Digestion of food starts from the mouth. While eating your food, chewing with your teeth, a lot of things are going on there. The saliva mixing with the food contains enzymes such as amylase that breaks down the carbohydrate content in the food.
The semi-digested food is then transported via the esophagus into the stomach where it is mixed with stomach acid and other enzymes. This breaks the food into small digestible pieces which is then circulated round the body system.
The stomach is lined by a mucous membrane called gastric mucosa. This forms a barrier between the food and the stomach.