Use of Selected Chemical and Physical Treatments to Reduce Listeria monocytogenes Biofilms Formed on Lettuce and Cabbage

Introduction: Produce is usually consumed raw to benefit human health. However, it also contributes to many outbreaks of Listeria monocytogenes, which can form a mature biofilm within 24h. Recent control strategies have proved ineffective in ensuring safe food production.

Purpose: The present study investigated the efficacy of chemical treatment (chlorine, citric, lactic, and malic acid) and physical treatment (ultrasonication, ultraviolet-C, and cold oxygen plasma) on L. monocytogenes biofilm on lettuce and cabbage stored at 4°C for 24h.

Methods: The inoculated samples were exposed to 200 ppm chlorine, 2% citric acid, 2% lactic acid, 2% malic acid, 32 Hz ultrasonication for 5 min. Each side of samples was also subjected to 390 mJ/cm² of ultraviolet-C (UV-C) and 750 mJ/cm²of cold oxygen plasma (COP).  

Results: Two hundred ppm chlorine reduced 0.68 log CFU/lettuce coupon which was statistically (P < 0.05) significant different compared to control. Lactic and malic acid also achieved a similar reduction 0.56 log CFU/lettuce coupon whereas citric acid and US did not give a statistically (P > 0.05) significant reduction compared to control sample. On the other hand, UV-C and COP significantly reduced (P < 0.05) L. monocytogenes on lettuce, a reduction of 4.06 and 3.85 log CFU/lettuce coupon was achieved, respectively. However, the chemical treatments seem to be more effective when applied on cabbage. Chlorine with the same concentration eliminated up to 3.35 log CFU/cabbage coupons whereas 2 % lactic and malic achieved a similar reduction (approx. 3.40 log CFU/cabbage coupons). These results were higher than that observed on lettuce. Furthermore, citric acid, which showed no reduction on lettuce, showed a 2.68 log CFU/cabbage coupon reduction effect; while US gained a lower reduction (0.77 log CFU/cabbage coupon). In addition, L. monocytogenes biofilm was reduced with a similar value by UV-C and COP (3.97 and 4.09 log CFU/cabbage coupon, respectively) compared to that on lettuce. The highest reduction for chemical treatment was achieved by chlorine, lactic and malic acid; and the highest reduction for physical treatment was accomplished by the UV-C and COP treatment. However, when compare between the chemical and physical treatment, UV-C and COP gave the best reduction on both lettuce and cabbage.

Significance: This study demonstrates that photosterilization such as UV-C and COP can be an effective mean to overcome the problem of L. monocytogenes biofilm on produce.