C₆H₁₂O₆ — Fructose

Fructose is a simple sugar or monosaccharide with the molecular formula \(C_6H_{12}O_6\). Commonly known as fruit sugar, it is one of the sweetest naturally occurring sugars and is found abundantly in fruits, honey, and some root vegetables. Along with glucose and galactose, fructose is one of the three major dietary monosaccharides absorbed directly into the bloodstream during digestion.

Fructose plays an important role in metabolism, serving as an energy source and a building block for complex carbohydrates. Industrially, it is used as a natural sweetener in the form of high-fructose corn syrup (HFCS), a popular additive in beverages and processed foods due to its high sweetness and solubility.

Fructose is a ketohexose, meaning it contains six carbon atoms and a ketone functional group. The open-chain structure of fructose can be represented as:

In aqueous solution, fructose primarily exists in cyclic forms through intramolecular reactions between the carbonyl group and hydroxyl groups, forming hemiacetal and hemiketal rings. The two main cyclic structures are:

• Fructofuranose: Five-membered ring structure.
• Fructopyranose: Six-membered ring structure.

Like glucose, fructose exhibits optical isomerism, existing as D- and L-forms. Naturally occurring fructose is the D-isomer (D-fructose), which rotates plane-polarized light to the left, hence called levulose.

Fructose occurs widely in nature as a component of many foods. It is found in:

• Fruits: Apples, pears, grapes, mangoes, and berries contain high concentrations of fructose.
• Honey: Honey is composed of about 40% fructose, 30% glucose, and smaller amounts of sucrose and other sugars.
• Vegetables: Onions, carrots, and sweet potatoes also contain fructose.
• Plant sap and nectar: Natural sources of simple sugars for bees and insects.

In plants, fructose is produced along with glucose during the hydrolysis of sucrose by the enzyme invertase:

1. From Sucrose (Cane Sugar):

Fructose can be obtained by hydrolyzing sucrose using acid or enzyme catalysts:

This reaction produces an equimolar mixture of glucose and fructose known as invert sugar, which is sweeter than sucrose and used in confectionery and beverages.

2. From Inulin:

Inulin, a polysaccharide found in Jerusalem artichokes and chicory roots, can be hydrolyzed to yield fructose:

This method is used in industrial production due to its efficiency and high yield of pure fructose.

3. From High-Fructose Corn Syrup (HFCS):

HFCS is produced by enzymatically converting glucose derived from corn starch into fructose using glucose isomerase. HFCS contains about 42–55% fructose and is used extensively in processed foods and soft drinks.

Physical Properties:

• Appearance: White, crystalline solid.
• Taste: 1.5 times sweeter than sucrose, making it one of the sweetest natural sugars.
• Solubility: Highly soluble in water due to hydrogen bonding between hydroxyl groups and water molecules.
• Optical Activity: D-fructose is levorotatory, rotating polarized light to the left, hence called levulose.

Chemical Properties:

• Reducing Nature: Fructose is a reducing sugar. Although it has a ketone group, it can tautomerize to an aldehyde form under alkaline conditions, allowing it to reduce Fehling’s and Tollen’s reagents.

\(R-CO-CH_2OH \rightleftharpoons R-CHOH-CHO\)

• Fermentation: Fructose undergoes fermentation to produce ethanol and carbon dioxide in the presence of yeast:

\(C_6H_{12}O_6 \xrightarrow{yeast} 2C_2H_5OH + 2CO_2\)

• Oxidation: Mild oxidizing agents convert fructose into glycolic and formic acids, while strong oxidizers produce oxalic acid and carbon dioxide.
• Dehydration: On heating with concentrated sulfuric acid, fructose dehydrates to form hydroxymethylfurfural (HMF), an intermediate used in green chemistry.

Fructose serves as a vital energy source in human metabolism. It is absorbed from the small intestine and metabolized primarily in the liver, where it is converted into glucose, glycogen, or fatty acids for energy storage. Unlike glucose, fructose metabolism does not directly require insulin, making it useful for diabetic diets when consumed moderately.

In plants, fructose participates in the synthesis of sucrose and starch, supporting energy transport and storage. It also plays a role in osmotic regulation and cellular respiration. However, excessive consumption of fructose (especially from HFCS) can lead to metabolic disorders such as obesity, insulin resistance, and fatty liver disease.

• Food and Beverage Industry: Used as a natural sweetener in juices, jams, desserts, and soft drinks due to its high sweetness and solubility.
• Pharmaceuticals: Incorporated into syrups, tonics, and energy supplements for its rapid absorption and sweetening effect.
• Nutrition: Provides a quick source of energy and is used in low-calorie and diabetic-friendly products.
• Industrial Applications: Acts as a starting material in the synthesis of mannitol, sorbitol, and other sugar alcohols used in food and pharmaceuticals.
• Fermentation Processes: Used as a substrate for ethanol and organic acid production.

While fructose is naturally present in fruits and vegetables, excessive intake through added sugars like HFCS has been linked to several health issues. High fructose consumption may increase triglyceride levels, contribute to obesity, and impair insulin sensitivity. However, when consumed through whole fruits, it provides beneficial fibers, vitamins, and antioxidants that aid in metabolism and digestion.

Moderate fructose intake is considered safe and beneficial as part of a balanced diet, especially when derived from natural sources.