ABSTRACT

The air inlet area of an active indirect mode solar dryer and product size of cooking banana (Musa spp) were optimized in Michael Okpara University of Agriculture, Umudike, Nigeria for the purpose of achieving efficient drying of the product. Temperature and relative humidity data were taken on a two hourly interval, with use of a Extech Data logger. The air flow rate and drag force of the dryer was computed with knowledge of the air inlet area. The air inlet shapes were: square, rectangular, circular and triangular, while the product sizes were 4, 8, 12, 16 and 20 mm,  respectively. The experimental design was conditioned for the two independent variables at five levels using a Central Composition Rotatable Design (CRD) of Response Surface Methodology (RSM). A total of 52 dryers of different air inlet areas were developed based on the experimental design. The dryers were able to reduce the weight of the product from 4.53 to 1.57 kg, as moisture content reduced from 68.97 to 12.05 % (wet basis), within 9 to 26 hours of drying. The dryers were able to conserve between 26.67 to 37.84 % of the drying time for the products when compared to open sun drying. The drying rate of the product increased as air inlet area increased. The efficiency of the dryers ranged from 13.85 to 31.84%. The experiment was replicated three times with sun drying of the product as control.  Microbial analysis of the dried products revealed the presence of bacteria and fungi. Proximate composition of the dried samples revealed less than 10% of ash, protein, lipid and fibre in its crude state. There was strong presence of carbohydrate and high caloric value in the dried products. In modeling and optimizing the air inlet, the air inlet area and product size were considered as independent variables. The desired responses were: moisture content, air flow rate, drag force, drying rate and drying efficiency. Analysis of Variance (ANOVA) was used to evaluate the quality of fit of the model and significance of the model to the response 5 % probability of error. The optimum responses were obtained as 11.98 % (moisture content), 0.0275 m³/s (air flow rate), 0.0476 N (drag force), 4.85 g/hr (drying rate) and 34.51 % (drying efficiency). The optimal product size was obtained as 20mm, for all the responses while the optimal air inlet areas were obtained as 40 cm² (triangular), 100 cm² (square), 80 cm²  (rectangular), 100 cm² (square) and 100 cm² (square) for the respective responses. It was observed that the higher the product size and air inlet opening, the higher the volumetric air flow rate, drying rate and drying efficiency The predicted and experimental values for the responses showed low deviation, while validation of the model for the responses gave a deviation range of 1.30 to 7.35%. The models developed from the optimization process were efficient enough to predict the responses.

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