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In this article we will discuss about:- 1. Classification and Functions of Colloidal Systems in Food 2. Types of Colloidal System in Food 3. Stability.
Classification and Function of Colloidal Systems in Food:
A colloid is a type of chemical mixture where one substance is dispersed evenly throughout another. Colloids are formed when one substance is dispersed through another, but does not combine to form a solution. There are many types of colloidal systems depending on the state of the two substances mixed together.
A colloidal system consists of two separate phases: a dispersed phase (or internal phase) and a continuous phase (or dispersion medium). A colloidal system may be solid, liquid, or gaseous. The substance which is dispersed is known as the disperse phase and is suspended in the continuous phase. Egg white foam is an example of this. Air bubbles (disperse phase) are trapped in the egg white (continuous phase) resulting in a foam.
Colloidal dispersion is a two phase system in which the particles in the dispersed phase are between 1 and 100 nm in diameter. For this reason, most manufactured foods can be considered as food colloids and many contain hydrocolloids that are added to control stability and rheological properties.
Food hydrocolloids are high molecular weight hydrophilic biopolymers used in food products to control their texture, flavor and shelf life. Colloidal systems in foods can be classified into different groups based on the states of matter constituting the two phases. They are sols, gels, emulsion and foam. Emulsion and foam again can be categorised into solid emulsion/foam and liquid emulsion/foam.
Colloids are formed when one substance is dispersed through another, e.g., sols (a solid is dispersed in a liquid), gels (a liquid held in a solid network, e.g., jam or jelly), emulsions (oily and watery liquids mixed together, e.g., milk and butter), foams (bubbles of gas trapped in a liquid, e.g., whisked egg white or whipped cream), solid foam (bubbles of gas trapped in a solid, e.g., meringue, cake, bread).
The detailed classification of colloidal systems in food is given in Table 2.1.
Most colloids are stable, but the two phases may separate over a period of time because of an increase in temperature or by physical force. They may also become unstable when frozen or heated, especially if they contain an emulsion of fat and water.
Functions of Colloidal Systems in Food Products:
Colloidal systems give structure; texture and mouth-feel to many different food products, for example – Jam, ice cream, mayonnaise. Food colloid contains hydrocolloid that gives stability and rheological properties of food components. An emulsifying agent may be used to help the oil and water phases to mix permanently.
Types of Colloidal System in Food:
(i) Sols and Gels:
A sol can be defined as a colloidal dispersion in which a solid is the dispersed phase and liquid is the continuous phase. Gravy, stirred custard and other thick sauces are some of the examples of sols. When a jelly is made, gelatin is dispersed into a liquid and heated to form a sol. As the solution cools, protein molecules unwind forming a network that traps water and forms a gel.
If corn flour is mixed with water and heated, the starch granules absorb water until they rupture, the starch then disperses in the water and the mixture becomes more viscous and forms a gel on cooling. Other types of gel are formed with pectin and agar. Pectin, a form of carbohydrate found in fruits, is used in the production of jam to help it set.
However, for it to gel there must be at least 50% sugar and conditions should be acidic. Agar is a polysaccharide extracted from seaweed which is capable of forming gels. If a gel is allowed to stand for a time, it starts to ‘weep’. This loss of liquid is known as syneresis.
An emulsion is a mixture of two or more immiscible (they will not mix together) liquids. One liquid (the dispersed phase) is dispersed in the other (the continuous phase), i.e., material that keep fat globules in water droplet or water droplet in fat are emulsifiers. When water and oil are shaken together, they form an emulsion. This emulsion is unstable.
If left to stand, the oil will form a separate layer on top of the water, e.g., traditional French dressing. A stable emulsion is formed when two immiscible liquids are held stable by a third substance, called an emulsifying agent. An emulsion may be oil-in-water (o/w) in which case small oil droplets are dispersed through water, e.g., milk, or water-in-oil (w/o) in which case small water droplets are dispersed through oil, e.g., butter.
Foams are composed of small bubbles of gas (usually air) dispersed in a liquid, e.g., egg white foam. As liquid egg white is whisked, air bubbles are incorporated. The mechanical action causes albumen proteins to unfold and form a network, trapping the air. If egg white is heated, protein coagulates and moisture is driven off. This forms solid foam, e.g., a meringue. Ice cream, bread and cake are other examples of solid foams.
Stability of Colloidal Systems:
All colloidal systems have two phases a continuous phase and discontinuous or dispersed phase. The particles of the dispersed substance are suspended in the mixture and do not completely dissolved within. The substance which is dispersed is known as the disperse phase and is suspended in the continuous phase. Most colloids are stable. The stability depends on the interaction between the two phases. But the two phases may separate over a period of time because of an increase in the temperature or by physical force.
Stability of Sols and Gel in Food:
A sol is a colloidal system in which a solid is dispersed phase and liquid is the continuous phase. The proper ratio of the ingredients is necessary to achieve the desired viscosity of the sols at a certain temperature. Pectin is hydrophilic and attracts a layer of water that is bound tightly to the molecules by hydrogen bonds. So water forms an insulating shield for the pectin providing layers that inhibit bonding between the molecules of the colloidal substances.
Sols can be transformed into gels as a result of reduction in temperature. In pectin gel, the pectin molecules are the continuous phase and the liquid is the dispersed phase while in pectin sol, the pectin molecules are the dispersed phase and the liquid is continuous phase. Sols may be formed as a preliminary step in making a gel. Jams and jellies made with pectin are common examples that form a sol prior to the desired structure.
Stability of Emulsion in Food:
An emulsion is a mixture of two or more immiscible (they will not mix together) liquids. One liquid (the dispersed phase) is dispersed in the other (the continuous phase), i.e., material that keep fat globules in water droplet or water droplet in fat are emulsifiers. An emulsion may be oil-in-water (o/w) in which case small oil droplets are dispersed through water, e.g., milk, or water-in-oil (w/o) in which case small water droplets are dispersed through oil, e.g., butter.
An emulsifying agent is made up of two parts. One is hydrophilic (water loving) and the other is hydrophobic (water hating). The emulsifier holds the disperse phase within the continuous phase. This results in the emulsion becoming stable.
Mayonnaise is an example of a stable emulsion of oil and vinegar, when egg yolk (lecithin) may be used as an emulsifying agent. Stabilisers are often added to emulsions to increase the viscosity of the product. These help improve the stability of the emulsion, as over time the emulsion may separate. Stabilisers also increase shelf life, E461 methylcellulose, used in low fat spreads.