A recent experiment by a Japanese television program is prompting a major rethink of a common summer habit: sipping directly from a bottle and leaving it for later. The show What’s the Difference? (この差って何ですか), in collaboration with experts from the University of Tokyo, tested six popular bottled beverages to measure how bacteria levels change when drinks are consumed directly from the container and then left at room temperature for 24 hours. The findings reveal that while most drinks become unsafe, one product turns into a genuine bacterial breeding ground—while another emerges as surprisingly safe.
How a Single Sip Changes Everything
Professor Tsutomu Sekizaki from the University of Tokyo’s Department of Agricultural and Life Sciences explains that sealed bottled beverages are sterile. The contamination begins the moment a person drinks directly from the bottle. Oral bacteria—including Staphylococcus aureus—enter the liquid through saliva. Once the bottle is capped and left at warm temperatures (the experiment simulated a typical summer day at 28°C/82°F with 70% humidity), the bacteria feed on the drink’s nutrients and multiply rapidly. The professor warns that once bacterial levels exceed 100,000 per gram, the risk of acute food poisoning—including diarrhea and vomiting—becomes alarmingly high.
The Rankings: From Safest to Most Dangerous
Starting with an initial count of around 600 bacteria per milliliter, the six drinks went through a dramatic 24-hour journey. The results challenge common assumptions about what is safe to leave out.
The Safest Choice: Green Tea
The experiment’s biggest surprise was green tea. After 24 hours, its bacteria count actually dropped to 490. The reason lies in catechins, powerful antioxidants with natural antibacterial properties that actively suppress microbial growth. For those who leave drinks out, green tea is the only beverage tested that became cleaner over time.
Water and Barley Tea: Low Risk
Water showed minimal growth, climbing to just 1,100 bacteria. Lacking the sugars and proteins that bacteria need to thrive, it remained relatively stable. Barley tea grew to 3,700 bacteria—an uptick, but still in a low-risk range due to its limited nutrients.
The Danger Zone: Sugar Is the Fuel
The results took a sharp turn with sugary drinks. Cola skyrocketed to 2.49 million bacteria. Each 500ml bottle contains roughly 15 cubes of sugar, providing a perfect energy source for oral bacteria to feast on. 100% orange juice performed nearly identically, reaching 2.5 million bacteria, driven by its high fructose content.
The Shocking Winner (Worst Case): Coffee Milk
The runaway hotspot was coffee milk. Combining high protein from milk with significant sugar content, this drink offered the ideal environment for bacteria. Its count exploded from 600 to a staggering 5,175,900—a more than 8,000-fold increase. Researchers called it a “perfect breeding ground” and a serious food poisoning risk if consumed after being left out.
3 Essential Tips to Stay Safe
The good news is that preventing this bacterial explosion is simple. The experts offer three actionable rules for anyone who drinks bottled beverages:
- Avoid direct drinking from the bottle. Pour your drink into a clean glass whenever possible. This prevents oral bacteria from contaminating the entire container.
- Refrigerate immediately if you don’t finish it. Room temperature is the prime zone for bacterial growth. Storing an opened bottle in the fridge dramatically slows reproduction.
- Buy single-serve bottles. The easiest way to eliminate the risk is to finish the beverage in one sitting. Smaller portions prevent the need for long-term storage at unsafe temperatures.
A Wake-Up Call for Warm Weather
As summer heat drives more people to grab a bottle for quick hydration, this experiment serves as an urgent public health reminder. The findings highlight just how quickly a refreshing sip can turn dangerous when common habits are left unchecked. The standout victory for green tea drinkers aside, the core takeaway is simple: a small change in how we drink can prevent a significant health risk. By avoiding direct drinking and storing leftovers properly, consumers can keep their beverages both refreshing and safe.