Viscosity is the liquid-binding capacity of the raw materials. With a high viscosity, the feed becomes 'syrupy' in the digestive tract, which results in a longer digestion time. Enzymes may break down the nutrients less effectively when viscosity is too high. The muscular stomach contracts more often due to stimulation of coarse particles and rough fiber. The feed particles, once further reduced in size, move on to the small intestine.

The longer food stays in the stomach when particles are coarser. The particles influence the functioning of the muscular stomach. Particles such as ground grains provide a positive stimulation of the muscular stomach. By adjusting particle size, shape, rough fiber, and viscosity, the digestion rate can be regulated.