Mannitol is made primarily from simple sugars like glucose and fructose, which are themselves derived from corn starch or other plant-based starches. In industrial production, these sugars undergo a chemical process called hydrogenation, where hydrogen is added under pressure to convert the sugar into a sugar alcohol. Mannitol also occurs naturally in a wide range of plants, mushrooms, and seaweed, and some of the earliest commercial mannitol was extracted directly from seaweed.
How Mannitol Is Produced Commercially
The dominant industrial method involves taking monosaccharides (single-unit sugars) like glucose or mannose and converting them through catalytic hydrogenation. In simple terms, the sugar is dissolved in water, mixed with a metal catalyst (typically nickel), and exposed to hydrogen gas at high temperature and pressure. This breaks the sugar’s carbonyl group and replaces it with a hydroxyl group, turning it into a sugar alcohol. The same basic process is used to make other sugar alcohols like sorbitol and xylitol.
Because glucose and fructose are cheaply produced from corn starch through enzymatic processing, corn is the most common upstream raw material for mannitol in the United States. Invert sugar, which is a mixture of glucose and fructose made by breaking down table sugar (sucrose), is another common starting material. The hydrogenation of fructose yields mannitol directly, while hydrogenating glucose produces mostly sorbitol, with mannitol as a secondary product that must be separated out through crystallization.
An older production method uses electrolytic reduction, where an electrical current converts glucose into mannitol in solution. This approach is less common today but still referenced in chemical literature.
Fermentation as a Newer Method
A growing area of production uses microbial fermentation instead of chemical hydrogenation. Certain bacteria, particularly heterofermentative lactic acid bacteria, naturally produce mannitol as a byproduct of their metabolism when they break down fructose. Yeasts and fungi can also be used. This biological route appeals to manufacturers looking for non-chemical, food-grade production processes. Researchers have been refining the fermentation conditions, including the sugar concentrations and reactor designs, to push yields high enough for commercial viability. One practical application is “in situ” mannitol production, where bacteria generate mannitol directly inside food and beverages as a way to reduce their sugar content.
Where Mannitol Occurs Naturally
Mannitol is widespread in nature. It shows up in tree sap, fruits, vegetables, mushrooms, and marine algae. The manna ash tree is the historical source that gave mannitol its name: the sweet, crystalline substance that oozes from its bark was once collected and used as a mild laxative in traditional medicine. Olive trees and plane trees produce similar exudates containing mannitol.
Among everyday foods, mannitol is found in pumpkin, celery, onions, strawberries, and cocoa beans. But the highest natural concentrations appear in mushrooms. The common white button mushroom (Agaricus bisporus) contains mannitol at levels approaching 50% of its dry weight in the cap. Shiitake mushrooms accumulate 20 to 30% mannitol by dry weight in the cap and stipe, though their mycelium (the root-like growing stage) contains only about 1%. Brown seaweed was historically an important source for commercial extraction before hydrogenation became the standard method.
What Mannitol Actually Is
Chemically, mannitol is a six-carbon sugar alcohol, a polyol. It looks and tastes like sugar but behaves differently in the body. Your digestive system absorbs it poorly, which means it provides roughly half the calories of regular sugar: about 1.6 to 2.4 calories per gram depending on which country’s regulatory framework you use, compared to 4 calories per gram for sucrose. More notably, mannitol has a glycemic index of zero. It does not raise blood sugar or trigger an insulin response, which is why it’s permitted in foods marketed to people with diabetes.
This poor absorption is also why mannitol works as an osmotic agent. In medicine, it draws water into the intestines (producing a laxative effect at high doses) or pulls fluid out of swollen tissues when given intravenously. In food manufacturing, its cool, sweet taste and resistance to absorbing moisture make it popular as a coating for chewing gum, a bulking agent in sugar-free candies, and a dusting powder for pharmaceutical tablets to keep them from sticking together.
Why the Source Matters
If you’re checking a product label and wondering where its mannitol came from, the answer is almost always corn-derived sugar that has been hydrogenated. Fermentation-derived mannitol is growing but still a small share of the market. Naturally extracted mannitol from seaweed or tree sap is rare in commercial products today. For people with corn allergies, this origin can be relevant, though the final mannitol molecule is chemically identical regardless of the starting material. The manufacturing process removes proteins and other allergenic components, so reactions are uncommon but not impossible for highly sensitive individuals.