# Reducing sugar
A reducing sugar is any carbohydrate that contains an aldehyde group, or a hemiacetal in equilibrium with an aldehyde, so that the sugar can act as a reducing agent and be oxidized itself; ketoses such as fructose also qualify, because they isomerize to aldoses in basic solution.[1](https://www.chem.ucla.edu/~harding/IGOC/R/reducing_sugar.html) • [4](https://openstax.org/books/organic-chemistry/pages/25-6-reactions-of-monosaccharides) The property matters in two practical settings: it underlies classical laboratory tests such as Benedict's and Fehling's tests, and it makes these sugars the reactive partners in the [Maillard reaction](https://www.edgepedia.org/maillard-reaction), the non-enzymatic browning that shapes the aroma, taste, and appearance of thermally processed food.[2](https://onlinelibrary.wiley.com/doi/full/10.1002/anie.201308808)
| Key fact | Detail |
|---|---|
| Definition | A carbohydrate containing an aldehyde, a hemiacetal in equilibrium with an aldehyde, or a ketose that isomerizes to an aldose in basic solution, allowing it to be oxidized with reduction of an oxidizing agent[1](https://www.chem.ucla.edu/~harding/IGOC/R/reducing_sugar.html) |
| Common examples | Ribose, glucose, fructose, lactose, and maltose; sucrose is not a reducing sugar[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC12154226/) |
| Ketose exception | Fructose reduces Tollens' reagent despite containing no aldehyde group, because base converts it to a mixture of the aldoses glucose and mannose[4](https://openstax.org/books/organic-chemistry/pages/25-6-reactions-of-monosaccharides) |
| Disaccharide rule | Maltose and lactose are reducing; sucrose is nonreducing because both anomeric carbons are tied up in the glycosidic bond[5](https://guides.hostos.cuny.edu/che120/chapter6) |
| Detection | Benedict's reagent (copper sulfate, sodium carbonate, sodium citrate) gives a green-to-brick-red copper(I) oxide precipitate with reducing sugars[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC12154226/) |
| Food role | Reducing sugars initiate the Maillard reaction with amino groups, producing α-dicarbonyls, furans, Strecker aldehydes, advanced glycation end-products, and melanoidins[6](https://doi.org/10.1146/annurev-food-052720-104513) |
## Chemical basis
The reducing behavior comes from the open-chain form of the sugar. In water, glucose exists mostly as the cyclic glucopyranose hemiacetal, with traces of the acyclic aldehyde form also present; that small open-chain fraction is what reacts, and as it is consumed, more cyclic form opens to replace it.[1](https://www.chem.ucla.edu/~harding/IGOC/R/reducing_sugar.html) For a carbohydrate to be a reducing sugar, the molecule must therefore contain a free anomeric carbon, the carbon that can open to the aldehyde form.[7](https://chem.libretexts.org/Courses/Triton_College/Elementary_Organic_Chemistry_%28Lab_Manual%29/09%3A_Lab_9-_Tests_for_Carbohydrates)
All aldoses are reducing sugars because they contain an aldehyde group, but some ketoses are reducing sugars as well.[4](https://openstax.org/books/organic-chemistry/pages/25-6-reactions-of-monosaccharides) In mild alkaline solution, sugars with a free carbonyl group tautomerize through enediol intermediates, and glucose, fructose, and mannose interconvert through a common enediol in the Lobry de Bruyn–Van Ekenstein transformation.[8](https://www.minams.edu.pk/cPanel/ebooks/biochemistry/3.%20Textbook%20of%20Biochemistry%20for%20Medical%20Students%20by%20D.%20M.%20Vasudevan.pdf) This is why fructose gives a positive Tollens test: two base-catalyzed keto–enol tautomerizations convert it to a mixture of aldoses.[4](https://openstax.org/books/organic-chemistry/pages/25-6-reactions-of-monosaccharides) Enediols are highly reactive, which makes sugars powerful reducing agents in alkaline medium.[8](https://www.minams.edu.pk/cPanel/ebooks/biochemistry/3.%20Textbook%20of%20Biochemistry%20for%20Medical%20Students%20by%20D.%20M.%20Vasudevan.pdf)
**Glycosides and blocked anomers.** When the anomeric hydroxyl of a sugar is converted to an acetal, as in a glycoside, the compound becomes nonreducing because the acetal group is not hydrolyzed to an aldehyde under basic conditions.[4](https://openstax.org/books/organic-chemistry/pages/25-6-reactions-of-monosaccharides) The same principle distinguishes the common disaccharides. Maltose, two glucose units joined by an α-1,4-glycosidic linkage, leaves one anomeric carbon free to open into an aldehyde, so it is a reducing sugar; lactose, galactose joined to glucose by a β-1,4 linkage, is also reducing.[5](https://guides.hostos.cuny.edu/che120/chapter6) Sucrose, glucose joined to fructose by a link between the anomeric carbons of both sugars (C1 of glucose and C2 of fructose), has no free anomeric carbon, does not undergo mutarotation, and is nonreducing.