A recent Japanese television experiment has uncovered startling differences in how common bottled drinks breed bacteria when left overnight after being sipped directly from the bottle. Conducted by TBS’s popular show What’s the Difference? (この差って何ですか), the test simulated hot summer conditions (28°C, 70% humidity) and measured bacterial growth over 24 hours. The result: one drink saw bacterial levels skyrocket more than 8,000 times—while another actually became safer to consume.
The Science Behind Overnight Bacterial Growth
According to Professor Sekizaki Tsutomu from the University of Tokyo’s Department of Agricultural and Life Sciences, unopened bottled beverages are sterile inside. The danger begins the moment you drink directly from the bottle. Saliva introduces oral bacteria—including Staphylococcus aureus—into the liquid. Left at room temperature, these microbes feast on the drink’s nutrients, multiplying rapidly. The professor warns that once bacterial counts reach 100,000 per gram or higher, the risk of food poisoning—with symptoms like diarrhea and vomiting—becomes significant.
Ranking of 6 Bottled Drinks After 24 Hours
The experiment started each drink with an initial bacterial load of approximately 600 colony-forming units per milliliter (CFU/ml) after one sip. Here’s how they fared after a day in simulated summer heat:
6th place: Green tea – CFU dropped to 490. Thanks to catechins, powerful natural antioxidants with antibacterial properties, green tea was the only beverage that saw bacterial levels fall. It remains the safest choice.
5th place: Water – Rose to 1,100 CFU/ml. Water lacks the protein and sugar that bacteria need to thrive, so growth was minimal. Experts consider it still safe.
4th place: Barley tea (mugicha) – Increased to 3,700 CFU/ml. Its trace amounts of sugar and protein provide modest nourishment for bacteria, leading to notable but not alarming growth.
3rd place: Cola – Exploded to 2,493,800 CFU/ml. A typical 500 ml bottle contains about 15 sugar cubes, creating an ideal breeding ground for S. aureus. Bacterial numbers jumped into the millions.
2nd place: 100% orange juice – Reached 2,503,900 CFU/ml. The high fructose content accelerated bacterial reproduction nearly as aggressively as cola.
1st place: Coffee milk – Skyrocketed to 5,175,900 CFU/ml—an increase of more than 8,600 times. Combining rich protein from milk with high sugar content, this drink became the perfect bacterial incubator. Drinking it after 24 hours poses a serious risk of food poisoning.
3 Essential Tips to Prevent Bacterial Contamination
To protect yourself and your family this summer, follow these expert-backed recommendations:
- Pour into a clean glass instead of drinking from the bottle. This prevents oral bacteria from entering the remaining liquid. If you must drink directly, try to “pour” the liquid into your mouth without touching the rim.
- Refrigerate immediately after opening. Room temperature—especially 28°C—is the sweet spot for bacterial reproduction. Chilling slows growth dramatically.
- Choose smaller bottles and finish them in one sitting. The simplest way to avoid overnight contamination is to eliminate leftovers altogether.
A Summer Safety Wake-Up Call
With Hong Kong’s sweltering heat, bottled beverages are a daily staple for many. This experiment serves as a powerful reminder that convenience can come with hidden risks. While green tea offers a surprisingly safe option, sugary and milky drinks demand extra caution. Next time you reach for a half-finished bottle, think twice—or better yet, reach for the fridge or a fresh pour.
Curious about other food safety pitfalls? Read our articles on overnight rice dangers, listeria risks in pregnancy, and the truth about reheating leftovers.