In this lesson, we’ll learn about reducing and non-reducing sugars. We’ll learn how to identify a reducing sugar and explore some reactions that involve them.

What Are Sugars?

What do toast, browned steak, and caramel have in common? Besides all tasting delicious, they’re all possible because of science. Who’d have thought there could by more to sugar than just tasting good?

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So where does that sweet stuff come from? A sugar is a carbohydrate that is abundantly present in the sugarcane and sugar beet plants. Sugar is composed of hydrogen, oxygen, and carbon and exists in different forms. One of these groups is called a simple sugar, called monosaccharides or disaccharides. Monosaccharides include glucose and fructose, while disaccharides include sucrose and lactose.

There is another way to group sugars as well: whether they are a reducing sugar or a non-reducing sugar. A reducing sugar is a sugar that has a free aldehyde or ketone that can act as a reducing agent. A non-reducing sugar does not have a free aldehyde or ketone, so it cannot act as a reducing agent. In other words, a reducing sugar, when chemically altered, can donate electrons to another molecule, which will change the color and taste of food. Let’s look at this a little further.

Reducing Sugars

You can identify a sugar by looking for the anomeric carbon. An anomeric carbon is the first stereocenter of the molecule. If that stereocenter has an OH group coming off of it then it is a reducing sugar. This is because when the sugar is in the open configuration, that alcohol becomes a ketone or aldehyde which is able to reduce other compounds.

All monosaccharides are reducing sugars. A monosaccharide is the simplest form of a sugar. Disaccharides are two monosaccharides combined. Common disacccharides are maltose, lactose, and sucrose. These can either be reducing or non-reducing sugars.

Maltose is a combination of two glucose molecules. They are combined using the first carbon (the anomeric carbon) from one of the glucose molecules and the fourth carbon from the other glucose molecule. Since the second glucose molecule still has the anomeric carbon available with the OH group, it is still able to act as a reducing sugar.

Non-Reducing Sugars

The main non-reducing sugar is sucrose, or more commonly known as table sugar. Sucrose is a glucose carbon connected at the anomeric carbon to an anomeric carbon on a fructose. Since both anomeric carbons are involved in the bond, neither one has an OH group, so it is not a reducing sugar.

Reactions with Reducing Sugars

So, why is it important to know that there are reducing and non-reducing sugars? Reactions with reducing sugars can be used in many different ways. One way is to help diagnose if a person is diabetic. It is important to measure how much glucose is in the urine to diagnose diabetes. The Fehling’s Test uses the reducing nature of glucose to help determine how much glucose is present. By seeing how much of the Fehling’s reagent (a copper-based solution) is able to reduce, we can deduce how much glucose is in the sample.

The browning on steak, toast, and caramel all happen due to the Maillard Reaction. When a reducing sugar combines with a protein, it undergoes a complex set of reactions that leads to a brown color.

Lesson Summary

Reducing sugars are sugars where the anomeric carbon has an OH group attached that can reduce other compounds. Non-reducing sugars do not have an OH group attached to the anomeric carbon so they cannot reduce other compounds. All monosaccharides such as glucose are reducing sugars. A disaccharide can be a reducing sugar or a non-reducing sugar. Maltose and lactose are reducing sugars, while sucrose is a non-reducing sugar.

The Fehling’s Test measures how much copper can be reduced by a solution to determine how much reducing sugars are present. This is useful for diagnosing diabetes. Reducing sugars are important in many food reactions, such as browned toast, through a reaction called the Maillard reaction.