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Why does tea in a thermos taste bad?

More and more people are carrying their own bottles lately, thinking it’s eco-friendly and cost-effective. But have you ever found that the tea you put in a thermos and took to work ends up looking different in color, with a strange smell and taste when you drink it? In this article, we will explain the reasons why tea in a thermos may not taste good and how to address the issue.

Many people are somewhat aware that tea contains catechins. In fact, when catechins oxidize, they can change the color, aroma, and taste of tea.

While tea will naturally oxidize over time whether it’s in a thermos or not, catechins are more prone to oxidation at higher temperatures. Therefore, in a thermos with a heat-retaining function, oxidation of the tea progresses more quickly, leading to a decline in flavor relatively soon.

You might wonder why tea in plastic bottles from convenience stores remains good despite being kept warm for a long time. The reason is that vitamin C is added as an antioxidant to prevent oxidation, keeping the taste, aroma, and color stable.

Following this idea, adding a small amount of citric acid to tea in a thermos can help slow down oxidation. (However, be cautious not to add too much, as it can alter the taste of the tea.)

Besides adding citric acid, there are other methods to maintain the quality of tea. One recommendation is cold brewing. Cold brewing reduces the amount of catechins in the tea and is less prone to oxidation due to the low temperature. However, the downside is that the tea will be cold. (If you’re okay with cold tea or don’t mind transferring it to a cup and warming it in the microwave, this method can be a great choice.)

There’s something quite satisfying about enjoying a good cup of tea from a thermos when you’re out and about. If you have any recommendations for teas that taste great when enjoyed outside, please share them!