reducing and non reducing sugar experiment

Benedict’s Test- Principle, Composition, Preparation, Procedure and Result Interpretation

Benedict’s Test is used to test for simple carbohydrates. The Benedict’s test identifies reducing sugars (monosaccharide’s and some disaccharides), which have free ketone or aldehyde functional groups. Benedict’s solution can be used to test for the presence of glucose in urine. 

Some sugars such as glucose are called reducing sugars because they are capable of transferring hydrogens (electrons) to other compounds, a process called reduction. When reducing sugars are mixed with Benedicts reagent and heated, a reduction reaction causes the Benedicts reagent to change color. The color varies from green to dark red (brick) or rusty-brown, depending on the amount of and type of sugar.

Benedict’s quantitative reagent contains potassium thiocyanate and is used to determine how much reducing sugar is present. This solution forms a copper thiocyanate precipitate which is white and can be used in a titration. The titration should be repeated with 1% glucose solution instead of the sample for calibration

Principle of Benedict’s Test

When Benedict’s solution and simple carbohydrates are heated, the solution changes to orange red/ brick red. This reaction is caused by the reducing property of simple carbohydrates. The copper (II) ions in the Benedict’s solution are reduced to Copper (I) ions, which causes the color change.

The red copper(I) oxide formed is insoluble in water and is precipitated out of solution. This accounts for the precipitate formed. As the concentration of reducing sugar increases, the nearer the final color is to brick-red and the greater the precipitate formed. Sometimes a brick red solid, copper oxide, precipitates out of the solution and collects at the bottom of the test tube.

Sodium carbonate provides the alkaline conditions which are required for the redox reaction.  Sodium citrate complexes with the copper (II) ions so that they do not deteriorate to copper(I) ions during storage.

Complex carbohydrates such as starches DO NOT react positive with the Benedict’s test unless they are broken down through heating or digestion (try chewing crackers and then doing the test). Table sugar (disaccharide) is a non-reducing sugar and does also not react with the iodine or with the Benedict Reagent. Sugar needs to be decomposed into its components glucose and fructose then the glucose test would be positive but the starch test would still be negative.

Composition and Preparation of Benedict’s Solution

Benedict’s solution is a deep-blue alkaline solution used to test for the presence of the aldehyde functional group, – CHO.

Anhydrous sodium carbonate = 100 gm Sodium citrate – 173 gm Copper(II) sulfate pentahydrate = 17.3 gm

One litre of Benedict’s solution can be prepared from 100 g of anhydrous sodium carbonate, 173 g of sodium citrate and 17.3 g of copper(II) sulfate pentahydrate.

Procedure of Benedict’s Test

• Approximately 1 ml of sample is placed into a clean test tube.
• 2 ml (10 drops) of Benedict’s reagent (CuSO4) is placed in the test tube.
• The solution is then heated in a boiling water bath for 3-5 minutes.
• Observe for color change in the solution of test tubes or precipitate formation.

Result Interpretation of Benedict’s Test

If the color upon boiling is changed into green, then there would be 0.1 to 0.5 percent sugar in solution. If it changes color to yellow, then 0.5 to 1 percent sugar is present. If it changes to orange, then it means that 1 to 1.5 percent sugar is present. If color changes to red,then 1.5 to 2.0 percent sugar is present. And if color changes to brick red,it means that more than 2 percent sugar is present in solution.

Positive Benedict’s Test: Formation of a reddish precipitate within three minutes. Reducing sugars present. Example: Glucose Negative Benedict’s Test: No color change (Remains Blue). Reducing sugars absent. Example: Sucrose.

• National Institutes of Health, Testing for Lipids, Proteins and Carbohydrates- Benedict’s solution.
• Fayetteville State University- Biological Molecules: Carbohydrates, Lipids, Proteins.
• Harper College- Benedict’s Test.
• National Biochemicals Corp.- BENEDICT’S SOLUTION (MB4755).
• Science Olympiad- Use of Benedict’s Solution.
• Brilliant Biology Student 2015- Food Tests- Benedict’s Test for Reducing Sugars.
• BBC Bitesize- Chemistry- Carbohydrates.
• University of Manitoba- The Molecules of Life: Biochemistry- Carbohydrates.
• Northern Kentucky University- Benedict’s Reagent: A Test for Reducing Sugars.
• KNUST Open Educational Resources, Benedict’s Test – Qualitative Test in Carbohydrates.
• Mark Rothery’s Biology Web Site- Biochemical Tests.
• All Medical Stuff- Benedict’s test for reducing sugar.
• Hendrix College- Benedicts Test for Glucose.
• Info Please- Benedict’s solution.
• Mystrica- Benedict’s Test.
• Amrita Virtual Lab Collaborative Platform- Qualitative Analysis of Carbohydrates.

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29 thoughts on “Benedict’s Test- Principle, Composition, Preparation, Procedure and Result Interpretation”

What can I use to remove the orange copper stains left over in the test tube? Simple scrubbing with soap is hard to get rid of it. Thanks!

it is possible to get the same results if sucrose was used instead of glucose?

