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Reducing sugar

A reducing sugar is any carbohydrate that can act as a reducing agent, characterized by the presence of a free aldehyde (-CHO) or ketone (-CO-) functional group that enables it to donate electrons and undergo oxidation.^[1] These sugars are typically oxidized by weak oxidizing agents, such as alkaline solutions of copper(II) ions, without requiring strong conditions.^[2] The reducing capability of these sugars stems from their ability to exist in an open-chain form, where the carbonyl group can tautomerize to an aldehyde (in the case of ketoses like fructose) or directly participate in redox reactions.^[3] In solution, monosaccharides and certain oligosaccharides equilibrate between cyclic hemiacetal forms and the reactive open-chain form via mutarotation, allowing the free anomeric carbon to reduce metal ions like Cu²⁺ to Cu⁺ or Ag⁺ to Ag.^[1] This property distinguishes reducing sugars from non-reducing ones, such as sucrose, where both anomeric carbons are tied up in the glycosidic bond, preventing open-chain formation.^[3] Common examples of reducing sugars include all monosaccharides, such as glucose, fructose, and galactose, which inherently possess the required free carbonyl group.^[1] Among disaccharides, maltose (from starch hydrolysis) and lactose (from milk) are reducing due to one free anomeric carbon, while sucrose is non-reducing.^[2] Some trisaccharides, such as maltotriose, are reducing due to a free anomeric carbon, while others like raffinose are non-reducing.^[4]^[5] Polysaccharides like starch and cellulose generally do not act as reducing sugars unless hydrolyzed to expose reducing ends, though glycogen has limited reducing capacity per molecule.^[3] Reducing sugars are detected through qualitative tests that exploit their redox properties, such as Benedict's test, where a color change from blue to green, yellow, or red precipitate (cuprous oxide, Cu₂O) indicates their presence in alkaline copper sulfate solution.^[2] Fehling's test similarly produces a red precipitate and was historically used to detect glucose in urine for diabetes diagnosis. Other methods include the Tollens' test (silver mirror formation) and quantitative assays like the dinitrosalicylic acid (DNS) method for measuring concentrations in biological or food samples.^[3] In biological systems, reducing sugars play crucial roles as primary energy sources in cellular metabolism, fueling processes like glycolysis and fermentation, where yeast utilizes them to produce ethanol in winemaking.^[3] They also contribute to the Maillard reaction, a non-enzymatic browning process between sugars and amino acids that generates flavors, aromas, and colors in cooked foods, though excessive accumulation (e.g., in stored potatoes) can lead to undesirable discoloration.^[1] In clinical and industrial contexts, their quantification is vital for monitoring blood glucose levels in diabetes management, assessing food quality, and pharmaceutical analysis.^[6]

Fundamentals

Definition and Terminology

Carbohydrates, also known as saccharides, are organic compounds classified as polyhydroxy aldehydes or ketones, or substances that hydrolyze to yield such polyhydroxy aldehyde or ketone units.^[7] This structural feature provides carbohydrates with diverse chemical reactivities, including the potential to participate in oxidation-reduction reactions. A reducing sugar is defined as any carbohydrate capable of acting as a reducing agent, owing to the presence of a free aldehyde group in its open-chain form or a free ketone group that can tautomerize to an aldehyde, especially under alkaline conditions.^[8] The key structural prerequisite is an available anomeric carbon not engaged in a glycosidic linkage, allowing the sugar to exist in equilibrium with its reactive aldehydic form.^[9] The term "reducing sugar" originated in the 19th century amid early advancements in carbohydrate chemistry, particularly with the introduction of Fehling's test in 1849 by German chemist Hermann von Fehling, which demonstrated sugars' ability to reduce copper(II) ions. Subsequent observations by Emil Fischer in the 1880s and 1890s highlighted the reactivity of sugars through structural analyses and synthesis, solidifying the concept during this foundational period.^[10] Non-reducing sugars, by contrast, lack a free anomeric carbon, as seen in sucrose where both anomeric carbons form a glycosidic bond, rendering them incapable of reducing chemical reagents like those in Fehling's solution.^[11]

Oxidation-Reduction Properties

Reducing sugars exhibit distinctive oxidation-reduction properties due to their ability to act as reducing agents in redox reactions, primarily through the oxidation of their carbonyl group. In alkaline solutions, the ring structure at the anomeric carbon opens, exposing a free aldehyde group in aldoses or forming an enediol intermediate in ketoses via enolization; this reactive species then donates electrons to metal ions such as Cu²⁺ or Ag⁺, reducing them to Cu⁺ or Ag, respectively, while the sugar is oxidized to a carboxylic acid or equivalent.^[12]^[13] The general reaction for the oxidation of an aldose in alkaline medium with copper(II) ions can be represented as:

