Polysaccharides

Question 1

What is a polysaccharide?

Answer 1

A polymer of monosaccharides (10 or more)

Question 2

What is a homopolysaccharide? Give three examples

Answer 2

Polysaccharide composed of a single type of monomer

• starch = D-glucose
• glycogen = D-glucose
• cellulose = D-glucose

Question 3

What is a heteropolysaccharide? Give two examples

Answer 3

Polysaccharides composed of two or more different monomers

• alginate
• gellan gum

Question 4

What is the predominant food reserve in plants?

Answer 4

Starch

Question 5

What are the two main sources of starch?

Answer 5

Cereal grain = corn, wheat, rice

Tubers/roots = potatoes and tapioca

Question 6

What are the functional uses of starch in food?

Answer 6

Thickening sauces
Binding
Dusting surfaces
Gelling agent

Question 7

What is the structure of amylopectin?

Answer 7

D-glucose chains joined by alpha 1,4 and 1,6 glycosidic bonds
These chains form hydrogen bonds producing helices
These helices clump to form crystals
Crystallites line up to give crystalline and amorphous regions

Question 8

What must starch undergo in order to gelatinise it?

Answer 8

Heating in excess water

Question 9

Describe the process of gelatinisation of starch.

Answer 9

1. Water uptake
2. Granule swelling
3. Loss of bifringence
4. Crystallite melting
5. Unwinding of double helices
6. Amylose leaching
7. Starch solubilisation
8. Increased viscosity
9. Gelling of amylose on cooling

Question 10

Describe the process by which starch is used in sauces.

Answer 10

1. Flour is added to hot fat and cooked off to produce a roux
2. Liquid is added gradually
3. Heated gently until thickened
4. Left to cool

Question 11

Why is flour added to hot fat and cooked off?

Answer 11

In order to coat the starch granules so that they stay separated

Question 12

Why is liquid added to the roux gradually?

Answer 12

So that the swelling of starch granules is gradual

Question 13

Why is the roux heated gently until thickened?

Answer 13

Boiling would disintegrate the starch granules

Question 14

Why is the roux left to cool?

Answer 14

To allow amylose to assemble into a network

Question 15

Why does overcooked roux have less thickening power?

Answer 15

Because the sugars have been broken down

Question 16

What is the function of starch in pasta?

Answer 16

• Granules are trapped in a solid = less water access
• Proteins compete with starch for water
• Limited expansion of the granule

Question 17

What is the function of starch in boiled mash potato?

Answer 17

• Granules swell but only some burst

- Bulging cells separate to give a floury texture

Question 18

What happens if boiled mash potato is blended?

Answer 18

All cells will burst producing a slimy paste

Question 19

What is the function of starch in chips and roast potatoes?

Answer 19

• At high temperatures water evaporates and fat enters

- Dextrinisation forms colour, texture, flavour and crust

Question 20

What is naturally occurring starch?

Answer 20

Starch which has not been chemically processed

Question 21

How is naturally occurring starch labelled?

Answer 21

Maize starch

Question 22

Give four examples of naturally occurring starch.

Answer 22

Waxy maize
Rice
Tapioca
Hylon

Question 23

What are the properties of waxy maize?

Answer 23

Low amylose and high amylopectin content
Granular integrity is maintained
Freeze thaw stability

Question 24

What are the properties of rice?

Answer 24

Small, uniform granule size

Question 25

What are the properties of tapioca?

Answer 25

Clear solutions

Question 26

What are the properties of hylon?

Answer 26

High amylose content and therefore resistant to digestion

Question 27

How is pregelatinized starch produced?

Answer 27

Preheating with water to gelatinise and then rapid drying to make a powder

Question 28

What are the properties of pregelatinized starch?

Answer 28

Weak gels | Less viscous paste

Question 29

What does natural variation cause among starch?

Answer 29

Less consistency

Question 30

Name three ways in which starch may be modified.

Answer 30

Acid modification (depolymerised/thin boiling) Stabilised starch phosphate and esters Cross-linked starch

Question 31

How is starch acid modified and how does that affect it?

Answer 31

- Acid treatment at 22-25 degrees for 6-24 hours | - Hydrolysis breaks down some glycosidic bonds which therefore increases solubility

Question 32

What is acid modified starch used in?

Answer 32

Gummy sweets

Question 33

How is phosphate stabilised starch different to naturally occurring starch?

Answer 33

Increased viscosity

Question 34

How is ester stabilised starch different to naturally occurring starch?

Answer 34

Freeze thaw resistant

Question 35

How is cross-linked starch different from naturally occurring starch?

Answer 35

Increased strength | pH resistant

Question 36

What is cross-linked starch used in?

Answer 36

Canned fruit pie fillings

Question 37

Why might modified starches have E numbers?

Answer 37

Because they are considered additives

Question 38

What is cellulose?

Answer 38

A polysaccharide found in cell walls

Question 39

What is the major component of dietary fibre?

Answer 39

Cellulose

Question 40

What is the structure of cellulose?

Answer 40

D-glucose monomers joined by beta 1,4 connections

Question 41

What are cellulose microfibrils?

Answer 41

Individuals D-glucose chains connected by hydrogen bonds

Question 42

How does the structure of cellulose provide strength in all directions?

Answer 42

Adjacent microfibril layers are laid down in different directions

Question 43

Name six other polysaccharides found in foods.

Answer 43

``` Alginate = thickner and gelling agent Arabinoxylan Beta-glucan = reduces cholesterol Glucomannan = treating constipation Pectin = jam and fruit Xyloglucan = fruit and veg cell walls ```