06. Manna and mannite
Natural and artificial mannite
The "natural mannite" is obtained only from manna because it contains it in the highest percentage. "Artificial mannite" is also available on the market, which is obtained from the fermentation of beet molasses by some schizomycetes capable of reducing the exoses to mannitol. The "natural mannite" has a greater softness than the artificial one.
Mannite performs various functions in the human organism, however it is advisable to pay attention not so much to this pure and isolated principle as to manna as a whole. In fact, it contains many other active ingredients which, although present in a not very high percentage, on the one hand limit the action of mannite and on the other widen the field of action of the manna itself.
The fact that the percentage of mannite in the manna is not constant is a positive factor, since it allows you to use not a drug dosed in all its components, but a drug that, like all drugs of plant origin, has a certain variability qualitative and quantitative of the active ingredients contained.
Methods for extracting mannite from manna
Mannite is the main constituent of manna, from which it is obtained through various extraction methods developed in the second half of the 19th century.
Mannite was initially extracted by exploiting its solubility in boiling alcohol. This process being very expensive came pr abandoned after the spread of the extraction method developed by Ruspini, a pharmacist from Bergamo who used an aqueous solution for the extraction of mannite. In particular, the manna was dissolved in boiling water and egg white, and left to cool slowly in copper pots or wooden containers. After two days the marble solution was filtered and pressed, separating the molasses from the mannite cake. With this system, after treatment with boiling water and animal coal, very pure mannite was obtained with a yield of 25-30%. This method, being much cheaper than the previous one, spread quickly.
A variant, which allowed to increase the yield, consisted in dissolving the manna in a solution of water and oxalic acid at 3%. The amalgam formed was left to rest for 24 hours: in this way the plant residues spontaneously separated by surfacing while the earthy impurities containing clay precipitated to the bottom due to the action of the acid. What remained in suspension was pressed in order to separate the liquid component (molasses) from the more solid one from which the so-called "cake" was obtained.
The cake was dissolved in containing water (3%) and slaked lime (2%) in order to obtain a solution which, after filtering and cooling, deposited the mannite in crystals. Subsequent recrystallizations in the appropriate forms made it possible to obtain the classic cones or blocks that were dried in the sun and, finally, smoothed with suitable files to reduce them to the weight of 250 or 500 grams.