Dairy products, oxidation stability

KRISTENSEN D, krOger-ohlsen m V and SKIBSTED L H (2002) Radical formation in dairy products Prediction of oxidative stability based on electron spin resonance spectroscopy, in Morello M J, Shahidi F Ho CT Free Radicals in Food, Chemistry, Nutrition and Health Effects, ACS Symposium Series 807, Washington D C, 114-25. 

Milk contains trace amounts of SOD which has been isolated and characterized it appears to be identical to the bovine erythrocyte enzyme. SOD inhibits lipid oxidation in model systems. The level of SOD in milk parallels that of XO (but at a lower level), suggesting that SOD may be excreted in milk in an attempt to offset the pro-oxidant effect of XO. However, the level of SOD in milk is probably insufficient to explain observed differences in the oxidative stability of milk. The possibility of using exogenous SOD to retard or inhibit lipid oxidation in dairy products has been considered. 

It is claimed that commercially-available ultrasound equipment can measure the following quality parameters of dairy products levels of solids, solids non-fat (SNF), protein, water and fat solid fat content (SFC), colloidal stability, gelation point, adulteration with oil, particle size, particle size distribution, oil composition, protein denaturation and fat oxidation. This incomplete list represents an impressive contribution towards the solution of food quality measurement although the present authors are slightly skeptical regarding some of these claims. In this review only those applications will be addressed which are regarded as robust. 

Literature reports since the 1970 review (312) continue to show interest in stabilization of dairy products such as use of ascorbic acid and tocopherol in controlling oxidized flavor in sterilized cream (648). 

Red beet gives a nice strawberry colour in ice cream, dairy products, fruit preps, jams and jellies as well as sugar confectionery products, which are not subjected to heavy heat treatment. It is a very intense colom and dosage levels are usually low. However, the pigment is susceptible to heat degradation and oxidation, which limits its use but can, in part, be overcome by adding the colom after heat treatment. Stability is highest at pH 4.5, and red beet colour is not recommended for alkaline applications. 

Although in fluid milk the phospholipid fraction is more susceptible to oxidation than the triacylglycerol fraction, in dry milk products, the triacylglycerol fraction is more susceptible to oxidation and the phospholipids act as antioxidants. Thus, solvent-extracted milkfat containing phosphohpids is much more stable to oxidation than milkfat free of phospholipids, obtained by melting churned butter (also called butter oil). The susceptibility of milk phospholipids to oxidation appears to be dependent on whether they are suspended in water or fat. This difference of oxidahve stability influences the development of different flavor defects in various dairy products. With butter, which is a water-in-oil emulsion system containing an aqueous phase of phospholipids dispersed in fat, the phosphohpids oxidize more readily than the triacylglycerol components. 

Radical Formation in Dairy Products Prediction of Oxidative Stability Based on Electron Spin Resonance Spectroscopy... 

Radical-generating oxidation reactions that have been foimd to take place in milk and dairy products will be reviewed. In addition, different ESR-based techniques that have been employed in dairy science will be described. Finally, examples will be given of die application of ESR specfroscopy to studies of oxidation in such products the perspective will be prediction of storage stability based on the tendency of radicals to form during the early stages of oxidation. 

In spin trapping experiments, relatively stable ESR-active compounds, the spin adducts, are formed by reaction of radicals with ESR-silent compounds, the spin traps, added to the smnpie. The most commonly used spin traps are nitroxides and nitrones, which form stabilized radicals by reaction with other radicals (23). Based on the characteristics of the spin adduct (e.g. hyperfine pattern, coupling constants, and g-value), an assignment of the radical in question is often possible. However, due to lack of specificity of the often-used nitroxides, like N-r-butyl-a-phenylnitro-ne (PBN), a valid verification of the radicals trapped depends on identification by tecimiques such as HPLC-MS. Despite the lack of spectral resolution, spin tr q>ping seems to be a promising technique for prediction of the oxidative susceptibility of dairy products (see later sections). 

ESR spectroscopy is a new and promising technique in dairy science. In addition to numerous specific plications, it may prove to be useful in predicting the oxidative stability of dairy products. Areas for further investigation include the identification of radicals formed during production and storage of milk and dairy products, and also transfer of radicals between enzymes, lipids, proteins, etc. New spin traps, designed with specific hydrophilic/hydrophobic balance to trap specific radicals, provide good opportunities for evaluation of such radical transfer... 

The delicate balance between the overall anti- and pro-oxidative processes, which is also known to exist in most odier biological systems, determines the oxidative stability of milk. Any disturbance in his balance in advantage of pro-oxidative activity, most often ascribed as oxidative strras, will unavoidably mediate deteriorative processes which i) are already perceived in he fresh milk, e.g. oxidized flavor, (3) ii) are recognized by poor technological quality of the milk, e.g. altered rheological properties (4) or Hi) give rise to inferior sensory quality of highly processed dairy products (5). 

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