Determination of the oxidative stability of vegetable oils by rancimat and conductivity and chemiluminescence measurements

The oxidative stability of five different oils was determined by Rancimat analysis with conductivity and chemiluminescence measurements for evaluation of the induction periods. Samples of oil, taken at intervals from the Rancimat apparatus, were used for chemiluminescence measurements. The chemiluminescence results were plotted vs. time, and the resulting curves were evaluated with a graphical tangential procedure in the same way as the curves of the Rancimat method (conductivity measurement). Induction periods of the oils assessed by Rancimat and chemiluminescence methods showed a significant linear correlation (r=0.9865). The temperature dependence of the induction periods evaluated by chemiluminescence and by conductivity was investigated with walnut oil. A marked temperature dependence was observed for both.     

Measurement of the Oxidation Stability of Biodiesel Using a Modified Karl Fischer Apparatus

Oxidative stability is an important parameter in the characterisation of fats and oils. The determination of this parameter with a Rancimat apparatus is very costly. The alternative modified Karl Fischer (KF) apparatus works on the same principle as the Rancimat, i.e., a conductivity based determination of volatile degradation products and automatic plotting of the conductivity against time. The apparatuses were compared by taking five different samples of methyl esters at different temperature ranges. The results indicate that the modified KF apparatus can be used for the determination of oxidation stability of biodiesel with comparable values of sensitivity, repeatability and reproducibility.     

A comparison of oil stability based on the metrohm rancimat with storage at 20°C

The induction time for oxidative stability by the Rancimat method has been compared with peroxide development during storage at 20°C for six edible oils and rapeseed oil samples with added metal ions, antioxidants or phosphatidylethanolamine. The Rancimat method correlated highly (r=0.966;P=0.000) with oil stability measured by peroxide development for all samples except the oils containing added phosphatidylethanolamine or added butylated hydroxytoluene.     

Comparison of near‐infrared emission spectroscopy and the Rancimat method for the determination of oxidative stability

This work presents a comparison between a new method for the determination of the oxidative stability of edible oils at frying temperatures, based on near‐infrared emission spectroscopy (NIRES), and the Rancimat method at 110 °C. In the NIRES‐based method, the induction time (IT) is determined by means of the variation of the emission band at 2900 nm during heating at 160 °C. The comparison between the IT values obtained with the two methods for 12 samples of edible oils shows some correlation for samples of the same type once there is an agreement on the sequence of highest to lowest IT values between the methods, but a poor correlation considering all samples (correlation coefficient of 0.78). This lack of correlation demonstrates that the results obtained with the Rancimat method cannot be used as an indication of the oxidative stability, or the resistance to degradation, of edible oils at frying temperatures. The difference in the heating temperatures used in the two methods leads to 20–36 times higher IT values for the Rancimat method in relation to the NIRES‐based method, but with similar repeatabilities (2.0 and 2.8%, respectively).     

Several factors affecting the stability of biodiesel in standard accelerated tests

This study examined factors impacting the stability of biodiesel (B100) samples collected as part of a 2004 nationwide fuel quality survey in the United States. Biodiesel is significantly less stable than petroleum diesel, so an understanding of the chemical and environmental factors affecting its degradation is required. The survey included samples produced from soy, waste oils, and tallow. The 27 samples were assessed for stability using the ASTM D2274 test for insoluble formation and the OSI method (via Rancimat apparatus) for induction time. Additionally, the samples were characterized for fatty acid make up, relative antioxidant content, metals content, and total glycerin content (free glycerin plus glycerin bound as mono-, di-, and triglycerides). For the samples examined here the polyunsaturated content (or oxidizability) has the largest impact on both increasing insoluble formation and reducing induction time. However, the formation of insolubles is also measurably decreased by increasing relative antioxidant content and increased by increasing total glycerin content. The OSI or Rancimat induction time is also increased by increasing relative antioxidant content, as expected.     

Bestimmung der Oxidationsstabilität von Fetten und Ölen – Vergleich zwischen der Active Oxygen method (AOCS Cd 12–57) und der Rancimat-Methode

Determination of the Oxidative Stability of Fats and Oils-Comparison between Active Oxygen Method (AOM) and Rancimat Method Oxidative stability is an important parameter for the characterization of fats and oils. The Active Oxygen Method (AOM) can be used to determine this parameter. However, the periodic determinations of peroxide number required with the AOM render this method expensive and labour-intensive. The alternative Rancimat Method is based on the conductometric determination of the volatile decomposition products. As the periodic titrations for the determination of the peroxide number are not necessary, Moreover, in contrast to the AOM, the Rancimat Method does not require expensive chemicals. The induction times t_I determined with the two methods using six different fats and oils show a good correlation. This means that the Rancimat Method gives results equivalent to those of the AOM Method and offers an attractive alternative.     

