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

Abstract

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, T_g, 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 T_m‘. 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