\text{R-CHO} + 2\text{Cu}^{2+} + 5\text{OH}^- \rightarrow \text{R-COO}^- + \text{Cu}_2\text{O} + 3\text{H}_2\text{O}

This simplified equation illustrates the conversion of the aldehyde to a carboxylate anion, accompanied by the formation of cuprous oxide precipitate.^[14] Alkaline conditions are essential for this process, as they facilitate the initial ring opening and subsequent enolization by deprotonating the alpha-hydrogen, thereby stabilizing the enediol intermediate and preventing the reformation of the cyclic structure.^[15] Higher pH levels enhance the rate of enolization, making the reaction more efficient compared to neutral or acidic environments.^[16] In these redox reactions, reducing sugars donate electrons from their carbonyl or enediol forms, undergoing oxidation that transitions them to a lower potential energy state, such as the more stable carboxylate form, driven by the favorable redox potential difference with the metal ions.^[17]

Structural Features

Reducing sugars possess specific structural elements that confer their ability to act as reducing agents, primarily centered around the anomeric carbon and the nature of their carbonyl groups. The anomeric carbon is the carbonyl carbon in the open-chain form of the sugar, which becomes the reducing end capable of oxidation. In the predominant ring form, this carbon exists as part of a hemiacetal (for aldoses) or hemiketal (for ketoses) linkage, but it remains poised for ring opening to expose the reactive carbonyl group.^[17] Carbohydrates are classified as aldoses or ketoses based on the position of their carbonyl group, which influences their reducing potential. Aldoses feature an aldehyde group (-CHO) at carbon 1 (C1), directly providing a reducing end in the open-chain configuration. Ketoses, in contrast, have a ketone group (-C=O) at carbon 2 (C2), but they exhibit reducing properties through keto-enol tautomerism, which allows isomerization to an aldose form via an enediol intermediate, thereby generating a free aldehyde.^[18]^[19] In oligosaccharides and polysaccharides, the reducing capability is limited to the terminal sugar unit where the anomeric carbon is not involved in a glycosidic bond. This free anomeric carbon at the reducing end can equilibrate between ring and open-chain forms, while internal sugar units linked via both anomeric and other hydroxyl groups lack this reactivity.^[20] Fischer projections provide a conventional linear representation of these open-chain structures, emphasizing the carbonyl position and stereochemistry. For the aldose D-glucose, the projection shows an aldehyde at C1, followed by hydroxyl groups on chiral carbons C2 through C5, with the D configuration indicated by the hydroxyl on C5 oriented to the right. In the ketose D-fructose, the ketone appears at C2, with hydroxyls on C1, C3 through C5, and again the D designation from C5. These projections highlight how the aldehyde in aldoses and the tautomerizable ketone in ketoses enable the reducing functionality.^[21]

Classification

Monosaccharides

All monosaccharides are reducing sugars because they contain a free anomeric carbon that can equilibrate with an open-chain form featuring a carbonyl group (aldehyde or ketone), enabling them to act as reducing agents.^[22] This inherent property arises from their single-unit structure, allowing the ring form to open readily in solution.^[23] Aldoses, which possess an aldehyde group at the anomeric carbon, represent a primary class of reducing monosaccharides. Common examples include the aldohexoses glucose, mannose, and galactose, all of which are prevalent in the D-series configuration in nature.^[23] Glucose, the most abundant monosaccharide, serves as the primary energy source in blood and occurs freely in fruits and plants.^[24] Mannose is found in various plant sources, such as fruits and gums, while galactose is a component of milk sugars.^[25] Ketoses, featuring a ketone group, also exhibit reducing properties through tautomerization to aldose forms. Notable examples are the ketohexose fructose and the ketopentose ribulose.^[26] Fructose is abundant in fruits, honey, and vegetables, contributing to their sweetness.^[27] Ribulose plays a key role in photosynthetic pathways within plants.^[9] Monosaccharides exhibit stereochemistry defined by D and L configurations, based on the orientation of the hydroxyl group at the penultimate chiral carbon in their Fischer projections, with the D-series predominating in biological systems. However, this stereochemical distinction does not influence their reducing capability, as it stems solely from the free anomeric carbon present in all monosaccharides.^[28]

