Relevance of physical properties in the stability of plant-based food products.

Plant tissues are composed of a watery solution of low molecular weight species, mainly sugars, salts and organic acids, and of high molecular weight hydrocolloids, contained in a water insoluble matrix of macromolecules, mostly carbohydrates. All these constituents interact with water, thus reducing its thermodynamic vapour pressure (a(w)), with small molecules interacting through polar binding, and large biopolymers through surface and capillary effects. Similarly, some constituents will greatly affect kinetic glass transition temperatures (T(g)), while others will not. As regards stability, while microbial and chemical changes are mainly related to a(w), structure-related changes such as collapse are dependent on the glass transition temperature, T(g). In simple systems such as juices, both thermodynamic and kinetic approaches, employed respectively for high and low moisture systems, have predictive ability, which can be unified in the concept of "critical a(w)". However, in complex, multidomain, multiphase systems, such as vegetables and fruits, where insoluble polymeric phases are present, hydrocolloids such as soluble pectins will only slightly affect T(g) and a(w), but significantly increase the macro viscosity of the soluble fraction, thereby reducing the tendency to collapse. In such cases the use of T(g) as a predictive tool must be considered with care. The interrelationships among these aspects are discussed in detail below.