The sour and bitter tastes we find in food are due to the presence of acids and bases. Some naturally occurring acids are: vinegar (acetic acid), citric acid (present in orange and lemon), tartaric acid (present in tamarind), etc. Also our stomach produces hydrochloric acid which helps in the digestion of food. Some commonly used bases are baking soda and tooth paste. Tooth paste is a basic substance used for cleaning the teeth and it neutralizes the excess of acids present in the mouth and prevents tooth decay.
This video explains how to study the properties of acids and bases (HCl and NaOH) by their reaction with: (a) Litmus solution (blue/Red), (b) Zinc metal and (c) Solid sodium carbonate.