Oligosaccharides and Polysaccharides

Disaccharides consist of two monosaccharide units, while oligosaccharides contain three to ten units, both linked by glycosidic bonds; they display reducing properties if at least one anomeric carbon remains free, allowing ring opening to form an aldehyde or ketone group.^[29] Disaccharides exemplify this: maltose features two D-glucose units joined by an α-1,4-glycosidic bond, with the anomeric carbon of the terminal glucose free, enabling it to act as a reducing sugar.^[30] Lactose, another reducing disaccharide, comprises β-D-galactose linked to D-glucose via a β-1,4-glycosidic bond, where the reducing end resides at the anomeric carbon of the glucose unit.^[30] Sucrose serves as a key exception among disaccharides, being non-reducing due to its α-1,2-glycosidic linkage between the anomeric carbons of D-glucose and D-fructose, which eliminates any free anomeric carbon.^[31] Higher oligosaccharides, such as trisaccharides, follow similar principles; for example, maltotriose (three α-1,4-linked glucose units) is reducing due to its free anomeric end, whereas raffinose (galactose-α-1,6-glucose-α-1,2-β-fructose) is non-reducing as all anomeric carbons are involved in glycosidic bonds. Polysaccharides, with chains often exceeding hundreds of monosaccharide units, generally possess just one reducing end per linear polymer chain, stemming from the sole free anomeric carbon.^[32] Starch, a mixture of amylose and amylopectin composed of α-1,4- and α-1,6-linked D-glucose, and glycogen, a highly branched α-linked D-glucose polymer, each exhibit a single reducing end per molecule.^[33] Cellulose, formed by β-1,4-linked D-glucose units, also contains reducing ends, though its reducing reactivity is diminished compared to α-linked polysaccharides like starch due to the rigid, linear structure imposed by the β-glycosidic bonds, which reduces solubility and accessibility in standard assays.^[34] The impact of chain length on reducing properties is significant: as polysaccharide chains elongate, the overall reducing power per unit mass declines proportionally, since only the single reducing end contributes to the reactivity while the majority of units are locked in non-reducing glycosidic bonds.^[35]

Analytical Methods

Qualitative Tests

Qualitative tests for reducing sugars exploit their redox properties to produce visible changes, such as precipitates or color shifts, upon reaction with specific reagents. These straightforward procedures allow for the detection of reducing sugars in samples like food extracts or biological fluids without requiring advanced equipment. Common tests include Benedict's, Fehling's, and Tollens', each relying on the reduction of metal ions by the free aldehyde or ketone groups in reducing sugars.^[36]^[37]^[38] Benedict's test uses an alkaline solution of copper(II) sulfate stabilized by sodium citrate to prevent premature precipitation of copper hydroxide. To perform the test, 2 ml of the sample is mixed with 2 ml of Benedict's reagent in a test tube and heated in a boiling water bath for 3-5 minutes. A positive result appears as a red precipitate of cuprous oxide (Cu₂O), indicating the presence of reducing sugars at concentrations exceeding 0.5%, while a blue color or green tint signifies a negative or weak response, respectively.^[36]^[39]^[40] Fehling's test employs a similar principle but uses Fehling's solution A (aqueous copper(II) sulfate) and solution B (alkaline potassium sodium tartrate). Equal volumes of the sample, solution A, and solution B are combined in a test tube and heated gently for 1-2 minutes. The initial deep blue solution turns to a brick-red precipitate of cuprous oxide upon reduction by sugars like glucose or fructose, confirming the presence of reducing agents; no color change indicates non-reducing sugars.^[37]^[41]^[42] Tollens' test involves preparing the reagent by adding ammonia to silver nitrate and sodium hydroxide to form the diamminesilver(I) complex. The clean test tube containing 2 ml of the sample is treated with 2 ml of Tollens' reagent and warmed in a water bath for about 1 minute. A positive reaction yields a shiny silver mirror deposited on the tube's inner surface due to the reduction of Ag⁺ to metallic silver by the aldehyde group in reducing sugars; a grey or black precipitate may form if the surface is not clean, while no change indicates absence of reducing sugars.^[38]^[43] These tests share limitations, including lack of specificity, as other reducing substances can interfere and produce false positives. For instance, ascorbic acid, creatinine, and certain drugs like penicillin or salicylates can reduce the reagents, mimicking the response of sugars. Additionally, the tests cannot distinguish between different types of reducing sugars and may require confirmation with more specific methods for accurate identification.^[44]^[40]^[38]

Quantitative Determination

The quantitative determination of reducing sugars is essential in food analysis, biochemistry, and industrial processes to assess their concentration accurately. One widely adopted colorimetric method is the dinitrosalicylic acid (DNS) assay, which relies on the reduction of DNS by reducing sugars under alkaline conditions to form 3-amino-5-nitrosalicylic acid, a chromophore with maximum absorbance at 540 nm.^[45] The intensity of the orange-red color is proportional to the reducing sugar concentration, and quantification follows Beer's law:

