Mannitol is classified as a
sugar alcohol; that is, it can be derived from a sugar (
mannose) by reduction. Other sugar alcohols include
xylitol and
sorbitol.
Industrial synthesis Mannitol is commonly produced via the
hydrogenation of fructose, which is formed from either
starch or
sucrose (common table sugar). Although starch is a cheaper source than sucrose, the transformation of starch is much more complicated. Eventually, it yields a syrup containing about 42%
fructose, 52%
glucose, and 6%
maltose. Sucrose is simply hydrolyzed into an
invert sugar syrup, which contains about 50% fructose. In both cases, the syrups are chromatographically purified to contain 90–95% fructose. The fructose is then hydrogenated over a nickel
catalyst into a mixture of isomers
sorbitol and mannitol. Yield is typically 50%:50%, although slightly
alkaline reaction conditions can slightly increase mannitol yields. It is a natural constituent of most microorganisms and plants, having roles in maintaining cellular water balance, and responding to drought or low temperature. A class of
lactic acid bacteria, labeled heterofermentive because of their multiple fermentation pathways, convert either three fructose molecules or two fructose and one glucose molecule into two mannitol molecules, and one molecule each of
lactic acid,
acetic acid, and
carbon dioxide.
Feedstock syrups containing medium to large concentrations of fructose (for example,
cashew apple juice, containing 55% fructose: 45%
glucose) can produce yields mannitol per liter of feedstock. Further research is being conducted, studying ways to engineer even more efficient mannitol pathways in lactic acid bacteria, as well as the use of other microorganisms such as
yeast Traditionally, mannitol is extracted from the crude material in a
Soxhlet apparatus using a mixture of
ethanol, water, and
methanol. The mannitol is then
recrystallized from the extract, generally resulting in yields of about 18% of the original natural product. Another method of extraction is using
supercritical and
subcritical fluids. These fluids are at such a stage that no difference exists between the liquid and gas stages, so are more
diffusive than normal fluids. This is considered to make them much more effective mass transfer agents than normal liquids. The super- or subcritical fluid is pumped through the natural product, and the mostly mannitol product is easily separated from the solvent and minute amount of byproduct. Supercritical
carbon dioxide extraction of olive leaves has been shown to require less solvent per measure of leaf than a traditional extraction – CO2 versus ethanol per olive leaf. Heated, pressurized,
subcritical water is even cheaper, and is shown to have dramatically greater results than traditional extraction. It requires only water per of olive leaf, and gives a yield of 76.75% mannitol. Both super- and subcritical extractions are cheaper, faster, purer, and more environmentally friendly than the traditional extraction. However, the required high operating temperatures and pressures are causes for hesitancy in the industrial use of this technique. ==History==