Recomendaciones para el escalamiento de las SbN

Water plays an important role with respect to the properties of food systems. It influences the physical or textural characteristics of food products as well as its chemical stability [7]. Moisture loss or gain from one region to another would continue in order to reach thermodynamic equilibrium. The term water activity (Aw) is used to indicate an intrinsic parameter of a food and equilibrium relative humidity, a property of the surrounding atmosphere in equilibrium with the food system under consideration (Van den Berg and Bruin, 1981). The thermodynamic concept of activity of a component can be explained in terms of its fugacity (f) and the fugacity of the component in a standard state, [8, 9]. The ratio for a component is called activity (relative activity) ai of the component i (Eq. 1):

(1) Fugacity is considered as a measure of the tendency of a component to escape. At equilibrium between different phases, the fugacity of each component is the same throughout the heterogeneous system. In this case, the activity is the same throughout the system when the reference fugacities are defined equally for each phase [10]. Gal [11] showed that in

Glassy State: A Way to Extend Shelf-Life of Food 151 experimental terms there is small difference between water activity and the concept equilibrium relative humidity. Therefore Aw can be express in the following relation (Eq. 2):

(2) where pw is the equilibrium water vapour pressure over the system and pw

θ

the vapour pressure of pure water at the same temperature and pressure. Van den Berg and Bruin [10] mentioned the following considerations regarding the definition of water activity: i) Aw refers only to the true equilibrium state (real food systems do not always fulfil this requirement), ii) Aw is defined at a specific temperature and total pressure and iii) the reference state must be well specified. The relationship between the total moisture content and the water activity, over a range of values, at constant temperature, yields the sorption isotherm when expressed graphically. It can be obtained by absorption or desorption. In both cases, ―equilibrium‖ is achieved when there is no water migration from/to the sample during storage (no changes in material mass is detected). Brunauer et al. [12] classified the adsorption isotherms into five general types, from which type I, II and III are the more to relevant for food systems. Type I is related to the Langmuir sorption behaviour, eg. non-swelling porous solids [13], whilst Type II is a combination of Type I and III. Moisture sorption isotherms of most foods are generally sigmoidal in shape and they have been classified as Type II [14-19]. The interpretation of the sorption isotherm for food systems may be divided in three regions (Figure 1), although the distinction between the three areas can not be expressed in terms of precise ranges of water contents, rather is an indication of the differences in the overall nature of the interaction between water and the solids in the three stages [20]. Region I represents strong interactions with water with an enthalpy of vaporisation considerably higher than of pure water [10, 20]. Usually water molecules in this region are unfreezable and are not available for chemical reaction. Most dried products are empirically observed to display their greatest stability at moisture contents comparable to the monolayer [20]. In region II, the energy associated the water-polymer interactions is lower than in the region I. The enthalpy of vaporisation is little greater than the enthalpy of vaporisation of pure water. These water molecules sorbed near or on the top of the first molecules or penetrate into the newly created spaces of the swollen structure [10]. The water is available as a solvent for low-molecular weight solutes and for some biochemical reactions [18]. The initially hard and brittle material undergoes a glass transition, by the plasticising effect of water, becoming weak, plastic, or rubbery depending of the polymer. Compared to the region I, water molecules show a sharp increase in molecular mobility and therefore in diffusion [4], taking longer to reach ―equilibrium‖, as the rate of polymer swelling can become the limiting factor. In region III, water is present in macro-capillaries or as a part of the fluid phase in high moisture materials. Swelling is exactly in proportion to the volume of water sorbed [20].This moisture exhibits nearly all properties of bulk water and thus is capable of acting as a solvent. Microbial growth becomes the major deteriorative reaction in this region [18]. Rahman [13] discussed the stability of foods in terms of reaction kinetics and sensory attributes as a function of water activity (Aw) presenting the stability map based on Labuza‘s work presented in the 70‘s. The above diagram has been useful in relating the rates of reaction for different food stability

J. I. Enrione and P. Díaz-Calderón 152

parameters to the water activity. Nevertheless, it must be considered that the term ―activity‖ is based on thermodynamic equilibrium, and most real foods do not reach this state between their various components nor with their environment. Some critics [3] have suggested that since the reaction rates (kinetics) shown in the map do not represent a relationship with partial vapour pressure in true equilibrium the stability map is useful, at best, as a quality control tool applicable only for the same products at the same temperature. It has been suggested that the glass to rubber transition provided useful information related to food stability, as it account for water dynamic and molecular mobility of the matrix constituents under various environmental conditions.

Figure 1. Food stability as a function of water activity modified from Rahman [13].