Investigation of the effects of cryoprotectants on the quality of fish subjected to multiple freeze-thaw cycles

In this study, different cryoprotectants and thawing methods have been used to prevent the loss of quality due to multiple freezing and thawing processes in bonito (Sarda sarda) and whiting (Merlangius merlangus) fillets which have high and low fat contents respectively. Saccharose-sorbitol used commercially, sodium tripolyphosphate and sodium alginate were used as cryoprotectant agents. After immersing, the samples were subjected to rapid freezing at -40 º C. The samples were stored for 14 days at -18°C and thawed at ambient conditions (18 ± 2 ºC, about 4,5 hours), under running water (12±1 ºC, 2 hours) and in the refrigerator (4± 1 º C, 14-15 hours) after that the fillets were left in the freezer (-18 ºC) again. The freezing and thawing of the fish fillets were repeated three times. According to the results of TMA-N and TVB-N of fish fillets, the use of cryoprotectant agent in the multiple freezing and thawing processes significantly affected the TMA-N and TVB-N values of the fillets. After the third thawing process, the TMA-N and TVB-N values of the fish fillets were below the limit values for consumption. Sodium tripolyphosphate was found to be effective in retarding formation of TVB-N and control groups had the highest TVB-N values. The increase in the pH value with the multiple freezing and thawing process was found and the highest pH values of both fish species were observed in the sodium tripolyphosphate treated groups after last thawing processes. Using sodium tripolyphosphate significantly affected the K value and K values in this group had lower than the other groups. The use of different cryoprotectants did not affect the amount of total soluble protein, whereas the thawing methods significantly affected the amount of soluble protein. Thawing the fillets at ambient conditions damaging muscle proteins decreased the amount of total soluble protein decreased much more during the multiple freezing and thawing process than other groups. Initial amount of total free amino acid of fresh bonito and whiting was 1527,62±5,77 mg/kg and 168,66±1,29 mg/kg respectively. When different cryoprotectants were used in multiple freezing and thawing process, the total free amino acid value was significantly affected. After the third thawing, this value was 1742,67 ± 4,175 mg / kg for bonito fillets treated with sodium tripolyphosphate and thawed under water, and was 458.48 ± 0.66 mg / kg in the control group thawed at ambient conditions for the whiting fillets. When the free amino acid profile was assessed, the content of hydrophobic free amino acids reached to the highest value in the bonito control samples treated with sodium tripolyphosphate and thawed both under running water and in the refrigerator. In the case of whiting, the control samples thawed in the refrigerator reached the highest hydrophobic amino acid value. The use of cryoprotectants affected the formation of agmatine and putrescine significantly. The amount of agmatine in the control group was higher than the other groups. All control samples reached higher agmatine content compared to the other groups. It was observed that suppressive effect of sodium tripolyphospate on formation of putrescine. Similar results were observed with the analysis of water holding capacity. Sodium tripolyphosphate had good effect on preventing the loss of water holding capacity whereas thawing at ambient conditions showed adversely effect on the water holding capacity. As the freezing and melting cycle increased, the fillet hardness increased and after the 3rd dissolution, the hardest samples were determined in the control samples. As the freezing and thawing cycle increased, the fillet hardness increased and after the third thawing, the hardest samples were determined in the control samples. The cryoprotectants affected the hardness of the samples and the sodium alginate was successful in preventing the hardening of the fillets. Adhesiveness increased with freeze-thaw cycles, whereas samples treated with saccharose-sorbitol and sodium alginate were less adhesive. The cohesion value has not changed in the freeze thaw cycle. It was found that control groups had higher gumminess and chewiness values among the other groups. Cryoprotectants affected both parameters significantly in the freeze-thaw cycle but no difference was found between cryoprotectants. Color measurements indicated that fish species, freezing and thawing cycles and thawing methods significantly affected L* value but cryoprotectant treatment did not affect L* value. It was determined that thawing in running water maintained the L* value during multiple freeze-thaw process. The a* value indicating the redness, which has higher values in bonito fish, decreased with freezing and thawing processes. The cryoprotectant treatment did not affect the a* value and the a* values of the fillets thawed in the refrigerator were lower than the other groups. The b* value increased with the freeze thaw cycle, while the highest values were found in the control group and sodium alginate treated group. The fillets thawed in the refrigerator had higher b* values the other groups. Sensory evaluations have shown that the appearance of fish fillets treated with sodium alginate, thawed under running water and in the refrigerator, was much more preferred by the panelists. Similarly, texture analysis of fish fillets treated with sodium alginate and saccharose-sorbitol yielded more successful results. The odor of fillets was neither affected by cryoprotectant treatment nor by thawing methods. The results showed that sodium tripolyphosphate and sodium alginate were effective in maintaining the initial quality of both fish species. After multiple freezing and thawing process, the quality of all groups was below the consumption limit however, found that the freeze-thaw cycle influenced some functional properties of the proteins by causing physicochemical changes in the fish muscle.