What experience can prove that gastric juice digests proteins only in an acidic environment?

Gastric juice itself has an acidic pH.
Bring the pH not even to neutral or alkaline, but only to a value of 6 – the digestion of proteins will already stop.
Before entering the stomach, food is soaked in saliva. Saliva continues to act inside the food lump, gastric juice acts on it from the outer surface. The stomach lining contains many digestive glands. Gastric juice is made by the release of certain chemicals. It consists of water, mucus, hydrochloric acid, enzymes. Hydrochloric acid, in turn, creates an acidic environment for chemical reactions and disinfects food. An enzyme such as pepsin accelerates the breakdown of proteins. Amino acids are formed. In the small intestine, amino acids are absorbed into the bloodstream. Through the portal vein, they are delivered to the liver. From the amino acids received, only those that are needed by the body are selected. The rest are converted into carbohydrates or fats, or amino acids needed by the body are formed from them. There are amino acids that cannot be formed in the human body, they are called essential and it comes only with food. The blood delivers amino acids to the cells. This is where our own proteins are created.