Disaccharides are double sugars that yield two simple sugars called monosaccharides upon hydrolysis. The three main disaccharides are sucrose, maltose, and lactose. They differ in their solubility, with sucrose being very soluble, maltose fairly soluble, and lactose only slightly soluble. Disaccharides are formed through a dehydration synthesis reaction combining two monosaccharides. Their structures depend on the type of glycosidic linkage between the monosaccharides. This determines their properties such as whether they are reducing sugars or able to undergo fermentation.

1. Disaccharides
Disaccharides (di- means two) are double sugars. One
hydrolysis, they yield two simple sugars.
Disaccharides 2 monosaccharides
Three common disaccharides
1. Sucrose
2. Maltose
3. Lactose
* All of which are isomers with the molecular formula

2. General Reaction
O
A disaccharide A monosaccharide A monosaccharide
Sucrose Glucose Fructose
Maltose Glucose Glucose
Lactose Glucose Galactose

3. In terms of Solubility
Sucrose – very soluble in water
Maltose – fairly soluble
Lactose – only slightly soluble
The disaccharides, just like monosaccharide are
white crystalline, sweet solids. The disaccharides
are also optically active; they rotate the plane of
polarized light. However, even though they are
soluble in water, they are too large to pass
through cell membranes.

4. Disaccharide Formation and Structure
• Disaccharides are formed when two
monosaccharides are joined by dehydration
synthesis reaction.

5. Disaccharide Formation and Structure
CH2OH CH2OH CH2OH CH2OH
H O H H O H H O H H O H
H20
+
O
OH OH HO OH OH OH
α- GLUCOSE α- GLUCOSE MALTOSE

6. Disaccharide Structure
Sucrose Maltose

7. Consider the combination of a molecule of a-
glucose with a molecule of b-glucose. The
products such as a reaction are b-maltose
B-maltose
The linkage in b-maltose is between carbon 1 of
one glucose and carbon 4 of the other glucose.
Such a linkage is caalled a-1,4 linkage; -
maltose has an –OH on carbon 1 of the second
glucose molecule below the plane rather than
above. Both - & -maltose exist, but the

8.  if a molecule of -glucose combines with
another molecule of -glucose, cellobiose, a
compound with a 1-4 glycosidic linkage is
formed.
Enzymes are specific in the type of glycosidic
linkage ( or ) whose hydrolysis they can
catalyze.
Conversly, an enzyme that contains
catalyszes the hydrolysis of glycosidic linkages
catalyzes the hydrolysis of cellobiose. Such -
glycosidic linkage hydrolysis enzymes are not
found in the human digestive system. That is
why human cannot digest cellulose and
cellobiose, both of which have glycosidic
linkages.

9. Reducing properties
In maltose
The aldehyde groups are at carbon 1 in each of the
original glucose molecules. Since the linkage is 1.4, one
free aldehyde group remains. Therefore, maltose acts as
a reducing sugar.
In sucrose
The glucose part had the aldehyde at carbon 1, and the
fructose part had the ketone group at carbon 2. since the
linkage is 1,2, neither group is free. Therefore, sucrose is
not a reducing sugar.
In lactose
Which has a 1,4-linkage, acts as a reducing sugar
because both of the original aldehyde group wre on

10. Fermentation
 Sucrose and maltose will ferment
when yeast is added because
yeast contains the enzyme sucrase
and maltase.
 Lactose will not ferment because
yeast does not contain lactase.

11. Testing for disaccharides
 The chemical reactions of these sugars
can be used to distinguish them in the
laboratory.
 If you have 2 test tubes containing a
disaccharide,C12H22O11.To determine if
it is sucrose lactose or maltose.
 We can use the alkaline Cu complex
reaction of glucose and the principle of
fermentation.

12. Sucrose
 Is a sugar used at home
 Also known as the cane
sugar
 When hydrolyzed, it forms
a mixture of glucose and
fructose.
 This 50:50 mixture of
glucose and fructose is
called invert sugar
because it reverses the
rotation of polarized light.

13. Maltose
 Commonly known as malt sugar.
 Present in germinating grain
 Produced commercially by hydrolysis of
starch.

14. Lactose
 Commercially known as milk
sugar.
 Of animal origin
 Bacteria cause fermentation of
lactose forming lactic acid.
 When these reaction occur ,it
changes the taste to a sour one.