State diagram of freeze-dried sardines (Sardinella longiceps, Valenciennes)

Abstract

State diagram is used to determine the stability of dried and frozen foods during their processing and storage. In this study, state diagram of sardine was developed by measuring the freezing curve, glass line, solids melting line, and maximal-freeze-concentration conditions. Glass transition temperature decreased with the increase in water content and it was adjusted using a modified Gordon-Taylor equation. The glass transition of dry-solids and critical temperature were 184.1 and 12.5oC, and the model parameter of the modified Gordon-Taylor equation was estimated as 4.2, respectively. Solids melting temperature was modeled by Flory-Huggins equation; and dry-solids melting temperature and Flory-Huggins solids-water interaction parameter were determined as 199.4oC and 0.956, respectively. Freezing point of fresh sardine was observed as −1.3oC with ice melting enthalpy of 186.3 kJ/kg, and it decreased with the increase of solids due to the freezing point depression with solutes. Freezing point was adjusted by Chen equation based on Clausius-Clapeyron equation. The ultimate maximal-freeze-concentration temperatures, (Tm′)u and (Tg′′′)u were determined as −18.6 and −25.1oC, respectively and maximal-freeze-concentration solids were estimated as 0.90 g solids/g sample (i.e. un-freezable water 0.10 g water/g sample). This indicated that frozen sardines could be most stable if stored below −25.1oC, and dried fish stored below its glass transition.