Consumption of processed and frozen fish has increased in recent years as a result of an increasing consumer demand for convenient high-quality food products. Fish lipids are a natural and good source of polyunsaturated fatty acids (PUFA) which have been reported to have several beneficial health effects. However, due to the high amount of PUFA, fish lipids are highly susceptible to lipid degradation.
Freezing and frozen storage is an effective method of preserving physicochemical properties and to prolong shelf life of fish products. However, some deterioration in fish quality occurs during frozen storage. Therefore, a project was executed to gain more understanding of different oxidative processes taking place in frozen fish products, and to investigate how two lean fish species with similar type of commercial utilization, differ in oxidative stability during prolonged frozen storage. The effects of different storage temperature and initial raw material quality on oxidative stability were studied, as well as the impact of cooking and subsequent cooked frozen storage on the lipid quality. Furthermore, the applicability of various chemical lipid quality markers and alternative non-invasive approaches to monitor fish lipid degradation was evaluated.
The studies performed provided deeper understanding on different oxidative mechanisms and stability of frozen fish products as influenced by different storage conditions and material diversity. Storage temperature and time proved to be very important factors with regard to oxidative stability of frozen fish products since more progressive lipid autoxidation, loss of essential fatty acids and enzymatic lipid hydrolysis of the fish was observed at higher storage temperatures. Oxidative stability of raw fish during frozen storage was also highly species- and muscle-type dependent due to variations in chemical properties, e.g. lipid content and profile, ratio of phospholipids and amount of heme-iron present.
The results demonstrated that even though white lean fish species are from the same order, their storage life during frozen storage can differ considerably. Furthermore, prolonged storage of raw material before cooking was the most important factor with regard to lipid stability after cooking. The results showed that cooking and subsequently frozen storage of the cooked products affected the oxidative mechanisms of the fish muscles. Cooking generally inhibited enzymatic lipid hydrolysis and simultaneously induced the formation of primary (PV) and secondary (TBARS) lipid autoxidation products. This ‘cooking effect’ was more expressed when the fish was cooked after long raw storage periods, denoting that important physicochemical changes are occurring in raw muscles stored for prolonged periods.
The results demonstrated the applicability of chemical analytical methods to monitor lipid degradation of frozen and cooked fish products. There are several methods that exist for the analysis of lipid degradation in aquatic food products, among which formation of oxidation products are the most commonly used. The results indicated that both tertiary lipid oxidation and hydrolysis products are appropriate for assessing lipid deterioration of lean fish during frozen storage, but they were, however, not useful to evaluate lipid deterioration of cooked fish products. It is of great importance for the seafood industry to have a rapid and robust method to monitor quality degradation. The results showed that NIR spectroscopy and colorimetric analysis have the potential to be non-invasive tools to follow quality deterioration of the frozen fish muscle.
For additional information, please contact dr. Magnea Guðrún Karlsdóttir at Matís.