The Interaction Between Lipoxygenase‐Catalyzed Oxidation and Autoxidation in Dry‐Cured Bacon and a Model System

A model system was conducted to characterize the interaction between lipid autoxidation and enzyme‐catalyzed oxidation in dry‐cured bacon. This involved the use of a hydroxyl radical (HO?) generating system and the extraction and purification of lipoxygenases (LOX) from pork belly. The results showed that LOX activity rapidly (P < 0.05) increased during the curing of dry‐cured bacon. This may be because of the hydroxyl‐radical‐mediated oxidation from LOX‐Fe²⁺ to LOX‐Fe³⁺, which activates LOX. In addition, experiments of the model system also showed that LOX activity could be inhibited by increasing the substrate concentration, although substrate type and concentration had no effect on autoxidation. Moreover, LOX enzyme‐catalyzed oxidation and autoxidation could act synergistically to promote lipid oxidation irrespective of the substrate (linoleic or arachidonic acid). These results provide useful information for regulating lipid oxidation during the production of dry‐cured pork products.     

Effect of thioanisole and trimethylene sulfide on the oxidation and yellowing of methyl linolenate

The effect of thioanisole and trimethylene sulfide on the autoxidation and yellowing of methyl linolenate has been investigated by nuclear magnetic resonance (NMR) and ultraviolet visible spectroscopy, respectively. The progress of autoxidation was followed by measuring the NMR integration of vinylic protons with respect to methoxy protons, which served as the internal standard, as a function of time. The degree of yellowing was determined by measuring the difference in absorbance at 400 nm and 450 nm as a function of time. Both thioanisole and trimethylene sulfide enhanced the autoxidation of methyl linolenate. Inhibition of yellowing was observed only with trimethylene sulfide.     

Aldehydes in grain sorghum wax

Improved knowledge of the properties, composition, and analysis of grain sorghum wax would assist in efforts for industrial application of this product. Wax extracted from grain sorghum, harvested in 1996 in Nebraska, using hot hexane was fractionated with silica gel column chromatography using a series of mixtures of hexane, chloroform, methanol, and acttic acid. During TLC analysis of the sorghum wax, a dark band, which did not appear in carnauba wax, was found between was esters and TAG. This dark band fraction was the primary component, representing more than 40% of the total sorghum wax weight. The purpose of this study was to chemically characterize the dark band. The fraction containing the dark band was subjected to borohydride reduction and autoxidation by exposure to air. The borohydride reduction gave a dark band at the fatty alcohol position on TLC, whereas the oxidized sample showed a dark band at the FA position, strongly suggesting the original dark band contained aldehydes. NMR and GC-MS data confirmed that this fraction contained a saturated C₂₈ aldehyde.     

Examination of Liquid-Phase Oxidation of Jet Fuel at Operational Temperatures

To gain a more complete understanding of the relevance of laboratory testing to predict fuel stability in advanced jet engine designs, the Navy single tube reactor (STR) was employed to examine the impact of copper contamination on fuel oxidation at fuel temperatures closer to operational limits. Oxygen consumption during autoxidation, in the presence of dissolved copper, was similar in both the STR and the standard jet fuel oxidation test at higher temperatures, although differences in flow rates and residence times were evident. By contrast, copper was innocuous in promoting the formation of thermal deposits at lower temperatures in the STR, up to approximately 170°C, after which rapid deposition occurred. This deposition was accompanied by depletion of copper in the fuel, except in the presence of selected additives. In addition to suppressing oxidation, limiting available oxygen also suppressed copper depletion. The findings of this study indicate that as bulk fuel design temperatures continue to increase, copper contamination may begin to be a greater factor in contributing to the accumulation of deleterious thermal deposits within advanced jet engine fuel systems. However, the results also indicate that additives containing metal deactivators could play a useful role in mediating the impact of copper contamination on thermal deposition within fuel system components.     

Destruction of polyunsaturated alkyl/acyl and alkenyl/acyl glycerophosphocholine of plasma lipoproteins during incubation with group V and X secretory phospholipase A2s