No, because sucrose is a table sugar and as this article states, table sugar is not reductive.

How do lipids react with ethanol

I want to test how freezing and thawing of food can affect its carbohydrate content. when pasta is frozen and thawed the starch turns into resistant starches so will the benedicts test be able to detect that?

the benedict test is not fantastic at testing starches in general due to their complexity. resistant starches, even less so. i’d recommend using an iodine test instead. the iodine test is designed to detect complex carbohydrates so it would detect your starches much better than benedict’s solution would.

I performed this particular experiment according to the outlined steps. The result l had was a dark brown color. What might be the cause of this result?

High concentration of reducing sugar

Which will be the colour of protiens if we add benedict’s solutoin 2 to 3 drops and caustic soda

If you add Benedict’s solution to caustic soda (sodium hydroxide), which is a strong base, without any reducing sugars present, you are likely to observe little to no color change. Benedict’s solution requires the presence of reducing sugars to undergo a chemical reaction that leads to the formation of a colored precipitate.

What is the differences between Benedict solution and Fehling’s solution

What are the precautions to take during the experiment

Use clean test tubes

Fehling’s reagent contains sodium potassium tartrate (Rochelle’s salt) in place of sodium citrate.

Hmm….Benedict’s Solution consists of copper sulfate, sodium citrate, and sodium carbonate. The copper ions in Benedict’s solution are reduced by the reducing sugars present in the test sample, leading to the formation of a colored precipitate.

Fehling’s Solution comes in two separate solutions, Fehling’s A and Fehling’s B. Fehling’s A contains copper(II) sulfate, while Fehling’s B contains sodium potassium tartrate and sodium hydroxide. The two solutions are mixed in equal proportions before use.

What is the different between Benedict’ and barfoe’d test

1 ml is approximately 20 drops. So 2 ml would be 40 drops. 10 drops would be 0.5 (1/2) ml. What should the protocol say?

There is one major problem in this writing….you’ve said Hydrogens are electrons. Hydrogens are protons…often, having given away an electron, they acquire a positive charge.

Id say this… reduction as we know it can also be the gain of hydrogen since it reduces non metals which would otherwise not lose electrons. N₂+2H₂->2NH₃

We can heat directly so what’s effect on solution

Will it detect the presence of lactobionic acid?

What happens if you keep on heating the solution in boiling water bath for more than 5 minutes?

I made this mistake while working with Benedict’s Reagent, it burns the reagent and the substance you are testing creating an odd off colour that should not be used as sound results in a report. It is also quite difficult to clean any glassware after that mistake.

Do it is also known as fehling’s test for reducing sugars????

Hello Akash, the Benedict’s test is much more sensitive than the Fehling’s Test but they’re both tests for reducing sugars. 🙂

– fellow pre-med student trying to pass pharmacy

Does Formica acid give benedict test??

ohh no worries at all, hope you are enjoying your results XDXDXD

Hydrogens are not electrons, they are protons and often have a positive charge. Is it possible that the sugars are reducing sugars because they accept hydrogens instead of give them up?

I would like to know the precautions while using the solution.

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Biochemical Tests: Reducing Sugars & Starch ( OCR A Level Biology )

Revision note.

Biology Lead

Biochemical Tests: Reducing Sugars & Starch

• There are a number of tests that can be carried out quickly and easily in a lab to determine if a sample contains a certain type of sugar
• The following tests are qualitative - they do not give a quantitative value as to how much of each type of molecule may be present in a sample
• OILRIG in Chemistry

The Benedict’s test for reducing sugars

• Copper (I) oxide is not soluble in water, so it forms a precipitate
• Add Benedict's reagent (which is blue as it contains copper (II) sulfate ions) to a sample solution in a test tube
• Heat the test tube in a water bath or beaker of water that has been brought to a boil for a few minutes
• If a reducing sugar is present, a coloured precipitate will form as copper (II) sulfate is reduced to copper (I) oxide which is insoluble in water
• It is important that an excess of Benedict’s solution is used so that there is more than enough copper (II) sulfate present to react with any sugar present
• This test is semi-quantitative as the degree of the colour change can give an approximate indication of how much reducing sugar is present without knowing the exact quantity

Benedict's test for reducing sugars produces a colour change from blue towards red if a reducing sugar is present

Reducing & Non-reducing Sugars Table

The test for non-reducing sugars

• Add dilute hydrochloric acid to the sample and heat in a water bath that has been brought to the boil
• Use a suitable indicator (such as red litmus paper) to identify when the solution has been neutralised, and then add a little more sodium hydrogencarbonate as the conditions need to be slightly alkaline for the Benedict’s test to work
• Add Benedict’s reagent to the sample and heat in a water bath that has been boiled – if a colour change occurs, a non-reducing sugar is present

Explanation

• The addition of acid will hydrolyse any glycosidic bonds present in any carbohydrate molecules
• The resulting monosaccharides left will have an aldehyde or ketone functional group that can donate electrons to copper (II) sulfate (reducing the copper), allowing a precipitate to form

The iodine test for starch

• The iodine is in potassium iodide solution as iodine is insoluble in water
• If starch is present, iodide ions in the solution interact with the centre of starch molecules, producing a complex with a distinctive blue-black colour
• This test is useful in experiments for showing that starch in a sample has been digested by enzymes

Iodine will change colour in the presence of starch

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The Benedict's Test for Reducing and Non-Reducing sugars

In this A-Level Biology Lesson "The Benedict's Test for Reducing and Non-Reducing sugars” you'll learn what makes a Reducing sugar a Reducing sugar.

