RESUMO
The aim of this study was to compare the oxidative stability of linseed oil using the pressure differential scanning calorimetry (PDSC) and Rancimat methods, and to determine the kinetic parameters of linseed oil oxidation. Five cold pressed linseed oils were oxidized at different temperatures under PDSC (90-140 °C) and Rancimat (70-140 °C) test conditions. The oxidative stability of the linseed oils was calculated based on induction times (PDSCτmax, Rancimat τon), the Arrhenius equation and activated complex theory, frequency factors (Z), the reaction rate coefficient (k) for all temperatures, activation energies (Ea), Q10 numbers, activation enthalpies (∆H++), and activation entropies (∆S++). The PDSC method was more convenient for the determination of the induction time of linseed oils than the Rancimat method. During oxidation measurement by Rancimat method, the linseed oil polymerized, which affected the measurements. The reaction rate coefficient increased with rising temperature during measurement by both methods. The activation energy values of linseed oil oxidation using the PDSC and Rancimat methods ranged from 93.14 to 94.53 and 74.03 to 77.76 kJ mol-1, respectively. The Q10 , ∆H++, and ∆S++ values for the analyzed linseed oils were between 2.11-2.13, 90.54-91.30 kJ mol-1, -33.20 to -30.90 J mol K-1 calculated by PDSC measurements, and 2.23-2.32, 71.03-74.76, -59.42 to -49.08 J mol K-1 by Rancimat measurements, respectively.
RESUMO
The effect of specific oil surface (SOS) during pan frying of rapeseed oil on its thermal stability and antioxidant capacity (AC) was evaluated. Rapeseed oils with different oil layer heights (OLH = 0.5, 1.0, 1.5, 2.0, and 2.5 cm) were heated on an electric frying pan coated with Teflon at 180 ± 10 °C until a selected end point of 25 % total polar compounds (TPC) was reached. The changes of chemical parameters of oil samples such as peroxide value, p-anisidine value, Totox value, free fatty acids, TPC and AC using the 2,2-diphenyl-1-picrylhydrazyl assay were determined. Irrespective of the applied methods, the highest changes in oil with OLH = 0.5 cm were observed. Heating in low OLH also led to the fastest time of TPC formation in rapeseed oil; the 0.5-cm layer reached 25 % TPC in a relatively short time (71.5 min) compared to the highest OLH = 2.5 cm (t = 315.1 min). The SOS and the rate of change in the heated oils decreased with increasing OLH. Crucial effects of SOS on physicochemical oil changes were observed. The present study demonstrated the protective effect of increasing the OLH on the quality of the heated rapeseed oils.
RESUMO
The aim of this work was to examine the quality of shortenings available on Polish market, produced home or imported. The quality of twelve 100% vegetable fats and lard was estimated. Both chemical (fatty acids composition, especially trans isomers content, acid value, peroxide value, anisidine value, Totox, iodine value and oxidative stability--Rancimat test) and physical (melting point, solid fat content--at temperatures from 5 to 50 degrees C) properties were measured. The fats were subject to sensoric examination. The parameters defining the freshness of examined fats and their shelf life for all examined samples were good and proved the good quality. Induction time (150 degrees C) for examined fats varied from 1,79 to 4,29h. Examined fats differed significantly in saturated fatty acids content (from 14,0 do 60,2%) and trans isomers (from 0,1 to 56,6%). Fats produced from palm oil are also present and there are fats with smaller trans fat acids content. Examined shortenings contained very small content of essential fatty acids (from 0,5 to 10,4), and they showed very different melting points (from 19,6 to 42,1 degrees C) and solid phase contents. In general the examined fats were of good sensoric value. Summing up the received results, it should be underlined that large disparity in the content of trans isomers in analysed samples was observed and definitely TFA content should be lowered.
