Development And Validation Of A Drying Model For Rastrineobola Argentea Fish In An Indirect Forced Convection Solar Dryer

ABSTRACT

Rastrineobola argentea, (R. argentea) is a commercially important fish species in Lake Victoria,

with a characteristically smaller size compared to the other fish and high surface moisture

content. However the prevalent preservation method of the fish is open sun drying, in which the

fish is laid uncovered on the ground for a prolonged period exposed to; contamination, attack by

bacteria, molds, rodents, birds and adverse weather conditions. Often huge losses estimated at

between 20 % and 50 % occur especially during the rainy season, because of the slow drying

process that lasts several hours or even days depending on the prevailing weather. Although

studies have been conducted on solar drying of R. argentea fish, none has been reported on

modeling of its drying process. Moreover existing drying models of food products have been

formulated based on internal moisture diffusion as the sole mass transfer mechanism, which is a

falling rate period phenomenon. Studies have however shown that high moisture products exhibit

both constant and falling rate drying periods, and therefore a linear section in their drying curves,

that is not accounted for by the diffusion models. Thus in the present study, a drying model

based on evaporation was formulated for R. argentea fish in an indirect forced convection solar

dryer. The model results were compared with experimental data obtained from a prototype solar

dryer and the tests revealed a reduction in moisture content of the fish from 73 % (w. b.) to

between 8 % and 10 % (w. b.) after a period of 11 hours in the solar dryer whereas the open sun

samples took 18 hours to reach the same moisture levels. Effective moisture diffusivities of

(1.65 0.02) 10 m / s 3 2   and (0.91 0.05) 10 m / s 3 2   were obtained for the fish in the solar

dryer and open sun, respectively. The activation energy of the drying process of R. argentea fish

was also found to be: 15.954kJ /mol , a much lower value than for other similar high moisture

food products thus indicating that the energy threshold for drying the product that can be

provided by the available solar energy. Statistical analysis carried out based on the moisture ratio

of the fish in the solar dryer gave: 0.946, 2 R  RMSE 0.0383, and 0.839, 2 R 

RMSE 0.0533, for the top and bottom trays, thus confirming the prediction reliability of the

model. The simulations of the model further revealed that the drying time of this product can be

reduced to 5 hours (the period after which deterioration of the product occurs if adequate drying

has not occurred) in the solar dryer by increasing the air mass flow rate to 0.06 kg / s