colloidal chemistry in foods

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<p>PowerPoint Presentation</p> <p>COLLOIDAL CHEMISTRYNeetu GoudaMsc. 1st yr</p> <p>INTRODUCTION</p> <p>The word colloids has actually derived from Greek word kola where it means glue like. The science of colloids was founded about 1861 by Thomas Graham. In his summary of work , he classified substances into two groups viz: crystalloids and colloids ,depending on their ability to diffuse through parchment membrane.According to Graham , crystalloids readily passed through parchment membranes while colloids do possess a size of diameter greater than 1 mu and less than 100 mu.</p> <p>Colloids are the particles that are too large to dissolve but remain dispersed in the liquids. The term colloid doesnt indicate a kind of matter, but a state of matter. All colloidal systems have two phases: A continuous phase extends throughout the system, surrounding all parts of the other phase of the system.The discontinuous phase or the discontinuous phase is, as the name implies, distributed in isolated or disconnected fashion throughout the entire colloidal system.COLLOIDS</p> <p>Name of colloidal systemDispersed phaseContinuous phase Example in foodsEMULSIONLIQUIDLIQUIDSALAD DRESSINGSOLSOLIDLIQUIDGRAVYGELLIQUIDSOLIDBAKED CUSTARDFOAMGAS LIQUIDEGG WHITE FOAMSUSPENSOIDGASSOLIDCONGEALED WHIPPED CREAMColloidal systems in foods:</p> <p>Name of colloidal systemDispersed phaseContinuous phase Example in foodsEMULSIONLIQUIDLIQUIDSALAD DRESSINGSOLSOLIDLIQUIDGRAVYGELLIQUIDSOLIDBAKED CUSTARDFOAMGAS LIQUIDEGG WHITE FOAMSUSPENSOIDGASSOLIDCONGEALED WHIPPED CREAM</p> <p>Properties of colloids:Particles are visible only by ultra microscope or electron microscope Brownian motion.They do not sediment, and pass through common filters ( but not through semipermeable membrane).Dispersion of passing light (Tyndall efect).Produce osmotic pressure.Colloids are everywhereIn the human bodyWashing powder, soup, tooth paste, etc.Many foods ( yogurt, butter, milk)Nanotechnologies are based on chemistry of colloids</p> <p>TYNDALL EFFECTS: In a colloidal solution, the dispersed particles are sufficiently large to scatter and polarize the incident light to same extent. This is known as the tyndall effect. The effect is more pronounced in lyophobic sols than in lyophilic sols.</p> <p>NATURAL TYNDALL EFFECTWhen the sunrays enter the forest canopy, TYNDALL effect can be observed due to the dust particles in the atmosphere which helps in scattering of light.</p> <p>BROWNIAN MOTION: When viewed through an ultra microscope , colloidal particles appear to be in a state of rapid and irregular motion called the Brownian movement. The movement is caused by the constant bombardment of the dispersed particles by the molecules of the dispersion medium .It is zigzag vibration of the dispersed particles .The smaller the size of the colloidal particles. The more vigorous is its Brownian motion.</p> <p>CLASSIFICATION OF COLLOIDS:Colloids are classified into two groups :1.] Lyophobic colloids: Includes those colloids which have affinity for water. Eg. Inorganic colloids.</p> <p>Negative charge of S2- or HS- is compensated by opposite charge from the (H+).</p> <p>2.] Lyophilic colloids: Includes organic colloids such as protein , starch , which have great affinity to water.</p> <p>CHARGE ON COLLOIDAL PARTICLES:All colloids possess an electrical charge which may be either positive or negative. In a given colloidal system, all particles have like charges and hence tend to repel one another and remain in suspension. To be in suspension state, the colloidal particles distribute themselves uniformly throughout the liquid in which they are dispersed.Colloidal systems with two phases can occur in eight different combinations so colloidal systems are not restricted to the dispersion of solid in liquid only. Each of the three states of matter can be dispersed in a medium which may be gaseous, liquid or solid. </p> <p>1.EMULSION: A mixture of two or more immiscible liquids. one liquid (the dispersed phase) is dispersed in the other (the continuous phase). Three substance are required to produce an emulsion: two immiscible liquids and an emulsifying agent. Prepared by shaking oil/water (milk), water/ oil (butter).</p> <p>Structure of emulsions: When a liquid e.g. oil is dispersed in another immiscible liquid e.g. water(continuous phase) by mechanical agitation , an emulsion is obtained.Most of the dispersed droplets in food emulsions have diameters between 0.1 and 10 micrometer.Classification of emulsions emulsions can be broadly divided into two groups :Oil in water type Oil in water type emulsions consist of fat dispersed in water e.g. milk.2.) Water in oil type water in oil type emulsions are cream , butter , mayonnaise etc.</p> <p>Some examples of food emulsions and uses:Milk is an emulsion of oil in water type and consists of fat dispersed in water; the emulsifying agent being albumin and casein . Cod liver oil is an emulsion of oil in water type in which egg yolk , casein , gums have been added as emulsifier.Butter is an emulsion of water dispersed in fat.