Digestive juice is the name for all juices secreted by the organs from the digestive system such as salivary glands, stomach, pancreas and liver. All of them contain enzymes that are responsible for food (macromolecule) break down. Digestive juice from the stomach contains enzyme protease (pepsinogen) which breaks down proteins and stomach acids which is involved in chemical breakdown of food materials. So, that is why the wheat grains are broken into tiny particles.