Antioxidant activity of lipid-soluble phenolic diterpenes from rosemary

A high-performance liquid chromatography method for analyzing the phenolic diterpenes present in rosemary (Rosmarinus officinalis L.) and commercial rosemary extracts is reported. Carnosic acid was the major phenolic diterpene present in rosemary leaves, with lesser amounts of 12-methoxycarnosic acid and carnosol. Several commercial rosemary extracts also were analyzed by this method, and in addition to these three compounds other phenolic diterpenes, such as 7-methoxyrosmanol, 7-methoxy-epirosmanol, and rosmanol, were found in some samples. These latter three compounds seem to be artifacts, produced from carnosic acid by oxidation and cyclization. The major phenolic diterpenes were isolated, and their relative antioxidatn activities in soybean oil were measured by the Rancimat. The potency of carnosic acid was more than twice that of any other compound. The antioxidant activity of pure carnosic acid was compared to butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), and tertiary butylhydroquinone (TBHQ) and was several times greater than BHT and BHA but less than TBHQ. Nuclear magnetic resonance data for several of the compounds that were incompletely characterized in previous literature are reported.     

Validation of the rancimat test for the assessment of the relative stability of fish oils

The induction periods for the peroxidation of various fish oils at 55–90°C were studied by the Rancimat test. The natural logarithms of the induction periods varied linearly with respect to temperature, with a mean coefficient of −7.5×10⁻²°C⁻¹, which was significantly different from that reported for vegetable oils. The activation energy for the formation of volatile acids had a mean value of 38.9 kJ/mol and was independent of the fish oil source. Peroxide formation under Rancimat test conditions followed first-order kinetics. The same kinetics were followed under Schaal Oven test conditions (forced-air oven, 60°C). On the basis of the results obtained, the Rancimat test appears to be useful in determining the relative stabilities of fish oils without the change in peroxide decomposition kinetics that may occur at elevated temperatures.     

Comparative Oxidative Stability of Fatty Acid Alkyl Esters by Accelerated Methods

Several fatty acid alkyl esters were subjected to accelerated methods of oxidation, including EN 14112 (Rancimat method) and pressurized differential scanning calorimetry (PDSC). Structural trends elucidated from both methods that improved oxidative stability included decreasing the number of double bonds, introduction of trans as opposed to cis unsaturation, location of unsaturation closer to the ester head group, and elimination of hydroxyl groups. Also noticed with EN 14112 was an improvement in oxidative stability when a larger ester head group was utilized. Methyl esters that contained ten or less carbons in the fatty acid backbone were unacceptable for analysis at 110 °C (EN 14112) due to excessive sample evaporation. With respect to PDSC, a correlation was noticed in which the oxidation onset temperature (OT) of saturated fatty esters increased with decreasing molecular weight (R ² 0.7328). In the case of the monounsaturates, a very strong inverse correlation was detected between molecular weight and OT (R ² 0.9988), which was in agreement with EN 14112. Lastly, a strong direct correlation (R ² 0.8759) was elucidated between OT and oil stability index (OSI, EN 14112, 80 °C). The correlation was not as strong (R ² 0.5852) between OSI obtained at 110 °C and OT. Disclaimer: Product names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable.     

Comparison between free radical scavenging capacity and oxidative stability of nut oils

Several works have measured free radical scavenging capacity of nut oils, since they may become a significant source of dietary fat. However, they have not considered kinetic parameters, what was the first aim of this work. Also, it was studied the possible relation between values of free radical scavenging capacity DPPH and oxidative stability (Rancimat method) in different nut (hazelnut, peanut, pistachio, walnut and almond) oils. The ranking of antioxidant capacity of nut oils, by both assays, was: pistachio > hazelnut > walnut > almond > peanut. A significant correlation was found between DPPH and Rancimat methods assays. Tocopherols appear to be the responsible compounds of this antioxidant capacity being neglictible the contribution of polyphenols. An interference effect of phospholipids, present in methanolic fraction of nut oils, was observed in the determination of polyphenols in nut oils by Folin and ortho-diphenols assays.     

Comparison of rancimat evaluation modes to assess oxidative stability of fish oils

Two Rancimat evaluation modes, the induction period (IP), and the time needed to achieve a selected difference in conductivity (tΔK) were compared for assessing relative stability of anchovy, sardine, and hake liver oils. Mean coefficients of variation were 2.5 and 2.4% for IP and tΔK values, respectively, for oils oxidized in the range 55–90°C. Natural logarithms of IP and tΔK values varied linearly with temperature (P<0.001). A linear relationship (r=0.999) was established between the IP and tΔK values (P<0.001). Relative oxidative stability of fish oils was determined with the same degree of confidence by either IP or tΔK values.     

The influence of antioxidants on the oxidation stability of biodiesel

Oxidation stability of bodiesel is an important issue because FA derivatives are more sensitive to oxidative degradation than mineral fuel. Therefore, in the most recent European Specifications for biodiesel, a minimum value of 6 h for the induction period at 110°C, measured with a Rancimat instrument, is specified. To guarantee this value at the filling station, the use of additional antioxidants will be necessary. In this paper we show the influence of different synthetic and natural antioxidants on the oxidation stability, using the specified test method. Biodiesel produced from rapeseed oil, sunflower oil, used frying oil, and beef tallow, both undistilled and distilled, was investigated. The four synthetic antioxidants pyrogallol (PY), propylgallate (PG), TBHQ, and BHA produced the greatest enhancement of the induction period. These four compounds and the widely used BHT were selected for further studies at concentrations from 100 to 1000 mg/kg. The induction periods of methyl esters from rapeseed, oil, used frying oil, and tallow could be improved significantly with PY, PG, and TBHQ, whereas BHT was not very effective. A good correlation was found between the improvement of the oxidation stability and the FA composition.     

