Final answer:
The stomach breaks down food into a semi-liquid mixture called chyme, through mechanical and chemical digestion, initiating the protein digestion process.
Step-by-step explanation:
The stomach breaks down food into a soft, partially digested mixture known as chyme. This thick, semi-liquid consistency is achieved through both mechanical and chemical digestion processes that occur in the stomach's environment.
The action of peristalsis, which is the contraction and relaxation of stomach muscles, helps to churn the food and mix it thoroughly with digestive juices. Protein digestion begins here, catalyzed by an enzyme called pepsin, in the highly acidic conditions created by gastric acid, primarily hydrochloric acid.
Protein digestion continues as the chyme is eventually passed into the small intestine, regulated by the pyloric sphincter, where further digestion and absorption take place.