A fluid ounce of SATURATED SYRUP weighs 5771⁄2 gr.; a gallon weighs 131⁄5 lbs. (avoir.); its sp. gr. is 1·319 to 1·321, or 35° of Baumé’s aërometer; its boiling-point is 221° Fahr., and its density at the temperature of 212° is 1·260 to 1·261, or 30° Baumé. The syrups prepared with the juices of fruits, or which contain much extractive matter, as those of sarsaparilla, poppies, &c., mark about 2° or 3° more on Baumé’s scale than the other syrups.
In most pharmaceutical works directions are given to completely saturate the water with sugar. Our own experience, which is extensive, leads us to disapprove of such a practice, since we find that, under all ordinary circumstances, a syrup with a very slight excess of water keeps better than one fully saturated. In the latter case a portion of sugar generally crystallises out on standing, and thus, by abstracting sugar from the remainder of the syrup, so weakens it, that it rapidly
ferments and spoils. This change proceeds at a rapidity proportionate to the temperature. Saturated syrup kept in a vessel that is frequently uncorked or exposed to the air soon loses sufficient water, by evaporation from its surface, to cause the formation of minute crystals of sugar, which, falling to the bottom of the vessel, continue to increase in size at the expense of the sugar in the solution. We have seen a single 6-gallon stone bottle, in which syrup has been kept for some time, the inside of which, when broken, has been found to be entirely cased with sugar candy, amounting in weight to 16 or 18 lbs. On the other hand, syrups containing too much water also rapidly ferment, and become acescent; but of the two this is the less evil, and may be more easily prevented. The proportions of sugar and water given above will form an excellent syrup, provided care be taken that an undue quantity be not lost by evaporation.
The decimal part of the number denoting the sp. gr. of a syrup, multiplied by 26, gives the number of pounds of sugar it contains per gallon, very nearly. (Ure.)
In boiling syrups, if they appear likely to boil over, a little oil, or rubbing the edges of the pan with soap, will prevent it.
Syrups may be decoloured by agitation with, or filtration through, recently burnt animal charcoal. Medicated syrups should not, however, be treated in this way.
The preservation of syrups, as well as of all other saccharine solutions, is best promoted by keeping them in a moderately cool, but not a very cold, place. “Let syrups be kept in vessels well closed, and in a situation where the temperature never rises above 55° Fahr.” (Ph. L.) They are better kept in smaller 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 syrups whilst boiling hot, and immediately corking down and tying the bottles over with bladder perfectly air-tight, they may be preserved, even at a summer heat, for years, without fermenting or losing their transparency.
The ‘candying,’ or crystallisation, of syrup, unless it be oversaturated with sugar, may be prevented by the addition of a little acetic or citric acid (2 or 3 dr. per gall.).
The fermentation of syrups may be effectually prevented by the addition of a little sulphite of potassa or of limes. Chlorate of potassa has been proposed for this purpose by Dr Macculloch, on theoretical grounds. M. Chereau recommends the addition of some (about 3 to 4%) sugar of milk, with the same intention. Dr Durand asserts that by adding about 1 fl. dr. of ‘Hofmann’s anodyne’ to each pint of syrup, the accession of fermentation may not only be prevented, but arrested when it occurs, fermenting syrups may be immediately restored by exposing the vessel containing them to the temperature of boiling
water. The addition of a little spirit is ordered in the new ‘London Pharmacopœia.’