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Title: Diffusion in a medium containing a solvent and solutes, with particular reference to fish muscle
Author: Peters, G. Ross
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 1971
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The diffusion of sodium chloride from a saturated brine solution into cod muscle is characterized by a constant diffusion coefficient of 1.4 x 10-9 m2 sec-1. The diffusion is thermally activated with an activation energy of 17 kJ K-1 mole-1. Both the diffusion coefficient and the activation energy correspond closely to those of NaCl in water. The diffusion of water in fish muscle during brining occurs mainly by solvent counterflow in the aqueous phase of the muscle. If the exterior solution concentration is greater than about 2 g. NaCl/ g. H2O, there is a net loss of water from the muscle, and the diffusion is controlled by the mutual coefficient of the water - sodium chloride system. A previously known empirical linear relationship between salt and moisture contents of fish muscle during brining in saturated NaCl solution was derived from theoretical considerations. In dilute solutions there is a net water uptake by the muscle. This has to take place against the normal outward solvent counterflow, and leads to complications in the internal water distribution. The results support the early work of Reay (21) in which a transient reduction of water content was observed during brining. The likelihood of errors in interpreting diffusion results in dilute brines was pointed out. An examination of the water and sodium chloride concentration inside a drying slab of fish muscle revealed that there was an inward movement of NaCl in response to the aqueous phase concentration gradients set up by the outwardly diffusing water. The direction of this solute transport reversed when the aqueous phase became salt-saturated. A series of computer programs were developed to derive the diffusion coefficients and other parameters pertaining to water in the fish muscle during drying. Water transport occurred in two diffusion-controlled modes and modification to the usual diffusion equations was made to account for this. It was found that all drying data, for both salted and unsalted muscle was described by .the same form of equation. The onset of the second diffusion-controlled drying phase is simultaneous with the point of saturation of the muscle water by the NaCl. Evidence is presented that in the saturated solution the tightly bound unimolecular water layer on the protein becomes involved in the hydration of Na+ ions, and this has the effect of binding "clusters" of water molecules to the hydrophilic sites. An experiment with another solute (KCl) in which the cation is unhydrated supports the model. The effective diffusion coefficients for the two stages are 1.8 and .29 x 10-10 m2 sec -1 respectively (at 30°C.) and are independent of salt concentration. There is statistically significant evidence that the apparent diffusion coefficient in the first stage decreases with increased temperature. The effect of temperature on the diffusion coefficient in the second stage was not large enough to be significant over the interval from 15°C. to 30°C. The equilibrium moisture content of cod muscle containing NaCl shows a marked dependence on salt concentration during the first desorption. This is consistent with the "water-clustering" model proposed above. The equilibrium moisture content reaches a maximum at a salt concentration of approximately 0.52 g/g of salt free solids, and it is possible to deduce from this that the original total monolayer water capacity is about 0.16 g/g of solids.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available