Introduction: When a watermelon is too large to finish, many families' practice is to put it in the refrigerator and eat it the next day. However, a recent news story that has been widely spread on social media like Weibo and WeChat—"Overnight watermelon sickens a family of three, experiment proves bacteria count in the billions"—has caused quite a panic.
According to a report by Qilu Evening News, a family of three all suffered from diarrhea and were hospitalized after eating an overnight, refrigerated watermelon. Subsequently, a reporter from the newspaper sent half of an overnight watermelon, stored at room temperature and covered with plastic wrap, to the relevant department for testing. Staff took samples from 1 gram each at the surface of the watermelon's cut face, 4 cm below the surface, and 8 cm below the surface. After crushing the samples, they were placed in culture media, labeled as No. 1, No. 2, and No. 3, and put into an incubator.
After 12 hours, No. 1 culture medium was covered with a dense layer of bacterial colonies. The experimenter stated that the number of bacteria was too numerous to count and had to be calculated in the hundreds of millions; the amount in No. 2 was slightly less than in No. 1; and the count in No. 3 was also uncountable. The final conclusion was that the number of bacteria in an overnight watermelon is in the billions, and the public was urged to throw away any leftover watermelon. Is this really the case? A reporter invited authoritative experts to verify the claim.
He Jiguo, an associate professor at the College of Food Science and Nutritional Engineering at China Agricultural University, told the reporter that this experiment, while seemingly persuasive, made a critical mistake by lacking a comparison with refrigerated storage. This is because the storage conditions of the watermelon in the experiment were different from the usual practice in a typical household. We usually put the unfinished watermelon in the refrigerator and cover it with plastic wrap or a sealed container. The low-temperature environment inhibits microbial growth, while the surface cover prevents secondary contamination. However, the watermelon sent for testing was stored at room temperature overnight. In summer, high temperatures can easily allow bacteria to proliferate in such an environment.
Song Xin, a dietitian at Beijing Chaoyang Hospital, stated that the news claimed the overnight watermelon sickened the family, but they had eaten a refrigerated watermelon, whereas the one sent for testing was stored at room temperature. Therefore, the two are not comparable. Moreover, there are many factors that can cause diarrhea. The report did not clarify whether a bacterial infection was confirmed at the hospital. Chilled watermelon is inherently cold, and discomfort after eating it can also be related to an individual's physical constitution. Therefore, readers should not immediately jump to conclusions and become overly worried upon seeing the news.
Despite this, Zhong Kai, a researcher at the Department of Risk Communication at the China National Center for Food Safety Risk Assessment, stated that unfinished watermelon should not be stored for long. A cut watermelon should generally not be left at room temperature for more than 4 hours; prompt refrigeration is necessary for safety. Furthermore, it is best to cover the watermelon with clean plastic wrap or scoop the remaining flesh into a sealed container.
He Jiguo pointed out that the experiment also has a positive aspect. Before being cut, the interior of all watermelons is sterile. After cutting, the bacteria in the watermelon can come from the knife, the air, and the plastic wrap. If the experimenter had covered the watermelon with plastic wrap immediately after cutting, a large portion of the billions of bacteria on the surface could have been caused by a dirty knife or unclean plastic wrap.
This suggests that when cutting watermelon at home, one must first wash the knife thoroughly, and immediately cover the cut surface with clean plastic wrap before placing it in the refrigerator. When out, it is best to avoid buying pre-cut watermelon. If you must buy it and are concerned, you can cut off the top layer of the watermelon before eating.