</p> <p>Properties of emulsions: The opacity of an emulsion is determined by droplet sizes , droplet concentration and difference in the refractive indices of the two phases.</p> <p>Emulsion is transparent when the refractive indices of the two liquids are the same or when droplet diameter is 0.05micrometer or less.</p> <p>When the dispersed phase is increased to a volume greater than that of the continuous phase the viscosity of the emulsion increases. </p> <p>Formation of emulsions: A stable emulsions is not formed by a mere mixing of the liquids .</p> <p>Because when it will be left for sometime the droplets in the dispersed phase coalesce due to surface tension.</p> <p>Therefore emulsifier is added to the emulsions,which reduces the interfacial tension and enhances the coalesce.</p> <p>Ex: Eggs yolk is incorporated in the emulsions since it contains lecithin , a natural emulsifying agents.</p> <p>Stability of emulsions: Stability changes in food emulsions can occur through the processes of creaming , flocculation and coalescence. Emulsions can be stabilised against creaming , flocculation and coalescence by introducing a strong interfacial film around each droplets , adding electric charges to the droplets surfaces and increasing the viscosity of the continuous phase.E.g Hydrocolloids, plant gums and gelatin , act as stabalizers in oil-in-water emulsions by increasing the viscosity of the continuous phase. sometimes by forming a strong interfacial film around droplets of thr dispersed phase. Pectins , alginates , plant and seed gums, and cellulose derivatives, such as carboxymethayl, hydroxypropyl and methylcellulose gums are used as stabilizers.</p> <p>Norevo- Acacia gum</p> <p>1.SOLS : Sols are the colloidal systems in which solid of colloidal dimensions is dispersed throughout a liquid .</p> <p>This type of system has flow properties , which may range from rather fluid to extremely viscous , barely flowing .</p> <p>Sol possessing a continuous aqueous phase , the colloidal particles have an electrically charged surface.The ionized groups of protiens and phospholipids can be sources of this charges.</p> <p>The resulting potential difference between the surface of the particles and the solution is known as zeta potential.</p> <p>In lyophobic sols, there is little interaction between the dispersed phase and the dispersion medium.</p> <p>The viscocity of the lyophilic sols is greater than that of the dispersion medium and it increases in particle concentration and decreases in increasing of teperature.</p> <p>GELS:Sols that have fairly high concentration of dispersed solids change spontaneously into gels. </p> <p>The liquid phase is entrapped in the interstitial ares of this structure, due to which the sols loses its fluidity and becomes a gel.</p> <p>Bonds present in the junction zones of gel are electrostatic, hydrophobic ,covalent and hydrogen bond.</p> <p>Some of the free liquid ma be released if the gel structure is cut.Drainage of free liquid from a gel is termed as Syneresis. </p> <p>Characteristics of gels:The concentration of the jelling agent ,the salt content , the pH and temperature determine the strength , elasticity and brittleness of the gel.</p> <p>All gels can be destroyed by mechanical action ,but some of them will set again after the agitation stops,such gels said to be thixotropic.</p> <p>FOAMS:Foams are dispersions of gas bubbles in a liquid which is in the continuous phase .</p> <p>The diameters of the foam bubbles range from about 1 micrometer to several centimeters. Depending on the bubble size and wall thickness, dense or light foams are formed.</p> <p> Food foams contain large amounts of entrapped gas.</p> <p>Formation of foams: Liquid capable of forming foams have low surface tension and thus can spread or stretch easily and so not coalesce readily. </p> <p>For the formation of a foam there must be a foaming agent in the continuous phase prior to dispersion of gas.</p> <p>The foaming agent must be absorbed at the surface to reduce surface tension and provide a distinct surface layer which resists the coalescence of gas bubbles.</p> <p>Surface active lipids, glucosides, cellulose derivatives and protein are used as foaming agents.</p> <p>Defoaming agents : Several liquids will destroy foams ; ether, toulene, the aliphatic alcohols , and some oil breaks most foams.</p> <p>Foam stability: Foam stability can be enhanced by increasing the elasticity of the bubble walls, by increasing the viscosity of the solution. </p> <p>This can be done by introducing gums and proteins, such as gelatin. Foaming agents also helps in the stability of foam.</p> <p>Protein coagulum has a high water binding capacity and thus contributes a decrease in the rate of drainage from foam.</p>


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