[9](https://openstax.org/books/organic-chemistry/pages/25-8-disaccharides)
## Detection tests
Historically, oxidation of an aldose with Ag+ in aqueous ammonia ([Tollens' reagent](https://www.edgepedia.org/tollens-reagent)) or Cu2+ with aqueous sodium citrate ([Benedict's reagent](https://www.edgepedia.org/benedicts-reagent)) formed the basis of simple tests for reducing sugars, named for the reduction of the metal oxidizing agent by the sugar.[4](https://openstax.org/books/organic-chemistry/pages/25-6-reactions-of-monosaccharides) In Benedict's test, the aldehyde reduces blue Cu2+ ions to Cu+, which forms a deep red copper oxide precipitate while the aldehyde is oxidized to a carboxylic acid.[1](https://www.chem.ucla.edu/~harding/IGOC/R/reducing_sugar.html) The observed color runs from green through yellow and orange to red depending on the amount of reducing sugar present.[7](https://chem.libretexts.org/Courses/Triton_College/Elementary_Organic_Chemistry_%28Lab_Manual%29/09%3A_Lab_9-_Tests_for_Carbohydrates)
The reduction is much more efficient in alkaline medium than in acid, because the alkaline conditions generate the reactive enediol forms.[8](https://www.minams.edu.pk/cPanel/ebooks/biochemistry/3.%20Textbook%20of%20Biochemistry%20for%20Medical%20Students%20by%20D.%20M.%20Vasudevan.pdf) Benedict's reagent contains sodium carbonate, copper sulfate, and sodium citrate, and was long a standard laboratory test for glucose in urine in the follow-up of diabetes mellitus; some simple diabetes self-test kits sold in drugstores still use it, though more modern methods have largely replaced it.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC12154226/) • [4](https://openstax.org/books/organic-chemistry/pages/25-6-reactions-of-monosaccharides) Because any sugar with a free aldehyde or keto group reduces the reagent, the test is not specific for glucose.[8](https://www.minams.edu.pk/cPanel/ebooks/biochemistry/3.%20Textbook%20of%20Biochemistry%20for%20Medical%20Students%20by%20D.%20M.%20Vasudevan.pdf) To test for a nonreducing sugar such as sucrose, the sample is first hydrolyzed with dilute acid and neutralized, and Benedict's test is then run on the resulting monosaccharides.[10](https://cognito.org/blog/reducing-and-non-reducing-sugar-tests)
## Role in the Maillard reaction
The Maillard reaction, described in 1912 by Louis-Camille Maillard, is the reaction of reducing carbohydrates with amino compounds and is responsible for the aroma, taste, and appearance of thermally processed food.[2](https://onlinelibrary.wiley.com/doi/full/10.1002/anie.201308808) It is initiated by a condensation of amino groups on proteins, peptides, and amino acids with the carbonyl groups of reducing sugars, forming a [Schiff base](https://www.edgepedia.org/schiff-base) that rearranges to an Amadori product (or, for ketose sugars, a Heyns product) via a 1,2-enaminol.[11](https://pubs.acs.org/doi/10.1021/acs.jafc.7b00882) Downstream, the reaction yields a heterogeneous mixture including α-dicarbonyls, furans, Strecker aldehydes, advanced glycation end-products (AGEs), and melanoidins, the brown pigments.[6](https://doi.org/10.1146/annurev-food-052720-104513)
Reactivity within the sugars is not uniform: reducing five-carbon sugars are more reactive than reducing six-carbon sugars, and among the amino partners, lysine and arginine, with primary amino groups, are highly reactive toward reducing sugars, while alanine, glycine, and proline are less so.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC12154226/) • [12](https://res.mdpi.com/d_attachment/foods/foods-10-01998/article_deploy/foods-10-01998-v2.pdf) Sucrose, lacking a reducing end, does not participate directly in the Maillard reaction under standard conditions.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC12154226/)
## Reducing sugars in food processing
In baking and confectionery, reducing sugars such as glucose undergo non-enzymatic browning on heating in the presence of free amino groups, developing color and flavor; caramelization of milk solids and lactose, by contrast, requires higher temperatures of 160–170 °C (320–340 °F).[13](https://asbe.org/article/reducing-sugar/) Milk is a naturally reactive system because it contains both protein and the reducing sugar lactose, so browning and nutritional change occur even under mild heating.[14](https://ndl.ethernet.edu.et/bitstream/123456789/62440/1/2009_Book_AdvancedDairyChemistry.pdf) Hydrolyzing lactose into glucose and galactose increases the monosaccharide content and accelerates the reaction; in one commercial lactose-hydrolyzed skim milk powder analyzed in kinetic work, the carbohydrate fraction was 21% glucose, 21% galactose, and 10% lactose, and the presence of monosaccharides enhanced the reaction rate.[15](https://www.sciencedirect.com/science/article/pii/S0308814613008704)
