How is a hamburger digested in the stomach?

Bread contains starch, it is digested since you put it into your mouth. Your saliva contains amylase, thus breaking down the starch molecules. When it reaches your duodenum, pancreatic amylase digests the remaining starch molecules. Even into the intestine, it keeps being digested and absorbed.

The lettuce contains fibers which helps the movement(peristalsis) of your gastrointestinal gut by rubbing its wall, in turn results in increased of enzymes secretion. The absorption of vitamins carries out when it reaches your small intestine.