A wet load is a steam sterilizer load that comes out of the cycle with visible water or residual moisture on surfaces, in cavities or inside packaging. In pharmaceutical manufacturing, wet loads and wet packs are deviations to investigate, because residual condensate can weaken the sterile barrier, open a route for recontamination or put the load outside the acceptance criteria set during validation.

This article is written for production, engineering and validation teams in pharmaceutical and biotech plants running saturated steam autoclaves under GMP. Hospital sterile processing deals with the same physics but works to different guidance and different loads.

The causes fall into two groups: too much water entering or forming during the cycle, or water that the drying phase cannot remove. Telling them apart comes first, because a longer drying phase only fixes the second group.

For the full sequence of cycle phases, including development and validation, see our guide on sterilization cycles in cGMP autoclaves. Here the focus is one symptom and how to trace it back to its cause.

What a wet load is, and how it differs from a wet pack

Condensate is normal and necessary during the cycle: steam transfers its heat to the load by condensing on it. The problem is condensate that is still on the load after the drying phase, when the door opens.

Two terms are used in English, and they are close but not identical. Wet load describes any load that comes out wet, including unwrapped items such as metal parts, tubing or filter housings. Wet pack refers specifically to moisture on or inside a wrapped item or sterile barrier system. In hospital sterile processing, “wet load” is also commonly used when more than one pack in the same load is wet.

How much residual moisture matters depends on what is being sterilized. An unwrapped metal part, a silicone hose, a filter cartridge and a component sealed in porous packaging do not carry the same risk:

• Porous packaging and sterile barrier systems. A wet wrap can carry microorganisms from the outside to the inside by capillary action (wicking). This is where moisture most directly affects how well the load stays protected after the cycle.
• Product-contact components. Here residual moisture can have effects of its own. A typical example is elastomeric stoppers for lyophilized products, where water retained by the stopper can migrate into the product.
• Every validated load. A load that misses the acceptance criteria defined during validation is a deviation to investigate, with the time and documentation that come with it.

The same issue runs through component preparation upstream of filling, where washing, sterilization and drying of stoppers can become the line constraint, as explained in our article on how to tell if stopper preparation is limiting your filling line.

EU GMP Annex 1 (2022 revision) addresses dryness directly. Items to be sterilized, other than products in sealed containers, should be dry when they leave the sterilizer, and load dryness should be confirmed by visual inspection as part of sterilization process acceptance (paragraph 8.56). For porous (hard goods) cycles, each item should be inspected on removal for damage, packaging integrity and moisture, and any item not fit for purpose should be removed from the manufacturing area and investigated (paragraph 8.57). PIC/S adopted the same revised Annex 1, so these expectations reach well beyond the EU.

Where the water comes from: the condensate balance

Understanding a wet load starts with a simple calculation. Every kilogram of material heated from room temperature to sterilization temperature absorbs heat, and that heat comes from steam condensing on its surface.

The amount of condensate depends on load mass, the specific heat of the material and the temperature rise. Theoretical values for heating from 20 °C to 121 °C (68 °F to 250 °F):

Simplified calculation: sensible heat absorbed divided by the latent heat of condensation of steam at 121 °C (about 2,200 kJ/kg). These figures explain the mechanism; they are not process data. The real balance also depends on load geometry, heat losses, chamber and jacket temperature, and the duration of each phase.

A cart carrying 40 kg (about 88 lb) of stainless steel in baskets, shelves and containers therefore produces about 0.9 kg (about 2 lb) of water just to heat up. That water has to go somewhere: part of it drains to the chamber floor, part of it stays on the load and has to be re-evaporated during drying.

Then comes the second, less intuitive mechanism. Under vacuum drying, pressure falls and water evaporates at a lower temperature, but it still needs heat to evaporate. That heat comes partly from the load, which cools down as it gives it up, and partly from the chamber walls heated by the jacket.

Two practical consequences follow:

• The heat stored in the load cannot, on its own, re-evaporate all the condensate it produced. One kilogram of stainless steel cooling from 121 °C to 50 °C (250 °F to 122 °F) releases enough heat to evaporate about 15 g of water, less than the 23 g condensed during heating. This is why drainage matters as much as drying: water trapped in a cavity or inside packaging is the water the cycle struggles to remove.
• What counts is the effective thermal mass of each item, not only the material. A light or thin-walled item stores little heat, and if the material is a poor conductor, as plastics and silicone are, it also releases that heat slowly. Condensate reaching it, often dripping from metal masses nearby, finds little heat available to evaporate.

First group of causes: too much water in the cycle

In these cases the drying phase may be working correctly, but it has to remove more water than was assumed when the cycle was developed. Extending it lengthens the cycle without removing the source of the problem.

Wet steam and condensate in the supply line

If steam reaches the chamber carrying too much liquid water, that water lands directly on the load. It is one of the first things to check when moisture is spread across the whole load and repeats from cycle to cycle.

