Diffusion of Water in Foods during Heating

Sammanfattning: The overall aim of this research was to improve the knowledge of diffusion of water in foods during heating, so that the final quality of foods after heat processing could be predicted and controlled.

The work consisted of three main parts. The first part involved the development of a method to study the diffusion. Using this method experimental studies on different types of foods were performed. The experimental results resulted in the development of a physical model. A computer simulation program was developed to verify the physical model. Using the simulation program the material parameters, such as the diffusion coefficients, were estimated. This work is presented in the five papers, which are included at the end of this thesis, and are briefly described below.

A method and an instrument for measuring local water content inside foods during heat processing was developed and evaluated. The method involves the use of a fibre optic NIR-instrument. The evaluation was mainly based on investigations of the influence of structure and temperature. The instrument was found to be very sensitive to both the structure and the temperature of the sample. The dependence on the temperature of the sample can be incorporated into the calibration and is thereby not an impediment to the measurements. The structural dependence makes quantitative measurements at different places in the sample difficult. However, measuring dynamic changes in the water content at one particular spot has proved successful. This is probably the best application of the instrument.

Three studies involving different foods were conducted. The first study was on the re-heating of meat to investigate the physical mechanisms behind the diffusion and to study the influences of fibre orientation on the diffusion rate. The main mechanisms behind the diffusion were found to be the squeezing of liquid water due to shrinkage and vaporisation-condensation of water vapour. The diffusion was also found to be slower in the direction perpendicular to the meat fibres as a result of the tortuous path the water had to take around the fibres.

The second study was on bread baking. The diffusion of water inside a loaf during the whole baking process was investigated, i.e. starting when the loaf was placed in the oven, through the introductory rise, the crust formation and the structural transformation from dough to bread, until the final finished loaf was taken out of the oven. No diffusion in the dough was found since the pore system consisted of discrete pores. During the transformation from dough to bread the pores were opened up and the water vapour could start moving.

The third study focused on the heating of pasta dough and involved an investigation of the influence of the diffusion, i.e. different water contents, and the heating rate on the structural and rheological properties. Two different water contents and heating rates were compared. Both the water content and the heating rate affected the swelling of the starch granules. In a high water content medium it is easier for the starch granules to absorb water than in a low water content medium, where they have to compete to a much greater extent with the gluten network. In the samples with low water content swelling was therefore delayed and occurred at a higher temperature, which resulted in different rheological properties.

Based on the experimental studies involving meat, bread and pasta a physical model for the transport mechanisms in porous materials was developed, i.e. a model for the vaporisation-condensation mechanism. The model is based on Fourierís law and Fickís second law, i.e. the differential equation of diffusion. Three differential equations are used simultaneously in the model, one for heat transfer, one for liquidwate r transport and one for water vapour diffusion. The novelty of this model is that the three equations are connected by the saturated partial water vapour pressure. The heat transfer into the sample is used on the one hand to raise the temperature and on the other hand to evaporate free water. A simulation program was written to verify the physical model. The simulation program can also be used to determine the diffusion coefficients and several other material parameters and their dependence on concentration and temperature. The simulated water content levels and temperatures conformed well to the experimental values and showed that the evaporation and condensation model presented a good description of the diffusion mechanisms in a porous food.

The results showed that the diffusion of water during a heating process strongly affects the quality of the final food product. Consequently, food quality can be better controlled through improved knowledge of diffusion.