Effects of processing on the oxidative stability of soybean oil produced in Mexico

The stability parameters of 22 samples of soybean oil produced in Mexico were determined. Samples were analyzed for moisture, color, free fatty acids, peroxide value, p-anisidine value, fatty acid profile, metals, flavor, and Rancimat test for oxidative stability. Results obtained were compared with the stability parameters of soybean oil sproduced in the United States and Costa Rica. The fatty acid profile in all samples analyzed corresponded to the expected profile for a 100% soybean oil. Sixty-four percent of the oils had oxidative stabilities similar to those reported for soybean oils from the United States and Costa Rica. This suggests that in spite of the good quality, the soybean oil production process in Mexico needs further improvement. Especially important is maintaining appropriate control during the degumming and bleaching steps. Special consideration should be given to preserving the natural antioxidants present in the oil.     

Evolution of Oxidative Values during Kinetic Studies on Olive Oil Oxidation in the Rancimat Test

A comparative study was carried out in order to evaluate the kinetics of the formation of a number of primary and secondary oxidation products during oxidation of olive oil in the Rancimat test at 100–130 °C. There were good correlations between the Rancimat index (OSI) and stability indices (IP) measured in the Rancimat test with no significant differences in kinetic parameters calculated from them. Mean values of the temperature coefficient, Q₁₀ number, activation energy (E_a), frequency factor (A), and free energy of activation (ΔG⁺⁺) for olive oil oxidation were calculated to be ?3.44 × 10^?2°C^?1, 2.21, 98.91 kJ/mol, 12.17 × 10¹² h^?1, and 128.25 kJ/mol, respectively. Each unit change in E_a was accompanied by an average 1.43 × 10¹² change in A, indicating a higher contribution for factor A than for E_a to the olive oil stability. The E_a and A correlated well with the values of enthalpy and entropy, respectively. The values of OSI or IP could be described well by the ΔG⁺⁺ values. Kinetic data indicated that olive oil stability is more affected by the indigenous antioxidants than by the fatty acid composition.     

Determination of oxidation stability of soybean oil with the oxidative stability instrument: Operation parameter effects

Operation parameters of the oil stability index instrument were evaluated to determine their effect on the oxidative stability of commercial soybean oil. A factorial design was developed to evaluate the following three parameters, each at two levels, sample weight (2.5 or 5.0 g), conductivity tube temperature (20 or 30°C), and air flow rate (12 or 20 L/h), for a total of eight observations. Significance testing indicated that sample size and air flow rate affected oil oxidative stability independently (P<0.001), but not in combination. The conductivity tube temperature did not affect the oxidation stability index. Presented at the 1993 American Oil Chemists’ Society Meeting in Anaheim, California.     

Comparison of the Rancimat and the ACL Instrument for Measuring the Oxidative Stability of Corn Oil

A comparison was undertaken between two machines that utilise accelerated methods to measure oxidative stability: the Rancimat (Metrohm, Switzerland) and the single channel chemiluminescence instrument (ACL Instruments, Switzerland). The results from the machines showed good correlation for the measurement of the oxidation induction time of corn oil. For the ACL Instrument, the effect of the sample holder material (aluminium or borosilicate glass) was also investigated and clear differences were noted. Finally, a stability study was carried out over a period of 6 months, with corn oil, stored in the dark at three temperatures: ~4, ~27, ~40 °C and it was observed that it was possible to track subtle changes in oxidative stability using chemiluminescence.     

Influence of Different Moisture Contents on the Stability of Tocochromanols in Bulk Oils at 25 °C Storage

The stabilities of tocochromanols including α‐tocopherol, α‐tocotrienol, γ‐tocopherol, γ‐tocotrienol, and δ‐tocotrienol in grape seed oil, palm oil, or stripped soybean oil with added tocotrienol mixtures (SOTT) were determined under relative humidity (RH) 0, 32, 75, and 93% at 25 °C for 8 months of storage. Stability of tocochromanols was significantly influenced by the presence of moisture and other tocochromanols. Tocochromanol stability in grape seed oil was high at RH 75%, whereas palm oil had significantly lower tocochromanol content at RH 75% compared to those under other RH (p < 0.05). Tocochromanol stability in SOTT was high at RH 0%. δ‐Tocotrienol had the highest stability followed by α‐tocotrienol, γ‐tocotrienol, and α‐tocopherol in SOTT. Moisture content in palm oil was the lowest while that in SOTT was the highest at the same RH. Oxidative stability of palm oil was the highest followed by grape seed oil and SOTT based on conjugated dienoic acid content and p‐anisidine values. Moisture in oils affects the stability of tocochromanols and oxidative stability in vegetable oils.     