Next Non-Reducing sugars will be discussed before moving on to the Benedict's Test and How Benedict's Reagent works.

You'll have to be able to "Describe how to carry out the Benedict's test for Reducing sugars and explain the expected results - so that's covered before moving on to describe how to carry out the Benedict's Test for Non-Reducing Sugars.

When you’re confident you know how to describe the Benedict's test procedure for both Reducing and Non-Reducing sugars and explain the expected results it's time for you to complete the accompanying “Benedict's Test for Reducing and Non-Reducing sugars” knowledge check and exam style questions in your lesson workbooklet. 

Don't worry, you’ll be able to check your answers with my properly written answers in the back of the workbook. (No vague mark schemes here either, you’ll see exactly how you need write your answers so that you gain maximum marks in the exams!)

A Level Biology: - The Benedict's Test for Reducing and Non-Reducing sugars

​ Here's a summary of the lesson: -

​Sugars can be classified as either Reducing or Non-Reducing. Monosaccharides and some disaccharides are reducing sugars – A sugar with a “free” Aldehyde [CO] or Ketone group [CHO]. These functional groups allow the sugar to donate electrons – making that sugar the “reductant” i.e. the “Reducing Sugar”. Thus, it is the reduction of Copper II Sulphate that gives the colour change when testing for the presence of reducing sugars in a test sample.

The reducing sugars you must know are: - 

 The only non-reducing sugar you must know is Sucrose .

You could be asked to Describe, Explain, Analyse and Evaluate the procedure testing for Reducing and / or Non-Reducing Sugars.

The Benedict’s test for reducing sugars: -

Heat the test sample with Benedict’s Reagent.

Observe the colour change.

A brick red precipitate indicates the presence of a reducing sugar.

The Benedict’s test for non-reducing sugars: -

Heat the test sample with dilute hydrochloric acid.

Neutralise the test sample by adding sodium hydrocarbonate.

Microbe Notes

Benedict’s Test- Principle, Procedure, Steps, Results, Uses

Table of Contents

Interesting Science Videos

What is Benedict’s Test?

Definition of benedict’s test.

Benedict’s Test is a chemical analytical method used for the detection of reducing sugar in a solution. Benedict’s Test is a qualitative test often used for the differentiation of carbohydrates (saccharides/sugars) into reducing and non-reducing types.

Reducing sugars are those sugars that have free aldose or ketose groups capable of donating electrons to other molecules oxidizing them. They have free carbon at the end of their molecules. All monosaccharides and some disaccharides, oligosaccharides, and polysaccharides are reducing sugar.

It is widely used to identify monosaccharides (simple sugars) and other reducing sugars. It is used as an alternative to Fehling’s test . Identification is based on the development of brick-red color due to the chemical reaction between Benedict’s reagent and reducing sugar. Based on the intensity of the reaction mixture, the concentration of sugar can be determined, but numerical value can’t be estimated. Hence, it is a qualitative and semi-quantitative test.

It is also used for detecting glucose in urine as a presumptive test of diabetes mellitus.

It was discovered by American Chemist/Biochemist Stanley Rossiter Benedict. 

Objectives of Benedict’s Test

• To detect the presence of reducing sugar in the sample solution
• To diagnose diabetes mellitus by detecting glucose in the urine sample
• To estimate the concentration of reducing sugar in the sample solution
• To differentiate and identify the extracted carbohydrates 

Principle of Benedict’s Test

Sodium carbonate in the Benedict reagent increases the pH of the sample-reagent solution mixture. Under warm alkaline conditions reducing sugars are tautomerism to strong reducing agents, enediols. These enediols reduce the cupric ions (Cu 2+ ) (present as Copper Sulfate (CuSO 4 )) of Benedict reagent into cuprous ions (Cu + ). The cuprous particles are present in form of insoluble Copper (I) oxide or cuprous oxide (Cu 2 O) which is of red color. These red-colored copper oxides get precipitated.

The concentration of reducing sugar in the sample differs from the intensity and shade of the color of the reaction mixture. This shade of color can be used to estimate the concentration of reducing sugar in the sample.  Color may vary from greenish to yellow to orange-red to brick-red. As the concentration of reducing sugar increases color gradually changes from greenish to yellowish to orange to brick-red.  