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.     

The effects of extraction methods on sesame oil stability

The oxidative stability of sesame oil, as measured by the Rancimat test, was shown to be dependent on extraction methods and seed pre-treatment. Oils extracted from whole seeds were more stable than those extracted from dehulled seeds by the same method. Extraction of the same seeds with polar solvents and effective seed crushing yielded more-stable oils (16.7–21.3 Rancimat hours) compared with extraction with nonpolar solvents and coarsely crushed or pressed seeds (4.5–6.4 Rancimat hours). Heptane-isopropanol (3:1, vol/vol) provided slightly more stable oils thann-hexane by the same method. Results are discussed in relation to some of the major anti- and prooxidants present in the oils.     

Oxidative stability of potato chips determined by Rancimat

During recent years the Rancimat apparatus has been used extensively to determine the stability of fats and oils. Most stability tests for fat-containing foods are performed on the extracted oil. The objective of this research was to observe the behavior of potato chips in the racimat without oil extraction. Determinations were carried out at 110°C and 20 L/hr air flow. The curves for the ground chips were completely different from those obtained with extracted oils. Curves for fresh potato chips showed two inflection points after approximately 5 and 20 hr, respectively. Sensory analyses (odor score) of samples withdrawn from the Rancimat after 0, 5, 10, 20 and 25 hr showed significant differences (P < 0.01). An odor score of 5 was considered the rejection point, and was equivalent to 10.8 hr and corresponded to 150 μs of electric conductivity. At this point, the curves started to level off between the first and second inflection points. Curves obtained with potato chips stored at 25°C for 2, 4, 6, 8, 10 and 12 wk showed the same basic pattern, although prolonged storage corresponded with lower induction time. Correlation of induction values between ground potato chips and extracted oil was high (r > 0.87). From these results, it seems that it is possible to estimate the oxidative stability of fatcontaining foods without prior extraction. Presented at 81st AOCS Annual Meeting, Baltimore, MD, 1990.     

Assessment of thin-film oxidation with ultraviolet irradiation for predicting the oxidative stability of edible oils

The oxidative stability of edible oils and samples of rapeseed oil with added antioxidants, metal ions, phospholipids and oxidized oil was assessed by a method involving oxidation of a thin film of oil with ultraviolet (UV) irradiation at 100°C. Induction times determined by this method were compared with those determined with the Rancimat at 100°C. The two methods agreed well in describing the effects of additives on the stability of the edible oil. Induction times were considerably shorter for the thin-film UV method, and the method may have potential as an accelerated test method for assessing the effect of additives on the oxidative stability of relatively stable oils and fats. The correlation between the Rancimat and the thin-film UV induction times also was assessed at 80°C for rapeseed oil containing additives, but there was no advantage in using the lower temperature alone because the induction times were 2–7 times longer than at 100°C. However, use of two elevated temperatures is likely to improve predictions of stability at lower temperatures, especially for samples containing copper, which have an exceptionally high-temperature coefficient. The thin-film UV method showed a poorer agreement with the Rancimat for comparing the oxidative stability of some fats and oils. For instance, corn oil was more stable than soybean oil in the Rancimat test but the order of stability was reversed in the thin-film UV test. Cocoa butter was much more stable in the Rancimat test than when assessed by the thin-film UV test.     

Triacylglycerols Oxidation in Oils and Fats Monitored by Easy Ambient Sonic-Spray Ionization Mass Spectrometry

Oxidation is the major cause of fat and oil deterioration which produces rancid and unpleasant flavors that drastically reduces their quality and nutritional value. The primary oxidation products are triacylglycerols (TAG) containing unsaturated fatty acyl hydroperoxides formed by a chain process occurring through free radical intermediates. The Rancimat method is currently the gold standard for measuring the oxidative stability of oils and fats. In this report, easy ambient sonic-spray ionization mass spectrometry (EASI-MS) is demonstrated to function as a direct (no pre-separation or sample preparation steps), fast and accurate method to monitor oils and fats oxidation, providing detailed overviews of the most immediate TAG hydroperoxide products. The EASI-MS results were compared to those obtained via the classical Rancimat test.     

