temperatures
of 50, 60, 80, 100 and 120 1C. The amount of cooked
rice of 0.8 kg was used for each drying test. Eighty percent
of exhaust air was recycled and then mixed with fresh air
before passing it through heating unit (no. 2) and drying
chamber.
The samples were weighted every 30 min. The drying
experiments were performed until the moisture content of
cooked rice reached the desire moisture contents of 7 and
10 g/100 g dry basis.
Drying rate curves of the cooked rice at different
temperatures are shown in Fig. 4. It is clearly indicated
that the constant drying period for cooked rice was not
found although the initial moisture content was very high.
The decrease of moisture content of cooked rice took place
in the falling rate period. The acceleration of drying at this
drying period could be accomplished by increasing
temperature. In addition, using high drying temperature
could improve the drying efficiency because the increase of
drying rate was relatively higher than the temperature
increase, especially at the range of high moisture content.
For example, the increase of temperature from 50 to 100 1C
resulted in the increase of drying rate from 1.0 kg water
evaporated/kg dry matter to 3.7 kg water evaporated/kg
dry matter (at 200 g/100 g dry basis.). However, the
temperature effect becomes less significant as the moisture
content reduced to low level. At the moisture content of
cooked rice below 30 g/100 g dry basis, the drying rates
were insignificantly different amongst high-and low-temperature
dryings, suggesting that low-temperature drying is
applicable to the low moisture range.