Study Material: Carbohydrates and Their Functional Properties

Learning Outcomes

Able to classify carbohydrates and write their chemical structures
Identify important food sugars, their chemical reactions, and their functional properties
Describe the functional properties of food polysaccharides including pectin, starch, and vegetable gums
Explain the Maillard reaction and ways of controlling this reaction
Differentiating between browning and Maillard reactions

Carbohydrates Overview

Carbohydrates are organic compounds made up of carbon (C), hydrogen (H), and oxygen (O) in a specific ratio. Typically, they have the empirical formula Cx(H2O)y, indicating that for every carbon atom, there are typically two hydrogen atoms and one oxygen atom.

Classification

Carbohydrates can be classified based on their chemical structure into three main categories:

Low molecular weight: Monosaccharides and disaccharides
Intermediate molecular weight: Oligosaccharides
High molecular weight: Polysaccharides (complex carbohydrates)

Monosaccharides

Definition

Monosaccharides are the simplest form of carbohydrates, consisting of a single saccharide unit.

Types

Monosaccharides can contain 3 (trioses), 4 (tetroses), 5 (pentoses), or 6 (hexoses) carbon atoms.
Examples:

Trioses: L-Glyceraldehyde, D-Glyceraldehyde
Pentoses: Ribose, Xylose
Hexoses: Glucose (dextrose), Fructose (levulose), Galactose

Classification based on Carbon Count

Number of Carbons	Category Name	Examples
4	Tetrose	Erythrose, Threose
5	Pentose	Arabinose, Ribose, Ribulose, Xylose, Xylulose, Lyxose
6	Hexose	Allose, Altrose, Fructose, Galactose, Glucose, Gulose, Idose, Mannose, Sorbose, Talose, Tagatose
7	Heptose	Sedoheptulose

Aldoses and Ketoses

Aldoses contain a carbonyl group (C=O) at C1, while ketoses have this group at C2:

Aldoses: D-Ribose, D-Xylose, D-Glucose
Ketoses: D-Fructose, D-Ribulose

Chemical Structure

The chemical structure of monosaccharides may differ in the orientation of hydroxyl groups, which affects their biochemical and sensory properties.

Aldoses and Ketoses Defined

Aldose: Monosaccharide with an aldehyde group.
Ketose: Monosaccharide with a ketone group.

Chain and Ring Forms

Monosaccharides can exist in both chain and ring forms.

Fischer projections represent the straight-chain form.
Haworth projections represent the cyclic form favored in aqueous solutions.

Epimers and Anomers

An epimer is a type of stereoisomer differing in configuration at one stereogenic center.
Anomers are isomers differing only in configuration about their anomeric carbon.

Oligosaccharides

Definition and Formation

Oligosaccharides consist of 2 to 10 monosaccharide units joined by glycosidic bonds.

Disaccharides are formed by the condensation of two monosaccharides, resulting in a 1,4-glycosidic linkage.

Common Disaccharides

Sucrose: Non-reducing sugar (from sugar cane/beets)
Maltose: Reducing sugar from starch hydrolysis.
Lactose: Found in milk, hydrolyzes into glucose and galactose.

Polysaccharides

Definition

Polysaccharides are large molecules formed from many monosaccharide units.

Classification

Homopolysaccharides: Composed of one type of monosaccharide.
Heteropolysaccharides: Composed of two or more types of monosaccharides.

Key Polysaccharides

Starch: Composed of amylose (20%) and amylopectin (80%); stores energy in plants.
Glycogen: Animal starch; energy storage in animals.
Cellulose: Structural component in plant cell walls.

Functional Properties of Carbohydrates

Reactions and Properties

1. Mutarotation

Mutarotation involves the change in optical rotation when sugars dissolve in water, creating an equilibrium between different forms.

2. Enolization and Isomerization

In alkaline medium, sugars can interconvert between aldose and ketose forms.

3. Crystallization

Crystallization is influenced by purity and concentration of sugar solutions.

4. Solubility

Solubility varies among sugars due to the presence of hydroxyl groups, affecting their ability to dissolve in water.

5. Humectancy

Some sugars and sugar alcohols maintain moisture in food products, enhancing texture and preventing dryness.

Humectants Examples and Functions

High-Fructose Corn Syrup: A blend of fructose and glucose, it offers moisture retention and sweetness.
Maltodextrins: Used for added texture and moisture retention in low-calorie products.

Inversion

The hydrolysis of sucrose produces invert sugar, a mixture of glucose and fructose, enhancing sweetness.

Oxidation-Reduction

Reducing sugars can be oxidized, leading to the formation of acids such as gluconic acid.

Sweetness and Sugar Alcohols

The relative sweetness varies among carbohydrates, with sugar alcohols providing fewer calories and are used for healthier alternatives.

Starch Behavior

Gelatinization and Retrogradation

Heating starch in water leads to gelatinization, where granules swell and become viscous.

Staling

Staling occurs post-baking due to the retrogradation of amylose, affecting freshness and texture.

Modified Food Starches

Modified starches undergo treatments to enhance functional properties, improving texture, stability, and gel formation.

Pectic Substances and Gums

Pectin

Pectin, found in cell walls, forms gels under certain conditions and is used for jams and jellies.

Food Gums

Gums, such as guar and carrageenan, are used for thickening and stabilizing food products.

Conclusion

Understanding carbohydrates, their classifications, structures, and functional properties is crucial for their application in food science and technology.

Carbohydrates and Their Functional Properties Study Material

Carbohydrates and Their Functional Properties Study Material