The transport of water in four EVOH copolymers commonly used in high barrier food packages was characterized through permeation (continuous flow) and gravimetric experiments at different rela tive humidities and 23 ± 2°C. Water sorption isotherms were fitted with the D'Arcy and Watts' equa tion. From these data, the value of the solubility coefficient (S, as defined by Henry's law) was deter mined and was found constant within a 0.2-0.75 water activity (a w) range. Water uptake at the same a w increased as the EVOH ethylene content decreased. The permeability coefficient (P) for water through EVOH was determined as a function of water activity. The permeability was constant within the range of 0.3-0.75 a w and decreased with EVOH ethylene content. At high relative humidities (a w > 0.75) the value of permeability increased by up to two orders of magnitude. In this range, the higher the ethylene content the lesser the value of P.

A new model of water sorption isotherm is developed on the basis of Raoult's law. It is assumed that water present in food occurs in two states, as free water with properties of the bulk water and as water of hydration. Hydrated molecules are considered as new entities with molecular weights larger than those of non-hydrated molecules. Hydration reduces the free concentration of water and thus affects water activity in solution. Application of the developed equation to food sorption data showed that it gives approximation of sorption isotherms much better than that offered by the GAB model. Moreover, it predicts infinite adsorption at a(w)=1, the property which is not offered by the GAB equation. The new equation makes it possible to interpolate isotherms at high water activities close to one. The probability that the new equation will fit the food isotherm with small RMS is higher than 90% and substantially exceeds that found for the GAB model. As a two-parameter model it makes substantial improvement over the three-parameter GAB equation.

This study has deduced a correlation between points of inflection of water activity and loss factor with respect to moisture content. A point of inflection in loss factor with respect to moisture content was found to coincide with the sorption isotherm point of inflection that defines the transition from multilayer to solution in every instance analysed, with an average difference of just 0.01 kg kg−1. Food can support microbial growth and chemical reactions in water activity levels above this critical transition. This correlation was discovered using published dielectric and sorption data for specific foods at similar temperatures. It was found that low sugar foods containing high levels of hydrocolloids generally exhibited different behaviour from fruits. This shows that microwave heating behaviour will be different in fruits compared to low sugar foods with high hydrocolloid content when drying to achieve a certain water activity and therefore shelf life.

In the present paper, a mathematical model able to predict the water barrier properties of cellophane film as a function of the water activity at the upstream and downstream side of the film is presented. To validate the model water sorption, and permeation tests were performed at 30°C and at several water vapor activities. Despite the approximations involved in deriving the model, its ability to predict the water permeability of the investigated film is quite satisfactory. The proposed model was then applied to hypothetical measuring conditions in which the water activity at one side of the film was set equal to zero (like in a permeation test) or equal to 0.6 (like in the real working conditions), while the water activity of the other side changes between 0 and 0.6. A substantial difference has been observed between the two examined cases, showing the need for a more accurate analysis of the transport phenomena in the case of water sensitive packaging films like cellophane.

Solution thermodynamics was applied to the water sorption isotherms of foods. First, we corrected Le Maguer's theory. By this correction, it was found that the same thermodynamic equations were applicable to both water sorption data measured at constant total pressure (P) and to those measured under the particular vapor pressure, P Pw, for a food material. Second, the change in the water sorption behavior of rice flour by defatting and extrusion cooking was analyzed by solution thermodynamics. It was confirmed that the principle of solution thermodynamics is more appropriate than that of conventional adsorption thermodynamics for evaluating the affinity of food for water. The enhanced affinity of rice flour for water by defatting and extrusion was quantitatively evaluated by solution thermodynamics

This paper presents a mathematical model able to predict the water barrier properties of nylon film as a function of the water activity at the upstream and downstream side of the film. To validate the model, water sorption and permeation tests were conducted at 25 °C. The fitting and predictive ability of the proposed model was successfully tested by fitting the model to the sorption data and by predicting the nylon water permeability coefficient, respectively. The implications of the adopted approach on practical aspects of packaging were pointed out by comparing the nylon water barrier properties as evaluated by means of a permeation test and that in the real working conditions.

In food packaging systems, moisture content influences chemical and physical film properties, also determining processes such as food spoilage, and properties of food texture and crispiness level. The study of water permeation and sorption processes of new materials intended to be used as packaging is very important to determine the best application conditions and to predict the film behavior under different moisture conditions inside and/or outside the packaging. In order to determine the suitable temperature and water activity (aw) application conditions for whey protein isolate (WPI)/polyvinyl alcohol (PVOH) blends as food flexible packaging, water permeation and water sorption thermodynamic behavior of these materials were evaluated. WPI/PVOH films and blends had solubility preponderant over the diffusion on the water permeation process. Water sorption experimental data were well described by the GAB model, and curves showed a more expressive increase of water sorption at aw > 0.75, with lower equilibrium moistures (Ye) at room than at chilled temperatures. Differential enthalpy decreased and differential entropy increased by the Ye gain, and the occurrence of enthalpy-entropy compensation was confirmed with enthalpy driving the sorption process. The addition of PVOH to the WPI matrix made the water sorption process more spontaneous. Water sorption thermodynamic analysis indicates that the application of WPI/PVOH blends as packaging is best suitable for foods and external environments with aw below 0.75 and at room temperature.

A novel method was used to estimate assimilation efficiencies (AEs) of dissolved and food associated PCBs (IUPAC 31, 49, and 153) by the Baltic Sea blue mussel (Mytilus edulis). Mussels were exposed to radiolabeled PCBs in a series of short-term toxicokinetic experiments at different algal food concentrations, both at apparent steady-state (ASS) and non-steady-state (NSS) conditions in respect to PCB partitioning between water and algae. The PCB AEs were calculated using a physiologically based bioaccumulation model where experimentally determined uptake and exposure rates at ASS and NSS conditions were combined into linear equation systems, which were solved for PCB AE from water and food. A positive relationship between PCB uptake and algae clearance by the mussels was observed for all three PCBs. The PCB AEs from both water and food increased with congener hydrophobicity (octanol/water partition coefficient [K(ow)]), but AEs decreased with increases in water pumping and filtration rate of the mussels, respectively, The average contribution of food-associated PCB to the total uptake also increased with K(ow) from approximately 30% for PCB 31 and PCB 49 to 50% for PCB 153, mainly as a consequence of increased sorption to the algal food.

There is currently no recognised equation, or set of equations, that can be used to adequately describe moisture dependant dielectrics. This study addresses this issue so that moisture dependant dielectric properties can be directly input into drying models, where the microwave or radio frequency source is always limited to a single frequency. This was achieved by adapting water activity equations to describe the moisture dependant behaviour of the loss factor and the dielectric constant of hygroscopic inhomogeneous materials at microwave and radio frequencies. These equations were fitted to thirty moisture dependant loss factor and dielectric constant data sets. The adapted water activity equations proved to be very effective at describing dielectric behaviour, with the best equation fits to the thirty moisture dependant dielectric data sets having an average Mean Relative Error of 2.99%. The suitability of the equations are discussed, and specific equations are recommended for fitting to different types of moisture dependant dielectric response.