Colloids

Question 1

What are colloids?

Answer 1

When one substance is dispersed through another but does not combine to form a solution

Question 2

What is the disperse phase?

Answer 2

The substance which is dispersed

Question 3

What is the continuous phase?

Answer 3

The substance the disperse phase is suspended in

Question 4

What is a sol?

Answer 4

A liquid colloid in which solid particles are dispersed in a liquid phase. It is usually an unset product and is more liquid than solid

Question 5

What is a gel?

Answer 5

A gel is a solid dispersed in a liquid but tends to be more solid than liquid and cool such as jelly

Question 6

What is an emulsion?

Answer 6

A liquid dispersed in a liquid to form a stable liquid usually. Ie oil and egg make mayonnaise

Question 7

What is a solid emulsion?

Answer 7

A liquid in a fat such as butter

Question 8

What is a foam?

Answer 8

A gas in a liquid eg whipped cream

Question 9

What is a solid foam?

Answer 9

Gas in a solid eg. Meringue, cake

Question 10

Why might colloids become unstable?

Answer 10

The two phases may separate over time due to increased temp and physical force particularly fat and water

Question 11

What is the continuous phase?

Answer 11

The substance the disperse phase is suspended in

Question 12

What is a sol?

Answer 12

A liquid colloid in which solid particles are dispersed in a liquid phase. It is usually an unset product and is more liquid than solid

Question 13

What is a gel?

Answer 13

A gel is a solid dispersed in a liquid but tends to be more solid than liquid and cool such as jelly

Question 14

What is an emulsion?

Answer 14

A liquid dispersed in a liquid to form a stable liquid usually. Ie oil and egg make mayonnaise

Question 15

What is a solid emulsion?

Answer 15

A liquid in a fat such as butter

Question 16

What is a foam?

Answer 16

A gas in a liquid eg whipped cream

Question 17

What is a solid foam?

Answer 17

Gas in a solid eg. Meringue, cake

Question 18

Why might colloids become unstable?

Answer 18

The two phases may separate over time due to increased temp and physical force particularly fat and water

Question 19

What is retrogradation?

Answer 19

A change which takes place once a gel has formed. The proportion of amylase in the starch affects the gelling properties. Amylose aids willing because the molecules are spiral shaped and form a network in which water is trapped
If a rigid gel is required high amylose starches are used
When foods are cooked and thawed or prepared for long term storage there is a risk of the amylase gel retrograding. The amylase molecules unwind and the gel becomes opaque and a pulpy sponge.
Manufacturers use high amylopectin starches to produce a strong gel that will not retrograde easily or a chemically modified starch

Question 20

What is gelatinisation?

Answer 20

A suspension of starch in water is heated
The water penetrates the outer layers of the granule and they begin to swell at temps between 60-80C. The granules swell until there volume is almost 5x the original causing the mixture to become viscous
At 80C the starch granules break up and the endosperm becomes dispersed throughout the water
The long chain molecules unfold and viscosity increases forming a sol.
On cooling a network is formed with the water enclosed which produces a gel

Question 21

Why do manufacturers use modified starch?

Answer 21

Prevent retrogradation and spoiling processed food products eg. Pie filling
Need processed foods to remain in good condition on shelves for several days - if they weep they are less desirable eg. Custard, white sauces
Two forms of starch - amylose = straight chain in coil, better gel but retrograde easier and amylopectin = highly branched reduces water loss and less retrogradation
Phosphoric acid gives amylose appearance but amylopectin properties
Modified starch is pre gelatinised and used in angel delight where thickening is instant on addition of cold water
Some starches are acid modified so clearer paste with less viscosity