Taste

We perceive taste though chemical reactions that take place on the tongue. The tongue has different receptors that react with different types on molecules and ions in the food. This gives us information about the chemical content of the food. 

We now generally recognize five categories of taste:

• Sweet taste is a reaction with dissolved glucose (or sucrose or some other similar molecule). This tells us the food has a lot of easily accessible energy content. If you're an animal this is a very good thing, so sweet tastes good.
• Salty taste is the detection of Na+ ions. Salt (NaCl) dissolves into Na+ and Cl- ions in water, so if our saliva has a lot of Na+ ions we must be eating something salty. Again, in nature salt can be scarce, but is critical for survival, so it tastes good. 

• Sour taste is detection of H+ ion concentration in the saliva. This means that sourness is the same as acidity. 
• Bitter is triggered by a variety of molecules, many are alkaloids which tend to be basic in pH. This usually comes from plants that are producing it as a defensive poison, so the body's first reaction tends to be to label the taste as bad or even gag. Bitter foods like coffee and beer tend to be acquired tastes.   
• Umami is detected by reacting with various proteins, and thus indicates protein-rich foods like meat. This is a savory taste.