Requirements of Benedict’s Test

• Sample solution of unknown carbohydrate (or urine sample)
• Test-tubes and test-tube holders
• Bunsen burner
• Benedict’s Reagent

Preparation of Benedict’s Reagent

• Measure 17.3 grams of copper sulfate (CuSO 4 ), 173 grams of sodium citrate (Na 3 C 6 H 5 O 7 ), and 100 grams of anhydrous sodium carbonate (Na 2 CO 3 ) (or 270 grams of sodium carbonate decahydrate (Na 2 CO 3 .10H 2 O)) 
• Put all the measured chemicals in a volumetric flask of 1000 mL.
• Pour distilled water up to 1000 mL marking.
• Dissolve all the components properly by shaking gently.

Procedure of Benedict’s Test

• In a clean test tube add 1 mL of sample solution (urine or carbohydrate solution).
• Add 2 mL of Benedict’s reagents over the sample.
• Place the test tube over a boiling water bath and heat for 3–5 minutes or directly heat over a flame.
• Observe for color change.

Result Interpretation / Observation of Benedict’s Test

Any change in color from blue to green or yellow or orange or red within 3 minutes indicates a positive Benedict test i.e. presence of reducing sugar in the sample.

For semiquantitative evaluation, the concentration of reducing sugar can be estimated based on the shade of developed color as follows; 

Blue	0	No reducing sugar
Green solution	< 0.5	Trace reducing sugar
Green ppt.	0.5 – 1	Trace reducing sugar
Yellow ppt.	1 -1.5	Low reducing sugar
Orange-red ppt.	1.5 – 2	Moderate reducing sugar
Brick-red ppt.	>2	High reducing sugar

Precautions of Benedict’s Test

• Measurement must be accurate.
• Don’t heat the mixture quickly. It is best to heat over a water bath slowly.
• During heating the solution, use a test-tube holder.
• Don’t face the test tube towards oneself or others during heating.
• Heating should be done at least thrice before reporting negative.

Applications of Benedict’s Test

• In biochemistry for analysis and identification of unknown carbohydrate extracts.
• In clinical diagnosis for rapid presumptive diagnosis of diabetes mellitus.
• In quality control for detecting simple sugar and their quantification.

Advantages of Benedict’s Test

• A simple test requiring fewer materials and less time.
• Non-toxic reagents.
• Inexpensive.
• Both qualitative and semi-quantitative evaluation.

Limitations of Benedict’s Test

• False-positive result due to reaction with drugs like penicillin, isoniazid, streptomycin, salicylates, and p-aminosalicylic acid.
• Chemicals in urine like creatinine, ascorbic acid, and urate retard Benedict’s reaction.
• The exact concentration of reducing sugar can’t be measured; only an estimated semiquantitative value can be indicated.
• Requires further test for identification of the carbohydrate  .
• Robert D. Simoni; Robert L. Hill & Martha Vaughan (2002). “Benedict’s Solution, a Reagent for Measuring Reducing Sugars: the Clinical Chemistry of Stanley R. Benedict”.  J. Biol. Chem .  277  (16): 10–11.  doi : 10.1016/S0021-9258(19)61050-1 .
• National Institutes of Health, Testing for Lipids, Proteins and Carbohydrates – Benedict’s solution.
• Northern Kentucky University- Benedict’s Reagent: A Test for Reducing Sugars.
• Shrestha B (2002). Practical biochemistry and biotechnology. First edition. 99933-665-1-X.
• Fayetteville State University- Biological Molecules: Carbohydrates, Lipids, Proteins.
• KNUST Open Educational Resources .
• Amrita Virtual Lab Collaborative Platform- Qualitative Analysis of Carbohydrates.
• Benedict’s Test – Reagent Preparation, Principle, Procedure, Reaction (byjus.com)
• Benedicts Test – Principle, Procedure, Result and Limitation (vedantu.com)
• Benedict’s Test- Principle, Preparation, Procedure and Result Interpretation (microbiologyinfo.com)
• Benedict’s test: Definition, Principle, Uses, and Reagent (chemistrylearner.com)

About Author

Prashant Dahal

9 thoughts on “Benedict’s Test- Principle, Procedure, Steps, Results, Uses”

Why we use 1.5 ml of Benedict’s reagent for 1 ml of glucose

Teacher’s notes or Teacher’s suggestion should be included for more perfect. Thank you for sharing. It is complete and simple explanation. Please kindly share another practical paper for IGCSE A level biology.

The concentration of reducing sugar (g%)….what is the meaning of g%?

I am trying to find out what it means too.

Hi Russell, g% means gram percentage. I mean the number of grams of solute per 100 ml or 100 grams of solution. 🙂 For Example- 2.5g% means there are 2.5 grams of reducing sugar in 100 ml of solution.

Thank you for responding this was a big help! 😀

Hi Sourav, g% means gram percentage. I mean the number of grams of solute per 100 ml or 100 grams of solution. 🙂 For Example- 2g% means there are 2 grams of reducing sugar in 100 ml of solution.

hi im syahmi iqbal, thank you for the web site. it really helps me a lot for my assignment.

