Abstract
The Nile tilapia (Oreochromis niloticus) fillets were frozen with static air (SA) at -20 °C, air blast freezing (AF) at -25 °C, combined method (CM - immersion in liquid nitrogen followed by AF) and cryogenic freezing (CF - immersion in liquid nitrogen). The effects of different freezing methods and of storage time at -18 °C (0, 30, 60 and 90 days) on fillet quality were evaluated. The temperature records were obtained, and the freezing times were determined by monitoring the temperatures of the geometric centers of the fillets. For CF, CM, AF and SA, the freezing times were 5.32, 127, 146 and 268 min, respectively. The quality of the frozen fillets was evaluated by measuring pH, volatile nitrogenous bases (TBV-N), thiobarbituric acid reactive substances (TBARS), and fatty acids and by obtaining a texture profile and performing scanning electron microscopy (SEM). The samples frozen by CF, after storage, had the highest levels of TBV-N because the high freezing rate produced superficial cracks that negatively affected the quality of the fillets. Freezing the samples with SA produced the worst quality indices and the microstructure was visibly damaged, with greater spaces between the fibers left by ice crystals. For all parameters evaluated, freezing with AF and CM resulted in fast freezing rates and better-quality tilapia fillets for 90 days of storage.
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