In Summary
- Heat can be used as a control against surfaces or niches that are suspected of harbouring L. monocytogenes. Especially in older equipment which are difficult to disassemble.
- For small pieces of equipment, submersion in a hot (>80°C) water bath with detergent will likely be effective.
- Larger equipment which is water sensitive may be heat treated in an oven. For equipment which is too large for an oven, a treatment may be to cover with a tarpaulin, protect/remove any electronic components, and treating with steam.
- Alternatives may be to treat with ozone or UV-C, although their effectiveness will need to be validated by surface material.
Below we provide information on the decontamination of equipment using heat. This information comes mostly from a review by Tompkin (2002) and validated modelling by Chmieliwski and Frank (2004).
Tompkin (2002) provided advice for the decontamination of processing equipment that was known or suspected to be contaminated with L. monocytogenes. It is apparent that to sanitise equipment harbouring L. monocytogenes, it would usually be sufficient to disassemble the equipment then clean and sanitise the parts using specialist chemicals. An important finding of the study was that although there is merit in exceptionally thorough cleaning using increased concentrations of chemicals, such an approach was not always effective. In older equipment, that may not have been designed for effective decontamination, there can be L. monocytogenes niches that are poorly penetrated by sanitisers. Examples of such niches includes hollow or ball bearing-filled rollers on conveyor belts, equipment support rods, the space between metal-to-metal joins, worn or cracked rubber seals around chiller doors, particularly if the chillers are prone to condensation, equipment controls and door handles and places that are wet.
Figure 1: Examples of niches with the potential to contain plant resident strains of L. monocytogenes. A) metal-to-metal joints that are not fully welded; B) wet and corroded collar and screws.
On those occasions when a chemical treatment was ineffective, Tompkin (2002) advised either the replacement of the contaminated item, or an equipment-specific application of heat. For small pieces of suitable equipment, immersion in a hot (>80°C) water bath containing detergent was reported as reliably effective (Tompkin 2002). For equipment that was larger, contained water-sensitive electronics or greased parts, heat could be applied by moving the equipment into a product-baking oven. Chmieliwski and Frank (2004) also assessed L. monocytogenes kill using heat using a dual methodology of experimental observation and modelling. Models for L. monocytogenes decline were constructed from labortatory test results, and the models used to predict total death with a corresponding confidence. The model predictions were then validated using laboratory testing. In broad agreement with Tomkin (2002), to obtain a 75% probability for the total inactivation of L. monocytogenes growing in a five-strain biofilm, a hot water heat treatment of 80°C for 11.7 min was required (Chmieliwski and Frank, 2004).
For equipment that was too large for an oven, Tompkin (2002) advised that any electronics were removed, the equipment was covered in a heat resistant tarpaulin and steam applied from the bottom. The previous experiences of the project team were that inexpensive (£20-30) steam generators of the type used to remove domestic wallpaper performed exceptionally well when used to decontaminate industrial equipment. Tompkin considered that a target temperature of 71°C should be achieved for at least 20 minutes in order to achieve effective equipment decontamination.
Harada and Nascimento (2021) also assessed dry sanitising methods on L. monocytogenes biofilms. Their conclusions were that UV-C and ozone had the best results against L. monocytogenes biofilm on polypropylene, although dry heat was the most effective against L. monocytogenes biofilm on stainless steel.