The reference parameter is the steam dryness value, also called dryness fraction. Annex 1 lists it, together with non-condensable gases and superheat, among the pure steam parameters to be assessed periodically against validated values (paragraph 6.17). EN 285, written for large steam sterilizers in healthcare, sets requirements and test methods for these parameters and is widely used as a technical reference in pharmaceutical manufacturing as well.

Typical sources of wet steam are:

• condensate separators that are missing, undersized or in poor condition on the supply line
• failed or blocked steam traps that let water accumulate in the pipework
• uninsulated, long or poorly sloped pipe runs with low points where steam condenses before reaching the chamber
• carryover from the generator, for example because of a high water level or sudden peaks in demand

In pharmaceutical plants the autoclave is normally fed with clean steam, and its quality depends on how it is generated and distributed. The differences from plant steam are covered in our article on the differences between clean steam and industrial steam in the pharma industry, while generator design choices are compared in the one on direct or indirect heating in cGMP clean steam generators.

Chamber, doors and jacket below temperature

The autoclave’s own surfaces are part of the balance. If walls, doors or jacket are colder than expected when steam is admitted, part of the steam condenses on them and adds to the condensate generated by the load.

Items in contact with walls or doors can pick up local condensate. When wet loads appear systematically, it is worth checking how the chamber behaves thermally both in the phases before exposure and during drying.

Load mass and loading pattern

More mass means more condensate, as the balance shows. For the same mass, though, the way the load is arranged decides how much water stays on it. Annex 1 asks for loads designed to support effective air removal and to be free draining, so that condensate does not build up (paragraph 8.61). The most common mistakes:

• Excessive density. Packs that are compressed or too close together hinder both air removal and the escape of moisture during drying.
• Containers facing upward. Trays, beakers and concave surfaces act as small reservoirs. They should be tilted or inverted, as defined in the validated loading pattern.
• Metal masses above porous or plastic items. Condensate forming on the metal drips onto the item below, which has less heat available to re-evaporate it.

Geometries that trap condensate

Tubing, flanges, blind cavities and concave parts need specific attention. Vacuum drying removes an exposed film of water well, but struggles with water standing in a point that cannot drain.

In these cases simply extending the vacuum may have limited effect. The fix lies in the orientation of the item in the loading pattern and, where possible, in supports that help it drain.

Chamber drain

Condensate that can leave by gravity should not be left to the drying phase. A partially blocked drain, a clogged drain strainer or sump screen, a faulty chamber trap: these are simple causes to check and often overlooked. The typical sign is moisture concentrated on the lowest shelves.

Loads that go in cold or already wet

A load coming from a cold area generates more condensate than expected. A load that goes in already wet, for example components not fully dried after washing, brings into the chamber water the cycle was not designed to handle.

Second group of causes: drying that cannot remove the water

Here the amount of condensate is compatible with the cycle, but the drying phase cannot remove it completely. This is where adjusting drying parameters is most directly relevant, once upstream causes have been ruled out.

Drying phase parameters

Vacuum drying works if it reaches a low enough pressure, holds it long enough and has enough heat available. Cycle-related causes are therefore:

• vacuum not deep enough for what the load requires
• drying time too short for the actual load, typical when the cycle was developed on a lighter load
• jacket below temperature during vacuum, which reduces the heat available
• no air pulses where the load would need them: alternating vacuum phases with sterile filtered air admissions helps carry moisture out of porous materials

Vacuum system performance

If pressure does not fall to the set level, evaporation slows down. The cause may lie in the vacuum pump, a chamber leak, valves or gaskets. A useful check is to compare the vacuum reached in recent cycles with historical data: a gradual decline points to wear or a leak, not to a recipe problem.

On liquid ring vacuum pumps, the achievable vacuum also depends on the seal water temperature: water that is too warm limits the lowest pressure the pump can reach.

Packaging and wrapping materials

With wraps, filters and textiles, water does not only have to evaporate: the vapor formed must also be able to get out of the packaging. Number of layers, permeability of the wrap and packing density all slow this down.

This is why even an apparently minor change, such as a new packaging supplier with the same cycle, should be assessed against the validated cycle: it can be enough to make the problem appear.

A separate case: recondensation after unloading

A load that is dry in the chamber can become wet afterwards if it is placed on cold surfaces or exposed to air currents while still hot. The sign is clear: the load is dry when the door opens and moisture appears only later.

How to diagnose a wet load: where the water is

The position, distribution and repeatability of moisture are the most useful clues. Before changing the cycle, record exactly where the water appears, on which items and in which cycles.

A first distinction is between moisture outside the pack and moisture inside it. Water on the outer surface usually arrives from somewhere: wet steam, drips from items above, cold chamber surfaces. Water inside the pack usually formed on the item itself and could not get out, which points to packaging, configuration or drying.

