Liquid foods diffusion

A. M. Gil, I. Duarte, E. Cabrita, B. J. Goodfellow, M. Spraul, R. Kerssebaum 2004, (Exploratory applications of diffusion ordered spectroscopy to liquid food an aid towards spectral assignment), Anal. Chim. Acta 506 (2), 215—223. 

The liquid state has a minimum of order, and the molecules have considerable freedom of motion. A drop of food coloring placed in water demonstrates the rapid diffusion that can occur in the liquid state. The solid state will exhibit no detectable diffusion. If this experiment is tried with a material such as iron, the liquid food coloring will merely form a drop on the surface of the metal. 

With large K values, that is low solubility of component i in a liquid food, the material transport through A can also be determined from the contribution of diffusion in L under conditions of thorough mixing. Van der Waals attractive forces between the package surface and the molecules of L in intimate contact with P lead to the formation of a thin but immobile layer in which the diffusion coefficient of i in L, DL, controls mass transport (the Nernst diffusion layer). 

PVC is often used in food packaging and blood bags. This study concerns mass transfers between plasticised PVC, having been subjected to a treatment, and liquid food or food simulants. The treatment reduces the diffusion of the plasticiser and the influence of some factors of this processing were investigated. A mathematical model, able to simulate these mass transfers and to quantify treatment parameters, is proposed to quantify the diffusion rate in terms of an average diffusion coefficient. 16 refs. 

One of the most important migration problems occurs if a liquid food or food simulant F with the volume Vp and density pp comes in contact with a plastic layer P of thickness dp and density pp. The mass transfer takes place across an interface with area A between two different media with different characteristics, e.g., with different diffusion coefficients Dp and Z>p of the migrant. If the value of a quantity is desired, for example, the concentration of the substance transported across the interface in one of the two media, then a mass balance must be considered that takes into account the ratio of the contact surface area and the volume of the corresponding medium. The model describing this process is based on the following assumptions ... 

A liquid food of not very high viscosity can be stirred to speed up transport of heat or mass. Even if it contains dispersed particles, these mostly are small enough to allow rapid diffusion in or out of them (cf. Table 5.3). Many foods, however, are solidlike, and there are even some that contain a lot of water (cucumbers, for example, contain about 97% water) transport generally is by diffusion and in some cases by—greatly hindered—flow. Some examples are... 

Thus from this recall of heat transfer, with the similarity between the two processes of heat transfer and mass transfer controlled by diffusion, the necessity of admitting without ambiguity that the course for the mass transfer should emerge as follows diffusion through the thickness of the sheet associated with the convection into the liquid. Finally, the parameters of main importance for a polymer package in contact with a liquid food are the diffusivity and the coefficient of convection. The diffusivity is concentration-dependent, as for example the case of highly plasticised polyvinylchloride where the plasticiser concentration may reach up to 50% of the polymer, but in the present case of the low concentration of the additives distributed in the polymer of the packaging -which are necessary to provide its qualities - the diffusivity can be considered as constant. 

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 theoretical treatment of the process of mass transfer controlled by diffusion-convection with liquid food or even by diffusion-diffnsion when the food is solid is described precisely. Emphasis is placed on the fact that an infinite rate of convection cannot exist at the package-liquid interface in our finite world, and conseqnently we have to deal with more complex equations. The question of the volume of the package as a fraction of the volume of the liquid is also considered. In fact, the problems of diffnsion are not simple to resolve, either by considering the experiments or by making calcnlations. 

This is the most common case of release of some additives taking place from the polymer package into a liquid food. The process, controlled either by diffusion through the thickness of the package or by convection at the solid-liquid interface, has been described in Chapter 1, as well as in Section 4.2. 

Various studies have been made by considering either the diffusion of the contaminant through the polymer package or its release into the liquid food. The basic question has also been considered with the two possibilities of the transport into the food which can be driven either by diffusion through the solid food or by convection into the liquid food [4]. 

The results obtained in this case have already been established in Chapter 1. They are expressed in terms of the profiles of concentration of the diffusing substance developed through the thickness of the package (Figure 4.2 and Figure 4.3), and of the kinetics of release of the substance in the liquid food (Figure 4.4 and Figure 4.5). 

Figure 4.11 Kinetics of transfer of diffusing substance in the liquid food with a = 166, with the bi-layer system with H = L/2 and different values of the dimensionless number R.

Figure 4.15 Profiles of concentration of the diffusing substance developed at various times (dimensionless time D-t/L ) when the three layers have the relative thicknesses shown in the figure. The package is in contact with a liquid food with a = 55.3 and R = 5.

Figure 4.17. Scheme of the bi-layer package made of a recycled layer and a virgin layer in contact with liquid food, by taking into account the transfer of the diffusing substance during the co-extrusion stage. 

Figure 4.24. Kinetics of transfer of a diffusing substance in the liquid food of finite volume such as a = 55.3, with various values of the dimensionless number R. The initial profile of concentration of the diffusing substance is that attained at the end of the co-moulding stage described in Figure 3.30 for the three-layer bottle. Dimensionless numbers are used for the co-ordinates.

The diffusion of the liquid food into the polymer bottle over a long time period (experimentally, this time can be reduced by considering that the time of diffusion necessary for a given transport is proportional to the square of the thickness of the sample). 

The variation of the diffusivity of the surrogates with the concentration of the liquid food. 

Meerdink, G 1993. Drying of liquid food droplets Enzyme inactivation and multi-component diffusion. Diss., Agricultural University Wageningen, The Netherlands. 

Diffusion of Liquids Like gases, the two liquids in this beaker diffuse over time. The green liquid food coloring from the drop will eventually fonn a unifonn solution with the water. 

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