Difference Between Sucrose and Lactose

FOOD CHEMISTRY: CARBOHYDRATES

Diagram

Figure 1: Sucrose and lactose are both disaccharides, but each is built from a different pair of monosaccharides. Drawn in our own style from the disaccharide discussion in the FTN Food Chemistry notes, Unit 3.

Introduction

Sucrose and lactose are both disaccharides, which means each is made of two monosaccharide units joined by a glycosidic bond, with a molecule of water lost in the joining. The difference is the pair: sucrose is glucose joined to fructose, and lactose is glucose joined to galactose. That difference decides where each sugar is found, how sweet it tastes, and how the body splits it.

Quick comparison

Table 1: Sucrose compared with lactose

Scientific explanation

Definition

Sucrose is a disaccharide of glucose and fructose, and it is ordinary table sugar. Lactose is a disaccharide of glucose and galactose, and it is the sugar of milk. Both are formed when two monosaccharides join by a glycosidic bond, with a molecule of water lost in the joining.

Working principle

The glycosidic bond is the weak point of both sugars: acid and the enzyme that matches it can split it, a reaction called hydrolysis. Sucrose splits into glucose and fructose, a mix called invert sugar, which is sweeter than sucrose itself and holds water better, which is why it is used in jams and sweets to stop them crystallising. Lactose splits into glucose and galactose under the enzyme lactase; people who make little lactase cannot split lactose well, and the undigested sugar ferments in the gut.

Apparatus

The books used here do not name a specific apparatus for telling sucrose from lactose in the laboratory, so no apparatus list is given. The split of either sugar is followed by measuring the freed monosaccharides as reducing sugars.

Measurement procedure

Each disaccharide is split by hydrolysis with acid or with its matching enzyme, and the freed monosaccharides are then measured as reducing sugars: the sugar is heated with a copper reagent, the copper is reduced, and the amount is read from the colour or by titration. The result is reported as glucose or as the sugar in question.

Factors affecting readings

Acid, the matching enzyme, heat, and time decide how fully the glycosidic bond is split. If the split is not complete, the reading understates the sugar present. The sweetness values also depend on weight: all sweetness figures here are on a weight basis.

Formulas and calculations

Sweetness comparison, worked from the book values. Lactose is about 0.2 times as sweet as sucrose, so 100 g of sucrose has the sweetness of about 100 / 0.2 = 500 g of lactose. That is why milk tastes only mildly sweet even though it holds 4.5 to 5 percent lactose by weight.

This topic has no other standard formula in the books used here: there is no single formula for sucrose or for lactose like the DE formula for starch syrups, so no further formula is given.

Industrial applications

• Sucrose: table sugar for drinks, baking, jams, and sweets.
• Sucrose: invert sugar in jams and sweets, to stop crystallising and to hold water.
• Lactose: the natural sugar of milk, milk powders, and milk sweets, giving a mild sweet taste.
• Lactose: a food for the bacteria in fermented milk foods, which split it during fermentation.

Advantages and limitations

• Sucrose, advantage: the standard sweet taste, cheap and widely available from cane or beet, and it makes invert sugar for jam making.
• Sucrose, limitation: it is the reference sweetness, so it sets the bar other sugars are judged by, and on heating alone it browns and can turn bitter if boiled too long.
• Lactose, advantage: a mild, clean sweetness that suits milk foods, and a natural part of milk.
• Lactose, limitation: only about 0.2 times as sweet as sucrose, so it cannot sweeten a food on its own, and people who make little lactase cannot digest it well.

FAQ

What is sucrose made of?

Glucose joined to fructose, by a glycosidic bond.

What is lactose made of?

Glucose joined to galactose, by a glycosidic bond.

Why does milk taste only mildly sweet?

Milk holds about 4.5 to 5 percent lactose, but lactose is only about 0.2 times as sweet as sucrose.

What is invert sugar?

The mix of glucose and fructose made when sucrose is split by hydrolysis. It is sweeter than sucrose and holds water better, so it stops jam crystallising.

Why do some people have trouble with milk sugar?

They make little lactase, the enzyme that splits lactose, so the undigested sugar ferments in the gut.

References and further reading

• FTN subject: Food Chemistry, Unit 3: Carbohydrates.
• Zeece, Michael. Introduction to the Chemistry of Food. Academic Press, 2020.
• Coultate, Tom. Food: The Chemistry of Its Components. 7th ed., Royal Society of Chemistry, 2023.
• Damodaran, Srinivasan, and Kirk L. Parkin. Fennema’s Food Chemistry. 5th ed., CRC Press, 2017.
• Wang, Dongfeng, Hong Lin, Jianqian Kan et al. Food Chemistry. De Gruyter, 2015.
• Google Books pages read for this article:
• Fennema, Owen R., ed. Food Chemistry, 2nd ed., Marcel Dekker, 1985: carbohydrate chapter on monosaccharides, disaccharides and polysaccharides (Google Books search result, full text read via the search record)
• Provost, Joseph J. et al. The Science of Cooking. John Wiley and Sons, 2016 (Google Books page read)