Foods thermodynamic stability

Oakenful, D. 1984. A method for using measurements of shear modulus to estimate the size and thermodynamic stability of junction zones in noncovalently cross-linked gels. J. Food Sci. 49 1103-1104,1110. 

The stability of an emulsion denotes its ability to resist changes in its properties over time (i.e., higher emulsion stability implies slower change in emulsion properties). When considering the stability of an emulsion, it is of major importance to distinguish between thermodynamic stability and kinetic stability. Thermodynamics predict whether or not a process will occur, whereas kinetics predict the rate of the process, if it does occur. All food emulsions are thermodynamically unstable and thus will break down if left long enough. 

A variety of rheological tests can be used to evaluate the nature and properties of different network structures in foods. The strength of bonds in a fat crystal network can be evaluated by stress relaxation and by the decrease in elastic recovery in creep tests as a function of loading time (deMan et al. 1985). Van Kleef et al. (1978) have reported on the determination of the number of crosslinks in a protein gel from its mechanical and swelling properties. Oakenfull (1984) used shear modulus measurements to estimate the size and thermodynamic stability of junction zones in noncovalently cross-linked gels. 

Very often, the microstructure and the macroscopic states of dispersions are determined by kinetic and thermodynamic considerations. While thermodynamics dictates what the equilibrium state will be, kinetics determine how fast that equilibrium state will be determined. While in thermodynamics the initial and final states must be determined, in kinetics the path and any energy barriers are important. The electrostatic and the electrical double-layer (the two charged portions of an inter cial region) play important roles in food emulsion stability. The Derjaguin-Landau-Verwey-Oveibeek (DLVO) theory of colloidal stability has been used to examine the factors affecting colloidal stability. 

This argument is logical and scientifically accurate with respect to the transfer of iron in the body. However, it does not address the potential importance of the thermodynamic stability constants of the two common valence forms of iron (II and III) with ligands in food. 

When iron is added to a food the environment is going to affect the valence state as has been discussed previously. One of the parameters which might maintain a particular valence state in the face of adverse environmental conditions, such as pH or redox potential, is the stability of its complex. Therefore, either the Fe+ or Fe+3 form might be maintained if one formed a complex with a greater thermodynamic stability than the other as discussed previously. Therefore, it would seem that the importance of the thermodynamic stability constant should not be discounted because it could have a great deal of relevance with respect to solubility and maintenance of a specific iron valence within a given food system. 

Fats and oils constitute a very large portion of the food industry. Fats are multicomponent systems containing many different triglycerides (and other lipids in smaller amounts). They crystallize in different polymorphic forms, often termed a, P and P in order of thermodynamic stability and melting point. The proportions of these phases in final products largely determine basic quality attributes of the material, such as palatability, appearance or shelf life. 

Microemulsions, like micelles, are considered to be lyophilic, stable, colloidal dispersions. In some systems, the addition of a fourth component, a cosurfactant, to an oil-water-surfactant system can cause the interfacial tension to drop to near-zero values, easily on the order of 10 - 10 mN/m, allowing spontaneous or nearly spontaneous emulsification to very small drop sizes, 10 nm or smaller. The droplets can be so small that they scatter little light, and the emulsions appear to be transparent and do not break on standing or centrifuging. Unlike coarse emulsions, microemulsions are thought to be thermodynamically stable. The thermodynamic stability is frequently attributed to transient negative interfacial tensions, but this, and the question of whether microemulsions are really lyophilic or lyophobic dispersions are areas of some discussion in the literature. As a practical matter, microemulsions can be formed, have some special qualities, and can have important applications in areas such as enhanced oil recovery, soil and aquifer remediation, foods, pharmaceuticals, cosmetics, herbicides, and pesticides (13,16,45,59-61). 

The potential technical and commercial applications of microemulsions are mainly linked to their unique properties such as thermodynamic stability, optical clarity, and high solubilization capacity. However, the most critical problem regarding the use of microemulsions in the food, cosmetic, and pharmaceutical fields is the toxicity of their partial components. Formulation and characterization of nontoxic microemulsions based on naturally occurring amphiphiles and different oils have been studied for almost two decades. The first attempt to use natural biodegradable surfactants for the formulation of nontoxic microemulsions was reported by Shinoda et al. [ 155,156]. In this regard, soybean lecithin is a combination... 