Thank you, Happy that our website is helping you in your assignment.

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O O + e → Cu + e → Cu + e → Ag OH OH Glucose is also known as blood sugar because it is present in a our bloodstream. O in the presence of a reducing sugar.

Footnotes: (1) Procedure for making Benedict's solution: Solution A: Measure out about 350 mL of water. Add 86.5 g of crystallised sodium citrate and 50 g of anhydrous sodium carbonate to the water. Stir to dissolve as much of the solids as possible. Filter the solution. Solution B: Make a solution of copper sulfate by dissolving 8.65 g of crystallised copper sulfate to 50 mL of water, stirring constantly. Benedict's Solution: Pour both solutions (solution A and solution B) into a 500 mL volumetric flask and make up to the mark with water. Label this solution Benedict's solution. The Benedict's solution should be clear. If it looks cloudy, filter it. (2) Procedure for making Fehling's solution: Solution A: dissolve 34.64 g of crystallised copper sulfate in water containing a few drops of dilute sulfuric acid in a 500 mL volumetric flask and make up to the mark with water. Solution B: dissolve 60 g of pure sodium hydroxide and 173 g of pure sodium potassium tartrate in water. Filter if necessary. Dilute the solution to 500 mL with water. Keep each of the two solutions in separate tightly stoppered vessels. When you want to use them, combine equal volumes of each solution and mix. (3) Procedure for making Tollen's reagent: Solution A: Dissolve 3 g of silver nitrate in 30 mL of water. Solution B: Dissolve 3 g of sodium hydroxide in 30 mL of water. When you need to use the reagent, mix 1 mL of each solution (A and B) together in a test tube and add dilute ammonia solution drop by drop until the silver oxide is just dissolved. To clean the test tube, rinse with dilute nitric acid.

Reducing vs Non-Reducing Sugar- Definition, 9 Key Differences, Examples

Table of Contents

Interesting Science Videos

Reducing Sugar Definition

Reducing sugar is a type of sugar that consists of a free aldehyde group or a free ketone group, allowing the molecule to act as a reducing agent.

• All monosaccharides are reducing sugars, and so are some disaccharides and oligosaccharides.
• Reducing monosaccharides can further be classified into two groups; aldoses and ketose. Aldoses are sugars consisting of an aldehyde group as the reducing component, whereas ketoses are sugars consisting of a ketone group as the reducing component.
• Ketoses can only reduce other components after they tautomerize into aldoses.
• All disaccharides are not reducing sugars as the aldehyde or ketone group of the molecule might be involved in the cyclic form of the molecule.
• In the case of reducing disaccharides, only one of the two anomeric carbons is involved in the glycosidic bond formation, allowing the other to be free that can convert into an open-chain structure.
• Reducing the property of sugars is important in the case of food as it determines the flavor of the food. The reducing sugar reacts with amino acids in the Maillard reaction when cooked at high temperatures, which are responsible for the flavor of the food.
• The detection of reducing sugars in a sample can be done by one of the two methods; Fehling’s reaction and Benedict’s test.
• The reducing sugar reduces the copper (III) ions in these tests into copper (I) ions resulting in the formation of a brick-red copper oxide precipitate.
• Some commonly encountered examples of reducing sugars are glucose, fructose, galactose, ribose, etc.

Non-Reducing Sugar Definition

Non-reducing sugar is a type of sugar that doesn’t have a free aldehyde or ketone group, as a result of which the sugar cannot act as a reducing agent.

• All polysaccharides are non-reducing sugars, and so are most disaccharides and oligosaccharides.
• Non-reducing sugars are either dimers, trimers, or polymers, which are formed of many reducing monomeric units by the formation of a glycosidic bond.
• The aldehyde and ketone present on the monomers are involved in the formation of the glycosidic bond in the case of most disaccharides and all polysaccharides.
• One of the most prominent properties of non-reducing sugars is that they do not generate any compounds with an aldehyde group in a basic aqueous solution.
• Non-reducing sugar can be differentiated from reducing sugars through tests like Benedict’s test and Fehling’s test.
• The test is based on the principle of reduction of copper sulfate into copper oxide, which results in the formation of a red brick precipitate.
• In the case of polymeric sugars, the anomeric carbons of all the sugar units are involved in the formation of a glycosidic bond. Thus, these molecules cannot convert into an open-chain form with an aldehyde group.
• Some of the examples of non-reducing sugars include sucrose, trehalose, starch, etc.

