This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
For this purpose the fluid extracts, compound or plain, are made of various strengths, as specially directed in the different formulas. Consistent extracts are scarcely made and employed for that purpose, and not practicable at all where the manufacturer prepares extracts for his own use only.
In this respect we refer to the directions given for home-made extracts, which apply likewise to purchased extracts, and require the same treatment and care in their preservation.
Manufacturers claim that fluid extracts represent the drug treated in such a way, and with a menstruum, as is best required to extract and hold in solution all its virtues, great discrimi-nation and care being necessary in selecting the proper menstruum in each separate drug. It is very evident that fluid extracts having for a menstrua ether or alcohol, which, by nature of their crude drug, will not give up their active principles to less volatile liquid, must of necessity be less staple than one that has for its body a liquid slow to evaporate. If, for example, ginger, which requires alcohol to extract, is left exposed, it will soon reduce by evaporation the quantity of menstruum necessary to a perfect solution, thereby rendering the extract unequal in its parts, and therefore unstable. In case the drug is in perfect solution, it will be stronger in proportion to the loss of its dissolving agent. Again, drugs containing volatile oils are liable to rapid change by evaporation, if left unstopped - such as peppermint, wintergreen, etc. One of the first indications we see of change in fluid extracts is their liability to precipitate, and with those containing gum or resin this often occurs, rendering them unsightly, and at least raising the question as to their trustworthiness.
This subject of precipitation is one of great importance, and has much to do with the manner with which they are made. It must not be taken as an evidence of bad extract that a precipitate is thrown down; changes of temperature of ten causing cloudiness and precipitating the drug, which upon a rise of temperature will be re-dissolved. Rather, it must be concluded that the solution was well saturated with the drug, and upon reducing the temperature could not be sustained in solution. Now, the truth is that many fluid extracts which undergo a change do so by virtue of their mode of preparation, and, while they may be worthless, they were not so in the beginning.
Light and air will perhaps do more to impair the virtues of extracts than all other elements combined, though heat and cold are no doubt important factors - a mean temperature being necessary to preserve their virtues. Most extracts, if unstopped, will lose by evaporation until a radical change in the menstruum takes place. Again, the direct rays of the sun will cause chemical changes that no doubt will eventually impair if not destroy their active principles. Concerning the subject, there is little known and much to learn. It may be concluded, however, that most extracts which contain little or no gum, and are reasonably free from volatile oil, if kept protected from light and air, will maintain their strength for several years.