NUMERICAL MODELLING OF COUPLED HEAT AND MASS TRANSPORT IN VACUUM FRYING OF POTATO CYLINDERS

Vacuum frying is an alternative technique to conventional deep-fat frying. Due to the lower pressure, the boiling point of the moisture in the foods decreases so that the process can be used to develop high quality fried products (reduced acrylamide formation, better colour and flavours). However, vacuum frying involves additional challenges. The frying at 20,000 Pa of cylinders of potato at 120 and 140 °C resulted in decreases (5 °C) in the temperature at the centre, as well as a change in the breaking force of the product. This could be because the pressure accumulation due to vapour generation causes the temperature saturation to increase. The point is reached where the internal structure collapses and sudden depressurization occurs. To verify this explanation, a mathematical model has been proposed that uses a 2-D model (cylindrical symmetry) of coupled heat and mass transport implemented with the commercial COMSOL Multiphysics CFD program. This includes two main equations, one for heat transfer (external convection, internal conduction, and phase change) and one for linking vapour transport and pressure accumulation (Darcy’s law). The solution of the model uses the experimental data of water loss and time variable permeability. The latter is related to textural changes and is proposed as the internal vapour modulator that regulates the phenomena. Although the model is simple in that it considers average values for internal pressure, it is capable of reproducing the temperature profiles encountered.