The interaction of combinations of food materials under different physical and chemical conditions

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  • Created by: AbbieKP56
  • Created 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
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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