Foods kinetic stability

Although most colloidal dispersions are not thermodynamically stable, a consequence of the small size and large surface area in colloids, and of the presence of an interfacial film on droplets, bubbles or particles, is that dispersions of these species, having reasonable kinetic stability, can be made. That is, suspended droplets or particles may not aggregate quickly nor settle or float out rapidly and droplets in an emulsion or bubbles in a foam may not coalesce quickly. Many food and personal care product emulsions and suspensions, for example, are formulated to remain stable for months to years. It is crucial that stability be understood in terms of a clearly defined process, and one must consider the degree of change and the time-scale in the definition of stability. 

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. 

Microstmctures are frequently present in a kinetically trapped nonequilibrium state, and their structures depend on the components and colloidal interactions based on their different chemical and physical properties, as well as on the procedure by which these components have been assembled. These structures are thermodynamically unstable and tend to reduce their free energy (surface area) with time. On the contrary, self-assembly nanostructures are thermodynamically stable, unless the molecules react with the environment or degrade. Most food systems are based on an interplay of kinetically stabilized and thermodynamic equilibrium structures. Some typical examples of structures at different length scales formd in food systems are shown in Figure 11.1. 

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. 

Emulsified systems can be classified aceording to their thermodynamie stability and their droplets size. Macroemulsions (or simply emulsions) are metastable systems, i.e., the system is not in thermodynamic equilibrium, and it will breakdown into two distinct phases if suffieient time is allowed. However, emulsions that keep their kinetic stability for periods of months or years ean be prepared by using appropriate components and amounts (McClements et al., 2007). This is the most common type of emulsion, and it is found in many food systems such as milk and salad dressing. Macroemulsions are usually polydisperse, with droplet sizes in the range of 1-100 pm. The main destabilization mechanisms in macroemulsions are droplets creaming, flocculation, and coalescence. 

Many products in the chemical and agrochemical, cosmetic, pharmaceutical, and food industries are emulsion-based. Their internal structure is composed of one or more fluids, with one being flnely dispersed as droplets within the other one. The size distribution of the droplets mainly influences characteristic product properties as color, texture, flow- and spreadability, viscosity, mouth-feel, shelf-life stability, and release of active ingredients. It therefore has to be maintained for the life-time of a product. Due to the extremely high interfacial area in these systems, this microstructure is thermodynamically unstable. By applying emulsiflers and thickeners, emulsions are kinetically stabilized for a certain amount of time. Elowever, shelf-life stability always is a big chal-... 

Emulsions are thermodynamically unstable structures given a degree of kinetic stability by an adsorbed interfacial layer of amphiphilic emulsifiers. The emulsifiers serve to lower the interfacial tension and provide some inter-droplet repulsive forces to stabilize the dispersions (e.g., steric and electrostatic). The interfacial layer is typically between about 1 and lOnm thick for food grade emulsifiers, such as surfactants, phospholipids, proteins, or polysaccharides, and the interfacial concentration is in the order of a few mg per square meter of surface (McClements, 2005 McClements and Decker, 2000). 

Emulsions are dispersions of two immiscible liquids into each other. They are thermodynamically unstable, but the addition of surfactant molecules can provide significant kinetic stability. Emulsions are extensively used in food, cosmetic, and pharmaceutical industries, just to name a few. Because of the thermodynamic penalty, emulsion formation requires an energy input. In bulk systems, this can most easily be achieved by vigorous stirring or shaking of the whole oil/water/surfactant system. This approach leads to an pulsion with broad droplet size distribution. Microfluidics allows for the minimization of polydispersity and the creation of droplets that are virtually identical in size. 

Kinetics can also be applied to the optimization of process conditions, as in organic syntheses, analytical reactions, and chemical manufacturing. This last example constitutes an important aspect of chemical engineering. Yet another practical use of chemical kinetics is for the determination and control of the stability of commercial products such as pharmaceutical dosage forms, foods, paints, and metals. 

The thermal degradation of anthocyanins, both in extracts and model systems, was reported to follow first-order reaction kinetics in all studies. The stability of anthocyanins and all pigments found in foods decreased with increases in temperature. 

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... 

Srinivasulu, S. and Rao, A.G.A. 1995. Structure and kinetic thermal stability studies of the interaction of monohydric alcohols with lipoxygenase 1 from soybeans (Glycine max). J. Agric. Food Chem. 43 562-567. ... 

Agboola, S.O. and Dalgleish, D.G. 1996. Enzymatic hydrolysis of milk proteins used for emulsion formation. 1. Kinetics of protein breakdown and storage stability of the emulsions. J. Agric. Food Chem. 44, 3631-3636. 

Carter, E.J.V., Paredes, G.E., Beristain, C.l. and Tehuitzil, H.R. (2001) Effect of foaming agents on the stability, rheological properties, drying kinetics and flavour retention of tamarind foam mats. Food Research International 34, 587-598. 

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