Atmospheric plasma inactivation of foodborne pathogens on fresh produce surfaces

by 1978- Johnson, Faith Michelle

A study was conducted to determine the effect of a one atmosphere uniform glow discharge plasma (OAUGDP) on inactivation of nalidixic acid resistant Escherichia coli O157:H7 and Salmonella, and Listeria monocytogenes on apples, cantaloupe, and lettuce, respectively, and culture media [tryptic soy agar (TSA) + 50 ppm nalidixic acid (TSAN) for E. coli O157:H7 and Salmonella; TSA for L. monocytogenes). A mixture of cultured test organisms was washed, suspended in phosphate buffer and spot inoculated onto produce or culture media (6 log CFU/sample). Inoculated produce or culture media (samples) were exposed inside a chamber affixed to the OAGUDP blower unit, operated at a power of 9 kV and a frequency of 6 kHz. This configuration allows the sample to be placed outside of the plasma generation unit, while allowing airflow to carry the antimicrobial active species, including ozone and nitric oxide, onto the sample. Cantaloupe and lettuce samples were exposed for 1, 3, and 5 min, while apple samples were exposed for 30 sec, 1, and 2 min. All culture media was exposed for 10, 30 sec and 1 min. After exposure, samples were pummeled in 0.1% peptone water containing 2% Tween 80, serially diluted, and plated in duplicate onto TSAN or TSA (both considered as non-selective) and selective media, and incubated as follows: E. coli O157:H7 (TSAN, modified EMB) and Salmonella (TSAN; XLT4), 48 hr, 37°C; L. monocytogenes (TSA and MOX); 48 hr, 32°C). Generally, survival curves for all pathogens as indicated by recovery on nonselective and selective media followed a biphasic pattern. Specifically, a sharp decrease in iii populations typically was observed after plasma treatment for the initial exposure time, followed by a decline in inactivation rate, or tailing effect, observed during after longer treatment times. This biphasic pattern was observed using both recovery media, although recovery on the selective medium was typically poorer than recovery on the non-selective medium. L. monocytogenes on lettuce did not follow this typical inactivation pattern when exposed to OAUGDP, although the biphasic inactivation pattern was observed when the organism was exposed to plasma on TSA. An approximate 3-log reduction was seen with E. coli O157:H7 on apples after exposure to plasma for two min, and similar levels of reduction were achieved with Salmonella and L. monocytogenes on cantaloupe rinds and lettuce, respectively, after three min of exposure to plasma. L. monocytogenes proved to be the slightly more sensitive to plasma treatment than E. coli O157:H7 and Salmonella. Populations of L. monocytogenes were reduced to undetectable levels and barely detected when lettuce and TSA, respectively, were exposed to plasma for 5 min. E. coli O157:H7 and Salmonella populations were never reduced to below 1 log CFU/sample. In all cases, substantially longer exposure times were required for reduction of pathogens exposed on produce as compared with exposure on culture media. Differences in recovery of pathogens on selective and non-selective media revealed that substantial portions of the surviving populations of all pathogens were sublethally injured by plasma treatment. Generally, injury development was greater when pathogens were exposed to plasma on produce than on culture media, with the exception that L. monocytogenes underwent greater injury when exposed on culture media. Plasma treatment of produce surfaces has the potential to be used in many areas of iv the food processing industry. The process can reduce bacterial populations by several log units with a few minutes without causing physical damage exposed produce. By combining plasma treatment with other antimicrobial treatments, the ability to obtain safe and wholesome produce may be improved. v