How to tell if it's a disaccharide?

December 13, 2025 •

3 min read

Approximately 75% of the world's population is lactose intolerant, highlighting the commonality of disaccharides like lactose in our food supply. Knowing how to tell if it's a disaccharide is crucial for chemists and food scientists, involving simple lab tests that reveal their unique chemical structure, a double sugar formed from two monosaccharides.

Quick Summary

This article details the chemical methods used to identify disaccharides, including distinguishing them from monosaccharides using Barfoed's test and classifying them as reducing or non-reducing sugars with Benedict's test. It covers the principle of hydrolysis to confirm a double-sugar structure.

Key Points

Start with Benedict's Test: Use Benedict's reagent to see if the sugar is a reducing sugar, which includes all monosaccharides and some disaccharides (like lactose and maltose).
Consider Non-Reducing Sugars: If Benedict's test is negative (solution remains blue), the sugar might be a non-reducing disaccharide like sucrose.
Perform Hydrolysis for Confirmation: If a negative Benedict's result occurs, hydrolyze the sample with dilute acid, neutralize it, and re-test with Benedict's reagent. A positive result confirms a disaccharide.
Distinguish from Monosaccharides: Use Barfoed's test, which works in a mildly acidic solution. Monosaccharides will react rapidly (1-2 minutes) to form a precipitate, while reducing disaccharides will react much more slowly (7+ minutes).
Identify the Structure: Disaccharides are composed of two monosaccharide units linked by a glycosidic bond, and this double-sugar structure is what the hydrolysis test ultimately proves.
Understand Bonding: The type of glycosidic bond (whether it involves the anomeric carbons) determines if a disaccharide is reducing or non-reducing, dictating its reactivity in tests like Benedict's.

Disaccharide Fundamentals: What You Need to Know

Before delving into testing methods, it is essential to understand what a disaccharide is. Disaccharides, also known as double sugars, are carbohydrates formed when two monosaccharides are joined together via a glycosidic bond. This bonding process, a condensation reaction, involves the removal of a water molecule. The most common examples are sucrose (glucose + fructose), lactose (glucose + galactose), and maltose (glucose + glucose). A key property of disaccharides is that they can be broken down into their constituent monosaccharides through hydrolysis, a reaction involving the addition of a water molecule.

Using Chemical Tests to Identify Disaccharides

Identifying a disaccharide often involves a series of chemical tests to confirm its properties and structure. These tests allow for differentiation between simple and complex sugars based on their reactivity.

The Benedict's Test: Identifying Reducing Sugars

Benedict's test is a foundational chemical test used to detect the presence of reducing sugars.

Methodology: A sample is mixed with Benedict's reagent (a solution containing copper(II) sulfate, sodium carbonate, and sodium citrate) and heated.
Principle: Reducing sugars possess a free aldehyde or ketone group (as a hemiacetal) that can reduce the blue copper(II) ions in the reagent to brick-red copper(I) oxide precipitate.
Interpreting Results for Disaccharides: Some disaccharides, like maltose and lactose, are reducing sugars and will give a positive result. However, a negative result does not rule out all disaccharides. Sucrose, a non-reducing disaccharide, will produce a negative result because its anomeric carbons are both involved in the glycosidic bond, leaving no free hemiacetal group.

The Barfoed's Test: Distinguishing from Monosaccharides

To differentiate a reducing disaccharide from a monosaccharide, the Barfoed's test is used. This test relies on the weaker reducing ability of disaccharides compared to monosaccharides.

Methodology: The test is conducted in a mildly acidic medium using Barfoed's reagent (copper acetate and acetic acid).
Principle: The acidic condition is less favorable for reduction. Monosaccharides react and produce a brick-red precipitate within 1–2 minutes, whereas disaccharides require prior hydrolysis and react much slower, taking 7–12 minutes.
Interpreting Results: The timing of the precipitate formation is key. An early positive reaction (within a few minutes) indicates a monosaccharide, while a delayed reaction suggests a reducing disaccharide.

