Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/17/2014
Publication Date: 3/26/2014
Publication URL: http://handle.nal.usda.gov/10113/60061
Citation: Rasooly, R., Hernlem, B.J., He, X., Friedman, M. 2014. Microwave heating inactivates Shiga Toxin (Stx2) in reconstituted fat-free Milk and adversely affects the nutritional value of cell culture medium. Journal of Agricultural and Food Chemistry. 62:3301-3305 DOI.org/10.1021/jf500278a. Interpretive Summary: Numerous foodborne diseases result from ingesting foods that are contaminated with microbial and plant toxins. Naturally occurring food toxicants can adversely affect the nutritional quality and safety of foods. In order to improve the quality and food safety of our food supply, research is needed to define conditions that inactivate or inhibit toxin production to minimize the levels of active toxic compounds in foods. Shiga toxins Stx1 and Stx2 produced by enterohemorrhagic strains of Escherichia coli (EHEC), such as O157:H7 are pathogens of major importance for food safety, causing foodborne illnesses, ranging from mild diarrhea to a life-threatening complication known as hemolytic uremic syndrome (HUS). In the present study, we demonstrated that exposure of a milk solution spiked with Stx2 to high level of microwave treatment or to 95 °C heat treatment for 5 min reduced Stx2 activity. We also observed that this treatment destroyed the nutritional value of the culture medium used to grow the cells. Our finding with the Shiga toxin and culture medium suggest the need to determine in detail the impact of microwave cooking on susceptible food ingredients and toxins.
Technical Abstract: Microwave exposure is a convenient and widely used method for defrosting, heating, and cooking numerous foods. Microwave cooking is also reported to kill pathogenic microorganisms that often contaminate food. Microwaves act by causing polar molecules in food, such as water, to rapidly rotate, thus energizing neighboring food molecules, which raises their temperature. In this study we tested if microwaves would inactivate the toxicity of Shiga toxin 2 (Stx2) added to 5% reconstituted fat-free milk administered to monkey kidney Vero cells. Heating of milk spiked with Stx2 in a microwave oven using a 10% duty cycle (cycle period of 30s) for a total of 165 kJ energy (65oC final temperature) or heat heating (pasteurization), widely used to kill pathogenic bacteria, and did not destroy the biological effect of the toxin in the Vero cells. However, conventional heating of milk to 95 C for 5 min or by increasing microwave energy to 198 kJ (78oC final temperature) reduced Stx2 activity. Gel electrophoresis showed that exposure of the protein toxin to high-energy microwaves resulted in the degradation of its original structure. In addition, two independent assays showed that exposure of the cell culture medium to microwave energy of 198 kJ completely destroyed the nutritional value of the culture medium used to grow the Vero cells, possibly by damaging susceptible essential nutrients present in the medium. These observations suggest that microwave heating has the potential to destroy the Shiga toxin in liquid food.