Hydrocyclone separator for starch refining is based on the difference in specific density of water, fibres and starch. The following is the density of starch, fibers and water:

In the strong gravitational fields of a hydrocyclone separator, starch settle quickly, while fibres (pulp residuals) just float in the water. The juice is directly diluted in the water and goes with the water phase.

Hydrocyclone separator

By creating a water flow moving towards the starch, lots of fibres just floating in the water may be forced into the overflow in hydrocyclone separator. Soil, sand and many fungi etc. are of equal density or heavier than starch and it is not possible to separate these particles from starch by centrifugal force - that is why it is so important to remove as many impurities as possible from the potato surface in the washing station.

Although some impurities go with the starch in the underflow, there is - by means of a sieve - a last chance to remove the larger particles - that is particles larger than 125 microns. The particles are not spherical. On the contrary, they are of irregular shape and may be forced through refining sieve, if the spray pressure is too high.

Impurities not removed this way are not removable by any known technique.

In the recovery steps of hydrocyclone separator,  all starch has to be retained in the underflow so only very little is wasted in the effluent (fruit water)