Leaching or solid-liquid extraction is a separation process where often no sharp interface for mass transfer can be defined. In many natural product extraction processes, the “ solid ” contains or is impregnated by a liquid phase. In the extraction of sucrose from beet the cell walls prevent undesired and high molecular constituents from being extracted. Thus the beet is prepared in long strips in order to minimize damage to the cell walls. If fatty oils are extracted the solute is itself a liquid and will diffuse more easily resulting in a faster mass transfer. The latter is markedly affected if the solute is solid or liquid, incorporated within, chemically combined with, adsorbed upon, or mechanically in a pore structure of the insoluble material.