Water adsorption isotherms of freeze-dried tuna meat

Water adsorption isotherms of freeze-dried tuna were measured at 5°C, 22°C, and 50°C using an isopiestic method. Water sorption modeling was carried out using Brunauer-Emmett-Teller (BET), Guggenheim-Anderson-deBoer (GAB), and Norrish equations. The BET equation predicted equilibrium moisture content (EMC) with a mean relative error of 9.6% (0.113 < a_w < 0.45). The GAB equation predicted EMC with a mean relative error of 9.6% and 12.3% without and with temperature dependence of C and K terms (constants related to heat of sorption of mono- and multilayers), respectively. The Norrish equation predicted the water activity with a mean relative error of 10.6%. Modeling with the BET equation predicted monolayer moisture content (M_o,b) of 4.8% to 6.0% depending upon the temperature ranging from 5°C to 50°C, while the GAB equation predicted monolayer moisture Content (M_o,g) from 3.8% to 8.8% for the same temperature range. Net isosteric heat of sorption, determined using the Clausius-Clapeyron equation, varied from 13.5 to 0.9 kJ/mol when moisture content changed from 4.8% to 33.5% dry basis (d.b).