What are critical temperatures and why are they important to the freeze drying process? An introduction to formulation analysis.

1. Freeze-drying Critical
Temperatures

2. Why are Critical Temperatures
important in freeze-drying?
• Freeze-drying above the • Freeze-drying too far
product critical below the product critical
temperature can lead to: temperature can led to:
– Loss of physical structure – Poor efficiency
– Incomplete drying (high – High running costs
moisture content) – Longer cycles than
– Decreased solubility necessary
– Reduced activity and/or
stability

3. 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.

4. 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

5. 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 Lyotherm2
Freeze-drying microscope DTA & Impedance Analyser

6. 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.

7. 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

8. Critical Temperature Application
Cycle Development
• 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

9. Case Study
Product Cycle Development
• 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

10. Case Study
Product Cycle Development
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

11. Case Study
Lyostat3 analysis
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
As the temperature increases to -39.6 C
the structure continues to weaken and
collapse becomes more evident

12. Case Study
Lyostat3 analysis
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
As the temperature increases to -30.8 C
the structure continues to weaken and
collapse becomes more evident

13. Case Study
Lyotherm2 analysis
1
2
3
4

14. Case Study
Lyotherm2 analysis
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

15. Case Study
Interpretation of analytical results
• 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

16. Case Study
Existing customer cycle – 70 hours
1 2 3
+20 C A
-15 C
-40 C Tc = -45.7 C
-50 C
Shelf Product Chamber
Temperature Temperature Pressure
1 – Freezing 2 – Primary Drying 3 – Secondary Drying
A – Product at risk of collapse

17. Case Study
BTL cycle – 42 hours (including annealing)
1 2 3 4
+20 C
-15 C
Tc = -31.4 C
-35 C
-50 C
Shelf Product Chamber
Temperature Temperature Pressure
1 – Freezing 2 – Annealing 3 – Primary Drying 4 – Secondary Drying

18. Case Study
Enlarged section of previous graph
3
+20 C The Sublimation Cooling Effect
The lowering of product temperature caused
by the sublimation of ice
-15 C
Tc = -31.4 C
-35 C
-50 C
Shelf Product Chamber
Temperature Temperature Pressure
1 – Freezing 2 – Annealing 3 – Primary Drying 4 – Secondary Drying

19. Case Study
The next steps
• 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

20. Case Study
End results
• A freeze-drying cycle with increased efficiency,
reduced costs and no product rejects
• Another very happy customer!

21. 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.

22. Biopharma House, Winnall Valley Road, Winchester SO23 0LD, UK
Tel: +44 (0)1962 841092 Web: www.btl-solutions.net