Conclusions on the Diffusion Process

After considering the various parts of the process of the matter transport controlled by diffusion, it seems necessary to recall the most important facts, in the form of a conclusion. 

Various parameters intervene in the process of diffusion when a film is in contact with a liquid food. First, those concerned with the film itself, with its thickness and the diffusivity of the substance, secondly, the volume of the food. The diffusing substance intervenes through the diffusivity which depends on the diffusing substance-polymer couple, as well as its solubility either in the polymer or in the food the partition factor results from the presence of the solubility in both these media which limits the concentration of this substance. The rate of stirring plays the major role, because of the presence of the convection stage at the liquid-package interface. 

The ratio of the volume and package, denoted eg is only of concern when it is lower than 20 it is clear that for a 1 litre bottle, whatever its shape, this ratio a being around 166, its effect becomes negligible except when a very high accuracy is needed. 

By contrast, the rate of stirring always plays the most important role in the process of transport of the diffusing substance, because of the presence of the convection at the liquid-film interface. This coefficient of convection h is of such great concern that it should always be considered. In fact, it is clear that when h is infinite, the concentration on the film surface reaches the value at equilibrium instantaneously as soon as the process starts. This fact makes the equations expressing either the kinetics (1.34 and 1.39) or the profiles of concentration through the film thickness (1.31) quite oversimplified. 

A strong difference also appears between the film playing the role of a package or used as a membrane. On the one hand, a package contains a finite amount of diffusing substance, which decreases during the process on the other hand, the concentration of the diffusing substance is kept constant on both sides of this membrane, whatever the convection on each side. 

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