الملخص الإنجليزي
In this study the characteristics of pores in dried tuna (Thunnus tongol) processed by air-, vacuum-, and freeze-drying methods were measured by mercury porosimetry, Apparent, substance and true density, porosity and pore size distributions of dried samples were measured to study the pores formed during the drying processes. The apparent density of fresh (before drying) tuna flesh was 1098 kg/m°, while the density of dried tuna by air-drying, vacuum drying, and freeze-drying were 960, 709 and 317 kg/m2, respectively. Porosity of freeze-dried sample was much higher than as I
the air-dried and vacuum-dried samples. Pores in different samples were
characterized as total intruded volume, total surface area, pore size range and average diameter, and nature of the pore size distribution curves.Peroxide value and color of dried tuna meat were also measured.
The state diagram of tuna meat was developed by measuring and modeling its glass transition temperatures and freezing points. Fresh tuna meat was dried in a freeze drier to vary the moisture content from 73.3 to 6.0% (wet basis). The glass transition temperature used in state diagram is taken from the literature. Cooling curve method was used to measure the freezing point and end point of freezing. The state diagram yielded maximally freeze-concentrated solutes at 61% solids with the characteristic temperature of glass formation being –54.2°C. The freezing curve and e-CO glass transition lines were developed by using Clausius-Clapeyron equation adjusted with un-freezable water and Gordon-Taylor models, respectively.
Water adsorption isotherms of freeze-dried tuna were measured at 0, 20, 40 and 60° C by using isopiestic method. Water sorption modeling was carried out by using Brunauer-Emmett-Teller (BET), Guggenheim-Anderson-DeBoer (GAB), and ZU Norrish models. BET and GAB monolayer moisture contents were varied from 4.46 to 6.10% and 4.34 to 9.44% (dry basis) respectively, when temperature varied from 60 to 0°C. Net isosteric heat of sorption, determined from the Clausius-Clapeyron
equation was found to vary from 4.491 to -0.016 kJ/g mol when moisture content was changed from 5% to 90% (dry basis).
