Simple Syrup Concentration: Sucrose and Purified Water as per IP and USP

What Is Simple Syrup and How Is Its Concentration Expressed?

Simple syrup is a concentrated aqueous solution of sucrose in purified water, with no medicament and no flavour. It is used as a sweet, viscous vehicle for medicated syrups and as the base of flavoured preparations. For the full overview of syrup types and the general manufacturing route, see the guide on liquid syrup types and manufacturing process.

Concentration is stated by weight or by volume. Weight per weight (w/w) means grams of sucrose in 100 g of finished syrup, whereas weight per volume (w/v) means grams of sucrose in 100 mL of finished syrup. Syrup is much denser than water, so the same syrup gives two quite different figures, and this is why the IP and USP values look unlike each other.

Syrups in general carry about 60 to 80% sucrose, as described by The Pharmapedia. Simple syrup lies at its upper end.

How Does a High Sugar Concentration Preserve Simple Syrup?

Simple syrup is self-preserving because its sucrose content creates high osmotic pressure and low water activity. Water activity is the measurable share of water that is free for microbial use. Dissolved sucrose holds most of the water, so bacteria, yeasts and moulds cannot take up enough of it to grow, and microbial cells placed in such a solution also lose water to it by osmosis.

A dilute sucrose solution behaves differently. It can ferment, turn turbid and change colour, because microbes grow in it. Our some facts checks found that 65% w/w as the minimum sucrose level that preserves a neutral syrup. This is a our figure and not a pharmacopoeial limit, and the IP strength of 66.7% w/w lies just above it.

The protection depends on the concentration of the final product, not on the concentration of the starting syrup. If an aqueous solution of a drug or buffer is added to simple syrup, the sugar fraction falls. For example, mixing 90 g of 65% w/w syrup with 10 g of a sugar-free aqueous solution gives about 58.5% w/w, which is below the teaching minimum. The formulation of such a medicated syrup must therefore be assessed for microbial risk, and a preservative is often added.

What Are the Stages of Simple Syrup Manufacture?

In a pharmaceutical plant, simple syrup is made in a fixed sequence of stages that are recorded in the batch manufacturing record (BMR). The sugar tank, the manufacturing tank and the storage tank are connected by dedicated pipelines.

1. Line clearance is completed in the dispensing and manufacturing areas, and Quality Assurance (QA) checks that no material, label or record of the previous batch remains.
2. Sucrose and purified water are dispensed and weighed as per the BMR.
3. Sucrose is transferred from the sugar tank to the manufacturing tank, where it is dissolved in purified water.
4. The syrup is filtered to remove particles and precipitates.
5. The filtered syrup is held in the storage tank.
6. The syrup is filled into pre-cleaned bottles and packed.

In-process controls (IPC) are carried out at the dissolving, filtration and holding stages, as described in the control section below. The equipment in this route (dissolver, tanks and filter housing) must be qualified. The process, the cleaning procedure and the test methods are also validated.

The operator weighs the materials and runs the dissolving step, while the production officer or shift in-charge supervises the stages and checks the entries in the BMR. The QA officer verifies each stage against BMR. Besides this, the QA officer collects or witnesses the IPC samples and approves any deviation. The store officer and Quality Control (QC) analyst are responsible for the sucrose, which must be received and released against the specification before it is dispensed.

How Do Simple Syrup IP and USP Differ, and How Are They Converted?

Simple Syrup IP is reported as 66.7 g of sucrose with purified water to make 100 g, which is 66.7% w/w. This is about 2 parts of sucrose to 1 part of water by weight, and the water portion is 33.3 g. An IP batch is therefore weighed.

Simple syrup USP is 850 g of sucrose in enough purified water to make 1000 mL, which is 85% w/v, and the batch is made up to volume. The specific gravity is taught as 1.313, so 1000 mL of the syrup weighs about 1313 g.

The conversion of USP syrup from w/v to w/w uses the weight of 100 mL of syrup.

