Difference Between Glucose and Fructose

FOOD CHEMISTRY: CARBOHYDRATES

Diagram

Figure 1: Glucose and fructose share the formula C₆H₁₂O₆ but carry the carbonyl group on a different carbon. Drawn in our own style from the monosaccharide discussion in the FTN Food Chemistry notes, Unit 3.

Introduction

Glucose and fructose are both single sugar units, and both share the formula C₆H₁₂O₆. The difference is the arrangement of their atoms: glucose carries its carbonyl group on the first carbon, while fructose carries its carbonyl group on the second carbon. That small change in shape changes the taste: fructose is about 1.7 times as sweet as sucrose, while glucose is about 0.7 times as sweet as sucrose.

Quick comparison

Table 1: Glucose compared with fructose

Scientific explanation

Definition

Glucose is a monosaccharide, a single sugar unit that cannot be split into simpler sugars. It is an aldose, with its carbonyl group on the first carbon. In solution it exists in two ring forms, alpha and beta, which differ in the position of one hydroxyl group and flip between each other until they settle into a balance. In the solid state glucose is usually the alpha form. Fructose is also a monosaccharide, and an isomer of glucose: it shares the formula C₆H₁₂O₆ but its atoms are arranged differently, with the carbonyl group on the second carbon, so it is a ketose.

Working principle

Both sugars are reducing sugars, because their free carbonyl group can reduce other compounds in chemical tests, and because it can take part in browning reactions with proteins. Both are very soluble in water, hold water well, and attract water from the air, which is why honey and glucose syrup feel sticky and keep a cake moist. In nature the D form of glucose dominates, and the enzymes of the body are built to handle D sugars, not their mirror images.

Apparatus

The laboratory test for these sugars uses a copper reagent, with heating and a titration or a colour reading. No other apparatus is named in the books used here for this pair, so no further apparatus is listed.

Measurement procedure

Reducing sugars are estimated by their power to reduce a copper reagent: the sugar is heated with the 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. The degree of breakdown of a starch syrup is reported by the dextrose equivalent, DE, which counts glucose as DE 100.

Factors affecting readings

The sugar mix matters: two syrups at the same DE can taste different, because fructose is sweeter than glucose. The temperature and the time of heating change how much browning and how much reaction with protein takes place. Water activity also matters, because these sugars hold water and lower water activity, so microbes grow more slowly where the sugar level is high.

Formulas and calculations

Dextrose equivalent (DE). DE = (reducing sugar as glucose / total solids) x 100. Glucose itself has DE 100, untouched starch has DE near 0, and a maltodextrin sits near DE 10 to 20.

Worked example: a glucose syrup sample weighs 80 g of solids and contains 16 g of reducing sugar counted as glucose. DE = (16 / 80) x 100 = 20.

Worked example from the sweetness values: to match the sweetness of 100 g of sucrose, a maker needs about 100 / 1.7 = 59 g of fructose, or about 100 / 0.7 = 143 g of glucose. A maker who swaps one sugar for another at the same weight must adjust the amount, or the sweetness and the browning will both change.

Industrial applications

• Glucose: glucose syrups for sweets and drinks, and as the reference sugar for the DE scale.
• Glucose: keeping cakes moist, because glucose syrup holds water well.
• Fructose: sweetening foods where a strong sweet taste is wanted at a lower weight, as in fruit products and honey.
• Fructose: jams and sweets, where its sweetness and water holding help stop crystallising when it is part of invert sugar.
• Both: browning and flavour in baked foods, through the Maillard reaction with proteins.

Advantages and limitations

• Glucose, advantage: a steady, well known sweetener, and the reference for DE and for reducing sugar tests.
• Glucose, limitation: less sweet than sucrose, so more weight is needed for the same sweetness, and it browns with protein on heating.
• Fructose, advantage: the sweetest common sugar, so less is needed for the same sweetness, and it holds water well.
• Fructose, limitation: it browns more readily than glucose, so heating must be watched, and it is a reducing sugar, so it reacts with proteins on storage and heating.

FAQ

Are glucose and fructose the same sugar?

No. They share the formula C₆H₁₂O₆, but they are isomers: glucose carries its carbonyl group on the first carbon and fructose on the second carbon.

Which one is sweeter?

Fructose, at about 1.7 times as sweet as sucrose, while glucose is about 0.7 times as sweet as sucrose.

Why does honey never dry out in the jar?

Honey is mostly glucose and fructose, and both sugars hold water well and attract water from the air.

What is a reducing sugar?

A sugar with a free carbonyl group that can reduce other compounds in chemical tests, such as the copper reagent test, and that can take part in browning with proteins.

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)