A = \epsilon b c

where A is the absorbance, \epsilon is the molar absorptivity of the chromophore, b is the path length (typically 1 cm), and c is the concentration of reducing sugars.^[46] This method, originally described by Miller in 1959, is simple and cost-effective for routine laboratory use, though it measures total reducing ends and may require calibration with standards like glucose for accuracy.^[45] Another classical approach is the Lane-Eynon titration, a volumetric method that determines reducing sugar content by oxidizing the sugars with a standardized Fehling's solution (alkaline copper(II) sulfate) to form cuprous oxide.^[47] The endpoint is detected using methylene blue as an internal indicator, which decolorizes upon reduction, allowing precise titration of the sample against the copper reagent. Fehling's solution is first standardized by titrating a known glucose concentration to establish the "Fehling factor," typically expressed as the volume of sugar solution equivalent to 0.05 g of copper.^[48] This method, developed by Lane and Eynon in 1923, provides direct equivalence to glucose and is particularly useful for syrups and food samples, with results reported as percentage reducing sugars on a dry basis.^[49] Modern instrumental techniques offer greater specificity and throughput for quantitative analysis. High-performance liquid chromatography (HPLC) separates reducing sugars based on their molecular size or charge, often using amine or ion-exchange columns, with detection via refractive index, evaporative light scattering, or amperometric methods for enhanced sensitivity.^[50] Enzymatic assays, such as those employing glucose oxidase, provide high selectivity for glucose (a key reducing sugar) by catalyzing its oxidation to gluconic acid and hydrogen peroxide, which is then quantified amperometrically or colorimetrically through peroxidase-coupled reactions.^[51] In the 2020s, advancements have focused on high-throughput platforms, including automated discrete analyzers that integrate enzymatic detection to process up to 350 samples per hour with minimal manual intervention, improving efficiency in food and biofuel industries.^[52] These methods often achieve limits of detection below 1 μM, surpassing traditional assays in precision.^[53] In industrial contexts, particularly for starch-derived syrups, the dextrose equivalent (DE) serves as a standardized metric for reducing sugar content, defined as the percentage of reducing power relative to pure glucose (DE = 100).^[54] DE is calculated from the measured reducing ends using titration methods like Lane-Eynon, where the value reflects the degree of starch hydrolysis: lower DE values (e.g., 20–40) indicate longer oligosaccharide chains with fewer reducing ends, while higher values (e.g., 60–100) signify greater free glucose content. This parameter guides syrup functionality in applications like confectionery and brewing.^[55]

Biological Role

Metabolic Functions

Reducing sugars, particularly glucose, serve as the primary energy source in cellular metabolism through glycolysis, a central pathway that oxidizes glucose to pyruvate while generating ATP and NADH. In this process, one molecule of glucose yields a net of two ATP molecules and two NADH under anaerobic conditions, providing rapid energy for cellular activities. Fructose enters glycolysis differently depending on the tissue; in the liver, it is primarily phosphorylated to fructose-1-phosphate by fructokinase and then enters the pathway at the triose phosphate level, while in other tissues like muscle, hexokinase phosphorylates it to fructose-6-phosphate, underscoring the versatility of reducing sugars in fueling this ancient metabolic route.^[56] The reducing end of polysaccharides such as starch and glycogen plays a crucial role in their enzymatic breakdown, enabling access for degradative enzymes. Glycogen phosphorylase sequentially cleaves glucose units from the non-reducing ends via phosphorolysis, but when branches are encountered, debranching enzymes—including oligo-1,4-1,4-glucantransferase and amylo-1,6-glucosidase—transfer chains and hydrolyze the exposed reducing end of the stub, releasing free glucose. This mechanism ensures efficient mobilization of stored glucose during energy demands, with the reducing end's free anomeric carbon facilitating the final hydrolytic step.^[57] Reducing sugars participate in non-enzymatic glycation reactions with proteins, initiating the formation of Schiff bases through nucleophilic addition of protein amino groups to the carbonyl at the reducing end. These unstable intermediates rearrange into more stable Amadori products, which can further degrade into advanced glycation end-products (AGEs) via oxidation, dehydration, and fragmentation. This process modifies protein structure and function, influencing cellular signaling and stability in various tissues.^[58] In plants, reducing sugars like glucose and fructose are key products of photosynthesis, generated through the Calvin-Benson cycle where CO2 is fixed into glyceraldehyde-3-phosphate and subsequently converted to these hexoses. Glucose serves as a building block for starch synthesis in chloroplasts, while fructose contributes to sucrose formation for transport, supporting growth and energy distribution across the plant. These reducing sugars thus link photosynthetic carbon fixation to broader metabolic networks.^[59] From an evolutionary perspective, the reducing properties of sugars likely facilitated early energy transfer mechanisms in primitive metabolism, as aldehydes and ketones in simple sugars could undergo redox reactions akin to those in glycolysis precursors under prebiotic conditions. Spontaneous formation of sugars via reactions like the formose process provided reactive carbonyls that supported the emergence of autocatalytic cycles, laying groundwork for glycolytic-like pathways in the origin of life.^[60]

