Leaving a cup of tea out overnight is a common habit, but it introduces significant concerns regarding both safety and quality. Letting brewed tea cool to room temperature and remain there for an extended period creates an environment where microorganisms can thrive. Understanding the science behind this spoilage is important for making informed decisions about what to consume the next morning.
The Primary Risk: Bacterial Growth
The main safety concern for any food or beverage left out is the rapid multiplication of bacteria. Brewed tea, even without additives, is a nutrient-rich liquid that provides an ideal medium for microbial growth once it cools. This risk is directly tied to the “Danger Zone,” the temperature range between 40°F and 140°F (4°C and 60°C) where foodborne bacteria flourish.
Room temperature falls squarely within this zone, allowing bacteria to double their population in as little as 20 minutes. Leaving tea out overnight provides ample time for these organisms to reach levels that can cause illness. One specific concern is Bacillus cereus, a spore-forming bacterium found in dried tea leaves that can survive the initial brewing process.
Once the tea cools, surviving spores can germinate and multiply rapidly, producing toxins that are not destroyed by reheating. Food safety guidelines recommend that perishable items should not remain in the Danger Zone for more than two hours. Therefore, tea left out overnight is considered unsafe to drink, regardless of whether it looks or smells fine.
Quality Degradation and Oxidation
Beyond the safety risk, leaving tea exposed to air at room temperature causes a noticeable decline in its quality. This change is primarily due to oxidation, a chemical process where compounds in the tea react with oxygen in the air. The polyphenols, which contribute to the tea’s flavor and color, begin to break down and transform.
This process causes the tea to become darker and often cloudier, a phenomenon sometimes referred to as “tea scum” or “tea film.” The flavor profile shifts dramatically, losing its fresh, aromatic notes and developing a stale, flat, or sometimes bitter taste. While this quality degradation is unpleasant, the chemical changes simply make the tea less enjoyable.
Factors That Influence Risk
The composition of the tea significantly influences how quickly it becomes unsafe to consume. The addition of dairy products, such as milk or cream, dramatically shortens the safe window. Dairy is highly perishable and provides a richer source of nutrients for bacteria, accelerating their growth rate.
Similarly, adding sugar or other sweeteners provides an additional food source for microorganisms, increasing the risk of spoilage. Even plain tea is susceptible, but these additives make the beverage a much more hospitable environment for bacterial proliferation. The type of tea also plays a minor role; herbal teas, especially those containing fruit pieces, may have different spoilage profiles due to their varying pH levels and nutrient content.
Safe Storage Guidelines
To ensure both safety and quality, brewed tea should be consumed or properly stored within a short timeframe. The general rule is to consume hot-brewed tea within two hours of preparation if it is left at room temperature. If you intend to save the tea for later, it must be refrigerated promptly.
For long-term storage, the tea should be cooled quickly to below 40°F (4°C) to minimize the time spent in the bacterial Danger Zone. This can be achieved by placing the container in an ice bath before transferring it to the refrigerator. Once cooled, the tea should be stored in a clean, airtight container to prevent it from absorbing odors and limiting further oxidation.
Properly refrigerated tea can maintain its quality and safety for approximately three to five days. If tea has been left out overnight, it is safest to discard it entirely.