Raisins are a common type of dried fruit with a sweet taste and a hint of tartness, making them very delicious. However, when used to cook congee or steeped in tea, they become very sour. So what's the deal with that?
Raisins are a very sweet type of dried fruit, and you usually don't find them too sour when eating them. But that doesn't mean they don't contain organic acids. For fruits, the sugar-to-acid ratio is a key indicator that determines the taste. If the ratio is high, the sweetness is much stronger than the tartness, and the flavor will be偏向甜 (biased towards sweet). Conversely, if the tartness is strong and the sweetness is weak, the taste will偏向酸 (biased towards sour). If the two are in a suitable proportion, it will be deliciously sweet and sour.
Raisins have a high sugar content, which masks the sourness of the organic acids, so they taste quite sweet. However, when boiled in congee or steeped in tea, the sweetness is greatly diluted, but the acidity does not decrease significantly with dilution. This is because after dilution, the organic acids increase their degree of ionization, releasing more hydrogen ions (which is the sour taste). Therefore, after using raisins to cook congee or steep in tea, the taste becomes sour.
In contrast, if you use raisins to steam steamed buns or make baked goods, the sugar in the raisins is not diluted, and the sweet flavor is preserved. It should be noted that most sweet and sour fruits and dried fruits have the same problem.