1. Weight of 100 mL of syrup = 100 mL × 1.313 = 131.3 g.
2. Sucrose in 100 mL = 85 g.
3. Percentage w/w = 85 ÷ 131.3 × 100 = about 64.7%, which textbooks round to 65% w/w.
4. Water in 100 mL = 131.3 − 85 = about 46.3 g.

The table gives both syrups on both bases. The USP per-100 g values are calculated from the figures above, and the IP volume basis is not given because no specific gravity is available for it.

Syrup	Basis	Sucrose	Water	Total
Simple Syrup IP	Per 100 g	66.7 g	33.3 g	100 g
Simple syrup USP	Per 100 mL	85 g	about 46.3 g	about 131.3 g
Simple syrup USP	Per 100 g	about 64.7 g	about 35.3 g	100 g

The two strengths differ by only about 2 percentage points on the weight basis, so IP and USP simple syrups are nearly equal in strength. For production planning, a 1000 kg IP batch needs 667 kg of sucrose and 333 kg of purified water. A 1000 L USP batch needs 850 kg of sucrose made up to 1000 L, and the syrup then weighs about 1313 kg.

How Is Simple Syrup Prepared in Practice?

Three methods are described for preparing syrup: solution with heat, agitation without heat, and percolation. Heat is the fastest method, but it is unsuitable for heat-labile ingredients. Agitation without heat takes longer and suits ingredients that must not be warmed.

In the heat method, purified water is warmed gently in a jacketed manufacturing tank, and sucrose is added with continuous stirring until it dissolves. Overheating must be avoided, because it causes caramelisation and darkening. The syrup is then cooled and brought to the final weight or volume. It is filtered after that.

The basis of the pharmacopoeia decides how the batch is finished. For a w/w syrup (IP), sucrose and purified water are weighed in the stated proportion, so the final weight is controlled directly. For a w/v syrup (USP), 850 g of sucrose per litre is dissolved and the syrup is made up to the final volume mark of a calibrated tank. Once the volume is adjusted, the specific gravity of the batch is checked against its intended strength.

A slight excess of water over the saturation amount keeps the syrup stable when the temperature changes. The UNC teaching page states that about 425 mL of water is enough to dissolve 850 g of sucrose, while the USP syrup holds about 450 mL per litre. This small margin helps to prevent crystallisation during cold storage.

Which In-Process Controls and Records Apply?

The strength of the syrup is verified by specific gravity at a stated temperature or by refractometer Brix. Brix is the percentage of sucrose by weight, so about 65 to 66.7 °Brix corresponds to the USP and IP syrups, although the equivalence is approximate. The result must be recorded in the BMR against the specification, together with the temperature at which it was read.

The IPC checks and BMR entries for a syrup batch normally include the following:

• SG or Brix of the finished syrup, compared with the specification.
• Clarity of the syrup after filtration.
• Filter identity and the integrity check recorded before use.
• Hold time in the storage tank, from the end of filtration to the start of filling.
• Yield and reconciliation of sucrose, purified water and bottles.
• The signatures of the operator, the production officer and the QA officer.

A deviation is raised whenever a result falls outside the specification or the hold time is exceeded. It is also raised if the syrup is turbid or discoloured, or if the yield and reconciliation do not agree. The batch stays on hold until QA completes the investigation.

The quality of the inputs also needs control. Purified water must meet the specification of its grade, and its microbial control depends on the water system, as explained in types of water in pharmaceutical use and water system validation and phases. The sugar tank and the storage tank must also be kept free from microbial growth.

What Are the Critical Process Parameters?

No pharmacopoeial or manufacturer value is available for temperature, mixing time or filter rating of a pharmaceutical syrup tank. These parameters are therefore listed without a value. They are set in the BMR from process validation and from the equipment qualification of the dissolver. Exact values depend on the manufacturer and model; always follow the Original Equipment Manufacturer (OEM) manual and site Standard Operating Procedure (SOP).

