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.
"The finished syrups should be introduced into dry bottles, so as to avoid diluting them with water; if bottled while warm, condensation of aqueous vapors in the neck of the bottle will cause dilution of the upper stratum and disposition to fermentation, unless the syrup be thoroughly mixed after cooling. Properly prepared syrups may generally be kept unaltered, even in partially filled bottles, at the ordinary temperatures, but it is best to keep them in a place where the temperature will not vary materially between about 15° and 20° C. (59° and 68° F.). The addition of small quantities of spirit of ether, Hoffmann's anodyne, chlorate of potassium, and more recently of salicylic acid, has been recommended for the purpose of preserving syrups; but all these additions are more or less objectionable". - N. D.
A dilute saccharine solution, such as weak syrup for instance, readil ferments. A full-strength simple syrup does not undergo active fermen-tation. It nevertheless deteriorates from a low type of ferment, which has its inception on the surface of the syrup immediately adjacent to the walls of the receptacle. No perceptible generation of carbonic acid gas occurs, but the whole body of the syrup soon becomes tainted by the disagreeable flavor of the ferment, which latter extensively accumulates as a grayish ring adhering to the vessel. Various but generally objection-able remedies have been applied to prevent the change. It is recommended to use ether vapor for an efficient preventive. This is readily secured by occasionally letting fall a few drops of ether upon the surface of the syrup. The ether does not appreciably penetrate the syrup, which therefore remains free from ether flavor, but it apparently disinfects the superjacent stratum of air.
Flavored syrups, such as are used at the soda counter, are better kept in small rather than in large bottles, as the longer a bottle lasts the more frequently it will be opened, and, consequently, the more it will be exposed to the air. By bottling them while boiling hot into previously heated and dried bottles to prevent their cracking, and immediately corking down perfectly air-tight, they may be preserved for years without fermenting or losing their transparency. Between the cork and the surface of the syrup should be left no space, the bottle being filled entirely. On cooling, the syrup contracts and might leave a little space, where the aqueous vapors would condense, and, as already stated, dilute the upper strata and thus cause a disposition to fermentation. To prevent this mix or shake the bottle after it has become cold.
"When fermentation sets in, the carbonic acid gas disengaged causes the syrups to assume a frothy appearance, and a vinous odor is noticed. As soon as these signs make their appearance the syrup should be heated to boiling, strained, and afterwards properly preserved. The medicinally active principles will rarely have undergone any alteration, but syrups which owe their virtues wholly or in part to volatile principles, or in which fermentation has proceeded beyond the incipient stage, cannot be thus restored without being impaired in their properties". - N. D.
For the manufacture of carbonated beverages it is best to prepare the syrups as occasion demands.