Polysaccharides Flashcards
What is a polysaccharide?
A polymer of monosaccharides (10 or more)
What is a homopolysaccharide? Give three examples
Polysaccharide composed of a single type of monomer
- starch = D-glucose
- glycogen = D-glucose
- cellulose = D-glucose
What is a heteropolysaccharide? Give two examples
Polysaccharides composed of two or more different monomers
- alginate
- gellan gum
What is the predominant food reserve in plants?
Starch
What are the two main sources of starch?
Cereal grain = corn, wheat, rice
Tubers/roots = potatoes and tapioca
What are the functional uses of starch in food?
Thickening sauces
Binding
Dusting surfaces
Gelling agent
What is the structure of amylopectin?
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
What must starch undergo in order to gelatinise it?
Heating in excess water
Describe the process of gelatinisation of starch.
- Water uptake
- Granule swelling
- Loss of bifringence
- Crystallite melting
- Unwinding of double helices
- Amylose leaching
- Starch solubilisation
- Increased viscosity
- Gelling of amylose on cooling
Describe the process by which starch is used in sauces.
- Flour is added to hot fat and cooked off to produce a roux
- Liquid is added gradually
- Heated gently until thickened
- Left to cool
Why is flour added to hot fat and cooked off?
In order to coat the starch granules so that they stay separated
Why is liquid added to the roux gradually?
So that the swelling of starch granules is gradual
Why is the roux heated gently until thickened?
Boiling would disintegrate the starch granules
Why is the roux left to cool?
To allow amylose to assemble into a network
Why does overcooked roux have less thickening power?
Because the sugars have been broken down
What is the function of starch in pasta?
- Granules are trapped in a solid = less water access
- Proteins compete with starch for water
- Limited expansion of the granule
What is the function of starch in boiled mash potato?
- Granules swell but only some burst
- Bulging cells separate to give a floury texture
What happens if boiled mash potato is blended?
All cells will burst producing a slimy paste
What is the function of starch in chips and roast potatoes?
- At high temperatures water evaporates and fat enters
- Dextrinisation forms colour, texture, flavour and crust
What is naturally occurring starch?
Starch which has not been chemically processed
How is naturally occurring starch labelled?
Maize starch
Give four examples of naturally occurring starch.
Waxy maize
Rice
Tapioca
Hylon
What are the properties of waxy maize?
Low amylose and high amylopectin content
Granular integrity is maintained
Freeze thaw stability
What are the properties of rice?
Small, uniform granule size
What are the properties of tapioca?
Clear solutions
What are the properties of hylon?
High amylose content and therefore resistant to digestion
How is pregelatinized starch produced?
Preheating with water to gelatinise and then rapid drying to make a powder
What are the properties of pregelatinized starch?
Weak gels
Less viscous paste
What does natural variation cause among starch?
Less consistency
Name three ways in which starch may be modified.
Acid modification (depolymerised/thin boiling)
Stabilised starch phosphate and esters
Cross-linked starch
How is starch acid modified and how does that affect it?
- Acid treatment at 22-25 degrees for 6-24 hours
- Hydrolysis breaks down some glycosidic bonds which therefore increases solubility
What is acid modified starch used in?
Gummy sweets
How is phosphate stabilised starch different to naturally occurring starch?
Increased viscosity
How is ester stabilised starch different to naturally occurring starch?
Freeze thaw resistant
How is cross-linked starch different from naturally occurring starch?
Increased strength
pH resistant
What is cross-linked starch used in?
Canned fruit pie fillings
Why might modified starches have E numbers?
Because they are considered additives
What is cellulose?
A polysaccharide found in cell walls
What is the major component of dietary fibre?
Cellulose
What is the structure of cellulose?
D-glucose monomers joined by beta 1,4 connections
What are cellulose microfibrils?
Individuals D-glucose chains connected by hydrogen bonds
How does the structure of cellulose provide strength in all directions?
Adjacent microfibril layers are laid down in different directions
Name six other polysaccharides found in foods.
Alginate = thickner and gelling agent Arabinoxylan Beta-glucan = reduces cholesterol Glucomannan = treating constipation Pectin = jam and fruit Xyloglucan = fruit and veg cell walls