Antioxidant activity of pilot‐plant alcoholic and supercritical carbon dioxide extracts of thyme

Leaves and flowering tops of thyme (Thymus vulgaris L.) were extracted with ethyl alcohol and supercritical carbon dioxide. Antioxidant activity was measured in sunflower oil at 0.3, 0.6 and 1% concentrations of extracts by oven test and Rancimat method. The activities of extracts were compared to those of butylated hydroxytoluene at 0.01 and 0.1% level. The ethanolic extract showed a slightly higher antioxidative effect than that obtained by supercritical fluid extraction. The effectiveness of both extracts added at 0.6% level were equal to that of 0.1% of butylated hydroxytoluene.     

Classification of Sunflower Oil Blends Stabilized by Oleoresin Rosemary (Rosmarinus officinalis L.) Using Multivariate Kinetic Approach

The sunflower oil–oleoresin rosemary (Rosmarinus officinalis L.) blends (SORB) at 9 different concentrations (200 to 2000 mg/kg), sunflower oil–tertiary butyl hydroquinone (SO_TBHQ) at 200 mg/kg and control (without preservatives) (SO_control) were oxidized using Rancimat (temperature: 100 to 130 °C; airflow rate: 20 L/h). The oxidative stability of blends was expressed using induction period (IP), oil stability index and photochemiluminescence assay. The linear regression models were generated by plotting ln IP with temperature to estimate the shelf life at 20 °C (SL₂₀; R² > 0.90). Principal component analysis (PCA) and hierarchical cluster analysis (HCA) was used to classify the oil blends depending upon the oxidative stability and kinetic parameters. The Arrhenius equation adequately described the temperature‐dependent kinetics (R² > 0.90, P < 0.05) and kinetic parameters viz. activation energies, activation enthalpies, and entropies were calculated in the range of 92.07 to 100.50 kJ/mol, 88.85 to 97.28 kJ/mol, ?33.33 to ?1.13 J/mol K, respectively. Using PCA, a satisfactory discrimination was noted among SORB, SO_TBHQ, and SO_control samples. HCA classified the oil blends into 3 different clusters (I, II, and III) where SORB₁₂₀₀ and SORB₁₅₀₀ were grouped together in close proximity with SO_TBHQ indicating the comparable oxidative stability. The SL₂₀ was estimated to be 3790, 6974, and 4179 h for SO_control, SO_TBHQ, and SORB₁₅₀₀, respectively. The multivariate kinetic approach effectively screened SORB₁₅₀₀ as the best blend conferring the highest oxidative stability to sunflower oil. This approach can be adopted for quick and reliable estimation of the oxidative stability of oil samples.     

Analysis of extra virgin olive oils from stoned olives

This paper focuses on comparing the main chemical characteristics of 16 fresh commercial samples of extra virgin olive oil obtained from four harvest years (1999–2002) and derived from both stoned and whole fruits. The qualitative and quantitative contents of minor polar compounds (MPCs) together with other reference analytical parameters (acidity, peroxides, UV absorption values and Rancimat test) were evaluated. An investigation of the MPCs and oil composition obtained from only stoned olives was also carried out. The acidity values of the oils from stoned fruits were always similar to or lower than those of the corresponding oils from whole fruits. For almost all the samples from stoned olives a better resistance to oxidation was revealed in comparison with the corresponding traditional oils. Five pairs obtained from the 2000 and 2001 harvests showed higher concentrations of both MPCs and hydroxytyrosol derivatives in the oils from stoned fruits, in agreement with their higher Rancimat values. Overall, our findings with regard to acidity values, % hydrolysis, the Rancimat test and the qualitative and quantitative distribution of MPCs suggest a higher antioxidant capacity of the oils from stoned olives. Copyright © 2004 Society of Chemical Industry     

Relationship of Rancimat method values at varying temperatures for virgin olive oils

Oxidative stability of VOO is widely determined by Rancimat test, an accelerated method that shortens analysis time. The most usual temperature used for Rancimat analysis is 98 °C; however several authors have reported oxidative stability values at different temperatures higher than 98 °C (110 °C, 120 °C) in order to reduce the analysis time. In this study the effect of temperature on Rancimat method for VOO analysis, measuring the method reproducibility at each temperature, has been evaluated. Method precision expressed as the relative standard deviation (RSD) was ranged from 1.14 to 5.84%, indicating that analytical method was repeatable even at 140 °C. A mathematical model is presented to transform the induction period inter-temperatures (98–140 °C). The results suggested 110 °C as the more adequate temperature since the analysis time was shortened by 63% compared to induction period obtained at 98 °C; the inter-conversion 98–110 °C presented low differences between theoretical value obtained from mathematical model and experimental value obtained by Rancimat apparatus. To shorten analysis time, 84 and 92% respectively, 120 and 130 °C can be used too; although some samples can present higher differences between theoretical and experimental induction period values because of the different oxidation mechanism at each temperature.