TOTAL POROSITY AND OPEN‐PORE POROSITY IN THE DRYING OF FRUITS

ABSTRACT A simple procedure has been implemented to measure porosity of apples as a function of moisture contents. The method has been applied to apples with change in moisture contents by conventional air drying. It is shown that the pores, which for biological reasons are open, i.e. connected to the outside, at the onset of dehydration, remain so until X = 1.5 g/g. As dehydration proceeds the pores are split into two families of pores: externally connected pores and locked‐in bores. Both total and open pore porosity can be predicted from experimental correlations. Alternatively, the total porosity can be estimated from data on bulk volume shrinkage and the average composition of the tissue by means of equations presented. The interpretation of experimental data using this approach confirms that there is a significant change in the three‐dimensional arrangement of the cellular tissue below X = 1.5 g/g. The two approaches lead to the conclusion that, probably due to cellular collapse, the open structure that characterizes the fresh foodstuff changes into a structure with locked‐in pores. This fact may help to explain the slow and difficult re‐hydration which characterizes air‐dried foodstuffs. This hypothesis will be followed up in future work. The information developed should be of help in qualitative and quantitative modeling of processes involving moisture changes.