Smoking out Foodborne Pathogens with SO2

Carter et al. (2015) have recently shown that a common food industry post-harvest treatment, sulfur dioxide (SO₂) fumigation, might also be beneficial in combating foodborne pathogens in fresh produce¹. This finding could prove useful with the current emphasis on eating healthily as consumers recognize the value of fresh fruits and vegetables in the diet. Since these food items have also been implicated in outbreaks of foodborne infections, food producers must be quick to acknowledge the importance of this mode of transmitting illness and proactively take steps to eliminate risk to customers.

Producers of table grapes harvest the crop at an optimal stage then transport the fruits immediately to cold storage. As one of the many steps taken to ensure quality and safety for the consumer, SO₂ fumigation is a standard step in the harvesting process. This step reduces the chances of mold spoiling the crop and maximizes quality while in cold storage.

Carter et al. (2015) wondered if this important post-harvest treatment might also prove beneficial in the ongoing battle against foodborne pathogens. Choosing three microbes commonly implicated in recent cases of foodborne infection, Salmonella enterica, Listeria monocytogenes and Escherichia coli 0157:H7, the researchers examined both in vitro and in vivo bacterial viability during cold storage with and without SO₂fumigation regimes.

For in vitro investigation, the researchers inoculated filter papers with two strengths of pathogen, using 10³ (low dose) or 10⁵ (high dose cells) per preparation. They then placed the papers on water agar plates before incubating them according to cold storage conditions at 0.2°C using a Fisher Isotemp Undercounter BOD refrigerated incubator (Thermo Scientific). They maintained the cultures for 14 days, harvesting them at two time points (days 8 and 14) to assess microbial survival.

The team used commercial table grapes for the in vivo survival experiments, taking the fruits post-harvest and after an initial SO₂ fumigation. The team removed the grapes from each bunch and placed them in 6-well or 24-well plates (depending on size of fruit) before inoculating them with a low count (10⁴pathogen cells) or high count (10⁶ cells) of the three microbes. Again, the research team assessed microbial survival in vivo by performing colony counts at specific times (days 8 and 14) during the experiment.

In addition to cold storage, the research team also subjected the inoculums to SO₂fumigation, using various concentrations of the gas (0, 100, 200, 300 parts per million per hour) at specific time points (day 0, harvest, and days 7 and 14 ) during the cold storage incubation.

Cold storage reduced viability and L. monocytogenes and S. enterica pathogen content in vitro, with higher survival levels seen with high dose inoculation. S. enterica was more resistant, especially at higher inoculations with approximately 50% of the high dose surviving at day 14. E. coli O157:H7 showed little change in viability with cold storage. The researchers also found similar results for in vivo inoculation although levels of E. coli contamination on the grapes did reduce with cold storage. The effect of cold storage was greatest with L. monocytogenes contamination.

When Carter et al. looked at the results following SO₂fumigation and compared with 0.2C incubation alone, they found that pathogen levels reduced further below the reduction in viability seen with cold storage alone for both the in vitro and in vivo test conditions. The initial fumigations rendered L. monocytogenes cells inactive although grapes treated with the higher dose inoculation required a higher concentration of the gas. E. coli and S. enterica showed more resistance to the treatment than L. monocytogenes, with two or three fumigations required to achieve eradication. SO₂fumigation at 200 or 300 ppm inactivated these pathogens completely by the end of the incubation periods for the contaminated grapes.

The authors conclude that SO₂fumigation in conjunction with cold storage is an effective antimicrobial treatment for eliminating three serious foodborne pathogens in table grapes.

Learn more about foodborne pathogens and microbiology testing in our Food and Beverage community.

Reference

1. Carter, M.Q. et al. (2015) “Effect of sulfur dioxide fumigation on survival of foodborne pathogens on table grapes under standard storage temperature“, Food Microbiology 49 (pp.189 -196) http://dx.doi.org/10.1016/j.fm.2015.02.002