Freeze-drying Critical Temperatures

Why are Critical Temperatures important in freeze-drying?

• Freeze-drying above the product critical temperature can lead to:– Loss of physical structure– Incomplete drying (high

moisture content)– Decreased solubility– Reduced activity and/or

stability

• Freeze-drying too far below the product critical temperature can led to:– Poor efficiency– High running costs– Longer cycles than

necessary

Critical Temperatures for freeze-drying • Collapse Temperature (Tc)

– This is the temperature at which the material softens to the point of not being able to support its own structure

• Eutectic Temperature (Teu)– This is the temperature at which the solute material

melts, preventing any structure from forming after the solvent has been removed

• All formulations can be described as having either a Collapse or a Eutectic Temprature.

Effect on formulation components on critical temperature

• Higher molecular weight components such as polymers tend to have higher critical temperatures

• Lower molecular weight components such as salts and small sugars tend to have lower critical temperatures

• Additionally, crystalline/amorphous mix can have a major impact on critical temperature:– Lactose + NaCl (1:1) Tc ≈ -30°C

– Lactose + NaCl (1:0.3) Tc ≈ -45°C

Critical Temperature Determination

• Using extensive knowledge and experience in the freeze-drying industry, BTL has developed two unique analytical instruments. These bring scientific understanding and a rational approach to freeze-drying cycle development:

Lyostat3

Freeze-drying microscope

Lyotherm2

DTA & Impedance Analyser

Lyostat3 – Freeze-drying microscope• Enables real-time observation of the behaviour of your

formulation during freeze-drying• Enables temperature control between -196°C and 125°C

to an accuracy of 0.1°C• By observing the sample structure during drying as the

temperature is raised, the exact point of collapse or eutectic melt can be observed under the microscope.

Lyotherm2 – DTA and Impedance Analyser• Provides and integrated Differential Thermal Analyser

(DTA) and Electrical Impedance Analyser (zsinφ) capability in one instrument

• Can measure critical events in the frozen material that are undetectable by standard thermal analysis techniques

• Enables characterisation of the required freezing parameters that are essential to a successful freeze-drying cycle

Critical Temperature Application

• From analysis of the product we can determine:– The maximum product temperature we can freeze-dry

at before the product is damaged, allowing us to set the primary drying temperature with confidence – by Lyostat3 analysis

– What events occur in the frozen state that affect the freezing stage of the cycle, allowing us to add in any thermal treatment steps such as annealing – by Lyotherm2 analysis

Cycle Development

Case Study

• A customer approached BTL with a product that was being freeze-dried using a cycle borrowed from another product

• They were discarding a high percentage of each batch due to defects occurring during freeze-drying

Product Cycle Development

Case Study

1. Information was obtained on the critical temperatures and thermal behaviour of the product using Lyostat3 and Lyotherm2

2. This data confirmed the lack of suitability of the existing freeze-drying cycle

3. Critical temperature information was used to create a ‘first approximation’ cycle, tailored to the needs of the product.

4. Date from this cycle was used to design a more optimised cycle until a safe and efficient cycle was achieved, minimising cycle time without jeopardising product quality

Product Cycle Development

Case Study

Sample dries well at -50°C, but collapse starts as the temperature is increased to -45.7°C. This can be identified by defects appearing in the dried material

Lyostat3 analysis

As the temperature increases to -39.6°C the structure continues to weaken and collapse becomes more evident

Case Study

The analysis is repeated but with an added heat annealing step – frozen to -50°C, warmed to -15°C and cooled back to -50°C. Defects don’t appear until -31.4°C upon drying

Lyostat3 analysis

As the temperature increases to -30.8°C the structure continues to weaken and collapse becomes more evident

Case StudyLyotherm2 analysis

1

23

4

Case Study

1. Exotherm in DTA and increase in Impedance indicating a stabilisation/rearrangement of the frozen structure

2. Increase in downwards gradient of Impedance curve indicating a softening of the frozen material

3. Onset of a sharp endotherm consistent with the melting of the ice

4. Minimum Impedance indicating complete mobility within the solute structure

Lyotherm2 analysis

Case Study

• From the results of these analyses, we can me the following deductions:– The inclusion of an annealing step resulted in an

increase in the collapse temperature from -45.7°C to -31.4°C, as well as increasing the ice crystal size and networking

– Therefore, the maximum allowable product temperature during sublimation (to avoid collapse) was raised by 14.3°C by annealing, thereby allowing drying at higher temperatures, for a more efficient cycle. The higher the product temperature, the faster the drying rate

Interpretation of analytical results

Case StudyExisting customer cycle – 70 hours

+20°C

-15°C

-50°C-40°C

Shelf Temperature

Product Temperature

Chamber Pressure

1 2 3

Tc = -45.7°C

1 – Freezing 2 – Primary Drying 3 – Secondary Drying

A

A – Product at risk of collapse

Case StudyBTL cycle – 42 hours (including annealing)

+20°C

-15°C

-50°C-35°C

Shelf Temperature

Product Temperature

Chamber Pressure

1 2 3 4

Tc = -31.4°C

1 – Freezing 2 – Annealing 3 – Primary Drying 4 – Secondary Drying

Case StudyEnlarged section of previous graph

+20°C

-15°C

-50°C-35°C

Shelf Temperature

Product Temperature

Chamber Pressure

Tc = -31.4°C

1 – Freezing 2 – Annealing 3 – Primary Drying 4 – Secondary Drying

3

The Sublimation Cooling Effect The lowering of product temperature caused

by the sublimation of ice

Case Study

• From the previous run, we now know:– The extent of sublimation cooling, allowing us to

increase the shelf temperature & chamber pressure as high as possible whilst sublimation cooling keep the product temperature below Tc

– When sublimation was complete in temperature probed samples

– The physical appearance of the cakes produced by the cycle

– Residual moisture was measured in the final product, in order to establish whether the extent of secondary drying was sufficient

The next steps

Case Study

• A freeze-drying cycle with increased efficiency, reduced costs and no product rejects

• Another very happy customer!

End results

What is BTL?

• Biopharma Technology Ltd was set up in 1997 to provide an international service in all aspects of freeze-drying technology.

• Our strength comes from a wealth of experience and knowledge of product formulation and process development, particularly in the field of pharmaceuticals and biotechnology.

Biopharma House, Winnall Valley Road, Winchester SO23 0LD, UK

Tel: +44 (0)1962 841092 Web: www.btl-solutions.net