A recent study examined the efficacy of three commonly used sanitizers—chlorine, quaternary ammonium compounds (QACs), and ultraviolet-C (UV‑C) light—against Listeria monocytogenes biofilms on different materials representing surfaces found in produce packing environments. The findings provided insight into how factors like biofilm age, surface material, and the presence of organic matter affect sanitizer efficacy against L. monocytogenes in produce packinghouses.

The study was conducted by Louisiana State University researchers and published in MDPI Foods.

The researchers cultivated single-species L. monocytogenes biofilms using two foodborne illness outbreak-associated strains diluted in apple juice. The biofilms were evaluated after one and seven days of growth. They were grown on stainless steel, polyethylene terephthalate (PET), and silicone rubber coupons, respectively representing equipment surfaces, packaging and brush beds, and gaskets and conveyor components in produce packinghouses.

Stainless Steel Supports Biofilm Attachment

Stainless steel supported the highest level of biofilm formation, consistent with other research showing that hydrophilic, smooth materials enable faster biofilm attachment. In contrast, the hydrophobicity and roughness of silicone rubber appeared to slow biofilm development.

Sanitizer Efficacy Drops as Biofilms Mature

Treating surfaces with 200 parts-per-million (ppm) of chlorine for two minutes resulted in a significant decrease in L. monocytogenes biofilms across surfaces. Most successful was chlorine applied to 1-day biofilms on stainless steel, achieving reductions of approximately 2.84 log colony forming units (CFU) per coupon. However, against 7-day biofilms on stainless steel, chlorine’s efficacy dropped to 1.9 log CFU/coupon reductions.

Similar trends were noted for QAC and UV-C. Specifically, pathogen reductions achieved by QAC (400 ppm) on stainless steel dropped from 2.42-log CFU/coupon for 1-day biofilms to 1.73-log CFU/coupon for 7-day biofilms. Reductions achieved by UV-C at 0.85 joules per square centimeter (J/cm²) on stainless steel dropped from 2.71-log CFU/coupon to 1.57-log CFU/coupon for 1- and 7-day biofilms, respectively.

The resiliency of older biofilms is potentially attributable to denser extracellular polymeric matrices that shielded cells from chemical penetration and UV‑C exposure. Mature L. monocytogenes populations also display stress‑adapted traits and, relevant to QAC, possible efflux‑pump-associated resistance.

Surface Properties Impact Sanitizer Performance

Across sanitizers, silicone rubber and PET showed consistently lower log reductions than stainless steel, likely due to roughness, hydrophobicity, and greater potential for microscopic surface defects that protect biofilms.

The study also found that chlorine and QAC treatments performed best on smooth, hydrophilic stainless steel, where liquids spread evenly and penetrate more effectively. However, UV‑C’s performance suffered on scratched or worn stainless steel, where microscopic grooves create shadowing that limits light penetration.

These findings reinforce the importance of equipment condition, surface integrity, and material selection in sanitation program design.

Organic Matter Reduces Sanitizer Efficacy

Apple juice was used in the study to emulate organic matter on surfaces. Consistent with available literature on the subject, its presence strongly reduced sanitizer performance.

The researchers believe the organic matter may have formed protective films around biofilms, neutralized chlorine and QACs, absorbed or scattered UV‑C light, and increased the thickness and complexity of biofilm matrices.

These findings reinforce the knowledge that organic matter must be removed through rigorous pre‑cleaning for sanitizers to work reliably.

Practical Implications for Produce Packing Facilities

Because none of the sanitizers tested achieved reductions greater than or equal to 3 logs on mature biofilms under realistic, soiled conditions, the researchers recommend:

• Prioritizing thorough pre-cleaning before sanitizer application
• Targeting high-risk surfaces
• Early biofilm intervention
• Combined or sequential treatments for mature biofilms (e.g., sanitizer plus UV-C or heat).

Overall, once Listeria biofilms mature, standard sanitizers and UV‑C treatments offer limited control. Effective sanitation must combine strong hygienic design and consistent surface maintenance with effective cleaning and sanitation protocols.

Limitations and Future Research

The researchers noted that their lab-grown biofilms were single-species, formed using only two strains, cultivated on new, unworn surfaces, and grown for only up to seven days. In contrast, real-world biofilms are older, multi-species, and exposed to flow, abrasion, and variable temperatures. Future work should integrate environmental isolates, worn surfaces, mixed communities, and expanded sanitizer concentrations.