Did you know? The claim that overnight tea causes cancer may be a myth. "Overnight tea" refers to tea brewed the previous day and left standing overnight. Scientifically speaking, "overnight tea" is essentially tea that has been left standing for an extended period, much like tea brewed in the morning and consumed in the evening. After prolonged standing, the chemical composition of such tea undergoes significant changes.Since daytime temperatures are higher than nighttime, tea left out during the day undergoes even more dramatic changes than "overnight tea."
The primary change in long-stored tea is the oxidation of its polyphenols, gradually forming yellowish-red or reddish-brown oxidation products that deepen the tea's color.Thus, a cup of clear, emerald-green tea, especially when left at higher temperatures, gradually loses its original green hue while increasing in yellow and red tones. The tea's clarity also diminishes, becoming increasingly cloudy. However, these oxidation products are non-toxic and pose no harm to human health.
There is a long-standing claim that "overnight tea should not be consumed" as it may increase cancer risk. This assertion is based on the idea that certain secondary amines in tea leaves can transform into carcinogens—nitrosamines—when left overnight. In reality, this claim lacks scientific basis.
First, secondary amines are not unique to tea liquor but are widely present in many foods, particularly abundant in cured and smoked products. Even in "overnight tea," the amount of secondary amines consumed from the tea liquor is extremely limited, far lower than what people ingest from any staple food or vegetable.
Furthermore, secondary amines are not synonymous with carcinogens—nitrosamines. Secondary amines themselves are not carcinogenic; they require specific conditions—coexistence with nitrates and chemical reactions—to form nitrosamines.These specific conditions encompass multiple factors including pH levels, microbial activity, and temperature. This means the mere presence of secondary amines and nitrates in tea does not automatically lead to nitrosamine formation.
Moreover, the carcinogenic process of nitrosamines also requires specific conditions: they only become carcinogenic when certain concentration thresholds are reached within the human body.Research indicates that nitrosamines must be absorbed at levels of 100 to 200 milligrams per kilogram of body weight to potentially cause cancer, and this requires sustained, high-dose exposure. Although "stale tea" may contain nitrosamines, their concentration is no higher than in other foods and poses no such significant risk.
Conversely, when consuming tea normally, individuals can absorb substantial amounts of tea polyphenols and vitamin C from the brew. These chemical compounds effectively inhibit the body's synthesis of nitrosamines, acting as natural inhibitors.
Nevertheless, this does not mean "overnight tea" is entirely free of safety concerns. When tea liquor is left standing for extended periods, proteins, sugars, and other substances within it provide ideal nutrients for bacterial and fungal growth. Consequently, prolonged storage increases the risk of harmful microorganisms proliferating, potentially leading to spoilage.
As tea cools during storage, most of its polyphenols and vitamins undergo chemical changes, diminishing its antioxidant capacity and reducing its nutritional and health benefits.From a scientific perspective on tea consumption, it is advisable to brew and drink tea fresh. Avoid consuming tea that has been left standing for too long, such as "overnight tea," to fully harness the benefits of tea leaves and provide the best nutritional and health components for people. That concludes our discussion on whether overnight tea causes cancer, which may be a rumor. We hope you now have a clearer understanding. Finally, we sincerely wish you good health and happiness.