Viscosity can be described as the inner friction of a liquid or the tendency of a liquid to resist flowing. It is measured on liquid or pasty foodstuffs, e.g. with the back extrusion test.

In this test, a die is moved up and down in a fluid test material. The measured forces at defined speeds determine the viscosity .

The back extrusion test device consists of a cylindrical container and a cylindrical compression die. The container is fixed to the machine to prevent lifting during the reverse stroke. A certain amount (min. 75 % of the container's volume) of the test material is filled into the cell. The die pushes into the specimen at a defined speed and continues until a pre-set position has been reached. When this position has been reached, the die drives back to the beginning position. During this movement, the specimen material flows through a ring slot between the container and the piston into the other container area. The force required for this action allows the viscosity to be determined.