Question

Why lime, orange juice, tamarind, etc. sour in taste.

Answer 1

Lime, orange juice tamarind, etc. taste sour because they contain very weak acids. Acids, in general all taste sour and have the ability to convert litmus paper from blue to red.

Complete answer:
Lime, orange juice, tamarind, etc. taste sour because all of these contain acids. Acids are generally sour in taste.
Acids can be defined by two methods
Bronsted-Lowry concept – According to the Bronsted-Lowry concept, an acid is an ion or molecule that can donate a proton (hydrogen ion - ${H^ + }$ ). In other words, a Bronsted or Arrhenius acid contains a hydrogen atom bound to a molecular arrangement that is energetically useful despite the loss of ${H^ + }$ ion.
Lewis concept - Lewis acids, which form a covalent bond with an electron pair, constitute the second group of acids. Lewis treated this as a generalization of the concept of Bronsted, such that an acid is a chemical species that either explicitly accepts electron pairs or releases protons ( ${H^ + }$ ) into the solution, and then accepts electron pairs. Hydrogen chloride, acetic acid, and most other Bronsted-Lowry acids, however, are not Lewis acids and do not form a covalent bond with an electron pair.
Most of the acids used in normal life are aqueous solutions, which may be dissolved in water, so the meanings of Arrhenius and Bronsted-Lowry are the most important.
Acids, with a sour taste, form aqueous solutions, can turn blue litmus red, and react to form salts with bases and certain metals (like calcium).

Note:
Arrhenius or Bronsted – Lowry acids are not all Lewis acids and vice versa. An acid is, in modern terminology, implicitly a Bronsted acid and not a Lewis acid, although chemists almost often refer to a Lewis acid directly as a Lewis acid. Remember this is just a convention and nothing more.