While no pathogen has been discovered that is immune to ultraviolet germicidal irradiation, biofilms present a unique challenge due to their structure. They allow harmful microbes a way to protect themselves against ultraviolet penetration and are ubiquitous on Earth.
Biofilms are microbial communities in a wet complex matrix attached to a solid surface. The microbes produce polysaccharides that allow them to attach to a surface, and they can be found all over the planet, including fossilized biofilms.
Biofilms in the Food Industry
One of the areas where biofilms pose a great threat is in the food and beverage industry, where contamination can spread throughout a facility.
According to Ultraviolet Light in Food Technology, "Biofilms pose a very real threat in the food industry, and contact of foods with biofilms invariably results in contamination as cells are shed from the biofilm to the food." Once a biofilm comes into contact with food, contamination can easily occur as pathogens transfer from the biofilm and then adhere to the food item.
As pathogens like E. coli and Salmonella enterica establish a biofilm on a food item, it can be very difficult to completely remove, other than by physically removing the biofilm from the surface. But the more surfaces that have been contaminated, the harder they can be to track down and eliminate entirely.
Biofilms can develop on a wide range of surfaces in the food industry, including the following:
- Air handling system surfaces, including cooling coils
- Architectural surfaces (ceiling, floors, walls)
- Cooking utensils and equipment
- Food and beverage containers
- Liquid storage tanks
They can also originate from a wide variety of sources, including the following:
- Animal feces
- Animal waste
- Irrigation water
- Municipal water
- Processing facilities
Pathogens from these sources can come into contact with food and quickly establish biofilms.
Biofilms on Commercial Cooling Coils
Cooling coils in commercial air handling systems can become a breeding ground for microbes, including build-up of biofilms. Due to the humidity of these areas, they can become a dangerous source of contamination in the ventilation system.
According to the Ultraviolet Germicidal Irradiation Handbook, "a biofilm on a cooling coil reduces the space between the coils fins (increasing pressure loss and reducing airflow) and decreases the heat transfer coefficient." Keeping them sterile with UV radiation can maintain design performance and increase energy efficiency.
It is the continuous exposure of the cooling coils to UVC light that makes it an effective strategy for controlling biofilm build-up, despite the protection microbes in a biofilm have to ultraviolet radiation.
Limited UV Effectiveness
Biofilms illustrate one of the limits of ultraviolet germicidal irradiation, as UVC light has a limited ability to penetrate biofilms, especially when time of exposure is limited. The microbes within biofilms are protected against environmental stresses, including the radiation from UV bulbs.
While UV light is effective against all pathogens, the light can only penetrate the top layers of the film. Microbes deeper in the biofilm are protected against the UV rays, and biofilms can endure high levels of ultraviolet light with little germicidal effectiveness.
This is why UVC disinfection of cooling coils works better than UVC disinfection of food surfaces. The cooling coil UVC system is never turned off, giving the lights enough time to penetrate and break down biofilms. But this might take so long that a food item would expire and decompose, a condition not present with cooling coils.
Currently, the advice to address biofilms in the food industry includes physically removing them from the surface, and then disinfecting the underlying surfaces. This is where ultraviolet germicidal light can be most effective as a supplemental disinfection method once the biofilm has been removed.
Prevention Better than Removal
Not surprisingly, prevention of biofilms in high-risk areas is more effective and less expensive than removal once contamination has spread.
Standard cleaning of surfaces should be used, and antimicrobial coatings of certain surfaces can also be considered.
HVAC UVC systems can be installed to decontaminate the cooling coils or the ventilation system, or in-room UVC air purifiers with a filter can be placed in a room to continuously treat air and remove larger particles that microbes could breed on.
For surfaces, mobile UVC units can provide very large doses of ultraviolet irradiation in a short amount of time, although they should be carefully evaluated for adequate coverage throughout a room.
Even handheld UVC sanitizers can be used on hard-to-reach surfaces, although they will have very limited effectiveness on any biofilms already present.
Disinfection of liquid storage tanks can also be done with headspace sanitizers (for any tank) or tank immersion units (for clear liquids).
Finally, if water is suspected as a source of contamination, in-line UVC water purifiers can provide high doses of ultraviolet irradiation. Combined with water filters, they can treat large volumes of water used in a food processing facility or beverage manufacturing plant.
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