We decided to verify the opinion that drinking tea that has been left at room temperature for a long time could lead to the formation of harmful substances. However, brewed tea’s shelf life and whether it remains beneficial over time remained unclear.
The film that forms on the surface of tea brew after some time has caused concern among scientists. Some argue that it results from the interaction between calcium bicarbonate and polyphenols, while others claim that it comes from essential oils released from tea leaves. Despite these differing opinions, we could not find scientific evidence confirming that the film is harmful to health.
Bacteria can start multiplying in yesterday’s tea left at room temperature, posing a potential danger. Although tea leaves themselves do not contain harmful substances, bacteria can enter during production, transportation, storage, and preparation. Therefore, specialists recommend brewing tea and herbal infusions, also known as teas, with boiling water, as it can help eliminate most of the harmful microorganisms. However, even with boiling water, tea brewed over 16 hours ago may have active E. coli growth, which can be harmful to health. Furthermore, if the brewing temperature was not high enough, the storage time may be even shorter.
The type of tea can influence its storage time. Biologists conducted an experiment using three types of tea – black, oolong, and white – brewed with boiling water and left in sterile containers at room temperature for two days. Even under laboratory conditions, where cleanliness is strictly observed, bacteria actively multiplied in white and oolong tea due to their lower oxidation levels. In contrast, no visible changes were observed in black tea, which was the most oxidized, after two days.
To avoid bacterial growth, it is advisable to store leftover tea in the refrigerator overnight instead of leaving it at room temperature. The temperature range of 4.5°C to 60°C provides an excellent environment for bacterial growth, and their numbers can double every 20 minutes. Additionally, if the tea has additives such as milk or sugar, the storage time outside the refrigerator becomes even shorter, as they create a more nutritious environment for bacteria.
Many tea portals advise not to store brewed tea for more than eight hours, citing the risk of bacterial growth and a decline in taste and aroma over time. Some even reference recommendations from the US Centers for Disease Control and Prevention to avoid drinking tea left out of the refrigerator for over eight hours, although the original source could not be located.
Tea can spoil and become harmful to health if left at room temperature for an extended period, especially if mold has begun to form. Bacteria multiply faster than mold and can pose a potential health risk. Although it is not recommended to drink tea brewed more than eight hours ago due to its diminished taste, calling it poisonous is an exaggeration.
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