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Single Layer Drying Characteristics of Half Fruit Tomato

M. A. Hossain
Farm Machinery and Postharvest Process Engineering Division, BangladeshAgricultural Research Institute, Gazipur, Bangladesh

K. Gottschalk
Postharvest Technology Department, Leibniz Institute for Agricultural Engineering,Potsdam, Germany

Abstract/ Executive Summary:

Drying kinetics of half fruit tomatoes were investigated in a laboratory dryer at the temperature of 35–65°C and air velocity of 0.13–1.00 m/s. The optimum drying conditions in terms of drying rate, colour and growth of microorganisms were also identified. Drying rate and shrinkage increased significantly with the increase in air temperature. But the red colour (a*) decreased significantly with the increase in air temperature. Drying rate increased with the increase of air velocity up to 0.75 m/s. The optimum drying air conditions was the temperature 55°C and air velocity 0.75 m/s, when drying time, colour, ripeness and growth of microorganisms were considered. Microorganisms (molds and fungus) were grown at or below 40°C temperature and it could be prevented by sodium metabisulphite pretreatment. The experimental data were fitted to six different single layer drying models and Midilli- Kucuk-Yapar model was found the best to predict the moisture content.

Keywords: Air velocity, Colour, Diffusivity, Shrinkage, Temperature.

Location: Leibniz-Institut für Agrartechnik, Potsdam-Bornim, Germany

Start Date: 01-10-2005

End Date: 31-03-2006

Program Area: Postharvest and Agro-processing

Commodity: Tomato

Objectives:

1. To study the half fruit drying characteristics of tomato in single layer,

2. To evaluate the suitability of some thin layer drying models, and

3. To optimise the drying conditions in terms of temperature, air velocity, colour and growth of visible microorganisms.

Methodology:

The experiments of single layer half fruit drying of tomato at various air temperature and air velocity were conducted in a laboratory scale dryer at the Leibniz-Institut für Agrartechnik, Potsdam-Bornim, Germany during October 2005 to March 2006.The dryer mainly consisted of a blower, heating unit, plenum chamber, drying chamber, sample container, a digital balance and data logger. A thermostat was used to control the temperature in the drying chamber. A controller was used to adjust the air velocity in the drying chamber. A series of sensors was inserted in the drying chamber to measure the air temperature and relative humidity. The diameter of each tomato was measured with slide calliper and mass was weighed with an electronic balance. The average diameter and mass of each tomato were found to be 58.2 mm and 124.6 g, respectively. The selected tomato was medium size (diameter; D: 57.2 ≤ D ≤ 64.3 mm) and mass (mass, W: 99.4 ≤ W ≤ 256.0 g). The ripeness of the tomato was determined by compression tests in term of softness using Zwick Universal Testing Machine (type: BZ2.5/TS1S) with a flat plate probe was pressed on the full tomato with a constant speed of 2 mm/min. The deformation was calculated for 10 to 30% range of the rupture stress. For determination of shrinkage the length, diameter and thickness of half cut tomato were measured by slide caliper before and after drying. The diameter and thickness were measured at different places and the average values were calculated. Also the initial and final mass of the tomato half was measured by an electronic balance. For our analysis, tomato shape is assumed as elliptical spheroid. Area of cross-section, volume, volume shrinkage, area shrinkage and thickness shrinkage were calculated, respectively. Single layer drying experiments were conducted at 35, 40, 45, 50, 55, 60, and 65°C drying air temperatures and air velocity of 0.13, 0.25, 0.50, 0.75, and 1.00 m/s. Experiments were also conducted on different maturity stages of tomato such as under, optimum and over maturity conditions. Six commonly used thin layer equations available in the literature were used to fit the experimental data by the direct least square method using SPSS 10.0 software.

Source of Information: International Journal of Food Properties, 12: 871–884, 2009; ISSN: 1094-2912 print / 1532-2386 online

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

Moisture loss increased with an increase of drying air temperature. Air temperatures above 55°C affected significantly the red colour of tomato. The overall performance of the tomato dried at 55°C and was the best, when both drying time, colour microbial infestation were considered. Tomato should be dried at an air velocity equal to or just above 0.750 m/s. Above this air velocity, moisture loss became independent of air velocity. There was no significant effect of air velocity on colour of tomato. Shrinkage of tomato increased with the increase of drying air temperature. There was no change of shrinkage observed for increase of air velocity. The moisture loss of over-ripe tomato was found higher than that of optimum ripe and unripe (green) tomato. The best to predict the moisture content of tomato at different drying air conditions. The effective moisture diffusivity of tomato increased linearly as drying air temperature increased. The activated energy of tomato was calculated as 37.716 kJ/mol. Hence, ripe half fruit tomato may be dried at an air temperature of 55°C and air velocity of 0.75 m/s and may be recommended to describe the single layer drying of tomato.

Knowledge Management: Journal