Plasma lipoproteins are carriers of various glycerophospholipids including diacyl, alkenyl/acyl, and alkyl/acyl glycerophosphocholines (GPCs), which become distributed among cells and tissues during metabolism. For metabolic function, these phospholipids require hydrolysis by phospholipases, but the responsible enzymes have not been identified. We had previously shown that after complete digestion of lipoprotein diacyl- and oxo-diacyl-GPCs, degradation of residual alkyl/acyl and alkenyl/acyl GPCs continues, despite the fact that ether lipids are resistant to hydrolysis by Ca²⁺-activated secretory PLA₂s and require the presence of the Ca²⁺-independent PLA₂. In the course of further investigation, we came across a report by Khaselev and Murphy in which the autoxidative degradation of plasmalogens in the presence of 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH) proceeded beyond the formation of dihydroperoxides, hydroxides and epoxides, and led to an attack on the enyl bond of the plasmalogen, resulting in formation of 1-OH/2-20:4-GPC and 1-formyl/2-20:4-GPC. Our preliminary investigation indicated that lipoprotein 16:0p/20:4ω6-GPC yielded the same autoxidation products as those reported for synthetic 16:0p/20:4ω6-GPC in the presence of AAPH. Such autoxidative degradation of lipoprotein plasmalogens had not been previously reported with or without AAPH. Subsequent study led to the conclusion that this reaction was not limited to arachidonates, but extended to other polyunsaturated eicosanoids, docosanoids, and tetracosanoids, as well as oligounsaturated octadecanoids. These observations led to a hypothesis that the autoxidative cleavage of the lipoprotein plasmalogens proceeded under the influence of apo-protein-derived free radicals as intermediates of oxidative processes.     

Reaction of N-Acetylcysteine with Cu2+: Appearance of Intermediates with High Free Radical Scavenging Activity: Implications for Anti-/Pro-Oxidant Properties of Thiols

We studied the kinetics of the reaction of N-acetyl-l-cysteine (NAC or RSH) with cupric ions at an equimolar ratio of the reactants in aqueous acid solution (pH 1.4–2) using UV/Vis absorption and circular dichroism (CD) spectroscopies. Cu²⁺ showed a strong catalytic effect on the 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate) radical (ABTSr) consumption and autoxidation of NAC. Difference spectra revealed the formation of intermediates with absorption maxima at 233 and 302 nm (ε₃₀₂/Cu > 8 × 10³ M⁻¹ cm⁻¹) and two positive Cotton effects centered at 284 and 302 nm. These intermediates accumulate during the first, O₂-independent, phase of the NAC autoxidation. The autocatalytic production of another chiral intermediate, characterized by two positive Cotton effects at 280 and 333 nm and an intense negative one at 305 nm, was observed in the second reaction phase. The intermediates are rapidly oxidized by added ABTSr; otherwise, they are stable for hours in the reaction solution, undergoing a slow pH- and O₂-dependent photosensitive decay. The kinetic and spectral data are consistent with proposed structures of the intermediates as disulfide-bridged dicopper(I) complexes of types cis-/trans-Cu^I₂(RS)₂(RSSR) and Cu^I₂(RSSR)₂. The electronic transitions observed in the UV/Vis and CD spectra are tentatively attributed to Cu(I) → disulfide charge transfer with an interaction of the transition dipole moments (exciton coupling). The catalytic activity of the intermediates as potential O₂ activators via Cu(II) peroxo-complexes is discussed. A mechanism for autocatalytic oxidation of Cu(I)–thiolates promoted by a growing electronically coupled –[Cu^I₂(RSSR)]_n– polymer is suggested. The obtained results are in line with other reported observations regarding copper-catalyzed autoxidation of thiols and provide new insight into these complicated, not yet fully understood systems. The proposed hypotheses point to the importance of the Cu(I)–disulfide interaction, which may have a profound impact on biological systems.     

脂类的空气氧化

本文对脂类的空气氧化,尤其是脂类的自动氧化作了全面、系统、深入的研究和讨论。对金属的催化氧化机制提出了本作者的一些见解。     

脂质氧化产生香味物质

综述了脂质自动氧化、热降解产生的挥发性物质及含有不饱和脂肪酸的脂质氧化在肉风味中的重要作用.着重叙述了脂质氧化生成重要的香味物质醛、酮、醇,并概述了猪肉脂肪受热生成的香味物质.     

A simple and quick determination of aldehydes in autoxidized vegetable and fish oils

A simple and quick method for the quantitative determination of aldehydes in oxidized oils was established. The analysis was based on the reaction of N,N-dimethyl-p-phenylenediamine (DPPD) with aldehydes in the presence of acetic acid. Reaction products were determined by visible absorption at wavelengths of 400, 460 and 500 nm. The errors of the DPPD method in the analyses of total aldehydes in authentic mixtures of various aldehydes were less than 5%, and this method could be used successfully to determine aldehyde contents in oxidized vegetable and fish oils. The reaction time of the DPPD method (10 min at 30°C) is much shorter than that of the conventional method that uses 2,4-dinitrophenylhydrazine as a reagent (30 min at 60°C). The simplicity of the procedure enables a quick determination of aldehyde contents in oxidized oil samples.