Photoinduced and thermal oxidation of rapeseed and sunflower oils

The aim of the present study was to compare oxidative stability of different sunflower and rapeseed oils. Ultra violet (UV) irradiation was used as an accelerator of the oil oxidation process. After UV irradiation, the formed volatile compounds were extracted by headspace solid‐phase microextraction HS‐SPME (DVB/CAR/PDMS fibre) and analysed by gas chromatography coupled with a flame ionization detector (GC/FID). At the same time, the same oil samples were thermally oxidized. The induction periods were determined on the basis of hexanal to 2‐trans‐nonenal ratio in the analysed samples. Finally, the obtained results were compared with induction period values obtained through the determination of peroxide and anisidine values, and from the Rancimat method, manostatic test and differential scanning calorimetry (DSC) method. The results obtained using the new method were well correlated with those achieved with the well‐established analytical techniques. The values of the induction period obtained after UV/HS‐SPME/GC/FID were up to three times higher than those from Rancimat, but the correlation between these two methods was on a very good level (correlation coefficient R>0.98). Similar correlation was also observed between these new methods and the DSC or manostatic test. In all cases, better results were obtained for rapeseed oils than sunflower oils.     

Chromatic evolution of virgin olive oils submitted to an accelerated oxidation test

Six samples of virgin olive oil obtained from several varieties of olive fruits (Picual, Manzanilla, Lechín, and Arbequina) were submitted to an accelerated oxidation process during a 63-h period under the conditions of the oil stability index (OSI), as measured by a Rancimat (100°C) apparatus. Spectra were measured every 3 h, and chlorophyll and carotenoid indexes and CIFLAB color ordinates were calculated. As oxidation time increased, remarkable changes in the spectral characteristics and color ordinates were observed. Oxidation provoked less vivid colors (lower values for chroma, C _* ^ab ) in all the samples; however, only some varieties became darker (lower values for lightness, L^*). The pigment loss calculated for oxidized oils was 67% for the carotenoid index and 58% for the chlorophyll index. Mathematical models are offered to predict color changes with time of storage at 20°C.     

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.     

Influence of the type of olive‐crusher used on the quality of extra virgin olive oils

An experimental investigation was carried out to evaluate the quality of virgin olive oils obtained when either a hammer‐crusher or a disk‐crusher were used for the olive paste preparation; the effect of the temperature rise caused by rapid olive crushing was also assessed. Oxidative degradation in the oils obtained from hammer‐crushed olives was significantly higher than in those obtained from disk‐crushed olives as shown by the levels of oxidised triacylglycerols and the results of the oven test. A significant inverse correlation (p <0.001) was found between the Rancimat induction time values and the amounts of oxidised triacylglycerols as determined by the high‐performance size exclusion chromatography analysis of polar compounds. These findings suggested that polar compound analyses, just as routine analyses, may be used as a suitable analytical tool to effectively evaluate the quality of virgin olive oils.     

Oxidative stability of olive oil flavoured by Capsicum frutescens supercritical fluid extracts

The effect of red pepper supercritical fluid extracts (SFE) on the oxidative stability of extra‐virgin olive oil was evaluated using accelerated stability tests [Rancimat and differential scanning calorimetry (DSC) methods] and by measuring the changes in the levels of polyunsaturated fatty acid primary and secondary oxidation products during storage under ambient conditions. SFE were produced according to a central composite rotatable design, at a constant temperature (40 °C), different pressures (15–23 MPa) and superficial velocities (0.04–0.08 cm/s). The results showed that the red pepper extracts produced at low extraction pressure and superficial velocity (e.g. 16.2 MPa and 0.046 cm/s) containing low/intermediate capsaicinoid levels did not affect olive oil stability. The extracts produced at higher pressure showed a slight pro‐oxidant activity. The K₂₃₂ and K₂₇₀ values always fell within the limit set by the European legislation for the quality characteristics of olive oil containing no additives. Evaluation of oxidative stability using DSC was found to be a useful methodology, which demands smaller oil samples and shorter times in comparison with the methodology using the Rancimat apparatus. Red pepper SFE obtained at low extraction pressures can be used in order to produce stable flavoured olive oils.     