The important properties unique to microemulsions - thermodynamic stability, ultra-low interfacial tensions, translucence, small and tunable microstructures - make microemulsions interesting for a variety of applications. Microemulsions find application as a reaction medium for formation of polymeric and inorganic nanoparticles, for the dispersion of drugs, food stuffs, agrochemicals, and cosmetic ingredients, and in detergency, the enhancement of oil recovery from reservoirs, and the extraction of contaminated solids (17). 

Studies of the stability and stabilization of anthocyanins are still required, based on the extreme importance of those pigments for food colors. Modem HPLC-MS equipment also allows us to easily follow the copigmentation reactions in detail, calculate their kinetic and thermodynamic parameters, identify the products formed during the reactions, and thus shed new light on the stability and stabilization of these pigments. Since anthocyanins play important roles as natural colorants for... 

An emulsion is a dispersed system of two immiscible phases. Emulsions are present in several food systems. In general, the disperse phase in an emulsion is normally in globules 0.1-10 microns in diameter. Emulsions are commonly classed as either oil in water (O/W) or water in oil (W/O). In sugar confectionery, O/W emulsions are most usually encountered, or perhaps more accurately, oil in sugar syrup. One of the most important properties of an emulsion is its stability, normally referred to as its emulsion stability. Emulsions normally break by one of three processes creaming (or sedimentation), flocculation or droplet coalescence. Creaming and sedimentation originate in density differences between the two phases. Emulsions often break by a mixture of the processes. The time it takes for an emulsion to break can vary from seconds to years. Emulsions are not normally inherently stable since they are not a thermodynamic state of matter. A stable emulsion normally needs some material to make the emulsion stable. Food law complicates this issue since various substances are listed as emulsifiers and stabilisers. Unfortunately, some natural substances that are extremely effective as emulsifiers in practice are not emulsifiers in law. An examination of those materials that do stabilise emulsions allows them to be classified as follows ... 

The two main assumptions underlying the derivation of Eq. (5) are (1) thermodynamic equilibrium and (2) conditions of constant temperature and pressure. These assumptions, especially assumption number 1, however, are often violated in food systems. Most foods are nonequilibrium systems. The complex nature of food systems (i.e., multicomponent and multiphase) lends itself readily to conditions of nonequilibrium. Many food systems, such as baked products, are not in equilibrium because they experience various physical, chemical, and microbiological changes over time. Other food products, such as butter (a water-in-oil emulsion) and mayonnaise (an oil-in-water emulsion), are produced as nonequilibrium systems, stabilized by the use of emulsifying agents. Some food products violate the assumption of equilibrium because they exhibit hysteresis (the final c/w value is dependent on the path taken, e.g., desorption or adsorption) or delayed crystallization (i.e., lactose crystallization in ice cream and powdered milk). In the case of hysteresis, the final c/w value should be independent of the path taken and should only be dependent on temperature, pressure, and composition (i.e.,... 

Nowadays it is well established that the interactions between different macromolecular ingredients (i.e., protein + protein, polysaccharide + polysaccharide, and protein + polysaccharide) are of great importance in determining the texture and shelf-life of multicomponent food colloids. These interactions affect the structure-forming properties of biopolymers in the bulk and at interfaces thermodynamic activity, self-assembly, sin-face loading, thermodynamic compatibility/incompatibility, phase separation, complexation and rheological behaviour. Therefore, one may infer that a knowledge of the key physico-chemical features of such biopolymer-biopolymer interactions, and their impact on stability properties of food colloids, is essential in order to be able to understand and predict the functional properties of mixed biopolymers in product formulations. 

In considering the impact of thermodynamically favourable interactions between biopolymers on the formation and stabilization of food colloids, a number of regular trends can be identified. One of the most important aspects is the effect of complexation on interfacial properties, including rates of adsorption and surface rheological behaviour. 

Adsorbed layers of mixed biopolymers are potentially non-equilibrium systems in terms of their structure and composition. Therefore one has to be aware that the impact of thermodynamical favourable interactions between biopolymers on the formation and stabilization of food colloids is dependent, not only on the total system composition, but also on the experimental procedure whereby the two interacting biopolymers are brought to the interface (McClements, 2004 Jourdain et aL 2008, 2009 Dickinson, 2008a). 

Water activity of foods is an important thermodynamic property affecting stability with respect to physical, chemical and microbiological changes. Water activity, av, is the ratio of the vapor pressure of water in a system, (Pl)sy, to the vapor pressure of pure water, (Pv)w, at the same temperature. It is equal to the equilibrium relative humidity (ERH) established in the surrounding air. Thus ... 

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