Overview of Freezing in Freeze drying

Hi pharma folks,

Till now we have gone through the freezing concept in freeze drying of pharmaceuticals.

Lets have a glance on what we have covered so far.

 If you can freeze it, you can freeze dry it. That is the importance of freezing in lyophilization.

Freezing phase: The principal dehydration step. Most of the solvent (typically water) is separated from the solutes to form ice.

Ok, lets freeze our formulation (In the form of aqueous bulk solution) by cooling in a lyophilizer at sub zero temperatures. 

As we all known from school days, water freezes at 0°C, let us keep the bulk solution on the shelves whose temperature is around -10°C and hold it for some time lets say 60 min. 

Guess, what might be the observation???

There is partially frozen vials.........

Why??? Because of freezing point depression.

The pure water freezes at 0°C. But the addition of Active ingredient, excipient (Bulking agent, preservative, pH adjuster, Buffer etc) may cause the depression of freezing point of the resulting bulk solution which is Pure water +Active Ingredient+ Excipient. 

How the freezing point depression happens?

As we all know that, Freezing point depression is a colligative property observed in solutions that results from the introduction of solute molecules to a solvent. The freezing points of solutions are all lower than that of the pure solvent and is directly proportional to the molality of the solute. 

Freezing point depression is nothing but difference between freezing point of the pure solvent and solution. It is calculated by the formula provided below,

 ΔT_f =T_f(solvent)− T_f(solution)=K_f×m 

Where,

ΔT_f  is the freezing point depression,

T_f  (solution) is the freezing point of the solution,

T_f  (solvent) is the freezing point of the solvent,

K_f  is the freezing point depression constant, and

m is the molality of non volatile solutes.

The freezing point depression constant of solvents per mole of a solute dissolved were provided below from literature. 

Solvent	Kf
Water	1.86
Acetic acid	3.90
Benzene	5.12
Phenol	7.27

The freezing point of a solution is less than the freezing point of the pure solvent. This means that a solution must be cooled to a lower temperature than the pure solvent in order for freezing to occur.

The freezing point of the solvent in a solution changes as the concentration of the solute in the solution changes (but it does not depend on the identity of either the solvent or the solute(s) particles (kind, size or charge) in the solution). 

Another reason for the freezing of pharmaceutical formulations at very low temperatures than 0°C is Super cooling.

·         Supercooling is the process of chilling a liquid below its freezing point, without it becoming solid. 

·         A liquid below its freezing point will crystallize in the presence of a seed crystal or nucleus around which a crystal structure can form.

·         In a clean room environment of pharmaceuticals, there is less probability of seed crystal (sub micron particles, undissolved drug particle etc).

·         Also this is the reason for difference in freezing temperature of lab scale and pilot scale.

·         This difference of temperature between  equilibrium freezing point and actual nucleation temperature (The point at which ice crystals begins to form) is known as Degree of super cooling. 

·         The degree of super cooling may change with cooling rate (low freezing rates, higher super cooling). Degree of super cooling  at Lab scale typically, in the range of -5°C to -15°C (Particulate contamination), and at Production scale Can be as low as -40°C (sterile environment). 

·         Also, there is impact of Rate of freezing on Crystal size of Ice. 

·         The crystal size formed during freezing can significantly affect the dissolution rate of the dried material.

A fast ice growth also helps to prevent the denaturation of proteins (If present) which may result from prolonged exposure to strong concentrations of salts because of slow ice growth.

Rapid cooling results in small ice crystals, useful in preserving structures to be examined microscopically, but resulting in a product that is, more difficult to freeze dry.

Slower cooling results in large ice crystals and less restrictive channel in the matrix during the drying process.

The main pores in the solid residue after freeze drying are those left by the sublimation of pure ice and they form the principal channels for the escape of vapor. 

After completion of freezing, that is after complete solidification of formulation, once ice sublimation begins, then the remaining structure should be maintained intact and should not collapse. 

This depends on the size of ice crystals (Large/small) and type of solutes in the formulation (Crystalline/amorphous/ partially crystalline). 

A frozen formulation contains, Ice crystal and solutes. The space between ice crystals and solutes is called Interstitial space (some authors described as  Interstitial state also). 

For the crystalline solutes, the interstitial material consists of a mixture of eutectic ice and crystalline solute. when ice is removed by sublimation, a crystalline solid with very little water is left.

For amorphous system, the interstitial glassy material must be rigid enough to support its own weight after the ice is removed in order to keep the micro-structure established during freezing. 

Examples of Crystalline substances:

• Glycine
• Mannitol
• Sodium carbonate
• Dibasic sodium phosphate
• Citric Acid

Examples of Amorphous substances:

• Dextran
• Fructose
• Gelatin
• Sorbitol
• Maltose
• Trehalose
• Lactose
• Glucose

For some solutes, it is observed that, there is incomplete crystallization of solutes during freezing stage and the complete crystallization occurs at sublimation step. This results in the breakage of vials. Examples Formulations containing Mannitol, glycine.

For those formulations, it is required to allow them for sufficient time at sufficient temperature to crystallize completely during freezing step only.   

Technically the phenomenon of holding the product at a temperature above the final freezing temperature for a defined period to crystallize the potentially crystalline components (usually, crystalline bulking agent) in the formulation during the freezing stage.

The temperature of Annealing is always above (Warmer) the Glass transition temperature of formulation and below (Colder) the Melting temperature (Eutectic melt) of formulation. 

Literature reveals that annealing has impact n primary drying time and product appearance, reconstitution time etc..

That’s it guys, this is my perception on freezing in pharmaceutical freeze drying.

Lets close today's write up and will catch you soon.

Till then,

Teja Ponduri signing off…….