Medical and Health Implications

Reducing sugars play a significant role in clinical diagnostics for diabetes mellitus, where their presence in urine, known as glycosuria, serves as an indicator of hyperglycemia. Glycosuria occurs when blood glucose levels exceed the renal threshold, leading to the excretion of glucose—a primary reducing sugar—into the urine, which can be detected through qualitative tests such as Benedict's reagent. This detection is particularly useful in monitoring diabetic control, as persistent glycosuria signals inadequate glycemic management and potential complications.^[61]^[62] The glycemic index (GI) further highlights the health implications of reducing sugars, with monosaccharides like glucose exhibiting a high GI of 100, causing rapid elevations in blood glucose levels post-consumption. In contrast, certain oligosaccharides, such as fructo-oligosaccharides, have a lower GI due to slower digestion and absorption, resulting in more gradual blood sugar rises and reduced risk of postprandial spikes. This distinction is crucial for dietary management in conditions like type 2 diabetes, where high-GI reducing sugars can exacerbate insulin resistance and cardiovascular risks.^[63]^[64] Advanced glycation end products (AGEs), formed via non-enzymatic reactions between reducing sugars and proteins or lipids, accumulate in tissues during hyperglycemia and contribute to pathology in diabetes, aging, and neurodegenerative diseases like Alzheimer's. These AGEs bind to the receptor for advanced glycation end products (RAGE), triggering inflammatory cascades that promote oxidative stress, endothelial dysfunction, and neuronal damage. Recent studies from the 2020s have strengthened links between AGE-RAGE signaling and chronic inflammation in Alzheimer's, where AGE accumulation correlates with amyloid plaque formation and cognitive decline.^[65]^[66]^[67] To mitigate these risks, including obesity and related non-communicable diseases, the World Health Organization recommends limiting free sugars—predominantly reducing monosaccharides and disaccharides—to less than 10% of total energy intake, with a further reduction to below 5% for additional benefits. This guideline, issued in 2015, emphasizes reducing intake from sources like sugar-sweetened beverages to prevent excessive caloric consumption and metabolic disruptions. Adherence to these limits has been associated with lower obesity prevalence in population studies.^[68]

Applications

Food Chemistry

In food chemistry, the Maillard reaction represents a key non-enzymatic process where reducing sugars react with amino acids or proteins during heating, leading to browning and flavor development in cooked foods. This reaction begins with the condensation of the carbonyl group of a reducing sugar, such as glucose, and the amino group of an amino acid to form a Schiff base, followed by the Amadori rearrangement to produce a more stable ketosamine known as the Amadori product. Subsequent degradation of the Amadori product generates reactive intermediates like dicarbonyl compounds, which further polymerize into advanced glycation end-products and ultimately melanoidins, the brown pigments responsible for color changes. The overall pathway can be summarized as: reducing sugar + amino acid → Schiff base → Amadori product → reactive intermediates → melanoidins.^[69]^[70] Distinct from the Maillard reaction, caramelization involves the thermal decomposition of reducing sugars in the absence of amino acids, occurring at temperatures typically above 160°C for glucose. During this process, sugars undergo dehydration, fragmentation, and polymerization to form volatile compounds and brown polymers, contributing to the characteristic flavors and colors in caramelized products like sauces and confections. This pyrolysis reaction is pH-dependent and favored in low-moisture environments, with glucose decomposing to produce hydroxymethylfurfural and other furan derivatives as key intermediates.^[71]^[72] In fermentation processes relevant to food production, such as baking and brewing, yeasts like Saccharomyces cerevisiae utilize reducing sugars, particularly glucose, as primary substrates for anaerobic metabolism, converting them into ethanol and carbon dioxide via glycolysis and alcoholic fermentation. This biochemical pathway starts with glucose phosphorylation and proceeds through pyruvate decarboxylation, enabling leavening in doughs and alcohol formation in beverages while depleting available reducing sugars.^[73]^[74] Early-stage Maillard reaction products, including certain Amadori compounds and reductones, exhibit antioxidant properties in processed foods by scavenging free radicals and chelating metal ions, thereby inhibiting lipid oxidation and extending shelf life in items like baked goods and dairy products. These effects stem from the redox capabilities of the intermediates, which donate electrons to stabilize reactive species without compromising food safety.^[75]^[76]