**Controlling browning.** Because the Maillard reaction can also make lysine unavailable and produce mutagenic compounds, food scientists have developed mitigation strategies, including plant polyphenols and vitamins that trap reactive intermediates, enzymes, non-thermal techniques, and additives, each with effects on food quality and safety.[16](https://www.sciencedirect.com/science/article/pii/S0956713523003110) • [11](https://pubs.acs.org/doi/10.1021/acs.jafc.7b00882) Lactose-free UHT milk is a product particularly vulnerable to Maillard-derived quality deterioration. One approach reduces the abundance of reducing sugar through a redox process, but this generates hydrogen peroxide, which can cause further protein modification and lipid peroxidation unless carefully controlled, for example with catalase.[11](https://pubs.acs.org/doi/10.1021/acs.jafc.7b00882) [Fermentation](https://www.edgepedia.org/fermentation) offers another route: adding starter cultures to cheese to ferment reducing sugar into nonreducing sugar can prevent browning.[17](https://pmc.ncbi.nlm.nih.gov/articles/PMC8472807/)
The same chemistry is not always a defect. Maillard reaction products are deliberately produced as flavor agents, and a well-controlled reaction can generate flavor-enhancing peptides and compounds that improve the acceptability of processed foods.[18](https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2022.973677/full)
## References
1. [Reducing sugar, Illustrated Glossary of Organic Chemistry, UCLA](https://www.chem.ucla.edu/~harding/IGOC/R/reducing_sugar.html)
2. [Hellwig, M. and Henle, T. (2014). Baking, Ageing, Diabetes: A Short History of the Maillard Reaction. Angewandte Chemie International Edition 53, 10316–10329.](https://onlinelibrary.wiley.com/doi/full/10.1002/anie.201308808)
3. [Maillard Reaction: Mechanism, Influencing Parameters, Advantages, Disadvantages, and Food Industrial Applications: A Review (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC12154226/)
4. [25.6 Reactions of Monosaccharides, Organic Chemistry, OpenStax](https://openstax.org/books/organic-chemistry/pages/25-6-reactions-of-monosaccharides)
5. [Chapter 6 – Carbohydrates, CHE 120, Hostos Community College](https://guides.hostos.cuny.edu/che120/chapter6)
6. [Chemical Stability of Proteins in Foods: Oxidation and the Maillard Reaction, Annual Reviews](https://doi.org/10.1146/annurev-food-052720-104513)
7. [Lab 9 – Tests for Carbohydrates, Chemistry LibreTexts](https://chem.libretexts.org/Courses/Triton_College/Elementary_Organic_Chemistry_%28Lab_Manual%29/09%3A_Lab_9-_Tests_for_Carbohydrates)
8. [Chapter 6: Chemistry of Carbohydrates, Textbook of Biochemistry for Medical Students (Vasudevan)](https://www.minams.edu.pk/cPanel/ebooks/biochemistry/3.%20Textbook%20of%20Biochemistry%20for%20Medical%20Students%20by%20D.%20M.%20Vasudevan.pdf)
9. [25.8 Disaccharides, Organic Chemistry, OpenStax](https://openstax.org/books/organic-chemistry/pages/25-8-disaccharides)
10. [Reducing and non-reducing sugar tests for Cambridge A-Level Biology, Cognito](https://cognito.org/blog/reducing-and-non-reducing-sugar-tests)
11. [Control of Maillard Reactions in Foods: Strategies and Chemical Mechanisms, Journal of Agricultural and Food Chemistry](https://pubs.acs.org/doi/10.1021/acs.jafc.7b00882)
12. [A Literature Review on Maillard Reaction Based on Milk Proteins and Carbohydrates, Foods](https://res.mdpi.com/d_attachment/foods/foods-10-01998/article_deploy/foods-10-01998-v2.pdf)
13. [Reducing Sugar, American Society of Baking](https://asbe.org/article/reducing-sugar/)
14. [Advanced Dairy Chemistry (Hynes and Zalazar chapter)](https://ndl.ethernet.edu.et/bitstream/123456789/62440/1/2009_Book_AdvancedDairyChemistry.pdf)
15. [The kinetics of Maillard reaction in lactose-hydrolysed milk powder and related systems, Food Chemistry](https://www.sciencedirect.com/science/article/pii/S0308814613008704)
16. [Maillard reaction in different food products: Effect on product quality, human health and mitigation strategies, Food Control](https://www.sciencedirect.com/science/article/pii/S0956713523003110)
17. [A Literature Review on Maillard Reaction Based on Milk Proteins and Carbohydrates in Food and Pharmaceutical Products (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC8472807/)
18. [Insights into flavor and key influencing factors of Maillard reaction products, Frontiers in Nutrition](https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2022.973677/full)
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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Organic substances › Carbonyl and carboxyl chemistry › Aldehydes and ketones › Aldehydes › Aldehyde derivatives: oximes, acetals, and related functional forms*
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