In this study, the effects of rosemary (Rosmarinus officinalis L.) plant (RP), its alcoholic extracts (REE and RME) and essential oil (REO) on the oxidative stability of hazelnut oil (HO) during deep frying were investigated. HOs containing rosemary additives had lower conjugated diene (CD) content, p-anisidine value (AV) and polar material content compared to control values. Additionally, changes in L*, a*, and b* color values and viscosity were lower than control values. RP (50 g/kg) caused the highest retention of a-tocopherol. At the end of the frying period, the induction periods (IPs) of HOs containing rosemary additives were between 9.43 and 35.01 min, much higher than the control IP (4.28 min). The ratios of C18:1/C16:0 and C18:2/C16:0 were 7.6 and 1.2, respectively, in HOs containing 50 g/kg RP, values which were higher than those of the control. The study indicated that HO containing 50 g/kg RP had the best frying performance. Rosemary additives had no adverse effects on sensory acceptability according to results of a sensorial analysis of French fries. The rosemary additives also had no significant effect in terms of texture and color of French fries (P > 0.05). Consequently, this study shows that rosemary plant or extracts can be used to extend the usage life of hazelnut oil for frying.Practical applications: Rosemary is an herb with a high antioxidant activity due to its phenolic compound content. After grinding, it can be directly added to frying oils or added as a solvent extract. In direct application, which does not require the use of any additional chemicals, it increases the stability of the frying oil when it is used in an appropriate amount.

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