Antioxidative Wirkung von Pflanzenextrakten auf Futterfette

Antioxidative Activity of Extracts of Plants on Feed Fats The antioxidative activity of ethanol extract from unripe Madura-fruits (Madura aurantiaca) Nutt./Osage orange) as well as extracts from overground plant parts was investigated. Tallow and lard (prepared by wet procedure) at 60° C and in Rancimat apparatus were used as substrates. The comparison was made with BHA as synthetic antioxidant. The plant extracts from Asclepias syriaca L. (sample A), Chenopodium ambrosiodes L. (sample B). Thymus marschallianus Willd, (sample C) showed only a moderate antioxidative activity with substrates tempered to 60^OC. which was not confirmed by the results obtained with Rancimat apparatus. The extract of unripe Maclura-fruits shows a very strong antioxidative activity, which is comparable with pure BHA and extends inductive period at applied substrates many times. The addition of citric and malic acid shows synergetic effect. Additionally, the experiments were made with application of pulver of unripe Maclura-fruits. The use of 0.1 -0.2% of this pulver at applied substrates (tallow, lard, heated to 60⁰Cj induces a very high prolongation of induction period, similar as by 0.01 % BHA. The measurements with Rancimat apparatus showed the same relations of prolongation of induction period of substrates for application of extract or pulver of Maclura-fruits.     

Relationship between rancimat and active oxygen method values at varying temperatures for several oils and fats

Determination of oxidative stability of different edible oils, fats, and typical fat products was made using the Rancimat method and the active oxygen method. Induction periods (IP) were recorded under controlled conditions at 110, 120, and 130 ± 0.1°C for all products and over a range of 100–160°C for selected fats. A general oil stability evaluation industrial shortenings and vanaspati to be the most stable fats, with IP ranging from 10.00 to 15.47 h. Margarine and butter samples (IP, 4.98–6.04 h) were also found to show fair oxidative stability. Among the extracted and open-market salad-grade cooking oils, rapeseed oil (IP, 4.10 h) and soybean oil (IP, 4.00 h) showed the highest oxidative stability, whereas Salicornia bigelovii oil (IP, 1.40 h) was the least stable. The induction periods of typical fat products ranged from 2.59 to 9.20 h. CV for four determinations were <5.2% for shortening and vanaspati products and <4.3% for various vegetable oils, margarine, butter, and typical fat products. Rancimat IP values obtained at 110, 120, and 130°C were 40–46, 20–25, and 9–13% of active oxygen method values, respectively, corresponding to a decrease in Rancimat IP by a factor of 1.99 with each 10°C increase in temperature. Similarly, in the temperature range 100–160°C, an increase of 10°C decreased the Rancimat IP by a factor of 1.99     

Oxidative Stability of Lipids by Means of EPR Spectroscopy and Chemiluminescence

The aim of this study was to investigate the oxidation of selected vegetable oils (CLSO, CCSO, CBO) at accelerated oxidation rates. Several seldom used analytical methods were applied including electron paramagnetic resonance (EPR), spin trapping with α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN) and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), Fe²⁺-induced chemiluminescence, Rancimat tests, and the determination of conjugated diens at λ = 233 nm. The antioxidative properties of POBN and DMPO were also investigated. The time required for each method was determined. EPR spectrometry of trapped radicals generated during oxidation turned out to be the fastest method to determine oxidative stability. Chemiluminometric determination of oxidation kinetics showed that POBN has a very strong anti-oxidative potential: it significantly (by 160–277%) lengthened the time to the chemiluminescence peak, as well as the induction time in the Rancimat test (by 110–140%). Photo-oxidation studies showed that superoxide anion radicals are the main factor responsible for the oxidation of lipids in the investigated oils.     

Utilization of high‐oleic rapeseed oil for deep‐fat frying of French fries compared to other commonly used edible oils

Changes in chemical, physical and sensory parameters of high‐oleic rapeseed oil (HORO) (NATREON?) during 72 h of deep‐fat frying of potatoes were compared with those of commonly used frying oils, palm olein (PO), high‐oleic sunflower oil (HOSO) and partially hydrogenated rapeseed oil (PHRO). In addition to the sensory evaluation of the oils and the potatoes, the content of polar compounds, oligomer triacylglycerols and free fatty acids, the oxidative stability by Rancimat, the smoke point and the anisidine value were determined. French fries obtained with HORO, PO and HOSO were still suitable for human consumption after 66 h of deep‐fat frying, while French fries fried in PHRO were inedible after 30 h. During the frying period, none of the oils exceeded the limit for the amount of polar compounds, oligomer triacylglycerols and free fatty acids recommended by the German Society of Fat Science (DGF) as criteria for rejection of used frying oils. After 72 h, the smoke point of all oils was below 150 °C, and the amount of tocopherols was reduced to 5 mg/100 g for PHRO and 15 mg/100 g for HORO and HOSO. Remarkable was the decrease of the oxidative stability of HOSO measured by Rancimat. During frying, the oxidative stability of this oil was reduced from 32 h for the fresh oil to below 1 h after 72 h of frying. Only HORO showed still an oxidative stability of more than 2 h. From the results, it can be concluded that the use of HORO for deep‐fat frying is comparable to other commonly used oils.