Carbohydrates
Introduction to carbohydrates
- one of the macromolecules in food
Classification
- carbohydrates have a ring structure (no double bonds) consisting of C-, H- and O- atoms
- carbohydrates contain different residues (glucose, galactose, fructose)
Roles of carbohydrates in food
1. Sweetener (sugar)
2. Humectant: lowers water activity
3. Energy supplier (glucose, lactose)
4. Texture: cell wall, thickeners, gelling agents
5. Colour: browning reactions
Monosaccharides
- Glucose and fructose (main monosaccharides in food)
- Other monosaccharides: galactose, mannose etc
Structural differences in monosaccharides
1. Aldose vs ketose
Double bonded oxygen becomes an OH-group
in the ring structure
,How to recognize an aldose/ketose by looking at the ring structure?
- anomeric carbon: a carbon between two oxygen atoms
- If the anomeric carbon has another Carbon bonded to it =
KETOSE
- No extra Carbon bonded to the anomeric carbon = ALDOSE
Pyranose vs furanose
Pyranose = 6-membered ring
furanose = 5-membered ring
Pentose vs Hexose
Hexose: 6 Carbon atoms
Pentose: 5 Carbon atoms
Alpha vs Beta
Alpha: OH-group points down
Beta: OH-group points upwards
QUESTIONS from slides
To understand why fructose unit in
sucrose is Beta-configurated you
need to look at the structure of the
whole sucrose molecule
(knowledge clip)
N-glycoside
O-glycoside
, When a carbohydrate reacts with another type of molecule, often a link at the anomeric
carbon is formed with the other molecule. This results in a glycoside, such as O-, N- or S-
glycosides. Glycosides are often better soluble than the molecule without the sugar unit, so
this is a way for molecules to increase their solubility.
Reducing properties
- One of the most important properties of carbohydrates
- When a saccharide is a reducing sugar, the ring structure can open at the anomeric carbon if
it has a FREE OH-group
- In the open form, the double bonded oxygen on the saccharide can now react with another
compound.
- However, this is only possible when the anomeric carbon is not involved in a glycosidic
linkage.
- This can also be represented with a double bonded oxygen on the first carbon in the open
line structure
Why is it called a reducing sugar?
The carbonyl group (C=O) of a sugar can reduce Cu2+ (blue colour) to a red colour
This property is used for the quantification of carbohydrates
- All monosaccharides and some oligo- and polysaccharides are reducing sugars.
- Cyclic carbohydrates are NOT reducing sugars
- A saccharide with a reducing end is reactive at elevated temperatures in for example
browning reactions (Maillard and caramelization)
Mutarotation: monomers in solution can convert from the alpha-anomer into the Beta-anomer and
vise-versa. ONLY reducing sugars have this property.
Glycosidic bonds – systematic name
Definition: glycosidic bond is a bond within the carbohydrate connecting the monosaccharides
Introduction to carbohydrates
- one of the macromolecules in food
Classification
- carbohydrates have a ring structure (no double bonds) consisting of C-, H- and O- atoms
- carbohydrates contain different residues (glucose, galactose, fructose)
Roles of carbohydrates in food
1. Sweetener (sugar)
2. Humectant: lowers water activity
3. Energy supplier (glucose, lactose)
4. Texture: cell wall, thickeners, gelling agents
5. Colour: browning reactions
Monosaccharides
- Glucose and fructose (main monosaccharides in food)
- Other monosaccharides: galactose, mannose etc
Structural differences in monosaccharides
1. Aldose vs ketose
Double bonded oxygen becomes an OH-group
in the ring structure
,How to recognize an aldose/ketose by looking at the ring structure?
- anomeric carbon: a carbon between two oxygen atoms
- If the anomeric carbon has another Carbon bonded to it =
KETOSE
- No extra Carbon bonded to the anomeric carbon = ALDOSE
Pyranose vs furanose
Pyranose = 6-membered ring
furanose = 5-membered ring
Pentose vs Hexose
Hexose: 6 Carbon atoms
Pentose: 5 Carbon atoms
Alpha vs Beta
Alpha: OH-group points down
Beta: OH-group points upwards
QUESTIONS from slides
To understand why fructose unit in
sucrose is Beta-configurated you
need to look at the structure of the
whole sucrose molecule
(knowledge clip)
N-glycoside
O-glycoside
, When a carbohydrate reacts with another type of molecule, often a link at the anomeric
carbon is formed with the other molecule. This results in a glycoside, such as O-, N- or S-
glycosides. Glycosides are often better soluble than the molecule without the sugar unit, so
this is a way for molecules to increase their solubility.
Reducing properties
- One of the most important properties of carbohydrates
- When a saccharide is a reducing sugar, the ring structure can open at the anomeric carbon if
it has a FREE OH-group
- In the open form, the double bonded oxygen on the saccharide can now react with another
compound.
- However, this is only possible when the anomeric carbon is not involved in a glycosidic
linkage.
- This can also be represented with a double bonded oxygen on the first carbon in the open
line structure
Why is it called a reducing sugar?
The carbonyl group (C=O) of a sugar can reduce Cu2+ (blue colour) to a red colour
This property is used for the quantification of carbohydrates
- All monosaccharides and some oligo- and polysaccharides are reducing sugars.
- Cyclic carbohydrates are NOT reducing sugars
- A saccharide with a reducing end is reactive at elevated temperatures in for example
browning reactions (Maillard and caramelization)
Mutarotation: monomers in solution can convert from the alpha-anomer into the Beta-anomer and
vise-versa. ONLY reducing sugars have this property.
Glycosidic bonds – systematic name
Definition: glycosidic bond is a bond within the carbohydrate connecting the monosaccharides
