Factors Affecting Moisture Content In Freeze-Dried Products And Key Control Points

I. The moisture content of the freeze-dried product, the performance of the freeze-dryer, and the duration and temperature of the desorption drying phase are the primary factors influencing final moisture levels. Freeze-dried products are required to have a moisture content of ≤3%. Excessive moisture can lead to product decomposition or shrinkage. The main causes of high moisture content include:

1. Excessive liquid fill volume. The thickness of the liquid layer is generally 10–15 mm and should not exceed 15 mm; if the fill volume is large, switching to a larger container to reduce the liquid thickness should be considered. Excessive thickness impairs heat transfer during drying, hindering moisture removal and resulting in high residual moisture.

2. Re-absorption of moisture after drying. For products prone to moisture absorption, improper post-drying handling can lead to moisture re-absorption and increased water content. If gas enters the freeze-drying chamber during the venting process, it must undergo sterile filtration and drying. The product temperature upon unloading also affects moisture content; the product temperature should be 2–3°C higher than the cleanroom temperature to prevent atmospheric moisture from condensing on the product. Additionally, the relative humidity of the cleanroom must be strictly controlled below 50%.

II. Product appearance; the appearance of freeze-dried products is closely linked to the freeze-drying cycle (curve) employed.

Poor appearance in freeze-dried products manifests as shrinkage, crystallization, or a lack of full, fluffy structure. Products with poor appearance are rejected and destroyed during inspection, significantly lowering the finished product yield and increasing production costs. Factors affecting product appearance include:

1. Solution concentration (high or low) and its effect on the liquid surface during the drying process. When the solution concentration is too high:

A dense surface layer forms during drying. This layer has tiny pores and poor permeability, making it difficult for internal moisture to escape; consequently, moisture migrates upward along the vial walls, causing the product to detach from the walls and shrink. Additionally, excessively high concentrations make the solution prone to agglomeration during freeze-drying, resulting in an uneven product appearance. Conversely, when the concentration is too low: mechanical strength is compromised; the product structure becomes loose or even turns into powder during packaging and transport, detracting from its appearance.

2. Excessive product thickness. The thickness should generally be 10–15 mm. Excessively thick products present issues similar to those caused by high concentrations—specifically, prolonged drying times and risks of collapse or agglomeration.

3. Temperature fluctuation rates. Rapid temperature changes can cause the product to detach from the walls. Regarding pre-freezing rates: a cooling rate of 10–15°C per minute is considered "rapid freezing," whereas a rate of 1°C per minute is considered "slow freezing." For the primary sublimation drying stage: the cooling rate should be controlled at 5–6°C.

Once the temperature is below the eutectic point, heating should be gradual—ideally at 5°C per hour—and the primary drying stage should remove over 90% of the moisture. Secondary drying: this stage primarily removes crystalline water and water adsorbed by the solid matrix. The heating rate can be increased to 5–10°C per hour, provided the final product temperature does not exceed the safe limit; otherwise, product activity may decline, agglomeration may occur, and solubility may be impaired.

4. Venting rate. Before unloading the chamber, the venting rate should not be too rapid. A sudden, drastic change in vacuum levels can create turbulent airflow rushing into the containers, turning the product into flocculent matter or powder and compromising its appearance.