9 Major Differences (Reducing Sugar vs Non-Reducing Sugar)

	Reducing sugar is a type of sugar that consists of a free aldehyde group or a free ketone group, allowing the molecule to act as a reducing agent.	Non-reducing sugar is a type of sugar that doesn’t have a free aldehyde or ketone group, as a result of which the sugar cannot act as a reducing agent.
	Reducing sugar are a good reducing agent.	Non-reducing sugars are poor, reducing agents.
	Reducing sugars have a free aldehyde or ketone group.	Non-reducing sugar does not have a free aldehyde or ketone group.
	Reducing sugars give a positive Fehling’s test.	Non-reducing sugars give a negative Fehling’s test.
	Reducing sugars give a positive Benedict’s test.	Non-reducing sugars give a negative Benedict’s test.
	Reducing sugars include all monosaccharides and some disaccharides.	Non-reducing sugars include most disaccharides and all polysaccharides.
	Reducing sugars generally have a sweet taste.	Non-reducing sugar generally has a less sweet taste.
	Reducing sugars have a lower molecular weight as these are usually of a smaller size.	Non-reducing sugars have a higher molecular weight as they are usually of a larger size.
	Some commonly encountered examples of reducing sugars are glucose, fructose, galactose, ribose, etc.	Some of the examples of non-reducing sugars include sucrose, trehalose, starch, etc.

Examples of Reducing Sugar

• Glucose is the most abundant monosaccharide on the plant, which is primarily produced by green algae and plants. 
• Glucose is a hexose with six carbon atoms and the molecular formula of C 6 H 12 O 6 .
• It is an aldose consisting of a free aldehyde group at one of the ends, making it a reducing sugar. 
• In the solid form, glucose exists in a ring or cyclic form, which converts into an open-chain structure in the aqueous solution. 
• The cyclic form of glucose is formed when the hydroxyl group on carbon 5 binds to the aldehyde group on carbon 1. 
• Glucose gives a positive Fehling’s, Benedict’s, and Tollen test, which is often used to differentiate glucose from other carbohydrates. 

Examples of Non-Reducing Sugar

• Starch is a polysaccharide composed of multiple monomeric units of glucose linked together by α-1,4 linkages. 
• Starch is a non-reducing sugar as it doesn’t have a free aldehyde or ketone group present in the structure. 
• The aldehyde or ketone groups on the monosaccharides are involved in the formation of glycosidic bonds that keeps the structure of the molecule. 
• Starch gives a negative Tollen’s, Fehling’s, and Benedict’s test as it is a non-reducing sugar. 
• Starch is an essential polysaccharide that is used in different industries as well as a source of nutrients in plants. Plants often store starch as a form of glucose storage.
• Gautum SD, Pant M and Adhikari NR (2016). Comprehensive Chemistry, Part 2. Sixth Edition. Heritage Publishers and Distributors Pvt. Ltd.
• https://pediaa.com/difference-between-reducing-and-nonreducing-sugar/
• https://vivadifferences.com/difference-between-reducing-sugar-and-non-reducing-sugar-with-examples/

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• Reducing vs Non reducing sugars

Understanding the differences between reducing and non-reducing sugars is crucial for your studies in the expansive field of chemistry. This article provides detailed insight into the key characteristics, examples, and chemical reactions associated with reducing versus non-reducing sugars. It outlines reliable methods to distinguish between the two and attaches fundamental uniqueness to each type. With a comprehensive comparison and separate discussions on distinct traits, identifying reducing and non-reducing sugars will not be a daunting task for you. Explore this in-depth analysis with each section elaborating on important aspects, designed to facilitate your learning.

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What characterises non-reducing sugars?

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What happens in the Maillard reaction involving reducing sugars?

What reaction are non-reducing sugars involved in and why?

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What is a reducing sugar and name two examples of reducing sugars.

What is a non-reducing sugar and name two examples of non-reducing sugars.

How can non-reducing sugars be converted into reducing sugars?

What is the role of the aldehyde or ketone group in reducing sugars?

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Understanding Reducing vs Non-reducing Sugars

Defining reducing and non-reducing sugars.

Reducing sugars are a form of carbohydrate that possess free aldehyde or ketone group(s) and can reduce other substances. These sugars have the ability to donate electrons or hydrogen atoms, acting as reducing agents. Examples of common reducing sugars include glucose, lactose , and maltose .

Non-reducing sugars lack a free aldehyde or ketone group and, therefore, cannot reduce other substances. They do not have the ability to donate electrons or hydrogen atoms. Sucrose is a typical example of a non-reducing sugar.

Characteristics of Reducing vs Non-reducing Sugars

• Contain free aldehyde or ketone groups
• Can donate electrons or act as reducing agents
• Can be oxidised
• Can participate in the Maillard reaction, a chemical reaction between amino acids and reducing sugars that gives browned food its flavour
• Lack free aldehyde or ketone groups
• Cannot donate electrons or act as reducing agents
• Cannot be oxidised
• Do not participate in the Maillard reaction

Reducing vs Non-reducing Sugars: A Comparison

	Contain free aldehyde or ketone groups	Lack free aldehyde or ketone groups
	Can donate electrons or act as reducing agents	Cannot donate electrons or act as reducing agents
	Can be oxidised	Cannot be oxidised
	Can participate in the Maillard reaction	Do not participate in the Maillard reaction

Consider Fehling's Test, which is a test to detect the presence of reducing sugars. The reducing sugar reduces the copper (II) ion in the Fehling's solution to a copper (I) ion, producing a brick-red precipitate of copper (I) oxide. Glucose, a reducing sugar, gives a positive result to this test, whereas sucrose , a non-reducing sugar, does not.

