Food systems, structure emulsion system

Food typically is a complicated system with diverse interfaces. Stable air-water or oil-water interfaces are essential for the production of food foams and emulsions. Interface phenomena, therefore, attract great interest in the food industry. AFM provides enough resolution to visualize the interface structures, but it cannot be directly applied on air-liquid or liquid-liquid interfaces. Fortunately, the interface structure can be captured and transferred onto a freshly cleaved mica substrate using Langmuir-Blodgett techniques for AFM scan. Images are normally captured under butanol to reduce adhesion between the probe and the sample. Then, sample distortion or damage can be avoided (Morris et al, 1999). 

New experimental techniques and several of their applications were presented which help in the understanding of structure, texture and stability of food systems. For future research, the mechanism of film stability by the microlayering of colloid particles seems to be the most promising - especially in food emulsions and foams. Work is in progress in our laboratory to calculate the oscillatory disjoining pressure inside liquid films containing microlayers [30],... 

Once shearing is stopped (the external force is no longer acting), the rate of the system structural recovery is the same as the rate of structural breakdown under steady shear. In foods, the time needed for recovery may vary significantly from one thixotropic product to another. The time dependency can be observed in food systems such as concentrated emulsions, sols, and gels. 

Most interfaces encountered in food systems will contain more than one protein. Commercial materials used to stabilize emulsions or foams are complex isolates rather than purified single proteins. To describe the behavior of protein isolates it is necessary to imderstand the types of structures formed by mixtures of proteins at the interface, and also how such mixed structures are displaced by surfactants. 

Time-dependent rheological properties reflect the nature of a system s structure and can be due to viscoelasticity, structural changes, or both (Cheng and Evans, 1965 Harris, 1972). Structure breakdown can result in a decrease in the viscosity of a substance. It occurs in emulsions, suspensions, and sols. The characterization of the time-dependent flow properties of food systems is important for process design and control, for product devel-... 

Fructans have humectant properties that reduce water activity and improve microbiological stability. When mixed with water, inulin can form a particle gel network, resulting in a white creamy structure with a short spreadable texture [32]. Emulsion of long-chain fructan in water has organoleptic properties similar to fat and has been used as a fat replacement in food systems. Inulin works in synergy with most gelling agents. 

McClements, D. J. Li, Y., Structured emulsion-based delivery systems Controlling the digestion and release of lipophilic food components. Advances in Colloid and Interface Science (2010) 159, 213-228. 

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-... 

As discussed above, for emulsion stabihzation in food systems lamellar liquid crystalline structures are ideal. At the interface, the liquid crystals serve as a viscous... 

Since William Seifriz described for the first time in 1925 these intricate liquid systems having ternary, quaternary, or more complex structures that he named multiple emulsions, the literature has been flooded every year with tens of new examples demonstrating release patterns and control of active ingredients using these systems. Multiple emulsions, at least in theory, have significant potential in many applications because the internal droplets can serve as an entrapping reservoir for active addenda that can be released by a controlled transport mechanism. Many of the potential applications would be realized in the fields of agriculture, pharmaceuticals, cosmetics, and food. 

One reason for the continued dominance of emulsion-based delivery systems is that they are so common in foods. Many foods are traditionally emulsions and if they are fortified with a lipophilic ingredient then that ingredient will partition into the lipid phase. The performance of these emulsions as delivery vehicles can be improved, particularly by modifying the structure of the surface and, to a lesser extent, the particle size. More recent research has examined the potential of improving the EBDS by modifying the crystal structure of the dispersed Upid phase. 

The term food colloids can be applied to all edible multi-phase systems such as foams, gels, dispersions and emulsions. Therefore, most manufactured foodstuffs can be classified as food colloids, and some natural ones also (notably milk). One of the key features of such systems is that they require the addition of a combination of surface-active molecules and thickeners for control of their texture and shelf-life. To achieve the requirements of consumers and food technologists, various combinations of proteins and polysaccharides are routinely used. The structures formed by these biopolymers in the bulk aqueous phase and at the surface of droplets and bubbles determine the long-term stability and rheological properties of food colloids. These structures are determined by the nature of the various kinds of biopolymer-biopolymer interactions, as well as by the interactions of the biopolymers with other food ingredients such as low-molecular-weight surfactants (emulsifiers). 

Most food products and food preparations are colloids. They are typically multicomponent and multiphase systems consisting of colloidal species of different kinds, shapes, and sizes and different phases. Ice cream, for example, is a combination of emulsions, foams, particles, and gels since it consists of a frozen aqueous phase containing fat droplets, ice crystals, and very small air pockets (microvoids). Salad dressing, special sauce, and the like are complicated emulsions and may contain small surfactant clusters known as micelles (Chapter 8). The dimensions of the particles in these entities usually cover a rather broad spectrum, ranging from nanometers (typical micellar units) to micrometers (emulsion droplets) or millimeters (foams). Food products may also contain macromolecules (such as proteins) and gels formed from other food particles aggregated by adsorbed protein molecules. The texture (how a food feels to touch or in the mouth) depends on the structure of the food. 

COLLOID SYSTEMS. Colloids are usually defined as disperse systems with at least one characteristic dimension in the range 10 7 lo ll> centimeter. Examples include sals (dispersions or solid in liquid) emulsions (dispersion of liquids in liquids) aerosols (dispersions of liquids or solids in gases) /inum (dispersion of gases in liquids or solids) and gels (system, such as common jelly, in which one component provides a sufficient structural framework for rigidity and other components fill the space between the structural units or spaces). All forms of colloid systems are encountered in nature. Products of a colloidal nature arc commonly found in industry and are notably extensive in the food field. Foams, widely used in industrial products, but also the causes of processing problems are described in entries on Foam and Foamed Plastics. 

Many food products (salad dressings, whipped toppings, ice cream etc.) are dispersed colloid systems, such as emulsions, suspensions or foams. Texture, structure and stability of these dispersions have fundamental importance for the food manufacturer. Our chapter presents new methods, most of them developed in our laboratory, and mechanisms which can be very helpful for the food researcher or developer. 

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