Samples of vegetable oils were oxidized at isothermal conditions in the cell of a Pressure Differential Scanning Calorimeter (PDSC) and in the Methrom Rancimat apparatus. The PDSC and Rancimat experiments were carried out at a temperature in the range of 90-160 7C. From resulting PDSC exotherms their times to reach the peak maximum (t PDSC ) were determined and used for the assessment of the oxidative stability of the samples. Similarly the measured Rancimat induction times (t Rancimat ) were used. As PDSC and Rancimat results were obtained at different temperatures the equations for temperature extrapolations of the t PDSC and t Rancimat values have been proposed. Using the Arrhenius type correlation between induction times and temperature and activated complex theory the reaction rate constants, activation energies, activation enthalpies and activation entropies for oils oxidation have been calculated.

Four samples of olive oil were oxidized under polythermal (dynamic) conditions in the cell of a normalpressure differential scanning calorimeter (DSC) and in the Metrohm Rancimat apparatus. The DSC experiments were carried out in an oxygen flow atmosphere using different linearly programmed heating rates in the range of 4-20 7C/min. Through DSC exotherms, the extrapolated onset temperatures were determined and used for the assessment of the thermal-oxidative stabilities of the samples. Using the Ozawa-Flynn-Wall method and the Arrhenius equation, the activation energies (E a ), pre-exponential factors (Z) and reaction rate constants (k) for oil oxidation under DSC conditions were calculated. The Rancimat measurements of oxidation induction times were carried out under isothermal conditions in an air atmosphere at temperatures from 100 to 140 7C with intervals of 10 7C. Using the Arrhenius-type correlation between the inverse of the induction times and the absolute temperature of the measurements, E a , Z, and k for oil oxidation under Rancimat conditions were calculated. The primary kinetic parameters derived from both methods were qualitatively consistent and they help to evaluate the oxidative stabilities of oils at increased temperatures.

A mixture of beef tallow and rapeseed oil (1:1, wt/wt) was interesterified using sodium methoxide or immobilized lipases from Rhizomucor miehei (Lipozyme IM) and Candida antarctica (Novozym 435) as catalysts. Chemical interesterifications were carried out at 60 and 90 7C for 0.5 and 1.5 h using 0.4, 0.6 and 1.0 wt-% CH 3 ONa. Enzymatic interesterifications were carried out at 60 7C for 8 h with Lipozyme IM or at 80 7C for 4 h with Novozym 435. The biocatalyst doses were kept constant (8 wt-%), but the water content was varied from 2 to 10 wt-%. The starting mixture and the interesterified products were separated by column chromatography into a pure triacylglycerol fraction and a nontriacylglycerol fraction, which contained free fatty acids, mono-, and diacylglycerols. It was found that the concentration of free fatty acids and partial acylglycerols increased after interesterification. The slip melting points and solid fat contents of the triacylglycerol fractions isolated from interesterified fats were lower compared with the nonesterified blends. The sn-2 and sn-1,3 distribution of fatty acids in the TAG fractions before and after interesterification were determined. These distributions were random after chemical interesterification and near random when Novozym 435 was used. When Lipozyme IM was used, the fatty acid composition at the sn-2 position remained practically unchanged, compared with the starting blend. The interesterified fats and isolated triacylglycerols had reduced oxidative stabilities, as assessed by Rancimat induction times. Addition of 0.02% BHA and BHT to the interesterified fats improved their stabilities.

European annual production of beef tallow is about 1.42 million tonnes (1) and its annual production in Poland is about 0.12 million tonnes. Because of high melting temperature and low level of polyunsaturated fatty acids beef tallow is considered as less valuable fat not suitable for direct consumption. Although some minor quantities of beef tallow are used as components of frying fats or shortenings for edible purposes tallow should be modified. One of the possible ways of tallow modification is its interesterification with vegetable oils. The incorporation of unsaturated fatty acids from vegetable oils into triacylglycerols of tallow can improve their nutritional and physical properties. The selection of vegetable oil depends on the regional production but in Poland only rapeseed oil can be taken into account, as its annual production is about 0.35 million tonnes (2).There are several published papers dealing with interesterification of beef tallow with vegetable oils. Chobanov and Chobanova (3) performed chemical 479 JOS

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