According to the American Diabetes Association, carbohydrates are the body’s main source of energy. Because they have carbon, hydrogen, and oxygen as chemical constituents, roughly (C.H₂O)n with n≥3, they are called carbohydrates. Examples of carbohydrates are sugars, fiber, and starches found in fruits, vegetables, grains, and dairy products.
- Monosaccharides are the building blocks of carbohydrates. Many of these substances are formed from simpler molecules through a process known as gluconeogenesis. Others are byproducts of photosynthesis. These are classified according to the number of carbon atoms, such as triose for 3 carbon atoms in the carbohydrate, tetrose for 4 carbon atoms, pentose for 5 carbon atoms, and so on. Some examples include glucose, fructose, galactose, etc.
- A small number of covalently linked monosaccharide molecules form an oligosaccharide. They often cooperate with proteins (glycoproteins) and lipids (glycolipids) and perform regulatory and structural functions. They contain 3 to 10 monosaccharide units linked together by glycosidic bonds. Examples are:
- Maltose (Glucose+Glucose)
- Sucrose (Glucose+ Fructose)
- Lactose (Glucose+Galactose)
- Raffinose (Glucose+Fructose+Galactose)
- Polysaccharides have molecular weights in the millions of Daltons and are made up of several covalently linked monosaccharide units. These are also known as glycans, which are further classified as homopolysaccharides or heteropolysaccharides, regardless of whether they consist of one type or more than one type of monosaccharide residue.
- Glucans (glucose polymer)
- Galactans (galactose polymer)
- Heparin (polymer of D-glucuronic acid, L-iduronic acid, N-sulfo-D-glucosamine)
- Hyaluronic acid (polymer of D-glucuronic acid and N-acetyl-glycosamine)
What are Reducing Sugars?
Reducing sugars are carbohydrates that can oxidize other substances by donating electrons while being reduced.
In reducing sugars, an aldehyde (-CHO) or ketone (-CO-) group is present. The presence of free carbon at the end of these reducing sugars is known as reducing ends. All categories of carbohydrates: monosaccharides, disaccharides, oligosaccharides, and polysaccharides include reducing sugars, while all monosaccharides, some disaccharides, some oligosaccharides, and some polysaccharides are reducing sugars.
- Two forms of sugars found in monosaccharides, aldose, and ketose, are reducing sugars since ketone groups are present in ketoses while aldehyde groups are present in aldoses. Examples of ketoses are fructose and aldoses are glucose and galactose.
- Tautomerization is the process by which isomers of a compound are converted into tautomers. Ketoses tautomerize to form aldoses and then act as reducing sugars.
- Disaccharides can be reducing or non-reducing. For example, sucrose and trehalose are nonreducing sugars because the glycosidic bonds between their anomeric carbons do not allow them to be converted to an open-chain form with an aldehyde group. Instead, they remain in the cyclic form. However, lactose and maltose are reducing disaccharides.
What are the Properties of reducing sugars?
The sugar that has the potential to donate an electron to another chemical or biological entity, reducing itself while oxidizing the other, is the reducing sugar. They serve as reducing agents. The following points summarize some of the most important properties of reducing sugars.
- A free aldehyde or ketone group is present in the structures of reducing sugars such as glucose and fructose.
- Reducing sugars are not usually oxidized, but some relatively weak oxidizing substances, such as metal salts, can be used to oxidize them.
- When carbon combines with some oxygen molecules to produce alcohol or ether, a hemiacetal structure develops.
- The osazones are generated when mutarotation is generated in the reducing sugars.
- Reducing agents in aqueous solution often produce one or more compounds with an aldehyde group.
- The Maillard reaction causes browning of foods such as cakes, sliced bread, chocolate, coffee, and processed and baked dishes due to the interaction between reducing sugars and amines. This reaction is evident when food is heated for a long time or left at room temperature.
- The reducing sugars reduce the copper ions in Fehling’s solution and Tollen’s reagent to copper ions to form brick-red precipitates.
What are examples of reducing sugars?
All monosaccharides are reducing sugars. These contain free aldehyde (aldose) and ketone (ketose) groups. The ketone tautomerizes in solution to form an aldehyde. Examples are glucose, galactose, fructose, ribose, glyceraldehyde, xylose, etc. The aldehyde group is easily oxidized to form carboxylic acid and at the same time reduces the other reactant in the process.
- Reducing disaccharides have a free hemiacetal moiety as an aldehyde group on one of their monosaccharides. For example, lactose, cellobiose, and maltose are reducing disaccharides in which a hemiacetal moiety is free. At the same time, the other is occupied by the glycosidic bond and can act as a reducing agent through the free hemiacetal unit.
- Non-reducing disaccharides and polysaccharides are acetals. Complex polysaccharides usually have a single hemiacetal unit, which for such a large molecule is insufficient to test positive for reducing sugars. The molecule that does not have hemiacetal groups is reducing in nature.
What are the Identifying Tests for Reducing Sugar
The tests commonly performed to determine whether or not reduced sugar is present in the sample are the Benedict test and the Fehling test.
The testing procedure begins by dissolving food samples in water to determine if reduced sugar is present. A very limited amount of Benedict’s reagent is added and at this point the solution begins to cool, followed by the start of cooling of the solution. The solution will start to change color after about 10 minutes. The presence of reducing sugars is indicated when the hue turns blue. However, if the hue changes to green, yellow, orange, red, and finally dark red or brown, this indicates that the food contains reducing sugars.
[Benedict’s solution is the aqueous solution of anhydrous sodium carbonate, sodium citrate, and copper(II) sulfate pentahydrate].
The sample in which the reducing sugar is to be detected is uniformly mixed in water, and then warm Fehling’s solution is added. The presence of reducing sugar is confirmed by the color change of the solution to a reddish-brown rust color.
[Fehlings’ solution is prepared from an aqueous solution of sodium potassium tartrate tetrahydrate and cupric sulfate pentahydrate combined in equal parts.]
Reducing Sugars, Properties, and Examples