Hydrolysis Followed by Benedict's Test

This two-step process is the definitive way to confirm a non-reducing sugar is a disaccharide. It involves breaking the molecule apart into its constituent monosaccharides.

Steps:

Acid Hydrolysis: Take a fresh sample that previously gave a negative Benedict's test. Add a small amount of dilute acid (e.g., hydrochloric acid) and heat gently. The acid will catalyze the hydrolysis of the glycosidic bond, breaking the disaccharide into its monosaccharide components.
Neutralization: Neutralize the acid by adding sodium hydrogen carbonate (sodium bicarbonate) until fizzing stops.
Second Benedict's Test: Re-test the neutralized solution with Benedict's reagent and heat. A positive result (color change to green, yellow, orange, or brick-red) indicates that the original compound was a disaccharide.

A Comparison of Identification Tests


Feature	Molisch's Test	Benedict's Test	Barfoed's Test	Hydrolysis followed by Benedict's Test
Carbohydrate Type	All carbohydrates	Reducing sugars (all monosaccharides, some disaccharides)	Monosaccharides (distinguishes from reducing disaccharides)	Disaccharides (both reducing and non-reducing)
Positive Result	Purple ring at interface	Color change (blue to brick-red precipitate)	Red precipitate forms rapidly	Color change (blue to brick-red precipitate) after hydrolysis
Negative Result	No purple ring	Stays blue	Red precipitate forms slowly or not at all	Stays blue (original sample contains no carbohydrate)
Key Principle	Dehydration by acid to form furfural derivatives	Reduction of copper(II) ions under alkaline conditions	Reduction of copper(II) ions under acidic conditions	Breakdown of a larger sugar molecule into smaller, reactive units

Conclusion

Determining if a compound is a disaccharide can be systematically achieved through a combination of chemical tests. An initial Benedict's test can identify reducing disaccharides like lactose, but a negative result for non-reducing sugars like sucrose necessitates a hydrolysis step. The Barfoed's test offers a crucial distinction between monosaccharides and reducing disaccharides based on reaction speed. Ultimately, confirming the breakdown of a compound into two simpler sugars via hydrolysis is the most conclusive evidence for identifying it as a disaccharide. This structured approach, using multiple tests, provides a reliable and comprehensive method for carbohydrate classification in a laboratory setting.

Learn more about carbohydrates and their roles

Frequently Asked Questions

A monosaccharide is a single sugar unit (e.g., glucose), while a disaccharide is formed from two monosaccharide units joined together. Monosaccharides cannot be hydrolyzed further, whereas disaccharides can be broken down into two simpler sugars.

Sucrose is a non-reducing sugar because the glycosidic bond links the anomeric carbons of both the glucose and fructose units. This means there is no free hemiacetal group available to act as a reducing agent, so it does not react with Benedict's reagent.

Barfoed's test distinguishes between monosaccharides and reducing disaccharides. Because the test is run in a mildly acidic medium, monosaccharides react quickly (1–2 minutes) to produce a precipitate, while the weaker reducing disaccharides react much more slowly (7+ minutes).

No. While a positive Benedict's test indicates a reducing sugar, it cannot distinguish between a monosaccharide and a reducing disaccharide. You need a further test, like Barfoed's or a hydrolysis and re-test, for confirmation.

During hydrolysis, a water molecule is added to break the glycosidic bond that links the two monosaccharide units. This process yields the two simple sugar molecules that make up the disaccharide.

A glycosidic bond is the covalent bond that joins two monosaccharide units together to form a disaccharide. The type and position of this bond are critical in determining the disaccharide's properties.

The most common reducing disaccharides are lactose (milk sugar) and maltose (malt sugar). They have a free hemiacetal unit that allows them to react as a reducing agent.

A non-reducing disaccharide is confirmed by a two-step process. First, it tests negative for Benedict's. Then, upon acid hydrolysis to break it into its monosaccharides, a second Benedict's test will yield a positive result.