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Experimental Investigations of Drying Kinetics of Cabbage

M A Rahman
Principal Scientific Officer
Farm Machinery and Postharvest Technology Division, Bangladesh Rice Research Institute, Gazipur-1701, Bangladesh.

M S Islam
Chief Scientific Officer
Farm Machinery and Postharvest Technology Division, Bangladesh Rice Research Institute, Gazipur-1701, Bangladesh.

S M M Rahman
Principal Scientific Officer
Farm Machinery and Postharvest Technology Division, Bangladesh Rice Research Institute, Gazipur-1701, Bangladesh.

AKM Saiful Islam
Senior Scientific Officer
Farm Machinery and Postharvest Technology Division, Bangladesh Rice Research Institute, Gazipur-1701, Bangladesh.

B C Nath
Scientific Officer
Farm Machinery and Postharvest Technology Division, Bangladesh Rice Research Institute, Gazipur-1701, Bangladesh.

MG Kibria
Senior Scientific Officer
Farm Machinery and Postharvest Technology Division, Bangladesh Rice Research Institute, Gazipur-1701, Bangladesh.

Abstract/ Executive Summary:

Thin layer drying of cabbage was conducted using four different thicknesses of 3, 5, 8, and 10 mm slices at three different temperatures of 55, 60 and 68°C. The drying process took place in the falling rate period and no constant rate period was observed. Drying rate of cabbage varied according to the product thickness as well as temperature differences. Results show that drying rate constant and thickness can be expressed as power law equations. The value of index 'n' of the second power law equations were found to be 2.19, 1.83 and 1.45 for cabbage indicating that the external resistance to mass transfer was highly significant. The effect of temperature on diffusion co-efficient follows an Arrhenius type relationship. The activation energy (Ea) of diffusion of water for cabbage at 5mm slice thickness was found to be 16.24 kcal /g-mole.

Keywords: Drying kinetics, Cabbage, Drying rate of cabbage

Location: FMPHT laboratory, BRRI, Gazipur

Start Date: 00-00-0000

End Date: 00-00-0000

Program Area: Postharvest and Agro-processing

Commodity: Cabbage

Objectives:

1. To study the drying kinetics of cabbage in single layer and

2. To examine the effect of slice thickness and temperature on different drying parameters of cabbage.

Methodology:

The experiments of a single layer drying of cabbage at different thicknesses and various air temperatures were conducted in a laboratory type mechanical dryer at the FMPHT laboratory, BRRI, Gazipur. The dryer mainly consisted of a blower, heating unit, drying chamber and sample container. The temperature in the drying chamber was adjusted with a thermostat. Freshly harvested mature cabbage (BARI-1, Provati) was collected from the horticultural farm of Bangladesh Agricultural Research Institute (BARI), Gazipur. Four different slice thicknesses (3, 5, 8 and 10mm) of cabbage were selected for this study. Accordingly the samples were sliced to these four different thicknesses by sterilized knife. Usually fruits and vegetables are being dried within the temperature range of 50 to 70°C. At lower drying temperature products were dried for longer time with resultant brown colour in the final dried products which is undesirable for consumer acceptance. Case hardening occurred at higher drying temperature generally above 70°C during drying in most of the fruits and vegetables. Outer surface was over dried causing interruption of moisture diffusion from the interior part of the product during drying. The final dried material could not be regarded as good quality due to non-uniform drying. Therefore, 55, 60 and 68°C temperature were selected for safe drying of cabbage. Sliced samples were placed on the tray in a thin layer in a mechanical drier. Samples were dried until the products were reached equilibrium moisture content. Air flow and RH were maintained constant in the drier. For analysis of experimental drying a number of researchers have used Fick's second law diffusion. To find a solution of the above unsteady state diffusion equation for one dimensional transport for the case of initial uniform moisture distribution in the sample and negligible external resistance, appropriate boundary conditions are assumed. The above relationship shows that if external resistance to mass transfer is negligible and if simultaneous heat and mass transfer effects are taken into account, the value of the exponent of the power law equation should be 2. But the above conditions are not always satisfied and experimentally determined "n' value is found to be less than 2. From equation it is apparent that plotting diffusion co-efficient (De) versus the inverse absolute temperature on semi-logarithmic co- ordinates would lead to the evaluation of activation energy for diffusion of moisture during drying.

Source of Information: Journal of Agricultural Engineering, 2009, Vol.36/AE, pp:69-76, ISSN:0379-4318.

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

Thin layer drying experiments of cabbage was conducted in a laboratory type mechanical dryer. The drying process took place in the falling rate period and no constant rate period was observed from the drying curves. Drying rate of cabbage varied according to the product thickness as well as the temperature differences. Drying time was the least for the lowest product thickness (3mm) and was followed by 5, 8 and 10mm thick product at similar drying temperature. For a specific moisture ratio, 68°C temperature was required for the least time to dry, followed by 60°C, while the highest drying time was required at 55°C to dry cabbage slice. The relationship between drying rate and sample thickness of cabbage was developed by three power law equations. From these equations the value of index 'n' of the power law equations were found to be 2.19, 1.83 and 1.45 for cabbage dried at 55, 60 and 68°C respectively. These values indicated that the external resistance to mass transfer was highly significant. Activation energy (Ea) for diffusion of water for cabbage was found to be 16.24 kcal/g moles for 5mm thickness product.

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