- Gadoid fish, like Alaska pollock and Pacific whiting, have a high level of trimethylamine-N-oxide demethylase (TMAOase) that catalyzes the breakdown of trimethylamine-N-oxide (TMAO) to formaldehyde (FA) and dimethylamine (DMA) even during frozen storage. FA reacts with amino acid residues to promote covalent cross-links in the formation of inter- and intramolecular linkages between protein chains via covalent methylene bridges, resulting in textural toughening when frozen storage extends. Heading and gutting (H&G) or filleting, which are a common practice for seafood processing, often affects fish quality during refrigerated and/or frozen storage due to freeze- and FA-induced protein denaturation followed by aggregation.
The primary goal of the study presented in the dissertation was to investigate physicochemical and textural properties of frozen Alaska pollock and Pacific whiting fillets as affected by postharvest processing and storage conditions, and further to measure the relationship between TMAOase activity and protein aggregation.
In the first part (Chapter 3), TMAOase activity, FA content and physicochemical properties of frozen Pacific whiting fillet during frozen storage (-18 and -80°C) were investigated. Whole fish and H&G fish were stored under refrigeration for 0, 2, and 5 days and subsequently filleted and frozen. Frozen fillets were analyzed during 24 weeks of storage. TMAOase activity and FA content, which affect the textural quality of frozen fillet, were measured. FA induced by TMAOase increased at all treatments at -18°C as frozen storage extended to 24 weeks, but it was near zero at -80°C up to 12 weeks of storage. This study showed that textural quality and freshness were significantly lower when fillets were prepared from H&G fish than whole fish and stored at -18oC.
Based on the finding from the first part, the second part (Chapters 4 and 5) evaluated various TMAOase-related (TMAOase activity, FA, DMA, and TMAO contents), physicochemical and textural properties of frozen Alaska pollock subjected to different freezing/thawing (F/T) cycles (0-12). TMAOase, FA and DMA contents significantly increased in fillets prepared from H&G fish as F/T cycles repeated. Interestingly, FA contents in fillets and drip were significantly reduced at 12-F/T, indicating that FA was possibly used for protein denaturation and textural toughness. F/T, which is often used to mimic long-term frozen storage, likely induced the rapid FA production by accumulated TMAOase in fish muscle as ice crystal formation was maximized.
The last part of the study (Chapters 6 and 7) was to investigate the effect of various sodium phosphates [sodium tripolyphosphate (STPP)/tetrasodium pyrophosphate (TSPP) mixture, STPP, TSPP, trisodium pyrophosphate, sodium hexametaphosphate, and disodium phosphate anhydrous] on TMAOase- and freeze-induced protein denaturation followed by aggregation using 2 step approach (lab and pilot scale). For the lab-scale approach, using natural actomyosin (NAM) extracted from fresh Pacific whiting, the phosphate treatments were evaluated at various concentrations (0.1, 0.3, and 0.5%), with and without cryoprotectants (CP), at various (0, 3, and 9) freeze/thaw (F/T) cycles. The NAM mixture containing 0.5% STPP/TSPP, STPP, and TSPP along with CP exhibited low TMAOase activity, low FA production, high salt-soluble proteins (SSP) concentration, and high turbidity. Among the phosphate treatments, STPP appeared to be the most effective compound in retarding both FA- and freeze-induced protein denaturation and aggregation. NAM without CP were more rapidly denatured and aggregated than NAM treated with CP as F/T was repeated, resulting in a rapid increase in the degree of aggregation (DA). The DA, which was calculated using photographical image analysis, was correlated well with biochemical properties. For the application of phosphates in Alaska pollock surimi and fillet, both phosphate-injected fillets and phosphate-added surimi along with cryoprotectant (5% sugar and 4% sorbitol) were subjected to 0, 3, and 9 F/T cycles. The concentration of sodium phosphate retained in fillet was adjusted to 0.3% using poking (6 times), soaking (10 min), and draining (2 min). The effect of various sodium phosphates on physicochemical properties of fillet and surimi was different during repeated F/T. The differences observed might be attributed by the functionality of the various phosphates used including their chelation ability, pH, and protein interaction. As STPP and TSPP were more effective to control TMAOase activity of fillet and surimi than other phosphates, FA content, SSP extractability, drip/cook loss, and texture properties were also better controlled.
In summary, even though H&G treatment before refrigeration has been commonly used to prevent cross contamination of microbes and TMAOase within fish flesh, this research concluded H&G treatment was not a necessary or favorable processing step for Pacific whiting and Alaska pollock. For the best quality, Pacific whiting whole fish should be stored at refrigerator for 2 days or less prior to filleting if stored at -18°C for 12 weeks, and Alaska pollock whole fish should be filleted and frozen within 3 days postharvest if fillets are to be stored frozen for longer periods. Quality reductions of fish muscle proteins by freeze- and FA-induced denaturation could be effectively retarded by the addition of STPP and/or TSPP as they inhibit TMAOase activity and reduce ice crystallization.