FOOD CHEMISTRY: SUGARS
Diagram
One reacts, one does not
Reducing sugar
Has a free aldehyde or ketone group. Glucose, fructose, lactose and maltose. Reduces Fehling solution and takes part in browning.
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Non-reducing sugar
Has no free aldehyde or ketone group. Sucrose is the main example: a non-reducing disaccharide.
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The test
A reducing sugar turns Fehling solution from blue to a brick red precipitate. A non-reducing sugar does not, until it is first split by hydrolysis.
Figure 1: Reducing sugars react in the Fehling test and in browning; sucrose does not until it is split. Drawn in our own style from Joslyn, Methods in Food Analysis, chapter on Reducing Sugars, and Honig, Principles of Sugar Technology.
Introduction
Reducing and non-reducing sugars look alike as white crystals, but they behave differently in chemical tests and in cooking. A reducing sugar has a free aldehyde or ketone group that can take part in a reaction, so glucose, fructose, lactose and maltose are reducing sugars. A non-reducing sugar has no free group, so sucrose, the common table sugar, is a non-reducing disaccharide. That one structural difference decides whether the sugar reacts in the Fehling test and whether it takes part in the Maillard reaction.
Joslyn gives a chapter to reducing sugars, and Honig states plainly that sucrose is a non-reducing disaccharide. This article follows those two books, with Coultate for the browning link.
Quick comparison
Table 1: Reducing sugars compared with non-reducing sugars
| Aspect | Reducing sugars | Non-reducing sugars |
|---|---|---|
| Definition | A sugar with a free aldehyde or ketone group | A sugar with no free aldehyde or ketone group |
| Examples | Glucose, fructose, lactose, maltose | Sucrose, the main example |
| Sucrose | Sucrose is not a reducing sugar | Sucrose is the standard non-reducing disaccharide |
| Fehling test | Reduces the blue copper solution to a brick red precipitate of cuprous oxide | No reaction, until the sugar is first split by hydrolysis |
| Benedict test | Gives a positive result, like the Fehling test | No result, until first split by hydrolysis |
| Maillard reaction | Takes part, because the free group reacts with amino groups | Does not take part until it is split into glucose and fructose |
| Browning | Browns readily on heating with protein | Browns by caramelisation when heated alone, not by the Maillard reaction |
| Sweetness and use | Glucose and fructose are used as sweeteners and in syrups | Sucrose is the main table sugar and the main sugar of confectionery |
| Hydrolysis | Already in a form that reacts | Sucrose splits by hydrolysis into glucose and fructose, which are both reducing |
| Measurement | Measured by copper reduction methods, as in Joslyn | Measured after hydrolysis, or by polarimetry, since sucrose rotates polarised light |
Scientific explanation
Definition
A reducing sugar is a sugar with a free aldehyde or ketone group that can reduce another compound, so it gives a positive Fehling or Benedict test. Glucose, fructose, lactose and maltose are reducing sugars. A non-reducing sugar has no free aldehyde or ketone group, so it gives no such test. Sucrose is a non-reducing disaccharide: its two parts, glucose and fructose, are joined in a way that blocks both free groups.
Working principle
In the Fehling test, the free aldehyde or ketone group of a reducing sugar reduces blue copper(II) ions to red copper(I) oxide, which falls out as a brick red precipitate. A non-reducing sugar cannot do this, because its free groups are blocked. If sucrose is first split by hydrolysis with acid or with the enzyme invertase, it gives glucose and fructose, and both of those are reducing, so the test then turns positive. The same free group is what reacts with amino groups in the Maillard reaction, which is why only reducing sugars brown with protein.
Apparatus
The books used here name the Fehling and Benedict reagents, a burette for titration, and a polarimeter for sucrose, as in Joslyn. No single named instrument decides the class: the test is the copper reduction itself.
Measurement procedure
Joslyn describes reducing sugars by copper reduction methods: the sugar solution is titrated against Fehling solution, and the brick red precipitate of cuprous oxide shows the reduction. Sucrose is measured after hydrolysis into its two reducing parts, or by polarimetry, because sucrose rotates polarised light. The books used here give no single worked food example with numbers for this comparison, so none is given.
Factors affecting readings
Acid and heat split sucrose into glucose and fructose, turning a non-reducing sugar into two reducing sugars. Temperature and time change how fast the Maillard reaction runs. Other reducing substances in a food can interfere with the copper test, so the sample must be prepared as Joslyn describes.
Formulas and calculations
Sucrose is split by hydrolysis into its two parts: C12H22O11 + H2O gives C6H12O6 + C6H12O6, glucose plus fructose. Both products are reducing sugars, so a solution that gave no Fehling reaction before hydrolysis gives a positive brick red result after it. The books used here give no worked food calculation with numbers for this comparison, so no worked example is given.
Industrial applications
- Reducing sugars are measured in fruit products, syrups and honey by the copper reduction methods in Joslyn.
- Sucrose is the main sugar of sugar confectionery, where Edwards notes it is a non-reducing disaccharide.
- Glucose and fructose syrups are used where a reducing sugar is wanted for browning and for keeping food moist.
- Lactose, a reducing sugar, takes part in the browning of baked milk products.
- Maltose, a reducing sugar, is formed when starch is broken down in brewing and baking.
Advantages and limitations
- Reducing sugars: advantage: react in browning, giving colour and roasted flavour. Limitation: can brown too far and darken a food that should stay pale.
- Non-reducing sugars: advantage: sucrose is stable in storage and does not react with protein. Limitation: it cannot take part in the Maillard reaction until it is split.
- Reducing sugars: advantage: easily measured by the Fehling test. Limitation: the test is affected by other reducing substances in the food.
FAQ
Is sucrose a reducing sugar?
No. Sucrose is a non-reducing disaccharide, because its two parts are joined in a way that blocks both free groups.
What happens if sucrose is boiled with acid?
It splits by hydrolysis into glucose and fructose, and both of those are reducing sugars.
Why do reducing sugars matter in baking?
They react with protein in the Maillard reaction, giving brown colour and roasted flavour to crusts.
Which sugars are reducing?
Glucose, fructose, lactose and maltose are reducing sugars.
References and further reading
- Maynard Joslyn, Methods in Food Analysis: Applied to Plant Products, Elsevier, 2012 (Google Books page read)
- Pieter Honig (ed.), Principles of Sugar Technology, Elsevier, 2013 (Google Books page read).
- Tom P. Coultate, Food: The Chemistry of Its Components, 5th ed., Royal Society of Chemistry, 2009 (Google Books page read)
- W. P. Edwards, The Science of Sugar Confectionery, Royal Society of Chemistry, 2000 (Google Books page read)
