Shrinkage and porosity of banana, pineapple and mango slices during air-drying

It is very important to monitor characteristics of fruits (e.g. volume, shrinkage and porosity) during drying. There is lack of study in comparing different methods to measure dried product's apparent volume. Specific volume, shrinkage and porosity of banana, pineapple and mango during air-drying were investigated. Banana, pineapple and mango slices were dried to different moisture contents down to approximately 5% wb in an oven dryer at 70 °C. The true volume was measured with gas pycnometry. Different methods were compared systematically to measure apparent volume, shrinkage and porosity of banana, pineapple and mango slices. Seven methods were tested: liquid pycnometry, liquid displacement and Archimedes principle, with two organic solvents (toluene and n-heptane), and displacement with glass beads. The application of the Archimedes principle with n-heptane to measure specific volume yielded the lowest coefficient of variation for banana and pineapple slices, the second lowest for mango slices, and therefore it was recommended for measuring the apparent volume of fresh and dried fruit samples. All fruit sample during drying showed a reduced degree of shrinkage at the later stage. With the fruit slices shrinking during drying, the specific volume of dried banana, pineapple and mango showed a minimum at approximately 24%, 6% and 30% wb moisture content, respectively. Porosity of banana and mango slices increased 3-folds up to 17% wb and 5-folds up to 12% wb from fresh sample, respectively. Porosity of pineapple increased with moisture content of dried pineapple from around 6% wb to 33% wb, and then kept decreasing till raw pineapple. Image analysis was successfully applicable to measure the diameter and perimeter and describe the structural changes of fruits during drying.