Kinetics of water vapor sorption in a model freeze‐dried food

Abstract

Sorption kinetics experiments were performed a t 37 C with microcrystalline cellulose held between dryness and 51% water activity in the absence of other gases. Effective diffusion coefficients (>lO-4 cm*/s) were greater than any previously reported for similar experiments. A model based on heat and mass transfer properties of the sample is proposed. The vapor space permeability calculated by application of this model is related to the structure of the microcrystalline cellulose as determined from water desorption isotherm analysis, mercury intrusion porosimetry, and steady state permeability techniques. Microcrystalline cellulose, like some freeze-dried foods, contains micropores and macropores. During sorption, the mocropores are important in determining the balance between internal and environmental control during the first half of sorption. The micropores, which account for less than 1% of the void volume of the porous matrix but approximately 40% of the surface area, control the mass transfer properties of the sample during the latter stages of sorption. Because of the great difference between their mass transfer properties, the macropores approach local equilibrium faster than the micropores; this phenomenon suggests that changes in the effective diffusion coefficient and permeability as sorption proceeds are related to the structure of the sample rother than to the moisture content itself.Drying of foods is a process of major industrial importance. The physical phenomena that occur during drying and the mechanisms of moisture transport are so varied that sorption experiments have been used to study mass transfer in porous and nonporous food materials. Generally, these investigations have been conducted without concern for the environmental resistances to moisture transport or for the conduction of the heat released during sorption from the sample food through the environment to a heat sink, although heat factors have been shown to affect the rate of moisture sorption considerably (1, 2).Many analyses of sorption have been used to calculate a diffusion coefficient based on an appropriate solution of Fick's second law of diffusion, Equation ( 1).Crank ( 3 ) has presented various solutions of Equation (1) with a constant diffusion coefficient based on appropriate boundary conditions and sample geometry, and he has outlined methods for determining the functional dependence of the diffusion coefficient on the moisture content. This implies, sometimes incorrectly, that the diffusion coefficient is mechanistically related to the moisture content. King ( 4 ) has shown that the effective diffusion coefficient ( D E F F ) is related to various physical properties of the sample (k, AH,, ps, etc.). This relationship occurs because sorption is a heat and mass transfer process. As such, any one rate parameter must be related to those properties important to both heat and mass transfer. The present study was undertaken to determine the importance of both heat and mass transfer in this process with attention given to the ...

Some characteristics of the heat of water vapour sorption in dried foodstuffs

“…The factor exp (cX') in eqn (6) (c < 0) provides a quick convergence of qg to the value of the heat of vaporization of pure water (10.38 kcal/g mole) when the moisture content is of the order of about 0.4 g water per g of dry matter. This agrees with the fact that at about this moisture value, water is present in most foods as unbound water (Duckworth, 1971) and consequently the net heat of desorption approaches zero. As is obvious, eqn (6) may be integrated between the monolayer value, calculated with the desorption branch of the isotherm, and any desired moisture level to obtain, either, 'integral' heats of desorption or average differential heats.…”

The Behavior of Invertase in Model Systems at Low Moisture Contents

“…Upon humidification a sample forms a cohesive cake, which is easily compressible and dispersible in water. Isothermal data show that the system has a (water) surface area of 114 m2/g, which is the same for the pure, untreated Avicel (Bluestein 1971) or the freeze-dried system with up to 20% sucrose (w/w). The building block microcrystals of cellulose contain virtually no amorphous regions ("hinges") and do not swell in the presence of water.…”

“…9) Experimental data for system 4/4. Assumed equal to 0.614 as estimated by Bluestein (1971). So = total surface area, (microns)2/g (from Labuza 1968).…”

Effect of freezing temperature on rehydration and water vapor adsorption characteristics of freeze-dried rice porridge

“…Most of theoretical analyses were based on the Fick's second law of diffusion (Bello et al, 2010;Hayakawa, 1974;Bluestein and Labuza, 1972;King, 1968) which usually involves numerous functions and parameters, made them difficult to describe the water vapor adsorption process in simple terms. In some cases, empirical models were preferred because of their relative ease of use.…”