Temperature effects on the determination of oxidative stability with the metrohm rancimat

Reproducibility of Oil Stability Index (OSI) values determined on the Metrohm Rancimat was measured with a single run and between experimental runs. Within a single experiment, a range of 0.13 h and a standard deviation of 0.066 h were determined. For multiple experiments, standard deviations of 0.24 and 0.26 were obtained for soybean and low-erucic rapeseed oil, respectively. The effect of temperature was determined for safflower, soybean, lowerucic rapeseed, corn, peanut and olive oils. A linear relationship was established between log (OSI) and temperature. The linear equation obtained for soybean oil was utilized to calculate variability of the OSI due to temperature differences in the heating block.     

Effect of Storage Conditions and Antioxidants on the Oxidative Stability of Sunflower–Chia Oil Blends

The mixture of different proportions of sunflower with chia oil provides a simple method to prepare edible oils with a wide range of desired fatty acid compositions. Sunflower–chia (90:10 and 80:20 wt/wt) oil blends with the addition of rosemary (ROS), ascorbyl palmitate (AP) and their blends (AP:ROS) were formulated to evaluate the oxidative stability during storage at two temperature levels normally used, cool (4 ± 1 °C) and room temperature (20 ± 2 °C) for a period of 360 days. Peroxide values (PV) of the oil blends with antioxidants stored at 4 ± 1 °C showed levels ≤10.0 mequiv O₂/kg oil; the lowest levels of PV were found for blends with AP:ROS. Values higher than 10.0 mequiv O₂/kg were observed between 120–240 days for oil blends stored at 20 ± 2 °C. Similar trends were observed with p-anisidine and Totox values. The oxidative stability determined by the Rancimat method and differential scanning calorimetry showed a greater susceptibility of the oils to oxidative deterioration with increasing unsaturated fatty acids content. The addition of antioxidants increased the induction time and decreased the Arrhenius rate constant, indicating an improvement in the oxidative stability for all the oil blends. Temperature had a strong influence on the stability of these blends during storage.     

Effects of free fatty acids on oxidative stability of vegetable oil

The effect of free fatty acid (FFA) content on the susceptibility to thermooxidative degeneration of vegetable oils was determined by Rancimat analysis. A prooxidant effect of FFA was observed in all filtered oils, independently of lipidic substrate and of its state of hydrolytic and oxidative alteration. The intensity of this effect was related to FFA concentration, but regression analysis of the experimental data did not show a general correlation law between FFA concentration and induction time (I _t). Different results were obtained for freshly processed virgin olive oils, characterized by postpressing natural suspension-dispersion: opposite behavior was observed of FFA content as regards oxidative stability, depending on the presence of suspended-dispersed material. This fact is of interest because the dispersed particles play a double stabilizing effect on both oxidative and hydrolytic degradation. These results showed that avoidance of oil filtration is highly desirable to extend olive oil’s shelf life.     

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.     

Long storage stability of biodiesel from vegetable and used frying oils

Biodiesel is defined as the mono-alkyl esters of vegetable oils. Production of biodiesel has grown tremendously in European Union in the last years. Though the commercial prospects for biodiesel have also grown, there remains some concern with respect to its resistance to oxidative degradation during storage. Due to the chemical structure of biodiesel the presence of the double bond in the molecule produce a high level of reactivity with the oxygen, especially when it placed in contact with air. Consequently, storage of biodiesel over extended periods may lead to degradation of fuel properties that can compromise fuel quality.This study used samples of biodiesel prepared by the process of transesterification from different vegetable oils: high oleic sunflower oil (HOSO), high and low erucic Brassica carinata oil (HEBO and LEBO) respectively and used frying oil (UFO). These biodiesels, produced from different sources, were used to determine the effects of long storage under different conditions on oxidation stability. Samples were stored in white (exposed) and amber (not exposed) glass containers at room temperature.The study was conducted for a period of 30-months. At regular intervals, samples were taken to measure the following physicochemical quality parameters: acid value (AV), peroxide value (PV), viscosity (ν), iodine value (IV) and insoluble impurities (II). Results showed that AV, PV, ν and II increased, while IV decreased with increasing storage time of biodiesel samples. However, slight differences were found between biodiesel samples exposed and not exposed to daylight before a storage time of 12 months. But after this period the differences were significant.