Industrial Uses

Reducing sugars play a central role in biofuel production, where the hydrolysis of starch from crops like corn or cellulose from lignocellulosic biomass such as agricultural residues generates fermentable sugars for ethanol fermentation. This process involves enzymatic or acid hydrolysis to break down complex carbohydrates into glucose and other reducing monosaccharides, which are then converted to ethanol by yeast.^[77]^[78] In starch-based systems, alpha-amylase and glucoamylase hydrolyze starch to produce reducing sugars, while for cellulosic feedstocks, pretreatment steps like steam explosion or alkaline treatment enhance accessibility for cellulase enzymes.^[79]^[80] Recent advances in the 2020s have improved enzymatic hydrolysis efficiency, with innovations like continuous hydrolysis systems achieving higher sugar yields—up to 90% glucose conversion from pretreated biomass—while minimizing enzyme inhibition and energy costs. These developments, including hydrodynamic cavitation-assisted processes, reduce the dosage of supplementary enzymes like sulfite pulping byproducts by 60% (e.g., from 25% w/w to 10% w/w) without compromising yields, supporting scalable second-generation biofuel production.^[81]^[82] In the pharmaceutical industry, reducing sugars such as glucose are integral to drug formulations, particularly in intravenous solutions where they serve as a primary source of calories and osmotic balance for patients unable to take oral nutrition. Dextrose injections, typically at concentrations of 5-50%, provide rapid energy replenishment and are compatible with amino acids or electrolytes, with glucose's reducing properties ensuring stability in sterile preparations.^[83]^[84] Additionally, reducing sugars contribute to vaccine adjuvants through glycation reactions, where the free aldehyde group at the reducing end of oligosaccharides like glucose or maltose enables conjugation to carrier proteins, enhancing immunogenicity in glycoconjugate vaccines against bacterial pathogens. This linkage mimics natural glycan structures, improving T-cell dependent responses and antibody production.^[85]^[86] Reducing sugars find applications in cosmetics as humectants, with fructose commonly incorporated to attract and retain moisture in skin care formulations, promoting hydration without irritation due to its natural compatibility with biological systems. Fructose, derived from fruit sources, functions by forming hydrogen bonds with water molecules, maintaining formulation stability in products like lotions and serums at concentrations up to 5%.^[87]^[88] In textiles, the reducing properties of sugars like glucose or fructose serve as eco-friendly alternatives to sodium dithionite in dyeing processes for sulfur and indigo dyes, enabling reduction of dye particles at room temperature and yielding comparable color strength (K/S values of 15-20) and wash fastness ratings (4-5 on a 5-point scale). These sugar-based reducers minimize environmental discharge of sulfides while achieving uniform dyeing on cotton fabrics.^[89]^[90] In the paper industry, reducing ends on cellulose chains in pulp influence bleaching outcomes by increasing fiber reactivity and potential for oxidative degradation, necessitating precise control to optimize lignin removal without excessive carbohydrate loss. During enzymatic bio-bleaching with xylanases, hydrolysis generates reducing sugars that correlate with delignification efficiency, facilitating the release of chromophores and reducing bleaching chemical requirements by 20-30%, while improving pulp brightness by 1-4 ISO points compared to untreated pulp. Quantitative management often involves the dextrose equivalent (DE), a measure of reducing sugar content in hydrolyzed starches used as pulp additives, where DE values of 10-20 ensure viscosity and binding properties that support uniform bleaching agent distribution.^[91]^[92]^[55]

