Effects of Freezing Temperature and Water Activity on Microstructure, Color, and Protein Conformation of Freeze-Dried Bluefin Tuna (Thunnus orientalis)

Freeze-drying of muscle foods including bluefin tuna (Thunnus orientalis) effectively restricted microbial growth and prolonged shelf life; however, microstructures and protein conformation changes possibly take place which contribute to functional and quality loss. This study aimed to investigate the effects of freezing and water sorption on microstructure, color, and conformation changes of protein in freeze-dried tuna. Tuna were frozen at various freezing temperatures prior to freeze-drying and stored at different awat 25 °C. X-ray computed tomography revealed microstructures of freeze-dried solids and reflected ice formation in parallel to the muscle fiber. Higher temperature freezing gave a slower nucleation and subsequent larger ice crystals formed, which enhanced aggregation of muscle fiber resulting in a thicker size but less integrity of fibrous structures. Water sorption induced structural changes of solids stored at above Tgin concurrent with the acceleration of browning which attributed to the increased molecular mobility. Attenuated total reflectance Fourier transform infrared spectroscopy indicated that freeze-dried tuna undergoing all freezing temperatures revealed a significant frequency shift and broadening of amide I band mainly associated with stretching vibrations of the C = O bond as awincreased. The increased intermolecular interaction via hydrogen bonding was found in high awsystems. The results indicated that a faster freezing increased protein stability in freeze-dried tuna but accelerated browning induced by water sorption which increased molecular mobility of solids above Tg.