Professor Hugh Pennington, an emeritus professor of bacteriology at the University of Aberdeen in Scotland -- who worked for the British, Scottish and Welsh governments as an expert on microbiology and food safety -- said demand for salad boomed because of healthily eating campaigns but salad is considered one of the products most likely to cause food-related illness.
That is largely because greens are grown directly in the soil, and some pathogens can only be killed by heat or strong detergents, not just water, Pennington said.
"Despite the recent horse meat and other scandals, the meat can be traced and through a rigorous process that checks for its quality etc.," Pennington told the Sunday Telegraph. "That does not exist to the same rigor for salad. You can only make vegetables safe by cooking and you can`t obviously do that with salad. You could irradiate it -- but that would be a 'no, no' with the public. You just can`t be absolutely sure that the bagged salad you are buying, which has been put through a chemical wash to kill the bugs, is actually free of them."
Food pathogens are very good at clinging on to salad and the risk from cryptosporidium, salmonella and listeria is very real, Pennington said.
"I would advise people to thoroughly wash salad even when it says it has been washed and is ready to eat," Pennington said.