Effects of changes in thermophysical and dielectric properties of pineapple juice on microwave power absorption density of the juice

Abstract

This study indicates the effects of temperature and concentration on the thermophysical and dielectric properties of pineapple juice concentrates (PJC) ranging from 10.9 to 50 ˚brix within a temperature range of 10–60˚C. Further experimental and simulation techniques were used to assess the microwave power absorption capacity of pineapple juices. The measured engineering properties of pineapple juices were influenced significantly by the different levels of concentration and temperature (p < 0.05). The density increased with concentration and decreased with temperature and ranged from 1056.57 to 1203.38 kg/m³. The thermal conductivity and specific heat decreased with concentration and increased with temperature and ranged from 0.425 to 0.605 W/m∙K and 2950.49–3860.24 J/kg∙˚C, respectively. The dielectric constants decreased with concentration and temperature and ranged from 73.91 to 48.58, and dielectric loss factor increased with concentration up to 45 ˚brix and further decreased for 50 ˚brix, and decreased with temperature, and ranged from 9.67 to 20.23. These properties of pineapple juice were correlated with temperature and concentration using polynomial regression equations (p < 0.0001). Non-uniform microwave power absorption was observed that was related with the dielectric properties of PJC. Transfer of the absorbed microwave heat within PJC was relatively efficient at lower concentration than that of at higher concentration because the thermal diffusivity of PJC decreased with concentration and increased with temperature and ranged from 1.52E-7–1.17E-7 m²/s. The measured and simulated values of microwave power absorption were ranged from 475.2 to 452.2 W and 464.3–449.9 W, respectively. The microwave power absorption at different concentrates, and corresponding average temperature change of the juices were not significantly different (p > 0.05). However, there was about 4.91 % and 3.09 % decrease in the measured and simulated microwave power absorption, respectively, as concentration of the juice increased from 10.9 to 50 ˚brix.

Practical application

The properties of fruit juice change with the progress of evaporation process. The method of concentration largely influences the way changes occur in the qualities of fruit juice concentrate especially in the case of thermal vacuum evaporation and microwave assisted vacuum evaporation due to difference in heat transfer mechanism. Pineapple juice can be suitably concentrated using microwave energy because microwave power absorption capacity of the juice does not drop significantly as concentration of the juice increases. The findings of this study facilitate in the design of microwave equipment, control and analysis of the process variables during microwave assisted vacuum concentration of pineapple juice.