Bagged tea turns bitter faster than loose-leaf tea because the leaves inside the bag are broken into very small pieces. These particles have more surface area in contact with the water, so their tannins dissolve much sooner.
How does leaf size affect steeping? Whole or coarsely broken leaves used for loose-leaf tea have less surface area in contact with the water, so their tannins and caffeine dissolve gradually. In a tea bag, the leaves are cut into pieces nearly as small as powder. This greatly increases the total contact area and lets much more tannin dissolve into the water in the same amount of time.
How do tannins create bitterness? Tannin is a compound that occurs naturally in tea and leaves a dry, slightly bitter sensation in the mouth. During the first few minutes of steeping, only a small amount of tannin dissolves into the water; that amount rises quickly as steeping continues. Because bagged tea has more surface area, this increase starts much sooner. As a result, bagged tea tastes noticeably more bitter than loose-leaf tea after the same steeping time.
How does water temperature affect it? A tea bag placed in boiling water releases tannin much faster than one steeped in warm water. As the temperature rises, both aromatic compounds and tannins dissolve into the water more readily. That's why leaving a tea bag in boiling water for a long time makes it bitter much sooner than loose-leaf tea steeped for the same amount of time.
Why does squeezing the bag increase bitterness? If you squeeze the bag with a spoon as you remove it, the water with the highest concentration of tannin inside the bag is pressed into the cup. This makes already fast-steeping tea even more bitter. To avoid that extra bitterness, lift the bag out and let it drain without squeezing.
How can you steep bagged tea without making it bitter? To take advantage of how quickly bagged tea steeps, use a shorter steeping time than for loose-leaf tea. About 2–3 minutes is usually enough to bring out good color and aroma in most tea bags. Steeping longer quickly increases bitterness because the finely cut leaves have already released most of their tannins.
Does every type of tea turn bitter at the same rate? Black tea has undergone oxidation, so its tannins are somewhat more forgiving, and steeping it for a few extra minutes usually does not make it seriously bitter. Green tea is much more sensitive: it should be steeped at a lower temperature, and even one extra minute can make it noticeably bitter. That makes steeping time more critical for bagged green tea than for bagged black tea.
Does re-steeping the same bag work? The leaves release a large share of their tannins during the first steep, so re-steeping the same bag usually produces a weaker color and less aroma. The risk of bitterness is lower, but much of the tea's character is lost. Repeating the steep usually does not produce tea comparable to the original steep.