Yingtai: Analysis of The Top Ten Common Poor Freeze-Drying Effects
Vacuum freeze-drying technology integrates various knowledge and specialized techniques, including refrigeration, vacuum, automation control, and aerodynamics. Consequently, the structure of freeze-dryers is more complex compared to general drying equipment.
Therefore, operating a freeze-dryer requires prior training for the operators and a certain level of experience and basic understanding of the freeze-drying process.
The complex structure and precise requirements of the freeze-drying process may lead to poor results or failures in freeze-drying experiments or process development due to various reasons.
Based on years of freeze-drying experiments, we have summarized several common poor effects, analyzed their causes, and proposed optimization measures. Let's explore them together:
Group One
Poor Effect:
Spraying during vacuuming of the product.
Cause Analysis:
This occurs because drying and vacuuming begin before the product is completely frozen. At this stage, the product’s temperature may not have dropped below the eutectic point, or it may be below the eutectic point but not for long enough, leaving some parts of the product still in a supercooled state, and freezing is not yet complete.
Optimization Measures:
Lower the pre-freezing temperature if the change in freezing rate does not affect the product’s freeze-drying. If the freezing rate does affect the product, extend the pre-freezing time to ensure the product is completely frozen.
Group Two
Poor Effect:
Product exhibits shrinkage and bubbling after freeze-drying.
Cause Analysis:
This is due to localized melting during the sublimation drying process, where moisture evaporates as gas, causing volume reduction, or dried product mixing with liquid, causing localized volume shrinkage. Severe melting can cause bubbling because the drying temperature is set too high, causing the product’s temperature to exceed the eutectic point or collapse temperature, leading to melting. After vacuuming, poor local vacuum causes the product to bubble.
Optimization Measures:
Lower the heating temperature during the sublimation drying phase and ensure good vacuum within the freeze-drying chamber; extend the sublimation drying phase to avoid premature transition to the desorption drying phase. Control the product temperature to keep it below the eutectic point or collapse temperature throughout the process.
Group Three
Poor Effect:
Product lacks a fixed shape.
Cause Analysis:
The product has too little dry matter, resulting in a low concentration and no formation of a supporting structure. The dried product might even be carried to the container’s edge by the sublimation gas flow.
Optimization Measures:
Increase the product concentration or add a bulking agent.
Group Four
Poor Effect:
Product’s moisture content is below standard.
Cause Analysis:
If freeze-drying ends while there is still frozen ice present, upon removal, the frozen parts may melt into liquid, which is absorbed by the dried product, creating “voids” in the product. If the liquid amount is significant, the dried product may dissolve into the liquid, resulting in a concentrated liquid. Products with this issue will feel cold at the bottom of the container and may not meet moisture content standards even if they appear to be fine.
Optimization Measures:
Increase heat supply during the drying phase, raise the shelf temperature, or use vacuum adjustments. It may also be necessary to extend the sublimation or desorption drying time.
Group Five
Poor Effect:
Product has crystallization patterns.
Cause Analysis:
Slow freezing rates can result in this phenomenon.
Optimization Measures:
Increase the freezing rate to form smaller crystals. However, excessive supercooling may prevent moisture from solidifying into crystals, leading to amorphous formation, which requires annealing.
Group Six
Poor Effect:
A dark ring of color is present around the product.
Cause Analysis:
A temporary increase in vacuum during the sublimation phase can cause this effect.
Optimization Measures:
Ensure continuity in the freeze-drying program and avoid interruptions like power outages.
Group Seven
Poor Effect:
Upper layers of the product are well-dried, while lower layers are poorly dried.
Cause Analysis 1:
The sublimation phase has not ended, and entering the desorption phase early by increasing shelf temperature can cause melting of the lower layers.
Optimization Measures 1:
Extend the sublimation phase.
Cause Analysis 2:
If the product thickness is too great or drying resistance is high, sublimation resistance increases as drying progresses to the lower layers, leading to localized vacuum issues and melting of the lower layers.
Optimization Measures 2:
For large product thickness, adjust the freeze-drying process by lowering shelf temperatures and increasing vacuum in the chamber.
Group Eight
Poor Effect:
Upper layers of the product have poor quality, while lower layers have good quality.
Cause Analysis:
A glassy structure may form on the product’s surface during pre-freezing without annealing. During the sublimation phase, as the product heats up, this glassy structure melts, causing surface shrinkage and rupture. After surface rupture, sublimation proceeds normally for the lower layers, resulting in poor quality of the upper layers and good quality of the lower layers.
Optimization Measures:
If this issue occurs, perform annealing during pre-freezing. Control the annealing duration carefully and ensure that the annealing temperature is below the product’s melting point.
Group Nine
Poor Effect:
Product has poor solubility.
Cause Analysis:
During drying, localized evaporation can lead to concentration in the product, such as internal hard lumps formed during sublimation that cause localized evaporation and concentration.
Optimization Measures:
Reduce shelf temperature, increase vacuum, or extend sublimation drying time to ensure complete sublimation.
Group Ten
Poor Effect:
Product loses vacuum.
Cause Analysis 1:
If the product loses vacuum after being stored, even if the vacuum was good during freeze-drying, it may be due to incompatible or improperly sealed bottle caps, causing air leakage.
Optimization Measures 1:
Replace bottle caps or adjust the tightness of the aluminum caps. Note that rubber stoppers can harden and lose elasticity at temperatures below -20°C, leading to vacuum loss.
Cause Analysis 2:
If the product’s moisture content is too high at the end of freeze-drying, water vapor pressure can cause vacuum loss.
Optimization Measures 2:
Extend the desorption phase or adjust vacuum levels to reduce product moisture content.
Cause Analysis 3:
Leakage of silicone oil or hydraulic oil in the freeze-drying chamber can cause vacuum imbalance and loss.
Optimization Measures 3:
Perform maintenance and inspection of the vacuum system.
This summary covers the most common poor freeze-drying effects encountered during experiments and their analysis.
