Introduction: Compared to red meat and many other animal protein sources, fish have a special place in human nutrition due to the unique composition of their lipid profile. This profile includes balanced and diverse amounts of saturated fatty acids (SFA), monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA), which are of high biological quality, especially in the case of omega-3 and omega-6. In fact, what distinguishes aquatic meat from other animal protein sources is not only the composition of fatty acids, but also the unique structure of their lipid profile. Two farmed fish species of significant economic and nutritional importance globally and regionally are rainbow trout (Oncorhynchus mykiss) and common carp (Cyprinus carpio). Trout, as a cold-water fish species, generally tend to have higher levels of marine omega-3 fatty acids (such as EPA and DHA), whereas carp, as a warm-water fish species, tend to store more plant-derived fatty acids (such as linolenic acid and linoleic acid). Despite the high nutritional value of fish meat, one of the main challenges in the processing and storage of these products is the occurrence of lipid oxidation, which by producing volatile compounds with an unpleasant odor, is one of the most important factors affecting the reduction of sensory and chemical quality and, as a result, the shelf life of marine products, and can lead to a decrease in consumer acceptance and a decrease in the market value of the product. In the meantime, the freezing method, as one of the common and effective strategies in the aquatic preservation industry, is used with the aim of reducing the speed of biochemical and microbial reactions. Preservation of fish at low temperatures and freezing conditions, while stopping the growth of most microorganisms, leads to a decrease in enzymatic and chemical activities and to some extent prevents the destruction of the protein and lipid structure of fish. The main objective of this study was to compare in detail the fatty acid profiles of two species, rainbow trout (Oncorhynchus mykiss) and common carp (Cyprinus carpio) before and after freezing at -18°C. This comparison was conducted to assess intrinsic species differences in lipid stability of both species.
Material and Methods: The studied species included farmed rainbow trout (Oncorhynchus mykiss) and common carp (Cyprinus carpio) from fish farms located in Savadkooh County, Mazandaran Province. Three pieces of them with an average weight of 250±1000 grams were transported to the laboratory in ice and inside ionolith boxes. After being transported to the laboratory, the fish were washed, gutted, and scaled in complete sanitary conditions. Then, they were divided into uniform pieces with an approximate weight of 60 grams. The sample pieces were stored in sterilized polyethylene packages and kept in a freezer at -18°C until preparation. After lipid extraction, the dried lipid fraction was subjected to transmethylation to prepare fatty acid methyl esters. In order to identify, determine, and measure fatty acids, the composition of fatty acids extracted from the samples was analyzed by GC-FID chromatography.
Results and Discussion: In the present study, the fatty acid composition of two farmed fish species, rainbow trout and common carp, was investigated during a 90-day storage period at -18°C. Identification and analysis of fatty acids were performed using GC-FID chromatography, and a total of 17 different fatty acids were identified, with monounsaturated fatty acids (MUFA) having the highest contribution (34.14% in trout and 39.83% in carp) and polyunsaturated fatty acids (PUFA) having the lowest contribution (27.72% in trout and 17.29% in carp) (Tenyang et al., 2019). Carp had the highest amount of saturated fatty acids (SFA) and MUFA, while trout had a significantly higher content of EPA, DHA and a better ratio of PUFA/SFA and omega-3 to omega-6 than carp (Öz, 2019). Also, during the storage period, the lowest rate of change against oxidation was related to saturated fatty acids. While a more significant decrease was observed in MUFA and especially in PUFA (Saberi et al., 2011). A significant decrease was also observed in the amount of omega-3 fatty acids, which is due to the high sensitivity of these fatty acids to oxidation. The results showed that freezing is an effective method in reducing the rate of fish fat degradation, but qualitative changes over time still exist. Also, the trout studied in this study had a higher consumption preference than common carp due to the presence of higher levels of unsaturated fatty acids before freezing and during storage at freezing temperature.
Conclusion: In this study, a significant difference in fatty acid composition was observed between the two species of rainbow trout and common carp; with trout containing higher levels of EPA and DHA fatty acids and a more favorable ratio of PUFA to SFA, which reinforces the preference for trout consumption from a nutritional perspective. Storage at -18°C resulted in a decrease in the content of saturated and unsaturated fatty acids, with the decrease being greater in polyunsaturated fatty acids than in other fatty acids. Freezing also proved to be an effective method in slowing down the rate of lipid degradation, but qualitative changes in PUFA and MUFA still persisted. Therefore, timely consumption and more appropriate storage, especially for trout, could lead to better utilization of the benefits of unsaturated fatty acids in fish. However, additional improvements such as antioxidants or oxidation management may be necessary to maximize the preservation of these fatty acids during long-term storage. |