Measure residual moisture, don’t just look for it. Visual inspection, which Annex 1 requires for load acceptance, tells you whether water is present but not how much. Weighing representative items before and after the cycle turns an impression into a repeatable figure you can compare over time. EN 285 includes load dryness tests with acceptance criteria expressed as a percentage increase in mass. On LAST Technology installations these tests are not part of FAT and SAT: they are carried out during PQ, by the user, on the actual production load.

One clarification: the Bowie-Dick test checks air removal and steam penetration under defined test conditions, not the dryness of a production load. A passed test does not rule out a wet load problem.

Loads coming out wet from your autoclave? Describe the load, the cycle and where the water appears: the LAST Technology team can review the application and help you assess the possible causes. Write to info@lasttechnology.it.

What to do when a wet load is found

The decision on the load belongs to the user’s quality system, based on the criteria defined during validation. In practice, investigations are faster when a few steps are taken straight away:

• keep the load out of use until the quality unit has decided, as Annex 1 expects for items not fit for purpose
• record where the moisture is, on which items, inside or outside the packaging, and keep the cycle printout or electronic record
• check whether the cycle parameters, the vacuum reached and the drying phase ran as validated
• avoid fixing the symptom by simply rerunning the load with a longer drying time before the cause is understood

Solutions: fix the cause, not the symptom

Solutions follow the diagnosis. Changing the cycle when the cause is the steam, or the other way round, adds time without solving the problem.

On the steam supply

• run steam quality tests (dryness, non-condensable gases, superheat) at the autoclave supply point
• check separators and steam traps on the line, and the slope of horizontal runs
• insulate exposed sections and remove low points where condensate collects

On the drying cycle

• review vacuum depth and duration against the actual load, not the nominal one
• introduce or optimize sterile filtered air pulses for porous loads
• check that the jacket stays at temperature during drying
• trend the vacuum reached over time, using cycle data as a maintenance indicator

On the load and loading patterns

The most effective solution often requires no change to the machine, just a better loading pattern:

• standardize orientation, density and weight distribution, and treat the loading pattern as part of cycle validation
• place metal masses below, not above, porous and plastic items
• use baskets and supports that drain, avoiding solid surfaces under the load
• manage unloading: cooling time and area, no contact with cold surfaces

In the design of the autoclave

Part of condensate management is decided at the design stage, before the cycle is even developed. On LAST Technology RSA autoclaves, a dimpled jacket surrounds the chamber for uniform heating, and the chamber floor converges toward the drain sump, with continuous condensate removal throughout the cycle. The software includes dedicated programs for porous loads and for rubber stoppers, plus automatic sterilization of the vacuum breaker filter with integrity testing.

Because the machines are built to order, the drying phase can be configured around the customer’s load. Elements that can be adjusted include:

• vacuum level
• number of air pulses
• jacket heating
• fans inside the chamber
• heat exchangers for the compressed air

A case from practice: filter cartridges packed in Tyvek

A customer needed to sterilize filter cartridges packed in Tyvek pouches and, with the autoclaves used before, could not obtain a load that was correctly sterilized and dry. The load combines two of the elements described above: a filtering component and packaging that the vapor formed during drying has to pass through to escape.

On the new autoclave supplied by LAST Technology, a cycle specific to this load was configured, and it solved the problem.

Changing the drying phase of a validated cycle

The drying phase follows exposure and is normally not included in the lethality calculation. That does not make it free of constraints: the cycle is validated as a whole, and any change to vacuum, duration, pulses or loading pattern should go through change control.

The points to assess are the effect on sensitive materials, the increase in cycle time, the repeatability of drying and the location of the hardest-to-sterilize point, which can shift with a different loading pattern. If load dryness is among the PQ acceptance criteria, the change may require a dedicated verification.

Conclusion

A wet load is almost always the result of a condensate balance that does not close: too much water coming in, with the steam or with the load, or too little capacity to drain it and re-evaporate it. Diagnosis starts from three questions, in this order: where the water is, when it forms, and why it cannot leave the load.

When the problem is structural, meaning heavy loads, difficult materials or cycle time constraints, the answer also lies in the design of the machine. LAST Technology cGMP saturated steam autoclaves are designed around the customer’s load, including condensate management and the drying phase.

Technical references

• EU GMP Annex 1 (2022), Manufacture of Sterile Medicinal Products: paragraph 6.17 on pure steam quality, paragraphs 8.56 and 8.57 on load dryness and inspection, paragraph 8.61 on free-draining loads. In force since 25 August 2023, also adopted in the PIC/S GMP Guide
• EN 285:2015+A1:2021, large steam sterilizers, written for healthcare: steam quality requirements and load dryness test methods, widely used as a technical reference
• ISO 17665:2024, development, validation and routine control of moist heat sterilization processes, written for healthcare products; its principles also apply to pharmaceutical sterilization

Massimo Castellarin President & CEO