English abstract
Water adsorption isotherms of freeze-dried tuna were measured at 5, 22 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 <aw <0.45). The GAB equation predicted EMC with a mean relative error of 9.6% and 12.3% without and with temperature dependence of
parameters C and K (constants related to heat of sorption of mono and multi-layers),
respectively. The Norrish equation predicted the water activity with a mean relative
error of 10.6%. Modeling with BET equation predicted monolayer moisture content
(Mo,b) of 4.8 to 6.0% depending upon the temperature ranging from 5 to 50 °C, while
the GAB equation predicted monolayer moisture content (Mo,g) from 3.8 to 8.8% for
the same temperature range. Net isosteric heat of sorption, determined using Clausius
Clapeyron equation, varied from 13.5 to 0.9 kJ/mol when moisture content changed
from 4.8 to 33.5% (dry basis. (The D-values decreased from 2.52 to 0.26 hr for moist-heating and 2.57 to 0.34 hr for dry-heating, respectively when temperature was maintained constant within 60 to 140 °C. In both cases, increasing temperature caused significant decrease in D-values (p<0.05), whereas the effect of heating methods was not significant (p>0.05). Thus the heat resistance characteristics of endogenous microorganisms in fresh tuna mince was not dependent on the changing. medium moisture content during drying.
