Antioxidant efficacy of amino acids in methyl linoleate at different relative humidities

Antioxidant efficacy of the amino acids methionine, tryptophan, aspartic acid, serine, alanine and arginine in methyl linoleate were compared to a methyl linoleate control at 2,50 or 79% relative humidity (RH) at 37°C. Antioxidant efficacy varied with RH and the individual amino acids. Arginine had the highest antioxidant efficacy at all RH values compared to the control. The efficacy of alanine was equal to that of arginine at RHs of 50 and 79% but was lower at 2% RH. The presence of aliphatic, alkaline amino, hydroxyl or thiol groups in the side chain of the amino acids increased the antioxidant efficacy at high RHs.     

1H-NMR study of bovine myoglobin autoxidation. Influence of muscle type and time post mortem

¹H NMR spectroscopy was used to determine first order rate constants at four temperatures (300, 304, 308, 312 K) and activation energies of the autoxidation reaction for oxymyoglobin. the haeminic pigment was purified from two bovine muscles with different colour stabilities ( psoas major (PM) and longissimus lumborum (LL)) at 2h (day 0) and 192 h (day 8) post mortem. to characterize this autoxidation reaction, we have focused attention on the time-temperature dependent disappearance of the Val-E11 methyl group signal. This study showed that, whatever the time post mortem , although the myoglobin autoxidation rate was greater for PM than for LL muscle, the activation energies were similar. It was also worth noting that, in the range 300–312 K, the average ratio of autoxidation rate constants between day 8 and day 0 was near 1.6 for the two muscles studied. It is reasonable to think that oxidative processes developed during 8 days meat storage have led to a structural change within the cavity of the heme pocket of the myoglobins. Moreover, only one orientation of the porphyrin within the heme pocket was noted for the two muscles studied.     

Are natural antioxidants better – and safer – than synthetic antioxidants?

Antioxidants are necessary in the Western diet as it is rich in polyenoic fatty acids, which are easily oxidized with formation of free radicals that are harmful if present in higher amounts. Consumers prefer natural antioxidants to synthetic antioxidants, mainly for emotional reasons. The common Western daily diet contains about 1 g natural antioxidants even if no natural antioxidants have been added for lipid stabilization. Their main sources are cereals, fruits, vegetables, and beverages. Only a part of the natural antioxidants is absorbed and used as free‐radical scavengers in vivo. Natural antioxidants should be added to food in larger amounts than synthetic antioxidants as they are less active, but the actual activity depends very much on particular conditions and food composition. Nevertheless, the addition of additional antioxidants is still negligible in comparison with the dietary supply of native antioxidants. The safety limits of natural antioxidants are mostly not known, but they are hardly safer than synthetic antioxidants. The best protection would be to replace high‐polyenoic oils in the diet with high‐oleic oils, and to use alternative methods of food protection against autoxidation.     

氧气液相氧化邻硝基甲苯合成邻硝基苯甲醛——溶剂对反应的影响

文章研究了几种有机溶剂对氧气液相氧化邻硝基甲苯制取邻硝基苯甲醛的影响,发现不同的有机溶剂对反应转化率和收率有很大的影响,其中以有机胺为溶剂,转化率和收率相对较高。以2-甲氧基乙胺为溶剂时,转化率为81.05％,收率17.90％结合反应机理,初步分析了溶剂影响的原因,发现有机胺能协助底物脱氢,从而起到一种助催化作用.     

氧气液相氧化法制备邻硝基苯甲醛

氧气直接液相氧化邻硝基甲苯一步合成邻硝基苯甲醛，成本低，污染少，腐蚀性小。反应以有机胺为溶剂，在5℃，催化剂加入量0．05g，反应时间4．5h条件下，邻硝基甲苯转化率为41％，邻硝基苯甲醛得率为36％，同时对反应机理也作了初步探讨，认为该反应是碳阴离子反应历程。     

Linalyl oleate as a frying oil autoxidation inhibitor

Linalyl oleate (LO), an interesterification product of linalyl acetate (LA) and methyl oleate catalyzed with sodium methoxide, was studied to determine its effectiveness in retarding oxidative changes in soybean oil heated continuously at 180±5°C for 32 h. The identity of LO was established by GC-MS and NMR. LO was tested at levels of 0.05 and 0.1% and compared with the more commonly used synthetic autoxidation inhibitor methyl silicone (MS) at levels of 5 and 10 ppm. FA changes and conjugated dienoic acid formation were monitored. First-order kinetic equations were used to model the decreases in linoleate (18∶2)/palmitate and linolenate (18∶3)/palmitate ratios. Plots of the data show an inflection point at ∼11 h. Oils with either level of MS and LO had lower reaction rate constants before the inflection points, and lower conjugated diene values and higher 18∶2 and 18∶3 percentages at the end of the 32-h heating period than did oil without additives and with LA. LO could replace methyl silicone in soybean oil during deep-fat frying but at levels about 100 times greater. [We propose to use the term “autoxidation inhibitor” for substances that inhibit autoxidation when added to fats and oils at low concentrations and whose mechanism of action may be unknown. Some may wish to call such substances “antioxidants” but others wish to reserve this term for substances that end free radical chains by hydrogen radical donation. Some refer to methyl silicone as a “polymerization inhibitor”, but this term suggests more about its mechanism of action than seems warranted.]     

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.     

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.     

Effect of water activity on secondary products formation in autoxidizing methyl linoleate

The role of water activity on the formation of peroxides and carbonyl compounds during lipid oxidation is important to know because there could be either beneficial or detrimental effects of water activity on lipid oxidation in stored foods. Therefore, methyl linoleate was chosen as a model lipid and was autoxidized to 1% at water activity ranging from 0.02 to 0.79 at 37°C. Oxygen uptake was monitored manometrically. The peroxide and carbonyl contents were determined upon termination of the autoxidation studies. Methyl linoleate autoxidation was characterized by three phases: i) an initial induction period of no oxygen absorption; ii) a slow rate of oxygen absorption, up to 0.15% oxidation; and iii) a relatively faster rate of oxygen absorption beyond 0.15% up to 1% oxidation. Water activity had considerable influence during the first phase. There was no induction period at or below water activity 0.22. The induction period begins at water activity 0.32 and could be extended to a limit with increase in water activity. Once the induction period was passed water activity had no influence on the rate of oxidation. However, during the second and third phases water activity becomes important in the stabilization of peroxides/hydroperoxides and decides the course of secondary reactions that follow peroxide decomposition. Higher water activity values, particularly water activity 0.67, tended to stabilize peroxides. Water activity had considerable influence on the formation of secondary products of autoxidation as evidenced by the variation in the type and quantity of carbonyl compounds at different water activity values.