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Rating 4.2 (373,000) Benedict's Test has some limitations: ... It's not specific to glucose; other reducing substances can also give a positive result. ... It is a qualitative or semi- ...
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Fehling Test - BYJU'S
The test developed by German chemist H.C. Von Fehling is also used to differentiate between ketone functional groups and water-soluble carbohydrates.
[34]
LAB 9 - TESTS FOR CARBOHYDRATES - Chemistry LibreTexts
Aug 11, 2025 · Observe the color change from green, yellow, orange, or red, depending on the amount of reducing sugar present in the test sample. 4. Fehling's ...
[35]
Tollens' Test - Chemistry LibreTexts
Jan 22, 2023 · Tollens' test, also known as silver-mirror test, is a qualitative laboratory test used to distinguish between an aldehyde and a ketone.
[36]
Limitations of Benedict's Test - BYJU'S
It is impossible to determine the exact concentration of reducing sugar; only a semiquantitative estimate may be given. Further testing is required to identify ...
[37]
Use of Dinitrosalicylic Acid Reagent for Determination of Reducing ...
Article March 1, 1959. Use of Dinitrosalicylic Acid Reagent for Determination of Reducing Sugar. Click to copy article linkArticle link copied! G. L. Miller.
[38]
Estimation of Sugars by DNS Method - BioTech Beacons
Oct 22, 2025 · Summary: The DNS assay quantifies reducing sugars (e.g., glucose ... The absorbance maximum (Amax) of this chromogen is near 540 nm.
[39]
[PDF] ISO-5377-1981.pdf - iTeh Standards
This International Standard specifies a Lane and Eynon cons- Titration of a prescribed volume of mixed Fehling's Solution tant titre method for the ...
[40]
[PDF] 114 Quantitative Analysis of Reducing Sugars in Sugar Preparations ...
Obtain the concentration of the direct reducing sugars, Ds (mg/100mL) from the titre, X (mL), by reference to the appended Lane-Eynon's Table (dextrose).
[41]
[PDF] 3 8 7 - On the Determination of Reducing Sugars by Titration with
In the determination of reducing sugars by direct titration a great advance was made by Lane and Eynon 4 when they discov- ered that methylene blue can be used ...
[42]
High performance liquid chromatographic method with post-column ...
Dec 20, 2021 · This method involves separation of reducing sugars on HPLC column at 30 °C and 0.8 mL min−1 flow rate, post−column reaction of sugars with ...
[43]
https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_%28Organic_Chemistry%29/Aldehydes_and_Ketones/Reactivity_of_Aldehydes_and_Ketones/Tollens_Test
[44]
[PDF] Combine selectivity and sensitivity for rapid multi-parameter sugar ...
With automated workflows, higher throughputs and a reduced skill requirement, laboratories can now produce up to 350 sugar assays per hour, a four-fold increase ...
[45]
https://pubs.acs.org/doi/10.1021/ac60147a030
[46]
[PDF] Dextrose Equivalent (Lane and Eynon) - Corn Refiners Association
The method may be standardized against a mixture of saccharides of similar composition to the test sample and the results may be expressed as mg reducing sugars ...
[47]
Dextrose Equivalent - an overview | ScienceDirect Topics
Dextrose equivalent (DE) refers to a measure of the reducing power of glucose syrups derived from starch, indicating the length of the chain of glucose residues ...
[48]
Biochemistry, Glycolysis - StatPearls - NCBI Bookshelf - NIH
During glycolysis, glucose ultimately breaks down into pyruvate and energy; a total of 2 ATP is derived in the process (Glucose + 2 NAD+ + 2 ADP + 2 Pi --> 2 ...
[49]
Biochemistry - Glycogenolysis - StatPearls - NCBI Bookshelf - NIH
Jan 27, 2024 · The formation of glycogen from glucose is known as glycogenesis, and the breakdown of glycogen to form glucose is called glycogen metabolism or glycogenolysis.
[50]
Advanced Glycation End-Products (AGEs): Formation, Chemistry ...
In general, they are formed non-enzymatically by condensation between carbonyl groups of reducing sugars and free amine groups of nucleic acids, proteins, or ...
[51]
Plant glucose transporter structure and function - PMC
Sucrose is synthesized in photosynthetically active cells from fructose and glucose and is then transported via the phloem to heterotrophic parts of the plant.
[52]
The Origin of Life - The Search for Life's Origins - NCBI Bookshelf
GOAL 1: To understand the origin and evolution of metabolism in primitive life forms. ... Sugars easily form spontaneously from formaldehyde ...
[53]
Physiology, Glycosuria - StatPearls - NCBI Bookshelf
Mar 13, 2023 · Glycosuria is a term that defines the presence of reducing sugars in the urine, such as glucose, galactose, lactose, fructose, etc.
[54]
Glucosuria - Clinical Methods - NCBI Bookshelf - NIH
Glucosuria, glucose in the urine, results from the glomerular filtration of more glucose than the renal tubule can absorb.
[55]
Carbohydrates and Blood Sugar - The Nutrition Source
The glycemic index ranks carbohydrates on a scale from 0 to 100 based on how quickly and how much they raise blood sugar levels after eating.
[56]
Can functional oligosaccharides reduce the risk of diabetes mellitus?
Functional oligosaccharides are attractive to patients with diabetes because of their low glycemic index.
[57]
Advanced Glycation End Products in Health and Disease - PMC - NIH
Sep 15, 2022 · Advanced glycation end products (AGEs), formed through the nonenzymatic reaction of reducing sugars with the side-chain amino groups of lysine ...
[58]
Advanced Glycation End Products-Induced Alzheimer's Disease and ...
May 30, 2024 · Advanced glycation end products (AGEs) accumulate in the brain, leading to neurodegenerative conditions such as Alzheimer's disease (AD).
[59]
Advanced Glycation End Products and Risks for Chronic Diseases
