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The coefficients of diffusion of water, Di and Djj, associated with states of ' high ' and ' low ' hydration respectively in fish muscle are sensitively dependent on the amount of fat present. The diffusion resistivity D -l for various species may be represented in both cases by straight lines of the form where I: is fat content and a and Although the results for various species appear t o fall about common lines for nj-1 and Dii-l, the dillusion of water in the muscle of dogfish (Squalus rccnnthias L.) is anomalous in certan respects.The experimental evidence is generally incompatible with a muscle microstructure consisting of a dispersion of fat in a non-fatty medium, as is commonly supposed, but is more in accord with one consisting of relatively non-fatty tissue entirely surrounded by fat, the thickness of ahicli increases linearly with fat content.n-1= a + p F are constants. IntroductionThe coefficient of diffusion of water in the muscle tissue of lean fish has been shown1 to be independent of species. The mechanism is one of thermal activation in which water molecules hop from one site of strong bonding to an adjacent site over a saddle point on a potential surface. The coefficient of diffusion D thus varies with temperature T in accordance with Boltzmann's energy distribution so that where E is the height of the potential barrier, R is the gas constant and Do M d2kT/3h, d being the separation between adjacent sites, k Boltzmann's constant and h Planck's constant.The coefficient of diffusion is independent of concentration over a wide range of water content. When the water concentration falls below a value corresponding to the unimolecular layer on the surface of the protein molecules, however, the diffusion-coefficient a t a given temperature is considerably reduced and there is a corresponding increase in the energy of activation, while Do remains practically unchanged. I t may be supposed that the higher activation energy in the drier condition results from strong double hydrogen-bonding between water molecules and reactive groups on the protein surfaces and that this is slightly reduced, perhaps by electrostatic screening, when successive layers of water molecules surround the unimolecular layer in the more hydrated state.Under practical conditions, the weight W, of a sample of fish at time t, drying under conditions in which the constant-rate period may be neglected, is given by the expression where We is the equilibrium weight of the sample when t -+ co, Wo is the initial weight and Aj, A;;, T ; , T~~ are constants. Ai > Aii and T~ < T~~~ T~ and T~~ are termed 'drying time-constants', the suffixes denoting values corresponding to states of ' high ' and ' low ' hydration respectively. For convenience, drying, as characterised by these parameters, in this, the falling-rate period, may be said to take place in two phases, phase i and phase ii respectively. Fig. I illustrating the typical drying behaviour of herring muscle, shows the unaccomplished weight loss W , -We of a small slab plotted on a logarith...

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Equations associated with a proposed analytical model involving bimodal diffusion of water in fish muscle are derived. In its simplest form the model is represented by the sum of two exponential terms containing diffusion coefficients in the exponents. A method is described which enables the parameters implicit in these equations to be obtained from experimental data. The technique is applied to the diffusion of water in drying muscle containing varying concentrations of sodium chloride. The treatment includes cases in which surface conditions may initially inhibit the exponential behaviour.