Experts warn that the salads and raw vegetables people choose for health may be a hidden cause of summer food poisoning. Contrary to the common belief that ingredients that look fresher are safer, vegetables that are not heated are more likely to retain a variety of bacteria.

Japan’s Fuji News Network (FNN) Prime Online recently introduced the risk factors for lunchbox-related food poisoning in summer through an interview with predictive microbiology expert Professor Shigenobu Goseki of Hokkaido University’s Graduate School of Agriculture.

Professor Goseki said, “From the standpoint of preventing food poisoning, it is better not to put brightly colored raw vegetables in summer lunchboxes.”Colorful, healthy items such as cherry tomatoes, lettuce, parsley, and shredded cabbage can increase the risk of food poisoning.

He explained, “Raw vegetables naturally contain a variety of plant-associated microorganisms,” and added, “In some cases, there are on the order of 100,000~1,000,000 bacteria per 1g of lettuce leaves.”

Indeed, studies in the field of food safety have reported similar findings. The U.S. Food and Drug Administration (FDA) and the U.S. Centers for Disease Control and Prevention (CDC) have identified leafy greens such as romaine lettuce and spinach as major sources of outbreaks of E. coli, Salmonella, and Listeria. The European Food Safety Authority (EFSA) likewise classifies raw vegetables and salads as a high-risk food category for food poisoning.

According to the U.S. CDC, in the 2018 outbreak of enterohemorrhagic E. coli linked to romaine lettuce in the United States, more than 200 people were infected across 36 states and five died. Since then, leafy greens have repeatedly been singled out as items associated with foodborne illness.

Professor Goseki explained, “Pre-cut vegetables sold on the market also undergo washing, but bacteria cannot be completely removed.” He pointed out that, on the contrary, only some hardy bacteria may survive and then rapidly multiply when conditions are favorable.

Cherry tomatoes are not exempt. He advised, “In cherry tomatoes, bacteria are particularly prone to adhere around the stem end,” and “when adding them to lunchboxes, it is best to remove the stems first.”

What experts emphasize most is, unexpectedly, not ‘washing’ but ‘heating.’ Both Korea’s Ministry of Food and Drug Safety and Japan’s Ministry of Health, Labour and Welfare recommend thorough cooking as a basic principle of preventing food poisoning. In particular, with ground meat, surface bacteria can be mixed into the interior during processing, so it must be cooked all the way through to the center.

Professor Goseki noted, “Many consumers think ‘fresh ingredients are safe,’ but that is not scientifically correct,” adding, “Food poisoning bacteria are completely different from spoilage bacteria.” When food spoils, noticeable changes such as odor or slime appear, but foodborne pathogens can exist without any odor or visible change. In other words, even if it looks fine on the outside, a small number of pathogenic bacteria can still cause food poisoning.

In the end, the key to preventing summer lunchbox food poisoning is not complicated. The most effective approach is to cut back on raw vegetables, use ingredients that have been heated thoroughly, then cool them quickly and keep them refrigerated.