Examples of Reducing and Non-Reducing Sugars

Instances of reducing sugars.

• Glucose : This is arguably the most common reducing sugar and is essential in biology as it provides energy for living organisms.
• Fructose : This is a ketonic monosaccharide found in many plants. Although it is a ketose, it is still a reducing sugar because it can isomerise to the aldose glucose.
• Lactose : This is a disaccharide sugar that consists of galactose and glucose. It is found most notably in milk and is a reducing sugar.
• Maltose : This is another disaccharide formed from two units of glucose. It is produced when starch sugar is broken down. Maltose is a reducing sugar.

Instances of Non-reducing Sugars

• Sucrose : Also known as table sugar, it is a disaccharide that consists of glucose and fructose.
• Trehalose : A non-reducing sugar that consists of two molecules of glucose. It is an important source of energy in insects and fungi.

Chemical Reactions Involving Reducing vs Non-reducing Sugars

Chemical reactions of reducing sugars, chemical reactions of non-reducing sugars, sorting reducing from non-reducing sugars, methods to identify reducing sugars.

Imagine conducting a Benedict's test for a solution containing glucose. Upon heating with Benedict's reagent, the solution will turn from blue (color of the reagent) to green, then to yellow and eventually to brick-red, indicating a strong positive result. This color change is due to the formation of red copper(I) oxide precipitate after the reducing property of glucose reduces the blue copper(II) ions in the reagent.

Methods to Identify Non-reducing Sugars

Consider a sugar solution of sucrose and you wish to test for its presence. Benedict's test at the onset will provide a negative result as no color change will be observed. Next, the test can be followed by heating the solution with dilute hydrochloric acid, which breaks down the non-reducing sucrose into its reducing constituents, glucose and fructose. On retesting the then hydrolysed solution with Benedict's reagent and upon seeing a brick-red precipitate form, you can confirm that the original sugar solution contained a non-reducing sugar.

Spotting the Difference between Reducing and Non-reducing Sugars

Revealing distinct characteristics.

Reducing Sugars : Reducing sugars are capable of acting as a reducing agent due to their free aldehyde or ketone group. They have the potential to donate electrons or hydrogen atoms, thus contributing to the reduction of other reactants. Examples include glucose, lactose, and maltose.

Non-reducing Sugars : Non-reducing sugars lack a free aldehyde or ketone group, making them incapable of acting as a reducing agent. They cannot donate hydrogen atoms or electrons because they often exist in a closed-ring structure where the oxygen atom is involved in a glycosidic bond, thus hiding the reactive carbonyl group. Primary examples of non-reducing sugars include sucrose and trehalose.

Identifying Unique Chemical Reactions

Reducing Sugars and Oxidation : Reducing sugars participate readily in oxidation reactions due to their ability to donate electrons or hydrogen atoms. In a classic oxidation reaction, glucose, a reducing sugar, reacts with oxygen to form carbon dioxide and water in our cells, releasing energy:

Non-reducing Sugars and Hydrolysis : Non-reducing sugars, unlike reducing sugars, do not participate in oxidation reactions or the Maillard reaction. Nonetheless, they are not inactive. A crucial reaction involving non-reducing sugars is hydrolysis. Hydrolysis splits the non-reducing sugar into constituent units under acidic conditions or in the presence of enzymes. For example, sucrose, a non-reducing sugar, can be broken down into glucose and fructose, both of which are reducing sugars:

Reducing vs Non reducing sugars - Key takeaways

• Reducing Sugars: These sugars contain free aldehyde or ketone groups, can donate electrons or act as reducing agents, can oxidise and participate in the Maillard reaction which gives browned food its flavour.
• Non-reducing Sugars: These sugars lack free aldehyde or ketone groups, cannot donate electrons or act as reducing agents, cannot oxidise and do not participate in the Maillard reaction.
• Examples of Reducing Sugars: Glucose (provides energy for living organisms), Fructose (found in many plants), Lactose (a disaccharide sugar found in milk) and Maltose (a disaccharide formed from two units of glucose).
• Examples of Non-reducing Sugars: Sucrose (table sugar), Trehalose (a disaccharide with two glucose molecules). These sugars can be converted into reducing sugars via hydrolysis.
• Chemical Reactions: Reducing sugars can undergo oxidation, participate in the Maillard reaction, as well as show positive results in Fehling's and Benedict's tests. Non-reducing sugars can undergo hydrolysis but cannot directly participate in oxidation or the Maillard reaction.
• Identifying Tests: Reduction tests like Benedict’s and Fehling's tests can help identify reducing sugar. Non-reducing sugars can be detected indirectly via hydrolysis followed by these tests again.
• Differences: The key difference infering upon Reducing vs Non-reducing sugars compare generally liesunder the carbonyl group. Where reducing sugars can act as a reducing agent due to their free aldehyde or ketone group, non-reducing sugars lack this group, making them incapable of acting as a reducing agent.

