As winter temperatures drop, more people are reaching for thermal flasks, but Dr. Yen Tsung-Hai, a toxicologist, warns that not all beverages are safe to store in them for long periods. Stainless steel flasks are generally safe for hot water, tea, or black coffee, but drinks like milk-based beverages (e.g., lattes) or sugary drinks can spoil if left too long. Once you reach your destination, pour these drinks into a regular cup and consume them quickly, or store them in the refrigerator. “For people with diabetes or liver cirrhosis, a bout of diarrhea and dehydration could lead to metabolic acidosis or sepsis, posing serious health risks,” Dr. Yen said.
Dr. Yen, director of the Clinical Toxicology Center at Linkou Chang Gung Memorial Hospital, told SET News that proper use of thermal flasks is crucial to prevent gastrointestinal infections. Using stainless steel flasks made from 304 or 316 grade steel is generally safe, but high temperatures combined with sugar or protein in the drink create a breeding ground for bacteria. “There’s a so-called ‘danger zone’ between 7°C and 60°C where microbes can multiply. Drinks containing sugar or protein kept in this temperature range can easily cause food poisoning,” he explained.
He added that office workers visiting clinics for acute gastroenteritis may forget that the latte or milk tea in their thermal flask from earlier in the day could be the culprit behind their diarrhea or vomiting. “Food poisoning caused by improper use of thermal flasks may actually be underestimated,” Dr. Yen noted.
His advice: if you buy milk- or sugar-based coffee or tea in a thermal flask, pour it into a regular cup once you reach your destination and drink it within an hour. If you can’t finish it immediately, refrigerate it to prevent bacterial growth.
