Antioxidant and antibacterial effects of sodium alginate coating enriched with thyme essential oil on rainbow trout fillets during refrigerated storage

Fish and fish products are highly perishable foods because of their high unsaturated fatty acids and high-quality protein contents. For this reason, preservatives are used in order to prevent or delay their spoilage. Sodium alginate is a natural antioxidant, and to increase its antibacterial properties one can add essential oils with antibacterial properties. The object of this study was to determine the antioxidant and antibacterial effects of sodium alginate enriched with thyme oil on rainbow trout fillets.

Solutions of sodium alginate (3%) and sodium alginate plus thyme oil (sodium alginate 3% + 0.5%, 1% and 1.5% thyme oil) were applied for coating the fish fillet samples. The control (with no coating) and the coated fish fillet samples were analyzed periodically (at 5-day intervals) to determine microbiological [(total viable (TVC) and psychrotrophic (PTC)) counts] and oxidation characteristics [(peroxide value (PV) and thiobarbituric. acid (TBA)] and free fatty acids (FFA) content.

As compared with the control samples, smaller changes were observed in the bacterial and oxidative indices in the samples treated with sodium alginate and sodium alginate plus thyme oil during storage. In the samples treated with sodium alginate enriched with 1% and 1.5% thyme essential oil, the magnitudes of change in PV, FFA, TVC, and PTC were less than in the other treated samples (p = 0.05), and there were no statistically significant differences between the 2 treatments (i.e., 1% and 1.5% thyme oil; p>0.05). The smallest change in TBA was seen in the sodium alginate sample enriched with 1.5% thyme oil (p<0.05).

The results indicate that the sodium alginate coating enriched with thyme oil has the potential to reduce bacterial growth and oxidation in rainbow trout fillets during refrigerated storage (4 ±10C).