Lyophilised: what freeze dried peptide powder means

What does lyophilised mean?

Lyophilisation is the technical name for freeze drying. The US FDA describes it as a process in which "water is removed from a product after it is frozen and placed under a vacuum, allowing the ice to change directly from solid to vapor without passing through a liquid phase" (FDA, Lyophilization of Parenteral).

You will see both spellings on labels and certificates: lyophilised in British English and lyophilized in American English. They mean the same thing. "Lyo" and "freeze dried powder" are common shorthand.

What are the stages of freeze drying?

The FDA guide names three linked stages: freezing, primary drying by sublimation, and secondary drying by desorption (FDA).

1. Freezing. The filled vials are cooled until the water in the solution forms ice.
2. Primary drying. Under vacuum, the ice sublimes: it turns into vapour without melting first.
3. Secondary drying. Water still bound to the solid is driven off, leaving a small amount of residual moisture.

The vials are then usually stoppered inside the drying chamber, under vacuum or an inert gas, before they are capped and crimped.

Why are peptides sold freeze dried?

The same FDA guide lists among the advantages of the process the enhanced stability of a dry powder and the removal of water without excessive heating (FDA). Many peptides keep better as a dry solid than dissolved in water, so freeze drying gives a product that can be stored and shipped.

Reference material makers use it for the same reasons. The United States Pharmacopeia has moved many of its peptide reference standards from powders to lyophilised forms "to eliminate the need for the user to determine counter-ions and residual moisture prior to use" (McCarthy et al., Pharmaceutical Research, 2023).

Freeze drying does not remove counter-ions. A peptide purified with trifluoroacetic acid and then freeze dried comes out as a TFA salt, and its weight still includes those salt partners. See counter-ion.

What is the cake, and what can it show?

The solid left in the vial is called the cake. A well-dried cake is usually an even, porous plug that keeps the shape of the liquid it came from. A cake that has shrunk, pulled away from the glass or turned glassy is worth noting.

The FDA guide singles out meltback, a form of cake collapse in which frozen material turned back to liquid instead of subliming. It links meltback to a change in the physical form of the substance or a pocket of moisture, which may mean greater instability (FDA).

Appearance is still a weak signal. A perfect cake can hold the wrong molecule, and a cake shaken into loose powder during shipping can be perfectly fine. The appearance and solubility method describes how a laboratory records what it sees.

Does lyophilised mean the powder holds no water?

No. Freeze dried powder keeps some residual moisture, and the FDA expects manufacturers to set moisture limits backed by stability data. Peptide powders are often hygroscopic as well, so they take up water from the air once a vial is opened. Water is measured by Karl Fischer titration, and the lesson on water content and stability goes further.

What does lyophilised not tell you?