Indian Journal of Experimental Biology Vol. 51, November 2013, pp. 895-904 Review Article Relevance of physical properties in the stability of plant-based food products Elena Venir* & Enrico Maltini Department of Food Science, University of Udine, Via Sondrio 2/A, 33100 Udine, Italy 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 (aw), 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 (Tg), while others will not. As regards stability, while microbial and chemical changes are mainly related to aw, structure-related changes such as collapse are dependent on the glass transition temperature, Tg. 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 aw”. 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 Tg and aw, but significantly increase the macro viscosity of the soluble fraction, thereby reducing the tendency to collapse. In such cases the use of Tg as a predictive tool must be considered with care. The interrelationships among these aspects are discussed in detail below. Keywords: Glass transition, Plant food, Stability, Water activity Food stability is influenced by chemical, physical and hydrocolloids4. The intracellular air spaces in enzymatic reactions, and is dependent on the system's parenchymous tissues of fresh plant food can be physical state−in relation to both thermodynamic and considered as structural elements, influencing the kinetic aspects−which can be described, respectively, perceived texture. In heterogeneous, multidomain, by water activity (aw) and glass transition temperature multiphase and more structured food systems (e.g. 1 (Tg) . In high-moisture homogeneous systems (e.g. dehydrated apple cubes), soluble/insoluble high concentrated juices, puree, etc.), the soluble molecular weight compounds, such as pectins and cell compounds, by interacting with water, are responsible wall components, have only a negligible effect on Tg for modifying aw, whereas in low-moisture dehydrated and aw, but can greatly increase the macro-viscosity of products (e.g. dried vegetables, fruit powders) the the system, and so delay time-dependent physical soluble components mainly affect mobility. In the changes to an extent that can no longer be predicted 5 latter case, the dynamics of change is influenced by only on the basis of Tg and aw . kinetic properties and may be better predicted by Tg 2 rather than by aw . The concept of “critical aw”, i.e. Interaction with water and food stability where the aw depresses Tg below environmental/ The physical properties of foods are affected by operative temperature3, can serve as a unifying water, which is more or less ubiquitous in food principle. Nevertheless, plant foods are mostly products. The influence of water on physical heterogeneous systems consisting of a water-insoluble properties is dependent on the state of water, which, cellular matrix of macromolecules, mostly in turn, depends on the interactions between water carbohydrates (including pectins and hemicelluloses) molecules and the constituents of food, with the and proteins, which are embedded in a watery solution chemical composition determining the extent and of low molecular weight species (mainly sugars, salts strength of interaction. and organic acids) and high molecular weight The thermodynamic parameter aw is mainly able to _______________ describe and predict the behaviour of reaction rates of *Correspondent author changes in intermediate high moisture systems, where Telephone: +39 0432 558100 (EV); +39 0432 558100 (EM) Fax. +39 0432 558160 (EV); +39 0432 558160 (EM) the soluble compounds interacting with water are E-mail: [email protected]; [email protected] responsible for lowering aw. 896 INDIAN J EXP BIOL, NOVEMBER 2013 In more dehydrated systems, which can be far from This description of water activity assumes that food thermodynamic equilibrium, metastable glasses may is in equilibrium with the surrounding atmosphere. form and aw can be less important with respect to the Under isobaric conditions and at moderate molecular mobility/macro viscosity of the system. In temperatures the deviation of water vapour from the such systems, the kinetic parameter Tg is the reference ideal gas is small, and water activity calculated by the parameter for the description and prediction of above equation varies from the thermodynamic value changes. by less than 5%11. Water activity is widely used as a reference Water activity is much more important for food parameter in most applications of food processing and stability than the total moisture content (MC). In fact storage for a variety of reasons: it is a determinant for MC alone does not provide information on the state of the growth of microorganisms; it is well related with water in a food, if it is “bound” or “free”, “inherent” most degradation reactions of a chemical, enzymatic, or “occluded”, etc., whereas aw gives an indication of and physical nature; it is easier to measure than the relationship of water with the binding sites of the moisture content, and measurement is non- food components. The equilibrium relationship destructive; the ‘‘monolayer’’ derived from the water between moisture content and the relative humidity of vapour sorption isotherm gives an indication of the the surrounding environment is described through the optimum moisture content in dried foods. moisture sorption isotherms12. Water sorption The role of aw in fruit-vegetable ingredients for isotherms may be either adsorption or desorption composite foods derives from the fact that a number of isotherms: adsorption occurs when food solids are formulations (ice cream, pie fillings, pastry, dessert, exposed to conditions where the vapour pressure of fruit yoghurt, etc.) are obtained from fruit/vegetable water in the atmosphere is higher than the water ingredients and moisture migration is critical for vapour pressure in the solid. Conversely, desorption obtaining end products which are attractive to the occurs when the water vapour pressure within the consumer. Indeed, moisture migration is frequently solid is higher than that of the atmosphere. implicated in quality loss in composite multiphase Water sorption isotherms systems when components or phases of different water 6−8 Sorption isotherm describes the thermodynamic activity are in contact . Water moves from the wet to relationship between water activity and the the dry components or phases, possibly leading to equilibrium moisture content of a food product, at irreversible loss in sensorial and microbiological 9 constant pressure and temperature (Fig. 1). quality and, of course, to shelf life reduction . According to the Brunauer’s classification13, the In multicomponent systems moisture loss or gain shapes of isotherms, which describe the sorption of from one region (or food component) to another will gases by solid materials, may have five basic forms. occur continuously until thermodynamic equilibrium is reached. Moisture migration in multi-domain foods obeys aw and not moisture content, with water activity being the controlling parameter. Water activity (Thermodynamic property) The state of water in a solution or in a solid is expressed by the activity coefficient (aw = water activity), which is a thermodynamic measure of the chemical potential of water in the system. In food science water activity is referred to as a measure of the moisture availability in a product expressed as the ratio of the water vapour pressure of the substance (p) to that of pure water (vapour pressure) (p0) at the 10 Fig. 1Schematic representation of adsorption and desorption same temperature and pressure . isotherms showing the amount of water adsorbed as a function of steady state relative vapour pressure (RVP) at a constant = P temperature. The difference between these two curves shows a W hysteresis, indicative of the irreversibility of water sorption during P0 dehydration and rehydration. VENIR & MALTINI: PHYSICAL PROPERTIES IN STABILITY OF PLANT-BASED FOOD PRODUCTS 897 Water adsorption isotherms of food materials are type The extra assumption of the GAB model states that II, i.e. sigmoid curves, which often exhibit hysteresis the sorption state of the sorbate molecules in the between adsorption and desorption. The sigmoid, type layers beyond the first is different to the pure liquid II curve is also the typical shape of the desorption and state22. In common with the BET model, the GAB adsorption isotherms of plant tissues14. Some equation has the monolayer moisture content and crystalline material, e.g. sugars, may display a fairly energy constant parameters, and introduces an low adsorption of water until water activity becomes additional constant (k) which is related to the enthalpy sufficient for solubilization, after which the difference between water molecules in the pure liquid adsorption increases. Such