The invention provides operating the dry or solvent fractionation of oils and fats in a crystallizer comprising a crystallization vessel (1,2), an agitator, cooling means and a drive, wherein said drive provides said agitator with an oscillating motion, and/or a rotating motion around an axis, with the proviso that each point of said agitator moves at substantially the same linear speed.
The invention also provides a crystallizer comprising a crystallization vessel (1,2) having a polygonal cross-section, an agitator and a drive, wherein said drive provides said agitator with an oscillating motion, and wherein said agitator is not inclined to the moving direction.