Laboratory Guide

What Is Lyophilisation?

How freeze-drying removes water from a frozen solution, why peptides are supplied as a dry cake, and what the cake’s appearance can and cannot show.

Updated 29 September 2026

[Research Use Only] This guide is for controlled laboratory research workflows only. It is not for human or veterinary use and does not provide applied-use guidance.

Drying without a liquid phase

Lyophilisation, or freeze-drying, removes water from a product after it has been frozen and placed under vacuum, so that the ice turns directly into vapour without passing through a liquid phase. It is three processes in sequence, and each can go wrong in its own way.

Stage	What happens	What it has to avoid
Freezing	The solution freezes; the ice crystals set the size of the pores the dried solid will have	A poorly chosen rate: slow freezing gives larger crystals and pores, but can shift the concentration of components
Primary drying	Under vacuum, with gentle heat, the ice sublimes	Warming past the product’s critical temperature; FDA guidance describes holding it at least 4–5 degrees below the eutectic point
Secondary drying	Remaining water is removed from the solid by desorption	Leaving more moisture than the product’s stability data support

The freeze-dryer is not a sterilising step. In pharmaceutical practice the solution is passed through a 0.22 micron bacteria-retentive filter and filled aseptically before drying, and vials are stoppered inside the chamber, commonly after a backfill of nitrogen or sterile air.

Why peptides are supplied dry

In solution, water is a reactant as well as a solvent. Hydrolysis is the breaking of a chemical bond by water, and a dissolved peptide is also exposed to decomposition in solution and to air–water interfaces; the dried form avoids those pathways. Removing water also restricts movement: molecules held in a rigid, glassy solid meet each other less often, which slows aggregation. Freeze-drying is the most common drying method for proteins and peptides, and an intact cake with very low residual moisture is associated with good long-term stability.

What the cake is

The solid left in the vial is called the cake. It is the skeleton of the frozen solution — the dissolved solids, arranged around the spaces the ice crystals occupied — so its size reflects how much solid was dissolved and how it was filled and frozen, not how much active substance it holds. Formulated medicines often add bulking agents: one authorised growth hormone product lists 5 mg of the protein and 30 mg of mannitol per vial. A vial holding a few milligrams of peptide and nothing else may dry to a thin layer rather than a tall cake.

What appearance does and does not tell you

Cake appearance is inspected in pharmaceutical manufacture, and it matters, though not always as readers assume. A non-ideal appearance is sometimes an inherent feature of a formulation, its presentation or its drying process, with no impact on quality. The case that does signal a problem is meltback: collapse caused by incomplete sublimation, where part of the product passed through a liquid state. It can leave a changed physical form or a pocket of moisture, both associated with greater instability and poorer solubility.

What you see	What it can indicate	What it cannot tell you
An intact, uniform cake	Drying completed without visible collapse	Identity, purity or the amount of peptide present
Cracks, shrinkage or a cake pulled away from the glass	Can be a feature of formulation or process, with no effect on quality	That nothing has changed chemically
Loose powder or fragments	A brittle solid broken up, for example in handling	Anything about the chemistry of the material
A collapsed or glassy residue	Possible meltback, with retained moisture	How much degradation, if any, has occurred
A thin film or barely visible deposit	Little solid in the vial, such as a small mass of peptide with no bulking agent	That the vial is under-filled
Visible liquid or a wet, sticky residue	The solid is no longer dry	Why, or what it has done to the peptide

Every entry in the right-hand column is a question for the certificate and for analysis. Appearance can raise a question about a vial; it cannot answer one about its contents.

Storing lyophilised material

The storage condition on a label is the one at which the product’s stability was established, and for a dried solid the variable that most undermines it is water.

Keep the vial sealed until it is needed; the closure is what separates a dry solid from room humidity
Store at the stated temperature, and avoid cycling in and out of it
Let a cold vial reach room temperature before opening, so that moisture does not condense on the solid
Treat dissolved material as a different material with a different stability, as the storage guide explains

Research Checklist

Confirm batch identity and records.
Document all preparation inputs.
Keep use within controlled laboratory workflows.
Do not infer applied-use suitability from guide content.

Frequently Asked Questions

Is lyophilised the same as freeze-dried?

Yes. Lyophilisation is the technical name for freeze-drying: water is removed from a frozen product under vacuum by sublimation, and desorption then removes the water that remains.

Does a cracked or powdery cake mean the material has degraded?

Not by itself. A cake is a brittle solid, and cracks or powder are physical changes that can have no effect on quality. Collapse or a wet-looking residue is different, because it suggests the solid did not stay dry. Neither appearance establishes what the vial contains; that is the certificate’s job.

Why is there so little visible material in some vials?

Because the cake is built from whatever solids were dissolved. Medicines often include bulking agents that make up most of the visible mass, while a few milligrams of peptide alone can dry to a thin layer. The size of a cake is not a measurement of its contents.

Does freeze-drying sterilise a product?

No. Sterility comes from filtering the solution and filling it aseptically before drying. The freeze-dryer itself has to be kept free of contamination; it is not a sterilising step.

Sources

The technical statements in this guide are drawn from the following. Where a definition is contested or a figure depends on method, the guide says so rather than picking one.

Lyophilization of Parenteral (7/93): Guide to InspectionsU.S. Food and Drug AdministrationUsed for the definition of freeze-drying and its three stages, sterile filtration through a 0.22 micron filter and aseptic filling before drying, stoppering in the chamber and nitrogen or sterile air backfill, the effect of freezing rate on ice crystals and voids, holding product at least 4–5 degrees below the eutectic point, moisture specifications tied to stability data, inspection of cake volume and appearance, and meltback with its association with moisture, instability and poor solubility.
Drying Technologies for the Stability and Bioavailability of BiopharmaceuticalsPharmaceutics, via PMC, National Library of MedicineUsed for freeze-drying as the most common drying method for proteins and peptides, intact cakes with very low residual moisture and good long-term stability, water-driven mobility and the glassy matrix, the influence of moisture content and temperature on degradation, freezing and dehydration stresses, and sugar stabilisers.
Instability Challenges and Stabilization Strategies of Pharmaceutical ProteinsPMC, National Library of MedicineUsed for hydrolysis as the breaking of a chemical bond by water, for the dried form avoiding hydrolysis pathways, in-solution decomposition and air–water interactions, and for glycine as a bulking agent in lyophilisation.
Lyophilized Drug Product Cake Appearance: What Is Acceptable? (Patel SM et al., 2017; PMID 28341598)Journal of Pharmaceutical Sciences, via PubMedUsed for cake appearance as an attribute that may or may not be critical to quality, and for non-ideal appearance sometimes being an inherent characteristic of formulation, presentation and process with no impact on quality.
Zomacton (somatropin) for injection: prescribing informationU.S. Food and Drug AdministrationUsed only as an example of formulation: a sterile, white lyophilised powder in which a 5 mg vial contains 5 mg of somatropin and 30 mg of mannitol.