Glass transition properties of frozen and freeze-dried surimi products: Effects of sugar and moisture on the glass transition temperature

Frozen surimi mixtures containing four types of sugar (sorbitol, glucose, sucrose, and trehalose) and freeze-dried surimi–trehalose mixtures were made from live carp, and their glass transition properties were investigated by differential scanning calorimetry. For comparison, a commercial frozen cod surimi was also investigated. The frozen carp surimi samples showed two freeze-concentrated glass transitions, at −42 to −65 °C and at −21 to −41 °C, depending on type and the sugar content of samples. These glass transition temperatures were denoted as Tg1′ and Tg2′, respectively. With increasing sugar content, the Tg1′ and Tg2′ of the samples increased and decreased, respectively. At each sugar level, the Tg1′ and Tg2′ of carp surimi–trehalose mixtures were higher than those of the other mixtures. From the sugar content effects on Tg1′ and Tg2′, the Tg1′ and Tg2′ of the carp surimi without sugar were estimated to be −55 to −65 °C and −15 °C, respectively. The commercial cod surimi showed Tg1′=-65°C and Tg2′=-22°C. These were reasonable values in comparison with those of carp surimi–sugar mixtures. From the result, it was suggested that the difference of fish stock did not greatly affect the Tg1′ and Tg2′ of surimi–sugar mixtures. The moisture content effect on the glass transition temperature of freeze-dried carp surimi–trehalose mixtures was investigated, and a state diagram of the surimi-moisture pseudo-binary system was developed. From the viewpoint of “glass transition” concept, it is expected that these results are useful in the production and storage of frozen and freeze-dried surimi products.