The kinetics and drying curve of the partial dehydration of oval cherry tomatoes, Lycopersicum sp. Mill., were analysed. The initial moisture content was 94.01% (15.7 kg water/kg dry mass) and the performances of both fixed and vibrating bed dryers were determined. In the latter case, a vibrated tray dryer with recycle was used, recycling the product until the tomato was left with only 15% of the initial amount of water, that is a moisture content of 70.15% (2.35 kg water/kg dry mass) on a wet weight basis. For identification purposes, the fruits sampled were arranged in five baskets made of polyethylene fabric with a mesh of about 0.01 m in diameter. The cherry tomatoes were cut into two halves perpendicular to the axis of greatest length. The temperature, air flow and the amount of tomato fed in were adjusted to 67.6 °C, 6 kg/min and 4.025 kg, respectively. The initial part of the drying process was a batch operation with no recirculation of the fruit, until the moisture content reached about 85% (5.67 kg water/kg dry tomatoes). Porcelain balls with a diameter of 0.0235 m were then added which forced the flow of tomato particles, and as from this point the process occurred with recirculation. Three periods were identified in the drying kinetics obtained using the experimental results: two periods during which the tomatoes showed steady state drying rates, intercalated with one period showing a variable rate. For the constant drying rates, correlation coefficients with values above 0.99 were quantified on fitting a straight line through the experimental points. A comparison of the behaviours of the fixed and vibrating beds showed that the application of vibration decreased the drying time by 30%. The sphericity of the tomatoes ranged from a value of unity (start of drying) to 0.53 at the end of drying. Shrinkage of the tomatoes during drying was 80%, the initial average particle diameter of 2.32 cm reducing to 1.34 cm at the end of drying. The results indicated that the simultaneous influence of vibration and porcelain balls intensified the drying process, resulting in non-conventional air drying kinetics.
drying; cherry tomatoes; vibration; drying kinetics; central composite design
