Mathematical Modelling of Solar Drying of Mango Slices
Mathematical Modelling of Solar Drying of Mango Slices
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Akoy, El-Amin Omda Mohamed
Three experiments were conducted through the cooperation among the Department of Agricultural Engineering, Faculty of Agriculture, University of Khartoum, Sudan and the Food Research Centre, Ministry of Science and Technology, Sudan and the Institute of Agricultural Engineering, University of Goettingen, Germany to study thin layer solar drying of mango and the related quality attributes resulted from the drying process and sorption isotherms of solar dried mango slices. The first experiment was conducted at the Food Research Centre under controlled conditions using an air oven. Only the influence of drying temperatures at 50°C, 60°C, 70°C, 80°C and 90°C on many drying behaviour and quality were studied. The change in mass of mango slices was recorded continuously at specified intervals of 30 min during the experiment using a sensitive balance. The desired drying air temperature was regulated by means of temperature controller and checked using a thermometer. Quality attributes studied included total sugar, reducing sugar, rehydration ratio and non-enzymatic browning. The total sugar and reducing sugar were evaluated by titration method whereas non-enzymatic browning by extraction method. Drying curves obtained from the experimental data were then fitted to three well-known semi-empirical thin-layer drying models namely; Lewis model, Page model and Henderson and Pabis model. Model constants and coefficients were determined by nonlinear regression method. Microsoft Excel spreadsheet software was used to simulate drying characteristics of mango slices. Results revealed that in order to produce high quality mango product, higher temperatures of 70°C to 80°C have to be applied as the optimum drying temperature. Results indicated that drying constant (k) increases as drying temperature increases. The effective moisture diffusivity (Deff) varied from 1.87 =620; 10-6cm2/sec to 3.67 =620; 10-6cm2/sec in temperature range from 50°C to 90°C. Also, the effective moisture diffusivity (Deff) increases as drying temperature increases. The drying took place in the falling rate period and increasing air temperature lessens drying times. All the models were validated using statistical parameters namely; coefficient of determination (R2), sum of square error (SSE), root mean square error (RMSE) and reduced chi-square (=539;2). Among the drying models investigated, the Page model satisfactory described the drying behaviour of mango slices. In the second experiment, a natural convection solar dryer was designed and constructed to dry mango slices. The constructed dryer consisted of a drying chamber and a solar collector combined in one unit. Inside the drying chamber there were two movable mesh wire trays for easy loading and unloading of the mango slices. In order to investigate the dryer performance several test runs were conducted at the Workshop of Department of Agricultural Engineering, Faculty of Agriculture, University of Khartoum. In these tests, drying air and ambient air parameters were recorded continuously at specified intervals of 30 min. Thermocouples connected to a data logger were used to measure temperature whereas digital hygrometers were used to measure relative humidity. To study solar drying characteristics of mango slices, the change in mass of mango slices was recorded at 30 minutes intervals until reaching constant weight using a sensitive balance. The same tested drying models used in the first experiment were employed for the experimental data of the second experiment. A computer programme was written using Turbo Pascal programming language, version 7 to simulate the thin layer solar drying of mango slices. Results revealed that the dryer attained a temperature 20ºC higher than ambient temperature. The heated air temperature was higher than ambient one and the difference was found to be significant (P < 0.05). The heated air relative humidity was lower than ambient one and the difference was found to be significant (P < 0.05). Furthermore results revealed that, the dryer attained a maximum temperature of 70ºC, which is considered as the optimum temperature for mango slices drying. Of the three tested drying models, Page model shows close agreement between predicted and measured moisture contents. In the third experiment water sorption behaviour of solar-dried mango slices at three temperatures of 20°C, 30°C and 40°C for a water activity (aw) range of 0.111 to 0.813 was conducted at the Institute of Agricultural Engineering, University of Goettingen. The sorption isotherms of solar dried mango slices were determined by the standard static gravimetric method; using saturated salt solutions inside air-tight glass jars to maintain a fixed relative humidity. To maintain the constant temperatures, incubators were used. Solar dried samples were weighed at intervals of two days using a sensitive balance until constant weight was reached. The experimental results were fitted to two well-known mathematical sorption models namely; BET model and GAB model. The validation of the two selected models was done by using a statistical parameter namely; mean relative percentage deviation (% P). Microsoft Excel spreadsheet software was used to simulate sorption isotherms of solar dried mango slices. Results indicated that the adsorption characteristic of solar dried mango slices is of typical shape of high-sugar foods i.e. it is type-III (J-shape). Intersection of isotherms occurred at aw>0.75 for investigated temperatures. It is clear that, the monolayer moisture content (Mo) decreases with increasing temperature. GAB model satisfactorily predicted moisture sorption isotherms of solar-dried mango slices.
University of khartoum