Background: Due to high content of unsaturated fatty acids, trout is susceptible to oxidative spoilage. In this study, the effects of Thymus vulgaris Essential Oil (EO) and extract on oxidative stability of cooked rainbow trout fillet during four months frozen storage period were investigated.

Methods: Three groups of fish fillets were treated with thyme EO and three other groups were treated with thyme extract and cooked by frying, oven, and steam. Fat hydrolysis was evaluated by measuring Free Fatty Acid (FFA) value and oxidation products were measured via Peroxide Value (PV) as well as Thiobarbituric Acid (TBA) value. Sensory analysis was evaluated by the overall acceptability using a 9-point hedonic scale. Statistical analyses were performed in SPSS, Inc, Chicago, IL software.

Results: Main components of T. vulgaris were thymol (60.54%), α-terpinen (9.47%), p-cymene (8.54%) and carvacrol (3.33%). The amount of FFA in oven baked samples (4.51–4.75% oleic acid) and steamed fillets (4.83–5.20% oleic acid) was significantly (p<0.05) higher than control and fried fillets. PV values showed an increase in all groups, especially fried fillets with the highest amount of PV (p<0.05). TBA values in the treated groups were significantly lower than control fillets (p<0.05). At day 0, steamed samples containing EO and extract showed lower scores of overall acceptability (8.64±0.31 and 8.64±0.64, respectively) compared to fillets cooked by frying and oven. However, at the end of the four months storage period, both treated and control groups had the lowest sensory scores with no significant difference (p>0.05).

Conclusion: Both thyme EO and extract effectively retarded the oxidation during frozen storage. However, the samples treated with thyme extract showed slower formation of free fatty acid, hydroperoxide and malonaldehyde than those of EO-treated or control samples. It is recommended to apply thyme EO and extract in producing ready-to-eat fish products.