Lipophilic components of three herbs, sage, thyme, and rosemary, were extracted into refined rapeseed oil by continuous stirring at 30°C for 24 h. The oxidative and frying stability of the flavored oil was assessed by Rancimat at 120°C and frying of French fries at 175°C, respectively. In comparison to the control with an induction period of 4.1 h in the Rancimat test, the treatment with thyme, rosemary, and sage resulted in induction periods of 5.3, 9.3, and 11.0 h, respectively, corresponding to stabilization factors of 1.1, 2.0, and 2.4, respectively. In contrast to the oxidative stability at 120°C, treated oils exhibited significantly lower frying stability, compared to the control. For instance, whereas rapeseed oils treated with plant materials exceeded the 12% regulatory limit for oligomeric triacylglycerols within 20 h of frying, the limit was only exceeded after 25 h of frying in the control. However, despite the significantly higher level of thermo-oxidative degradation in the flavored oils, the sensory quality of the French fries prepared in these oils remained still acceptable within the frying time of 32 h whereas fries prepared in the control oil were judged unacceptable. Further, endogenous tocopherols were better protected in the treated oils.
Practical applications: The prolongation of the shelf life of frying oils is of great economical and commercial importance. Thus, food processors are very interested in the improvement of the thermal stability of frying oils, which may provide remarkable savings. On the other side more and more flavored oils come to the market. Therefore it is important to have more information about the oxidative and thermal stability of such oils.