La phase de séchage : pourquoi un cycle de classe B n'est pas terminé quand la pression revient à zéro

Two separate jobs inside one cycle

A Class B sterilization cycle is often described as a single process, but it’s actually solving two distinct problems in sequence. The sterilization phase — fractionated pre-vacuum pulses, steam injection, holding at temperature and pressure for a validated time — is what actually kills microorganisms and their spores. Once that phase ends and the chamber pressure returns to atmospheric, the load is sterile in the microbiological sense. But it’s also, at that exact moment, sitting in a chamber full of condensed moisture, and the pouches or wraps around it are typically damp. The drying phase that follows — a final vacuum stage that evaporates and extracts residual moisture from the load and the chamber — is a separate mechanical problem: getting the water out, not killing anything further.

Why a wet pack fails even though the cycle succeeded

Sterility and dryness are validated as two separate requirements precisely because a load can pass the first and fail the second. A pouch or wrap that comes out of the chamber damp doesn’t stay sterile for long — moisture is exactly the condition that lets environmental microorganisms migrate through a porous sterile barrier material via capillary action, a failure mode known in the field as a “wet pack.” The load was genuinely sterile the moment the holding phase ended; by the time a damp pack has sat in storage, handling, or transport, that’s no longer a safe assumption. This is why EN 13060 includes a dryness test as part of Class B type-testing — the sterilizer has to demonstrate it removes moisture to a defined limit on a specified test load, not just that it reaches sterilizing temperature and pressure.

Where drying time actually gets shortened

Drying phase duration isn’t fixed across all cycles or all loads — most Class B sterilizers let an operator select or adjust drying time, and that’s exactly where the risk concentrates:

• Heavier or more densely packed loads dry more slowly than the light test load a sterilizer was originally validated against — a full tray of wrapped instruments has more mass and surface area holding moisture than a single test pouch.
• A drying phase manually shortened to speed up throughput on a busy day removes exactly the margin the validated cycle was built with, without any visible warning that the load is now leaving the chamber wetter than intended.
• A chamber or generator affected by feed water quality issues can produce more residual moisture to begin with, effectively demanding more from the drying phase than a well-maintained unit would need.

What this means in practice

• Drying time should be matched to actual load type and density, not treated as a fixed setting to minimize — heavier loads may need the longer end of a sterilizer’s drying range, not the shortest available option.
• A visibly damp pack coming out of the chamber is a processing failure, not a cosmetic issue — it should be treated the same as a failed biological or chemical indicator: the load doesn’t go into storage or use.
• Drying performance is part of what periodic sterilizer maintenance and validation should confirm, not just cycle temperature and pressure — a machine that reaches the right sterilization parameters but consistently under-dries loads is still failing at its job, even though every temperature log looks correct.