What is the Difference Between Reducing Sugar and Starch

The main difference between reducing sugar and starch is that reducing sugar can be either a mono- or disaccharide, which contains a hemiacetal group with a one OH group and one O-R group attached to the same carbon whereas starch is a polysaccharide,  consisting of numerous glucose units joined by glycosidic bonds. Furthermore, all monosaccharides and many disaccharides such as cellobiose, lactose, and maltose are reducing sugars while glucose polymers such as starch and starch-derivatives such as dextrin, glucose syrup, maltodextrin, etc. are macromolecules starting with a reducing sugar. 

Reducing sugar and starch are two types of carbohydrates, which are one of the three types of macronutrients in animals. Also, carbohydrates contain carbon, hydrogen, and oxygen; hydrogen to oxygen ratio of 2:1. 

Key Areas Covered 

1. What is a Reducing Sugar
     – Definition, Structure, Importance
2. What is Starch
     – Definition, Structure, Importance
3. What are the Similarities Between Reducing Sugar and Starch
     – Outline of Common Features
4. What is the Difference Between Reducing Sugar and Starch
     – Comparison of Key Differences

Key Terms 

Aldehyde Groups, Disaccharides, Monosaccharides, Reducing Sugar, Starch

What is a Reducing Sugar  

Reducing sugars are the carbohydrates, which can act as reducing agents. Therefore, they contain either a free aldehyde or free ketone group. Significantly, all monosaccharides are reducing sugars. In addition to these, some disaccharides, oligosaccharides, and polysaccharides also have some reducing properties. Generally, there are two types of monosaccharides; aldoses with an aldehyde group and ketoses with a ketone group. However, ketone groups have to convert into aldose group to become a reducing sugar. Usually, hemiacetal groups, containing one OH group and one O-R group attached to the same carbon have this ability to interconvert. Also, some of the examples of monosaccharides are glucose, galactose, and fructose. 

Furthermore, disaccharides have two monosaccharides connected by a glycosidic bond. However, in non-reducing disaccharides such as sucrose, glycosidic bond forms between anomeric carbons, making it convert into the open-chain structure without an aldehyde group. Still, in reducing disaccharides such as lactose and maltose, the open-chain structure has an aldehyde group. For instance, in starch such as starch and starch-derivatives, the polymer starts with reducing sugar, containing a free aldehyde. Significantly, more hydrolysis of starch makes more reducing sugar moieties.

Figure 1: Equilibrium between the Cyclic and Open Form of Maltose

Moreover, open-chain forms with aldehyde groups serve as reducing agents, oxidizing metal ions. Therefore, in Benedict’s test and Fehling’s solution, Cu2+ ions oxidize to form a brick red color by reducing sugars. Also, in the Tollen’s test, Ag+ undergoes oxidization. 

What is Starch 

Starch is the main storage form of carbohydrates in plants. Generally, it occurs in large amounts in staple foods like potatoes, wheat, maize (corn), rice, and cassava. Also, starch is a white color, which is tasteless, odorless, and insoluble in cold water and alcohol. For instance, in plants, glucose 1-phosphate is first converted into ADP-glucose, which undergoes polymerization via 1,4-alpha glycosidic bonds. Significantly, the addition of new ADP-glucose molecules occurs to the non-reducing end of the polymer. On the other hand, branching of the polymer occurs via 1,6-alpha glycosidic bonds. 

Figure 2: Amylose and Amylopectin

Moreover, based on the structure, there are two types of starch occurring in plants. They are linear amylose and branched amylopectin. In general, plants contain 25% of amylose and 75% of amylopectin. However, in industry, starch is important for the fermentation of ethanol to produce beer, whiskey, as well as biofuel. Also, starch has non-food uses in the industry as well. 

Similarities Between Reducing Sugar and Starch 

• Reducing sugar and starch are two types of carbohydrates made up of carbon, hydrogen, and oxygen. 
• Their hydrogen to oxygen ratio is 2:1. 
• They are the most common components of carbohydrate diets. 
• Significantly, they play an important role as fuels for cellular respiration. 

Difference Between Reducing Sugar and Starch 

Definition 

Reducing sugar refers to any sugar capable of acting as a reducing agent due to the presence of a free aldehyde or ketone group while starch refers to an odorless, tasteless, white substance, occurring widely in plant tissue such as cereals and potatoes. 

Structure 

Reducing sugar can be either a mono- or disaccharide, which contains a hemiacetal group with a one OH group and one O-R group attached to the same carbon while starch is a polysaccharide,  consisting of numerous glucose units joined by glycosidic bonds. 

Reducing Properties 

All monosaccharides and many disaccharides such as cellobiose, lactose, and maltose are reducing sugars while glucose polymers such as starch and starch-derivatives such as dextrin, glucose syrup, maltodextrin, etc. are macromolecules starting with a reducing sugar. 

Result for the Benedict’s Test 

Reducing sugars give positive results for Benedict’s test while starch reacts very poorly with Benedict’s reagent due to the presence of a small number of reducing sugar moieties.  

Conclusion 

Reducing sugars are either monosaccharides or disaccharides with a hemiacetal group with a free aldehyde or ketone group. Significantly, this group acts as a reducing agent, oxidizing metal salts. For instance, all monosaccharides and some disaccharides including lactose and moltose are reducing sugars. In contrast, starch is the main form of storage polysaccharide in plants. Also, it is made up of glucose units joined by glycosidic bonds. However, it begins with a reducing sugar with a free aldehyde group. Therefore, it also has reducing properties only to a small extent. Hence, the main difference between reducing sugars and starch is their structure and properties. 

References:

1. “What Are Reducing Sugars?” Master Organic Chemistry, 19 June 2019, Available Here.
2. “Starch.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., Available Here.

Image Courtesy:

1. “Maltose Gleichgewicht” By NEUROtiker – Own work (Public Domain) via Commons Wikimedia   
2. “Figure 03 02 06” By CNX OpenStax (CC BY 4.0) via Commons Wikimedia