Ácido is the regular term to describe the naturally sour taste of foods such as citrus fruits (e.g. lemons). Azedo is often used as a synonym of ácido, but it can also have a negative connotation, suggesting that the food went bad. For example, when we say that a bowl of soup or a glass of milk taste azedo/azeda, we mean that they have a sour taste that they shouldn’t have, so they’re not safe for consumption. This most likely isn’t the case when talking about lemons 
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