Disaccharides: Sucrose, Lactose and Maltose

Table sugar, milk sugar and malt sugar have something in common: each is made of two simple sugars joined together. These disaccharides are small enough to taste sweet and dissolve easily, but they have to be split into single sugars before your body can absorb them. How they’re joined, and how easily they’re split, explains everything from lactose intolerance to why sucrose doesn’t pass a classic test for sugars.

How two sugars join: the glycosidic bond

A disaccharide forms when two monosaccharides link in a condensation reaction. An –OH group on one sugar and an –OH group on the other react; a molecule of water is removed, and the two rings are left joined through an oxygen atom. This C–O–C link is a glycosidic bond.

For two hexoses:

C₆H₁₂O₆ + C₆H₁₂O₆ → C₁₂H₂₂O₁₁ + H₂O

So sucrose, lactose and maltose all share the formula C₁₂H₂₂O₁₁ (molar mass about 342 g mol⁻¹, which you can check with the molar mass calculator). They’re isomers that differ in which sugars are joined and exactly how.

The reverse reaction is hydrolysis: adding water across the glycosidic bond splits the disaccharide back into two monosaccharides. In the body, specific enzymes do this; in the lab, heating with dilute acid works.

Naming the link: α, β and carbon numbers

A glycosidic bond is described by:

• which carbon atoms it joins (for example, carbon 1 of one sugar to carbon 4 of the other: a 1→4 link), and
• whether the bond at the first sugar’s carbon 1 points α (down) or β (up) relative to the ring, as drawn in standard diagrams.

These details matter because enzymes are shaped to recognise one specific kind of link. An enzyme that cuts α(1→4) bonds won’t touch β(1→4) bonds.

The big three disaccharides

Sucrose: table sugar

• Made of: glucose + fructose.
• Link: α(1→2)β, joining carbon 1 of glucose to carbon 2 of fructose.
• Sources: sugar cane, sugar beet, maple syrup, most fruits.
• Digested by: sucrase (also called invertase), producing glucose and fructose.

Sucrose is the most familiar sugar, extracted industrially from cane and beet on an enormous scale.

Lactose: milk sugar

• Made of: galactose + glucose.
• Link: β(1→4), joining carbon 1 of galactose to carbon 4 of glucose.
• Sources: milk and dairy products; about 5% of cow’s milk and around 7% of human milk.
• Digested by: lactase.

Lactose is much less sweet than sucrose and less soluble, which is why it can sometimes crystallise in ice cream, giving a sandy texture.

Maltose: malt sugar

• Made of: glucose + glucose.
• Link: α(1→4).
• Sources: produced when starch is broken down, for example during digestion, in germinating grain (malting), and in brewing.
• Digested by: maltase.

Maltose is an intermediate in starch digestion: amylase in saliva and the pancreas chops starch into maltose and short chains, which maltase then splits into glucose.

Comparison table

	Sucrose	Lactose	Maltose
Monomers	Glucose + fructose	Galactose + glucose	Glucose + glucose
Glycosidic link	α(1→2)β	β(1→4)	α(1→4)
Reducing sugar?	No	Yes	Yes
Digestive enzyme	Sucrase	Lactase	Maltase
Relative sweetness (sucrose = 100)	100	about 15–40	about 30–50
Main source	Cane, beet, fruit	Milk	Starch breakdown, malted grain

(Sweetness figures vary with concentration and temperature; they’re rough guides.)

Why sucrose is a non-reducing sugar

Monosaccharides are reducing sugars because their rings can open to expose a reactive aldehyde (or a ketone that can rearrange into one). In a disaccharide, one ring’s carbonyl carbon is always tied up in the glycosidic bond, but the other ring usually still has a free one that can open. That’s why lactose and maltose are reducing sugars.

In sucrose, the glycosidic bond joins the carbonyl carbon of glucose (C1) to the carbonyl carbon of fructose (C2). Both reactive carbons are locked in the bond. Neither ring can open, so sucrose cannot reduce copper(II) ions and gives a negative Benedict’s test.

To detect sucrose, you first hydrolyse it by boiling with dilute hydrochloric acid, neutralise with sodium hydrogencarbonate, and then do Benedict’s test. The glucose and fructose released now give a positive result. The whole method is in Benedict’s test for reducing sugars.

Invert sugar

Hydrolysing sucrose gives an equal mixture of glucose and fructose called invert sugar. The name comes from the way the mixture rotates plane-polarised light: sucrose solution rotates it one way, but after hydrolysis the mixture rotates it the other way, because fructose’s rotation is stronger and opposite.

Invert sugar is sweeter than sucrose (because of the fructose) and much less likely to crystallise, so confectioners and bakers use it in fondants, syrups and soft-centred chocolates. Honey is largely a natural invert sugar, made when bees’ enzymes hydrolyse sucrose from nectar.

Lactose intolerance

Babies produce plenty of lactase to digest milk. In many people, lactase production falls sharply after weaning. When they drink milk, lactose reaches the large intestine undigested. There, bacteria ferment it, producing gases (hydrogen, carbon dioxide, methane) and short-chain acids, which cause bloating, cramps and diarrhoea.

How common this is varies widely between populations. Many people of northern European, some African and some Middle Eastern ancestry keep making lactase throughout life, thanks to genetic variants that spread in populations with a long history of dairy farming. In much of East Asia, most adults produce little lactase.

Management options include:

• lactose-free milk, made by adding lactase enzyme during processing (which also makes the milk taste slightly sweeter, since glucose and galactose are sweeter than lactose);
• fermented dairy products such as yoghurt and hard cheese, where bacteria have already consumed much of the lactose;
• lactase tablets taken with meals.

Other disaccharides worth knowing

• Trehalose (glucose + glucose, α(1→1)α) is found in fungi and insects and helps organisms survive drying out. Like sucrose, it’s non-reducing.
• Cellobiose (glucose + glucose, β(1→4)) is the repeating unit of cellulose. Humans can’t digest it, which shows how much the α/β difference matters.

Making sugar from plants

Almost all table sugar comes from two crops. In a cane sugar mill, the stalks are crushed and the juice is clarified with lime, then boiled down under reduced pressure until sucrose crystallises. Spinning the crystals in a centrifuge separates them from the remaining syrup, called molasses. Beet sugar is extracted from sliced sugar beet with hot water in a similar way. Refining then dissolves the raw sugar, removes coloured impurities (often with activated carbon), and recrystallises it as pure white sucrose. The chemistry is the same for both sources: cane and beet sugar are chemically identical sucrose, and only trace impurities in less refined sugars differ.

Key takeaways

• A disaccharide is two monosaccharides joined by a glycosidic bond, formed by condensation and broken by hydrolysis.
• Sucrose = glucose + fructose; lactose = galactose + glucose; maltose = glucose + glucose; all are C₁₂H₂₂O₁₁.
• Lactose and maltose are reducing sugars; sucrose is non-reducing because both carbonyl carbons are used in its glycosidic bond.
• Each disaccharide needs its own enzyme (sucrase, lactase, maltase); too little lactase causes lactose intolerance.
• Continue to starch vs glycogen vs cellulose to see what happens when many sugars join.