Antioxidant effect of hydroalcoholic extract of peppermint on chemical changes and sensory properties of rainbow trout fillet (Oncorhynchus mykiss) at refrigerated temperature

In recent years, the consumption of fish and seafood has increased and the demand for aquatic products is increasing due to population growth, increasing income and also the preference of fish and aquatic products over other foods. On the other hand, fish fats are very sensitive to oxidative damage due to having a significant amount of fatty acids with several double bonds and spoil faster than other meat foods (Rathod et al., 2021). Free radicals are the most important oxidizing agents of foods (which with a destructive process causes the loss of nutritional value and changes in their chemical composition). In addition to the undesirable sensory effects on food products, by eliminating vitamins and essential fatty acids from the body and creating toxic compounds, they can lead to undesirable effects such as heart and brain disease, cancer, immunodeficiency and aging in the human body. (Henry & Heppell, 2002). Adding antioxidants to foods is one of the most effective ways to slow down the oxidation of fats. Today, the use of natural antioxidants is used as a substitute for synthetic antioxidants (Shahidi & Ambigaipalan, 2017). Many studies have shown the presence of bioactive compounds with antioxidant properties, especially polyphenolic compounds and vitamins in foods of plant origin, is an effective factor in maintaining human health (Fraga et al., 2010). Peppermint with the scientific name of Mentha Spicata is a strong and durable plant that sometimes reaches a height of one meter, has various uses in traditional medicine and is used as a stomach tonic, analgesic, anticonvulsant, and nerve relaxant (Naidu et al ., 2012). Due to the possible effect of peppermint extract on the shelf life and quality of rainbow trout fillet (Oncorhynchus mykiss) at refrigerator temperature (41°C) due to antimicrobial and antioxidant properties of this extract and the lack of similar studies on the effect of peppermint extract on quality Fillets were considered during storage at refrigerator temperature.

For this purpose, to prepare the extract, 500 ml of 45% ethanol was mixed with 10 g of dry plant powder and placed in a shaker incubator at 35°C for 48 hours. After 24 hours, the extracts were filtered with Whatman No. 1 filter paper. The ethanolic extract was concentrated by vacuum rotary evaporator at 40°C and then powdered with complete freeze-drying to completely remove the solvent. To produce the research treatments, the fillets were divided into 3 groups. One group of fillets as a control sample (rinsing with distilled water), and the other two groups as experimental treatments, with concentrations of 1000 and 1500 ppm peppermint extract were immersed for 10 minutes (Farjami & Hosseini, 2015). The samples were then packaged and labeled in ordinary cellophane packages and stored at 41°C for 9 days. On days 1, 3, 5, 7 and 9, chemical tests (peroxide index (PV), thiobarbituric acid index (TBA), total volatile nitrogen base (TVB-N), Free fat acids (FFA), pH) and sensory properties were performed on the samples with three replications.

Results of this study showed that the amount of TBA in the control sample increased significantly during storage (p<0.05). The highest amount of TBA in the control sample was on the ninth day (2.04 mg MDA/Kg) which was significantly higher than the 1000 and 1500 ppm treatments of peppermint extract (p<0.05). The lowest level of TBA on day 0 in 1500 ppm treatment was equal to 0.24 mg MDA/Kg, which showed a significant difference with 1000 ppm treatment and control sample (p<0.05). The amount of TBA in 1500 ppm treatment on different storage days with increasing period of time was significantly less than 1000 ppm treatment and control sample (p<0.05). The amount of PV in the control sample and the concentrations of 1000 and 1500 ppm of peppermint extract increased significantly during storage (p<0.05). In samples treated with peppermint extract, the highest amount of PV in 1000 ppm treatment was equal to 3.82 meq/kg of fat on day 9 of storage, which was significantly less than the control sample and more the treatment was 1500 ppm (p<0.05). With increasing storage time, the amount of PV in 1500 ppm treatment on different days was significantly less than 100 ppm treatment and control sample (p<0.05). In the samples treated with peppermint extract, the highest pH value was recorded in the 1500 ppm treatment equal to 6.88 on day 5 of storage, which was significantly higher than the 1000 ppm treatment and the control sample (p<0.05). The lowest pH on day 9 in the control sample was 6.45, which was lower than the values of the treatments. The highest amount of TVB-N was observed on day 0 in the control group. On day 3, no significant difference was observed between the groups (P<0.05). On day 5, a significant difference was observed between the 1500 ppm treatment of peppermint extract with the other two groups (P<0.05), so that this treatment showed the lowest TVB-N. Comparing the levels of free fatty acids (FFA) between the two treatments containing extracts and the control sample, the control sample from the 5th day of storage period onwards showed a significant difference compared to the treatments containing the extract (p<0.05) and increased further. However, the two treatments of peppermint extract had less increase until 9 days of storage and only on the first day there was a significant difference between the treatments and the control sample (p<0.05). At the end of the storage period, the lowest amount of free fatty acids was related to the treatment of mint 1500 ppm. The results of the process of sensory changes of different treatments during 9 days of storage at refrigerator temperature showed that in both treatments and the control sample, it decreased over time, but in the control sample in terms of sensory properties as in other indicators, compared with fillets treated with peppermint extract, the quality decline was faster.

The results of chemical analysis and sensory evaluation of rainbow trout treated with natural peppermint extracts during the 9-day storage period showed; in general, peppermint extract slows down the increasing process of measured parameters and increases the shelf life of fish in the refrigerator. However, in the case of treatments containing plant extracts, despite the effectiveness of both concentrations, but in the case of treatments containing a concentration of 1500 ppm, antioxidant and sensory indices showed a stronger effect and accelerated the process of fish spoilage. Therefore, peppermint extract can be used as a natural preservative to reduce the antioxidant process during storage of rainbow trout fillets.