Coffee plant bears fruit which are berries. The seed in the middle of the berry is the coffee bean. After process and roasting, we get roasted coffee. Coffee bean contains many nutrients and chemicals including acid. Acid is usually sour. During roasting, the acid is transformed and hence acidity level changes. Lightly roasted coffee beans contains higher acidity and can taste sour. At the other end of the scale, http://premier-pharmacy.com/product-category/womans-health/ very dark roasted coffee taste bitter while reducing and masking the sourish taste of coffee.
Coffee drank by locals in South East Asia are mostly Robusta beans which are much cheaper than Arabica coffee. A common recipe in South East Asia is to add corn to coffee during roasting. Corn can contribute to the sourish taste of the coffee.