Changes of Taste Substances in Frozen Tilapia (Oreochromis niloticus) Fillets during Freeze-thaw Cycles

In order to explore the quality changes of frozen aquatic products during distribution in the cold chain, tilapia (Oreochromis niloticus) fillets were repeatedly frozen and thawed for up to seven cycles. The differences in the basic nutrients in eight samples (i.e., one frozen-thaw sample and seven repeatedly frozen-thawed samples) were evaluated. Then, the differences in overall taste profile were determined by electronic tongue, and the contents of free amino acids, flavor nucleotides and taste inorganic ions were also measured. In addition, the changes of flavor substances were analyzed by sensory evaluation. The results showed that the overall flavor profile of tilapia fillets could be effectively distinguished during freeze-thaw cycles. With the increase in the number of freeze-thaw cycles, the contents of umami and sweet amino acids and nucleotides decreased, while the contents of bitter amino acids and nucleotides increased, which was consistent with the trend of sensory scores. The total content of free amino acids showed a decreasing trend, and decreased significantly after the fourth freeze-thaw cycle (P < 0.05). The content of Na⁺, which had an adjunctive effect on umami taste, was also significantly decreased. The above results showed that the overall taste profile was gradually deteriorated as the number of freeze-thaw cycles increased. The results of this study may provide a theoretical basis for further research on quality control of frozen aquatic products.