What are Mannitol and Sorbitol?
Mannitol and sorbitol are both classified as sugar alcohols, or polyols, which are carbohydrates derived from sugars through a process called hydrogenation. Found naturally in various fruits and vegetables, they are also commercially produced for wide-ranging applications. As isomers, they share the same molecular formula, but their three-dimensional arrangement, or stereochemistry, is different. This seemingly minor distinction on a molecular level is responsible for the divergent characteristics of each compound.
The Fundamental Difference: A Twist in the Molecule
On a molecular level, the primary difference between mannitol and sorbitol lies in the orientation of the hydroxyl (-OH) group on the second carbon atom. This subtle structural twist results in two distinct molecules that cannot be superimposed on one another. This is known as stereoisomerism. Mannitol has a symmetric configuration that allows it to form more stable crystals, while sorbitol has a less symmetric structure. This fundamental structural variation is the root cause for all their contrasting behaviors and properties, including how they interact with moisture, melt, and are absorbed by the human body.
Contrasting Physical Properties
- Hygroscopicity: A key physical difference is their interaction with moisture. Sorbitol is highly hygroscopic, meaning it readily absorbs moisture from the air, making it useful as a humectant in products like chewing gum and toothpaste to prevent them from drying out. Mannitol, by contrast, is not hygroscopic, a property that makes it an excellent coating for hard candies and pharmaceuticals where moisture absorption is undesirable.
- Melting Point: Their melting points are notably different, a direct consequence of their molecular structure and crystal packing. Mannitol melts at a relatively high temperature (around 166-168°C), whereas sorbitol has a lower melting point (around 95°C).
- Taste and Cooling Sensation: Both provide a cool, sweet taste, but to varying degrees. Sorbitol is approximately 60% as sweet as sucrose (table sugar), while mannitol is around 50-70% as sweet. The cooling effect is due to an endothermic heat of solution, which is more pronounced in mannitol when fully dissolved.
Comparison of Mannitol vs. Sorbitol
| Feature | Mannitol | Sorbitol |
|---|---|---|
| Molecular Formula | $$C_6H_14O_6$$ | $$C_6H_14O_6$$ |
| Structural Difference | Hydroxyl group on C-2 is behind the plane, giving a symmetric structure. | Hydroxyl group on C-2 is out of the plane, giving a less symmetric structure. |
| Primary Medical Use | Osmotic diuretic to reduce intracranial/intraocular pressure. | Laxative, draws water into the colon. |
| Hygroscopicity | Very low; non-hygroscopic. | High; retains moisture effectively. |
| Calories per gram | ~1.6 kcal/g. | ~2.6 kcal/g. |
| Sweetness (vs. Sucrose) | 50-70%. | ~60%. |
| Melting Point | 166-168°C (331-334°F). | ~95°C (205°F). |
| Key Food Applications | Hard candy coating, chewable tablets. | Sugar-free gum, diet foods, preserves, humectant. |
| Absorption by Body | Poorly absorbed, excreted largely unchanged via kidneys. | Poorly absorbed, but more than mannitol; partly metabolized. |
Distinct Applications in Health and Food
Due to their different properties, mannitol and sorbitol are not used interchangeably, especially in medical contexts. Mannitol's poor metabolism and excretion by the kidneys make it an effective osmotic diuretic for reducing intracranial and intraocular pressure. Administered intravenously, it pulls fluid from the brain and eyes into the bloodstream, where it is then excreted. This poor intestinal absorption also means it functions as an oral laxative, though it is less potent for this use than sorbitol.
Sorbitol's higher moisture-retaining capacity makes it a valuable humectant and texturizing agent in the food and cosmetic industries. Its use in sugar-free gums, mints, and other dietetic products is widespread. In medicine, sorbitol is commonly used as a laxative, either orally or via enema, to relieve constipation by drawing water into the large intestine. In large amounts, it can also cause gastrointestinal distress, such as bloating and diarrhea.
Production and Natural Occurrence
Both polyols occur naturally in plants, but for commercial applications, they are typically produced by the catalytic hydrogenation of sugars from corn syrup or other starches. Sorbitol is synthesized from glucose, while mannitol is produced from fructose. While they are chemically manufactured, they are identical to their naturally occurring counterparts and are not considered artificial sweeteners.
Food Sources
- Mannitol: Natural sources include celery, olives, strawberries, and pineapples. It is also found in the secretions of the manna ash tree, from which its name is derived.
- Sorbitol: Abundant in fruits like apples, pears, peaches, and prunes, as well as berries.
Conclusion: Not Interchangeable Isomers
In summary, despite sharing the same chemical formula, mannitol and sorbitol are not the same compound. Their distinct stereoisomeric structures lead to crucial differences in physical properties like melting point, hygroscopicity, and sweetness. These variations dictate their specialized roles across different industries—mannitol as an osmotic diuretic and non-hygroscopic food coating, and sorbitol as a humectant and laxative. Understanding these differences is essential for both medical and food science applications, ensuring the correct compound is used for its intended purpose. For more detailed information on excipients, refer to sources such as Sigma-Aldrich's resources on sorbitol and mannitol.
Note: The use of either substance, particularly in medical contexts, should always be guided by a healthcare professional. Excessive consumption can cause gastrointestinal side effects.
