This section is from the book "The Manufacture Of Liquors, Wines, And Cordials, Without The Aid Of Distillation", by Pierre Lacour. Also available from Amazon: Manufacture of Liquors, Wines, and Cordials, Without the Aid of Distillation.
In the manufacture of syrups, the quality and quantity of the sugar employed are points of importance. Refined sugar should always be employed, as it often saves the necessity of clarification, and makes a clearer and better flavored syrup than the impure kinds. In relation to the quantity of sugar, if in too small proportion fermentation is apt to occur; if too abundant crystallization will ensue. The proper proportion is about two parts to one of the liquid. A somewhat smaller quantity will answer, where an acid such as lemon juice, etc, is used.
Syrup is apt to become scorched, or brown, by a continued application of heat; therefore, syrups should boil briskly over a lively fire, so as to accomplish the object as quickly as possible. It is important to be able to ascertain positively when they have attained the due consistence. An operator skilled in their preparation, can judge with sufficient accu racy by various signs, such as the slowness with which the parts of a drop of syrup part or break; for instance, if a stick is plunged in the syrup and withdrawn and waved around in the air a couple of times, then, if upon studying it, the particles of syrup should hang in large, round, heavy tears, and fall from the stick in long, ropy threads, this is an evidence of its having been boiled sufficiently. A pellicle forming upon the surface of the syrup when it cools, indicates that it has been too much boiled.
The easiest method of ascertaining the proper point of concentration is by the use of Baume's hydrometer - called a saccharometer. This should stand at 30° in boiling syrup (30 1/2 in hot weather), and at 35° when the syrup is cool.
When carefully prepared with the best double refined sugar, syrups usually require no other clarification than to remove any scum which may rise to the surface upon standing, and to pour them off from any dregs which may subside; but as the sugar employed is not always free from impurities, it would, as a general rule, be best to remove the scum as it rises, during the heating process, and, if required, to strain them while hot through muslin or flannel. Should they at any time want the due degree of clearness, they may be warmed and filtered through flannel, raw cotton, etc, or clarified by the whites of eggs.
Syrups are liable to undergo various alterations, according to their nature and mode of preparation. The acid syrups, when too much boiled, often let fall a copious white precipitate, which is said to be a saccharine matter, analogous to the sugar of grapes, produced by the reaction of the acid upon the sugar. At an ordinary temperature, acids slowly convert common sugar into grape sugar, which being less soluble than the former is gradually deposited in the form of crystalline grains. Syrups which contain too little sugar are apt to pass into the vinous fermentation, in consequence of the presence of matters which act a ferment. Those which contain too much deposit a portion in the crystalline state, and the crystals, attracting the sugar remaining in solution, gradually weaken the syrup and render it liable to the same change as when originally made with too little sugar. The want of a due proportion of sugar frequently gives rise to mouldiness, when air has access to the syrup.
Syrups bottled while hot are apt to ferment, owing to the watery vapor or steam rising to the surface and condensing, which diminishes the proportion of sugar so as to produce a commencement of chemical action, which gradually extends throughout the whole mass. If the bottles are well shaken, the result is obviated, and the syrup will generally keep better when thus treated. When syrups undergo the vinous fermentation, their surface becomes covered with froth, produced by the disengagement of carbonic acid, and acquire a vinous odor from the presence of alcohol, while their consistence is diminished by a loss of a portion of the sugar which has been converted into that liquid. When the alcohol has been increased to a certain point, the fermentation ceases or goes on more slowly, owing to the preservative influence of that principle, and as the active ingredient of the syrup may have undergone no material change, the preparation may be recovered by boiling so as to drive off the alcohol and carbonic acid, and sufficiently concentrate the liquid.
A syrup thus revived, is less liable afterwards to undergo fermentation, because the principles which acted as ferments have been diminished. It is obvious that syrups which depend for their virtues upon a volatile ingredient, or one readily changed by heat, cannot be restored to their original condition.
At best, syrups are apt to change, and various measures have been proposed for their preservation. A small portion of sulphate of potassa or chlorate of potassa, which is tasteless, prevents their fermentation, and sugar of milk has been effectual to the same end. The proportion employed, is thirty parts of sugar of milk, one thousand of syrup; but the best plan for the preservation of syrup, is to keep it excluded from the air, in well closed vessels, and pack ed in a cold olace.