This depends on the type of enzyme. The enzyme pepsin breaks down proteins in the acidic conditions of the stomach. Pepsin has an optimum of pH 2.5 and a working range of between pH 1-4.

Glands in the inner lining of the stomach make gastric juice. It contains stomach acid, water and a protein called pepsin. It begins to flow as soon as we smell something. The stomach muscle contracts ...

The esophagus empties into the stomach, a large, muscular chamber that mixes food up with digestive juices including the enzyme pepsin, which targets proteins, and lipase, which works on fats.