Rancimat method

A widespread method for determining the induction period for autoxidation of oils and fats consists of passing a continuous stream of air through the heated sample and collecting the volatile acids evolved in a water trap, where they are determined on a real time basis. The time plot usually presents a flat appearance for a certain period and then takes off in an accelerated manner. This test is the basis of several national and international standards (e.g. AOCS Cd 12b-92—oil stability index" ISO 6886—accelerated oxidation test for oxidative stability of fats and oils ) and the design of the Rancimat equipment, where the end determination is based on conductivity measurements . In addition to oxidation stability as determined by the Rancimat method and POV, which negatively affects virgin olive oil stability, other nonstandard properties were proposed for better assessment of the quality of this oil, namely LC determination of Vitamin E (21), colorimetric determination of total polar phenols and UVD of total chlorophyll. ... 

An earlier study (Stavinoha and Howell, 2000) examined the effects of TBHQ and a-tocopherol on oxidative stability of SME from four different sources by non-isothermal P-DSC in static (zero air-purge) mode. P-DSC curves were analyzed by measuring the OT where P = 2000 kPa, initial temperature = 25 °C, and (S = 5°C/min. Results for two of the SME samples showed that addition of 2000 ppm a-tocopherol increased OT by -20 °C while addition of 2000 ppm TBHQ increased OT by -30 °C. Addition of the same concentration of a-tocopherol and TBHQ to the other two SME samples increased OT by -30 °C and -40 °C, respectively. Interpretation of these results suggested TBHQ was more effective at increasing relative resistance to oxidation of SME than a-tocopherol, a conclusion that was in accordance with those by Mittelbach and Schober (2003) for the isothermal Rancimat method. 

Figure 7.13 shows the oxidative stabiiity of virgin oiive oiis from diverse oiive varieties. Figure 7.14 exposes the exceiient correiation of the US method with the oonventionai Rancimat method, and the drastic reduction in operating time (from, for exampie, 129 h to 50 min [74]). 

Figure 7.14. Comparison of the results of the US-assisted and conventional Rancimat methods (Reproduced with permission of Eisevier, Ref [74].)...

According to these results, the relative antioxidant effectiveness of BHT increased with decreasing temperature, and it decreased for GP. The Rancimat method underrated the stability of those samples containing BHT, perhaps because of volatilization of antioxidants during Rancimat testing. The relative activity of... 

Laubli, M.W. P.A. Bruttel. Determination of the oxidative stability of fats and oils Comparison between the Active Oxygen Method (AOCS Cd 12-57) and the Rancimat Method. /. Am. Oil Chem. Soc. 1986, 63, 792-795. 

Antiphotooxidative effect of carotenoids on the induction lime of soybean oil as measured by the Rancimat method. The effect of 100 ppm or 200 ppm capsanthin, P-carotene, or lutein on the photooxidative stability of soybean oil containing 200 ppm chlorophyll was measured by the Rancimat method after 4 hours light exposure at 2S°C. Preliminary studies showed that the induction lime of purified soybean oil containing 100 or 200 ppm carotenoids without chlorophyll was slightly lower than that of the blank sample which contained no carotenoids (data not shown. Therefore, there is no anti-autoxidative effect of carotenoids on the soybean oil- Table I shows that as the concentration of the carotenoids increased from 100 to 200 ppm, the induction time as well as the anti-photooxidation index (API) increased. The induction time of soybean oil containing the carotenoids was longer than that of (he control sample which contained no carotenoid however, it was still shorter than that of the blank sample which contained no carotenoid and no chlorophyll. 

TMble I. EfTect of carotenoids on the induction time detected by the Rancimat method of soybean oil containing 200 ppm chlorophyll during light exposure for 4 hours. 

The Rancimat method has been used to measure the antioxidant activity of synthetic and natural antioxidants (2-/>2d) and has correlated well with oil stability measured by the Active Oxygen Method (27) and peroxide value measurement (28). Our study showed that using the Rancimat method to study the antiphotooxidative effect of carotenoids on the soybean oil was in agreement with the results using the headspace oxygen depletion method (16) and the peroxide value method (16,JT). 

The development of the detection methods led to the emergence of other techniques based on oxygen adsorption and formation of volatile oxidation products to assist oxidative stability analyses, which can be assessed by different methods, such as the Rancimat method, used to determine the oxidative induction time or the oxidative stability, which is defined as the time required to reach a detectable level of rancidity. Therefore, this method determines the oxidative stability by the degradation and release of volatiles. The results are based on the principle of conductometric detection and provide a faster and accurate analysis of the oxidative stability of oils [ 11 -14]. 

According to some studies on other oilseeds, the differences between oxidative stability and the peculiar temperature and airflow parameters of the Rancimat method can be evaluated. 

A close relationship between the resxxlts obtained by the Rancimat method was observed in the hydraxilic pressing and solid-liquid extractions. When the DSC time parameters were standardized, the results found for the oils extracted by hydraulic pressing showed a correlation between the oxidative stability methods applied of around 72%, with OSI values of 12.73 h for the Rancimat and 550 min for the DSC method. 

The induced oxidative values of the oils obtained by the solid-liquid extractions with organic solvents, such as petrolexxm ether, using the Rancimat method showed a correlation around 92.66% when compared to that obtained using the DSC method. 

However, the behavior of the oils extracted with hexane in terms of the induction time obtained by the DSC was 25% higher than that obtained by the Rancimat method. 

Induction times determined for neat fatty acid esters using the Rancimat method are also given in Tables 2.3 and 2.4. It may be noted that no unsaturated fatty ester has an induction time >3 h, which is the minimum specification in the ASTM biodiesel standard. This observation implies that antioxidant additives will almost always be necessary to attain the minimum oxidative stability specification in biodiesel standards. 

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