TbAs 533 susceptibility

Storage Temperature. The role of storage temperature in the oxidative deterioration of dairy products is anomalous. Dunkley and Franke (1967) observed more intense oxidized flavors and higher TBA values in fluid milks stored at 0°C than at 4° and 8°C. The flavor intensity and the TBA values decreased with increasing storage temperature. Other conditions being equal, condensed milk stored at - 17°C is more susceptible to the development of oxidized flavor than is condensed milk maintained at -7°C (Parks 1974). 

Lipids are susceptible to oxidation and, as such, require analytical protocols to measure their quality. As described in vnitd2.i, autoxi-dation is one of the chief processes by which lipids degrade. The primary products from this reaction are hydroperoxides. These odorless and colorless transient species break down by various means to secondary products, which are generally odoriferous by nature. Being able to measure secondary oxidation products by simple spectrophotometric means is important for the food scientist so that he or she is able to characterize the extent of lipid oxidation. However, the researcher should be cautioned that one assay (e.g., TBA test) does not provide all the answers. To get a better picture of the story, both primary and secondary products of lipid oxidation should be assessed simultaneously by the different methods available (unitdu). 

Fig. 5 Plots of (top) Xm T, (center) xm"/Xm and (bottom) xm" against temperature T, where Xm, Xm" and xm are in-phase-ac, out-of-phase-ac and dc molar magnetic susceptibilities, respectively, for a powder sample of [Pc2Tb]- TBA+ (open marks) and that diluted in [Pc2Y]— TBA with the molar ratio [Pc2Tb] /[Pc2Y] = 1/4 (filled marks) measured in 3.5 G ac magnetic field oscillating at indicated frequencies...

In a study on butteroil held at a temperature ranging from —10 to +50°C, oxidation rate increased with increasing temperature but the same flavor was formed on storage and the reaction sequence for flavor formation was similar at all temperatures (Hamm et al., 1968). Dunkley and Franke (1967) reported a decrease in flavor intensity and thiobarbituric acid (TBA) values in liquid milk as storage temperature was increased from 0 to 4 to 8°C. Schwartz and Parks (1974) reported that condensed milk stored at — 17°C was more susceptible to oxidized flavor development than at — 7°C. 

In another study, Burbano et al. [106] investigated the elimination of the MTBE by-products tBA, tBF, methyl acetate and acetone at a concentration of 0.0227 mM (1.3 lo 2.3 mg/L). The reaclion rales were slower than that of MTBE. The compounds containing a lerl-butyl group were observed to be more susceptible to OH radical attack. 

It was shown by the magnetic susceptibility measurement and Mossbauer, ESR, and UN-visible spectroscopy that TBA-I shows an antiferromagnetic coupling of the two high-spin Fe centers. For example, the 8, AEq, and J values of TBA-I, diferric complexes, oxidized methane monooxygenase (abbreviated as MMOox), and oxidized ribonucleotide reductase (abbreviated as RRox) are shown in Table 1. The 5 and AEq values for TBA-I are close to those for MMOox, 1, 2, and 4 with the symmetrical iron centers, and different from those for RRox (5 = 0.53 mms, AEqi = 1.65 mms, and 82 =... 

Because of its convenience, the TB A method has become a common assay to determine the degree of peroxidation and oxidative susceptibility of a wide range of biological materials, including LDL. However, the validity of the TBA determination as an index of lipid peroxidation in biological samples has been a matter of considerable debate in the literature. The determination of TBARS inherently lacks specificity, and is subject to interference by many compounds including materials that are not due to lipid peroxidation (see Chapter 5.E). This method is also flawed by analytical artifacts, and is affected by the same factors as lipid peroxidation. 

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