Parameter	Setting or range	Why it matters	How it is monitored
Sucrose quantity	USP: 850 g per 1000 mL; IP: 66.7 g per 100 g	Decides the strength and the preservation	Weighing record in the BMR
Specific gravity of finished syrup	USP: 1.313 (teaching value)	Confirms that the syrup is at full strength	SG or Brix check against the specification
Water temperature	Set in the BMR from validation	Speeds up dissolving, but excess heat darkens the syrup	Tank temperature indicator, recorded by the operator
Heating limit	Set in the BMR from validation	Prevents caramelisation and darkening	Temperature record and visual colour check
Mixing time	Set in the BMR from validation	Ensures complete dissolution of sucrose	Visual check of clarity, IPC record
Final volume or weight	Per batch size in the BMR	Controls the batch strength	Calibrated tank or balance, with reconciliation

What Are the Common Problems in Simple Syrup and How Are They Fixed?

Crystallisation at the Neck and Cap

Possible causes: loss of water by evaporation, a formula close to saturation, or cooling during storage.

Corrective actions: make up to the correct volume or weight. Add a small amount of glycerin, sorbitol or propylene glycol to hinder crystallisation (cap-locking), as described by The Pharmapedia.

Darkening of the Syrup

Possible causes: overheating while dissolving, which causes caramelisation.

Corrective actions: heat gently and only as long as dissolving needs, or use agitation without heat when the formula contains heat-labile ingredients.

Turbidity or Fermentation

Possible causes: a syrup below the minimum strength or dilution after medicament addition. Microbial contamination from purified water, tanks or lines can also cause it.

Corrective actions: check SG or Brix, review the purified water results and hold the batch under a deviation until QA completes the investigation.

Sugar Residue and Microbial Growth in Tanks and Lines

Sugar residue left in the syrup tank, pipelines and filter housing can support microbial growth in the next batch. After each batch and especially at a product changeover, the tank, lines and filters must be cleaned as per the approved procedure. The equipment is released only after QA verifies the cleaning record. Wet equipment should be allowed to drain and dry so that no residual water dilutes the next batch.

Strength Out of Specification

Possible causes: weighing error, wrong final volume, or water left in the lines.

Corrective actions: recheck the weighing record and the calibration of the balance and the tank, then correct the batch only as per the approved procedure.

What Do the Regulations Say About Simple Syrup?

The strength of simple syrup is defined by the monographs of the Indian Pharmacopoeia (IP) and of the United States Pharmacopeia and National Formulary (USP-NF). The IP 2022 edition came into effect on 1 December 2022, as stated in the salient features published by the Indian Pharmacopoeia Commission.

Co-solvents such as glycerin, sorbitol solution and propylene glycol are high-risk components for contamination with diethylene glycol (DEG) and ethylene glycol (EG). The FDA guidance “Testing of Glycerin, Propylene Glycol, Maltitol Solution, Hydrogenated Starch Hydrolysate, Sorbitol Solution, and Other High-Risk Drug Components for Diethylene Glycol and Ethylene Glycol” (May 2023) alerts manufacturers to this hazard.

It reports fatal poisonings from liquid drug products made with contaminated components, and it expects DEG and EG testing with a safety limit of NMT 0.10% on a representative sample from all containers of all lots of each high-risk component. Where the DEG/EG test is not part of the identification test in the USP-NF monograph, the manufacturer should use a suitable equivalent method. A syrup plant that adds these components should therefore test them before use.

Frequently Asked Questions

References

1. Testing of Glycerin, Propylene Glycol, Maltitol Solution, Hydrogenated Starch Hydrolysate, Sorbitol Solution, and Other High-Risk Drug Components for DEG and EG, US FDA, May 2023
2. Salient Features of Indian Pharmacopoeia 2022, Indian Pharmacopoeia Commission
3. Lachman/Lieberman, The Theory and Practice of Industrial Pharmacy, textbook
4. Aulton’s Pharmaceutics: The Design and Manufacture of Medicines, textbook