The Influence of Operating Temperature on Mass Transfer Characteristics during a Diffusion Process in Bilimbi Fruit

Abstract

The effects of operating temperature on an osmotic sucrose solution with a selected concentration pattern (30-50-70 °Brix) on mass transfer characteristics during the diffusion process in bilimbi fruits in a dehydration system were investigated. Half-ripe bilimbi fruits were used. In the pretreatment step, all fruits were blanched in boiling water for 1 min to soften the fruit for better diffusion. The osmotic solutions were prepared without agitation. For each temperature level, bilimbi fruits were firstly immersed in a 30 °Brix sucrose solution for 3,000 min, then a 50 °Brix for 3,000 min, and finally a 70 °Brix for 3,000 min. The ratio of solution to raw materials was 2:1 and the pH was adjusted to 3.5 before each step. Samples from both solid and liquid phases were taken out at different periods in order to determine water loss, soluble solid gain, acid loss, corresponding diffusivities, as well as shrinkage. It was found that an operating temperature of 40 °C yielded the lowest mass transfer characteristics i.e. water loss and its rate, soluble solid gain and its rate, and acid loss and its rate while there was nearly no difference in the values obtained at 50 and 60 °C. Nevertheless, at the highest concentration and at all temperatures, though the water loss (%) and the acid loss (%) still increased upon time the soluble solid gain (%) decreased. The phenomenon was similar to what happened in mechanical squeezing. This result corresponded to continuous shrinking, which was clearly observed during dehydration. For bilimbi fruits, the final shrinkage was found to be 80 % of the initial volume. The final shrinkage value and also the rate of shrinkage for each period were lowest at 40 °C with no difference at 50 and 60 °C. Higher concentrations like 50 and 70 °Brix did not lead to a higher rate of water loss, soluble solid gain and acid loss; on the contrary it lowered these values. In addition, for all experiments, the diffusivity for each component i.e. water, soluble solids, and acid decreased as the concentration was increased (30-50-70 °Brix). Seemingly for blanched materials at higher concentrations, strong interactions among components were observed. The results lead to a conclusion that blanched materials in a system without agitation were accompanied by shrinkage, by a squeezing-like phenomenon. The concentration effect not only dominated the temperature effect but also stabilized the rate of mass transfer especially at the operating temperatures of 50 and 60 °C.

Keywords

• Averrhoa bilimbi
• bilimbi fruit
• temperature effect
• shrinkage
• osmotic dehydration
• diffusion rates