AGEs are a family of compounds that are the products of nonenzymatic reactions between reducing sugars and proteins, lipids, or nucleic acids.
[60]
Reducing free sugars intake in adults to reduce the risk of ...
Aug 9, 2023 · In both adults and children, WHO recommends reducing the intake of free sugars to less than 10% of total energy intake. WHO suggests a further ...
[61]
Maillard Reaction: Mechanism, Influencing Parameters, Advantages ...
The Maillard reaction is a chemical reaction between amino acid residues and reducing sugars that leads to the formation of melanoidins, contributing to the ...
[62]
Control of Maillard Reactions in Foods: Strategies and Chemical ...
May 23, 2017 · Maillard products arise from condensation reactions between amino acids or proteins with reducing sugars during food processing. As ...Introduction to Maillard... · Strategies for Control of... · Emerging Strategies for...
[63]
Comment on the Melting and Decomposition of Sugars
Sep 27, 2012 · is thermal decomposition; thus, the heating ... Such decomposition of liquid sugars is well-known and often referred to as caramelization.
[64]
Maillard Reaction in Flour Product Processing: Mechanism, Impact ...
Aug 3, 2025 · In addition, elevated moisture also promotes caramelization, which consumes reducing sugars through continuous dehydration and decomposition ...
[65]
The Role of Yeasts in Fermentation Processes - PubMed Central - NIH
Fermentation is carried out by yeasts (and some bacteria) when pyruvate generated from glucose metabolism is broken into ethanol and carbon dioxide.
[66]
Sugar Levels Determine Fermentation Dynamics during Yeast ... - NIH
During alcoholic fermentation, yeast converts sugars into carbon dioxide (CO2) and ethanol [1], with a preference for glucose consumption over fructose and ...
[67]
Food Processing and Maillard Reaction Products: Effect on Human ...
When foods are being processed or cooked at high temperature, chemical reaction between amino acids and reducing sugars leads to the formation of Maillard ...
[68]
Antioxidant and Functional Activities of MRPs Derived from Different ...
Nov 19, 2021 · The Maillard reaction represents a complex series of reactions in heated foods that initiate when a carbonyl group from a reducing sugar reacts ...
[69]
Hydrolysis of lignocellulosic materials for ethanol production: a review
The conversion includes two processes: hydrolysis of cellulose in the lignocellulosic materials to fermentable reducing sugars, and fermentation of the sugars ...
[70]
Bioethanol Production by Enzymatic Hydrolysis from Different ... - NIH
Feb 1, 2021 · This review investigates the very recent enzymatic hydrolysis pathways in bioethanol production from lignocellulosic biomass.
[71]
Bioethanol Production from Renewable Raw Materials and Its ...
For bioethanol production from starch-containing feedstocks, it is necessary to perform the starch hydrolysis (mostly by α-amylase and glucoamylase) into ...
[72]
Strategies of pretreatment of feedstocks for optimized bioethanol ...
Mar 13, 2023 · Pretreatment is required to disintegrate the lignin structure and to make the cellulosic complex more accessible for hydrolysis by enhancing ...
[73]
Advancing continuous enzymatic hydrolysis for improved biomass ...
Jul 25, 2025 · Continuous Enzymatic Hydrolysis aims to reduce greenhouse gas (GHG) emissions of SAF and bio-based chemicals production by increasing sugar– ...
[74]
Enhanced Sugar Yield from Enzymatic Hydrolysis of Cellulignin from ...
The results showed that hydrodynamic cavitation reduced the required SBP dosage while maintaining sugar yields, allowing 10% w/w of dried cellulignin cavitated ...
[75]
[PDF] DEXTROSE injection, for intravenous use - accessdata.fda.gov
Dextrose Injection is indicated as source of calories and fluid replenishment when mixed with amino acids or other compatible intravenous fluids for patients ...
[76]
[PDF] DEXTROSE INJECTION 20%, 30%, 40%, 50% and 70%
Dextrose Injection 20%, 30%, 40%, 50% and 70%, mixed with amino acids or other compatible intravenous fluids, is indicated as a source of calories for patients ...
[77]
Impact and Control of Sugar Size in Glycoconjugate Vaccines - PMC
Here, we examine and discuss the impact of glycan chain length on the efficacy of glycoconjugate vaccines and report the methods employed to size ...
[78]
Immunobiology of Carbohydrates: Implications for Novel Vaccine ...
In this review, we highlight historical and recent advances on the use of glycans either as vaccine antigens or adjuvants, and we review the use of currently ...
[79]
Fructose - Cosmetics Info
Fructose is a simple sugar found in fruit and honey. In cosmetics and personal care products, Fructose may be used in the formulation of bath products, ...Missing: reducing | Show results with:reducing
[80]
[PDF] Safety Assessment of Saccharide Humectants as Used in Cosmetics
Aug 21, 2020 · The cosmetic ingredient. Saccharide Hydrolysate contains fructose and glucose, and Saccharide Hydrolysate is also a direct food substance ...
[81]
https://biotechnologyforbiofuels.biomedcentral.com/articles/10.1186/s13068-025-02680-z
[82]
(PDF) Investigation of process parameters for eco-friendly indigo ...
Oct 2, 2025 · Previous studies have investigated that reducing sugars can be used as green alternatives to sodium dithionite in the indigo dyeing of cotton ...
[83]
Bio-bleaching of ankara pulp with xylanase-producing bacterial ...
Jul 14, 2024 · 3). Bio-bleaching efficiency measured in terms of release of colour, lignin, and reducing sugars was more in consortium as compared to ...Missing: manufacturing | Show results with:manufacturing
[84]
Influence of lignin content in cellulose pulp on paper durability - Nature
Nov 17, 2020 · This manuscript presents studies on the accelerated ageing of papers with different lignin contents that are produced in neutral environments.