the development of rancidity and deterioration of sensory attributes related to flavors and aroma and also function as oxidation inhibitors or retarders. The effectiveness of these additives depends on a number of factors, like intrinsic factors, such as the composition (lipids, carbohydrates and proteins), pH, water activity and oxide reduction potential extrinsic factors, such as temperature, storage time, and humidity and atmospheric conditions processing factors and microbial factors, such as the type and quantity of microorganisms, resilience microorganisms and cellular composition (Davidson and Taylor 2007). 

To fulfill the needs for long term stability of a biological material, the optimum range of water activity lies between 0.20 and 0.35, This area represents the best compromise between lipid oxidation and non-enzymatic browning. Enzymatic browning is suppressed under these conditions, and growth of microorganisms is impossible. 

The yield of HAs in food systems is affected by the concentration of substrates, enhancers and inhibitors, duration and temperature of heating, water activity, and pH. Some HAs are formed in mixtures of substrates heated for several weeks at relatively low temperature, about 37 to 60°C at 150 to 200°C the rate of reaction is much higher. However, in model systems prolonged heating may also bring about a decrease of the concentration of some HAs. Low water activity in the surface layers of the heated products favors the formation of HAs. In presence of lipids, Fe, and Fe, the rate of reaction increases, probably due to oxidation and generation of radicals (Jagerstad et ah, 2000). 

Effect of Water Activity. A preliminary study was done to determine the a at which encapsulated orange peel oil was the most stable to oxidation. Figure 1 summarizes the results of this study. The formation of the limonene oxidation product, limonene oxide, was the slowest for the powder adjusted over Mg(NO3)2 (a 0.536). While the levels of oxidation product do not follow in exact order of a, it is evident that better storage stability correlates with a higher a of the powder. This relationship was not anticipated. Literature on lipid oxidation (2, 2) indicates that there is an optimum a for product... 

Food deterioration desiccation/water activity, 61 -66 oil stability index, 541-545 primary lipid oxidation products,... 

Labuza (1971) has described the complex relationship between water activity (uw) and lipid oxidation, with a minimum observed at intermediate aw ( 0.4)... 

Karel, M. 1980. Lipid oxidation, secondary reactions, and water activity of foods. In Auto-oxidation in Foods and Biological Systems (M. Simic, M. Karel, eds.), pp. 191-206, Plenum Press, New York. 

This reaction is catalyzed by manganese ions at pH values from 6 to 7.5. S02 can also react with cystine to yield a series of oxidation products. Some of the possible reaction products resulting from the oxidation of sulfur amino acids are listed in Table 3-11. Nielsen et al. (1985) studied the reactions between protein-bound amino acids and oxidizing lipids. Significant losses occurred of the amino acids lysine, tryptophan, and histidine. Methionine was extensively oxidized to its sulfoxide. Increasing water activity increased losses of lysine and tryptophan but had no effect on methionine oxidation. 

Cause of denaturation. Many hypotheses have been proposed to explain the denaturation of muscle proteins (9-17). These hypotheses include 1) the effects of inorganic salts concentrated into the liquid phase of the frozen system 2) water-activity relations 3) reactions with lipids 4) reaction with formaldehyde derived from trimethyl amine (in fish) 5) auto-oxidation ... 

A plausible interpretation of the interaction between residual moisture and microorganism stability can be drawn from the works of Labuza et al. [18]. Figure 7 shows the progression of the lipid oxidation rate as a function of water activity (a ). This parameter reflects the availability of water for chemical reactions within the product. Its value varies from 0 for an anhydrous product to 1 for pure... 

There are various chemical reactions that proceed and may be accelerated at low values of water activity. Maillard reactions leading to lysine loss and brown color develop peaks at aw values around 0.5-0.8. Nonenzymatic lipid oxidation increases rapidly below aw = 0.4. Enzymic hydrolysis decreases with water activity to aw= 0.3, after which, it is negligible. 

---