The interaction of combinations of food materials under different physical and chemical conditions 0.0 / 5 ? Design & Technology: Food TechnologyA2 Food TechnologyA2/A-levelAQA Created by: AbbieKP56Created on: 06-06-17 09:35 Colloidal System Colloidal systems give structure, texture and mouthfeel to many different products, for example jam, ice cream and mayonnaise. Colloids are formed when one substance is dispersed through another, but does not combine to form a solution. The substance which is dispersed is known as the disperse phase and is suspended in the continuous phase. Most colloids are stable, but the two phases may seperate over a period of time because of an increase in temperature or by physical force. They may also become unstable when frozen or heated. Types of Colloids? Foams Emulsions Gels Sols 1 of 8 Foams Gas disperse phase, liquid continuous phase, e.g. whipped cream. SOLID FOAM gas disperse phase, solid continuous phase, e.g. meringue. Foams are composed of small bubbles of gas 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 a solid foam, e.g. meringue. 2 of 8 Emulsions Liquid disperse phase, liquid continuous phase, e.g. mayonnaise or milk. SOLID EMULSION liquid disperse phase, solid continuous phase, e.g. butter. When water and oil are shaken together, they form an emulsion. This emulsion is unstable. If left to stand, the oil will form a seperate layer on top of the water. A stable emulsion is formed when two immiscible liquids are held stable by a third substance, called an emulsifying agent. 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. 3 of 8 Gels Liquid disperse phase, solid continous phase, e.g. jelly Sometimes the mixture needs to be heated and stirred. When the solution cools, the sol changes into a gel, which resembles a solid rather than a liquid. Both protein and starch can be used in the formation of a sol or gel. 4 of 8 Sol Solid disperse phase, liquid continuous phase, e.g. uncooked custard or unset jelly A sol is a liquid colloid or mixture in which solid particles are dispersed in a liquid phase. The disperse phase is attracted to molecules of the continuous phase. Both protein and starch can be used in the formation of a sol or gel. 5 of 8 Gelatinisation of Starch When a suspension of starch in liquid is heated, the liquid penetrates the outer layers of the granules and the granules begin to swell. This occurs from 60 - 80 degrees. The granules swell until they are as much as 5 times the original. At 80 degrees the starch granules begin to break up dispersing the contents into the liquid. The long chain molecules unfold and the mixture becomes more viscous and thickens. 6 of 8 Retrograding of Starch Starch containing amylose gel best because the molecules are spiral shaped and form a network in which water is trapped. This produces a rigid gel. Amylose molecules tend to unwind in time however, producing a watery deposit, an apaque gel and a pulpy, spongy texture. This process is known as retrogradation and it occurs mainly when foods are frozen and thawed. High amylopectin starches do not retrograde easily. 7 of 8 Syneresis If a gel is allowed to stand for a time, it starts to 'weep'. This loss of liquid is known as syneresis. Syneresis is the shrinkage of a gel and the subsequent loss of liquid. Gels affected by syneresis can be based upon protein. Usually refers to eggs; if overcooked, the proteins shrink as they coagulate and seperate from the watery liquid. Syneresis is caused by excessive heat. Effects of Syneresis Over-hardens the protein shell Moisture inside expands upon heating. The hard protein shell pops, expelling the moisture. 8 of 8
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