The Pharmaceutics and Compounding Laboratory
Pharmaceutical Solutions I:
Simple, Saturated, Syrups

Preserving Syrups

Syrup, USP is protected from bacterial contamination by virtue of its high solute concentration. More dilute syrups are good media for microbial growth and require the addition of preservatives. Industrially formulated syrups often contain ingredients to improve solubility, stability, taste or appearance which also contribute to product preservation. It is necessary, from an economic standpoint, to consider the additive preservative effects of such ingredients as alcohol, glycerin, propylene glycol, and other dissolve solids. Syrup USP, having a specific gravity of 1.313 and a concentration of 85% w/v is a 65% w/w solution. This 65% by weight is the minimum amount of sucrose which will preserve neutral syrup. If one wants to formulate a syrup containing less sucrose, the quantity of alcohol, or other preservatives, may be estimated by considering the USP Syrup equivalent and the free water equivalent. One may assume that free water is preserved by 18% alcohol.

To calculate the free water equivalent, the volume occupied by the sucrose, the volume preserved by the sucrose, and the volume occupied and/or preserved by other additives must be subtracted from the total volume of the preparation. In Syrup, USP 850 g sucrose occupies an apparent volume of 550 ml; so each gram of sucrose will occupy 550/850 or 0.647 ml. If the 850 g sucrose preserves 450 ml of water, then each gram of sucrose will preserve 450/850 = 0.53 ml of water.

e.g. How much Alcohol USP is required to preserve 1L of syrup containing 500 g sucrose?

    Volume preserved by sucrose = 500 g x 0.53 ml/g = 265 ml | Volume occupied by sucrose = 500 g x 0.647 ml/g = 324 ml |Free water equivalent = 1000 ml - 265 ml - 324 ml = 411 ml | Volume of alcohol required to preserve the product is 411 ml x 18% = 74 ml | 74 ml Absolute alcohol ÷ 95% = 78 ml Alcohol USP

If other dissolved solids are present, their volume (often estimated) is subtracted from the free water volume. If glycerin is present, its volume preserves an equal volume of free water. If propylene glycol is present, it is considered equivalent to ethanol.