total food moisture shown in Fig. 3. Once wet bulb temperature
was attained, Cs exceeded the value of Cgb that,
depending on air temperature and its relative humidity
only, was assumed constant throughout all the process;
water evaporation, therefore, prevailed over vapour condensation
and heating rate decreases. Hence, while food
temperature slowly increased towards air temperature
value, food moisture content diminished at decreasing drying
rate. Fig. 3 showed that initially, when the process was
controlled by external heat transfer and the water leaving
the sample was not bounded to food structure (free water),
drying rate attained its maximum value. Afterwards, when
the process rate was controlled by mass transfer and the
water leaving the solid was bounded to the food structure,
a progressively decrease of drying curve slope was
observed. Figs. 4 and 5 gave a detailed microscopic description
of food moisture content profiles during the process.
They showed very clearly the above-mentioned variation
of drying rate; in fact at the beginning of the process, the
profiles appeared very well separated, whilst they tended
to overlap as soon as the drying rate decreased. It should
be noted that the asymmetry shown in Fig. 5 is to be
total food moisture shown in Fig. 3. Once wet bulb temperaturewas attained, Cs exceeded the value of Cgb that,depending on air temperature and its relative humidityonly, was assumed constant throughout all the process;water evaporation, therefore, prevailed over vapour condensationand heating rate decreases. Hence, while foodtemperature slowly increased towards air temperaturevalue, food moisture content diminished at decreasing dryingrate. Fig. 3 showed that initially, when the process wascontrolled by external heat transfer and the water leavingthe sample was not bounded to food structure (free water),drying rate attained its maximum value. Afterwards, whenthe process rate was controlled by mass transfer and thewater leaving the solid was bounded to the food structure,a progressively decrease of drying curve slope wasobserved. Figs. 4 and 5 gave a detailed microscopic descriptionof food moisture content profiles during the process.They showed very clearly the above-mentioned variationof drying rate; in fact at the beginning of the process, theprofiles appeared very well separated, whilst they tendedto overlap as soon as the drying rate decreased. It shouldbe noted that the asymmetry shown in Fig. 5 is to be
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