Flashcards in Reducing vs Non reducing sugars 15

Non-reducing sugars lack a free aldehyde or ketone group and, therefore, cannot reduce other substances or donate electrons or hydrogen atoms. They cannot be oxidised or participate in the Maillard reaction. Sucrose is an example.

Non-reducing sugars often undergo hydrolysis, a reaction that splits them into their constituent units under acidic conditions or in the presence of enzymes. This can convert them into reducing sugars, such as when sucrose is broken down into glucose and fructose.

The Maillard reaction is a heat-induced browning involving an amino acid and a reducing sugar. It starts with a reducing sugar molecule condensing with an amino acid to form glycosylamine, leading to a distinctive flavour in browned food.

Non-reducing sugars are involved in hydrolysis, due to their inability to reduce other substances. Hydrolysis breaks down compounds, like non-reducing sugars, due to reaction with water. This can convert them into reducing sugars.

Reducing sugars are carbohydrates that possess free aldehyde or ketone group(s) and can reduce other substances by donating electrons or hydrogen atoms. Examples include glucose, lactose, and maltose.

In Fehling's test, reducing sugars reduce the copper (II) ion in the solution to a copper (I) ion, producing a brick-red precipitate. However, non-reducing sugars like sucrose do not give a positive result.

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Test for Reducing Sugars

How to Hydrolyze Starch With Heat & Hydrochloric Acid

A wide variety of sugars exist in nature and have different chemical and biological properties. A reducing sugar is one that contains, or can form, an aldehyde or ketone and that can act as a reducing agent. The chemical properties of reducing sugars play a role in diabetes and other ailments and are also important constituents of some foods. There are several tests to qualitatively or quantitatively identify the presence of reducing sugars.

TL;DR (Too Long; Didn't Read)

Benedict's test and Fehling's test are two common tests for reducing sugars.

What is a reducing sugar?

Any sugar that forms an aldehyde or ketone in the presence of an alkaline solution is a reducing sugar. Types of reducing sugars include:

• glyceraldehyde

Sucroses and trehaloses are not reducing sugars. Ultimately, a reducing sugar is a type of sugar that reduces certain chemicals through an oxidation reaction.

Benedict's Test

To test for the presence of reducing sugars, a food sample is dissolved in boiling water. Next, a small amount of Benedict's reagent is added and the solution begins to cool. During the next four to 10 minutes, the solution should begin to change colors. If the color changes to blue, then no glucose is present. If a high amount of glucose is present, then the color change will progress to green, yellow, orange, red and then a dark red or brown.

How Benedit's Test Works

Benedict's reagent is made from anhydrous sodium carbonate, sodium citrate and copper(II) sulfate pentahydrate. Once added to the test solution, reducing sugars reduce the blue copper sulphate from the Benedict's solution to a red brown copper sulphide, which is seen as the precipitate and is responsible for the color change. Non-reducing sugars cannot do this. This particular test only provides a qualitative understanding of the presence of reducing sugars.

Fehling's Test

To carry out Fehling's test, the solution is diluted in water and warmed until fully dissolved. Next, Fehling's solution is added while stirring. If reducing sugars are present, the solution should begin to change colors as a rust or red colored precipitate forms. If reducing sugars are not present, the solution will remain blue or green.

How Fehling's Test Works

Fehling's solution is made by first making two sub-solutions. Solution A is made from copper(II) sulfate pentahydrate dissolved in water and solution B contains potassium sodium tartrate tetrahydrate (Rochelle salt) and sodium hydroxide in water. The two solutions are added together in equal parts to make the final test solution. The test is a detection method for monosaccharides, specifically aldoses and ketoses. These are detected when aldehyde oxidizes to acid and forms a cuprous oxide. Upon contact with an aldehyde group, it is reduced to cuprous ion, which forms the red precipitate and inedicates the presence of reducing sugars.

Practical applications

Reducing sugar tests such as Benedict's and Fehling's test can be used to determine whether sugars are present in urine, which can be indicative of diabetes mellitus. They can also be used in a qualitative manner, such as in a titration experiment, to determine the amount of reducing sugars in a solution.

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Melissa Kelly is a freelance writer from Indianapolis who focuses on scientific and medical topics. Kelly attended Marian College where she obtained a Bachelor of Science degree in chemistry. Recently, she completed her Master's in business communications & project management.

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Investigating and testing for reducing and non-reducing sugars

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Teacher reviews, here's what a teacher thought of this essay.

Ross Robertson

An adequate report of an investigation into the identities of three food samples. The writer has correctly described the appropriate sugar tests and draws relevant conclusions. However, to fully satisfy 'A' level requirements, the report needs to describe the methods used in more detail with special attention to volumes, times and temperatures. It would have been helpful if the writer had given details of the chemistry behind the Benedict's test (donation of electrons, etc.) and exactly why glucose is - and sucrose is not - a reducing sugar. 3 stars

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