The Question of High- or Low-Temperature Glass Transition in Frozen Fish. Construction of the Supplemented State Diagram for Tuna Muscle by Differential Scanning Calorimetry

The thermal behavior of fresh tuna muscle, rehydrated freeze-dried tuna muscle, and tuna sarcoplasmic protein fraction was studied by three types of differential scanning calorimetry (DSC): conventional DSC, alternating DSC, and sensitive micro-DSC. The relationship between glass transition temperature, Tg, and water content was established. Only a low-temperature glass transition was detected for fresh tuna and freeze-dried tuna rehydrated to high water contents, whereas for sarcoplasmic protein fraction both a low-temperature and an apparent high-temperature glass transition were detected for samples of high water content. Construction of the supplemented state diagrams for whole tuna muscle and for tuna sarcoplasmic protein fraction confirmed the low-temperature transition to be glass transition of the maximally freeze-dehydrated phase. The apparent upper transition of sarcoplasmic protein fraction was shown not to be a glass transition but rather to originate from the onset of melting of ice, and the temperature of this event should be denoted Tm‘. The glass transition temperature and the concentration of the maximally freeze dehydrated tuna muscle are −74 °C and 79% (w/w), respectively. Keywords: Glass transition; differential scanning calorimetry; state diagram; glass relaxation; ice melting; frozen fish