They have acids in them--citric, malic, or tartaric acids. Acids are sour.
malic acid
Plant acids are naturally occurring organic acids such as citric acid and malic acid produced by plants in their fruits and other locations. In fruits the acids give plants a tangy taste.
You can find acids in fruits, for example oranges this contains Citric acid
Fruits, particularly lemons.
papaya
There are naturally occurring sugars and ascorbic acids in fruits that cause this.
Yes, fruits contain acid.
The main acids in fruits include: malic acid (apples), tartaric acid (grapes), and citric acid (citrus fruits).
Citric Acid is common in many different types of fruits.
Yeah, citric acid for one.
No, not in the first place. More important to taste of fruits (and juices) are concentration and which kind of acids are in them.