The Design and Evaluation of a Greenhouse Type Solar Dryer and Its Performance on Two Food Commodities: Cassava and Cocoa.

Abstract

Drying, a process for food preservation involves subjecting a food commodity to high temperatures that aid in the removal of moisture thereby making the food shelf stable. The objective of this study was to design a greenhouse type solar dryer that would be used for the drying of cassava and other foods and evaluate the characteristics of the dried products. The rationale behind this study was that open sun drying has been the method of drying over the centuries but it is subjected to a lot of challenges such as contamination from bird droppings and rodents, high moisture content due to inconsistent temperature variations which contributes to microbial contamination as a result of the slow or intermittent drying and no protection from rain or dew. The greenhouse type solar dryer with a total volume and surface area of 20.335 m3 and 35.774 m2 respectively was designed and constructed with locally available materials. Temperature and humidity were monitored in the empty greenhouse type solar dryer (GTSD) and open sun drying (OSD) for a period of 14 days showed that the GTSD recorded the highest temperature of 60oC occurring between 12:00 and 13:00 hours GMT. The highest relative humidity was recorded at 84 % and this was observed at a later time in the day, 18:00 hours GMT. Cassava slices (0.47cm thickness) and chunks (4 cm length × 2 cm breadth × 1.5 cm) and cocoa beans were dried. The total drying period for the two food commodities was 156 hours. Generally, there was moisture removal in all the dried samples. However the GTSD gave a faster drying rate of 3.361 g/g h for the slices occurring after 12 hours of initial drying than the OSD. An Analysis of variance performed on the moisture data delineating time of drying, drying method and sample size showed that the removal of moisture of the cassava samples is dependent on the time for drying, the drying method and sample size. The data observed for both moisture and drying rate of the cassava and cocoa samples depicted that drying does not occur at nightfall but rather moisture absorption due to higher humidity levels observed at night. To further determine the influence of drying on the samples, some laboratory analyses such as pasting property, water activity, colour profile and particle size determination were performed on the resultant dried cassava flour samples and bean cut test and free fatty acid determination on the cocoa beans. The flours emanating from the two drying systems generally had similar characteristics. The cassava slices under the GTSD demonstrated a better quality than the chunks in terms of colour and particle size. Generally the SD50 values for flours from the slices were finer than those from the chunks. Values of 120 and 92 for the cassava flours from slices dried in the open sun and GTSD and 150 and 135 for the flours from the chunks were obtained. To further evaluate the efficiency of the GTSD, a mathematical modelling on Henderson and Pabis and Logarithmic models. From the data on the two models, the Henderson and Pabis better explains the drying concept of the dried samples than the logarithmic model. The Henderson and Pabis model gives a better efficient drying system for the dried cassava and cocoa beans because it provided the higher coefficient correlation constant for the cassava slices, chunks and cocoa beans of 0.9991, 0.9989 and 0.9990 respectively thereby depicting a better drying model to be used.