Effectiveness of UV Light as a Means to Reduce Salmonella Contamination on Tomatoes

Introduction: Salmonella-contaminated fresh produce has been linked to multiple foodborne illness outbreaks.  Fresh produce receives little microbial intervention other than washing.  An alternative strategy to reduce the pathogenic bacterial levels on fresh produce is to use ultraviolet-C light at a wavelength of 200-280 nm.  However, repair of bacterial injury by photoreactivation with visible light is a concern. 

Purpose: This study evaluated the effectiveness of UV-C light (254 nm) in reducing Salmonella contamination on tomatoes under different UV-C treatments and possible photoreactivation of injured Salmonella post-UV-C treatments with visible light.

Methods: Tomatoes were spot inoculated with approximately 5 log cfu/ tomato with rifampicin-resistant Salmonella and air-dried before exposing to UV-C light at different doses (0, 1, 2, 4, and 6 mJ/cm²) at a distance of 60 cm.  After the UV-C treatments, the tomatoes were exposed to fluorescent light at 0, 0.85, 4.26, and 20.42 J/cm².  Treated tomatoes were rinsed with 20 ml of 0.1% peptone and spiral-plated onto tryptic soy agar plus rifampicin (TSAR).  Plates were incubated at 37 °C for 24 h before enumerating surviving Salmonella.

Results: UV-C dosages of 1 and 2 mJ/cm² reduced the Salmonella population by 2 logs. Salmonella populations were reduced by >2.56 logs when the tomatoes were treated with 4 and 6 mJ/cm².  There was no evidence of injury repair by photoreactivation by visible light treatments.

Significance: Use of UV-C light may be a promising means to reduce Salmonella contamination on fresh tomatoes.