Starch is an industrial and food raw material of major importance. Starch for food use is commonly extracted from starchy plant constituents (cereal grains, seeds, tubers) using procedures that involve reducing the plant material to fine particles, washing the starch from the insoluble plant material as a suspension in water, then removing the starch from the suspension, usually by a process involving filtering or centrifugation in some form. To obtain starch of high purity, it is important that the non-starch plant materials have, or can be made to have, differences in solubility or density, compared to the starch.
The process of separating starch from insoluble and soluble components of plants is an important step for obtaining purified starch, but it is also important for the isolation of other plant components that may be valuable, where removal of the starch is a required step.
Industrial extraction of starch using procedures based on the above principle is applied in various forms. For example, starch is isolated from potatoes and cassava by rasping the raw material to open the plant cells, reducing the solubility of proteins by reaction with sulphur dioxide or sodium bisulphate, then washing and extracting the starch. Starch is isolated from maize (zea mays) by steeping then wet milling the corn to remove the germ and husk, fine grinding, then applying centrifugal separation and filtering techniques to separate the starch from the proteins. Starch is isolated from wheat by mixing milled wheat flour with water to a dough then washing the starch from the insoluble wheat gluten.
Starch may also be isolated from plant materials by non-aqueous extractions. For example, in a review of procedures for obtaining starch from barley, McDonald and Stark (1) identified several procedures, but considered none food safe or industrially applicable.
The aqueous separation processes described above are applicable to a limited number of plants, and thus most industrial starch is made from only four sources: potatoes, cassava, maize and wheat. Rice starch can be isolated from rice using a process involving steeping in alkaline solution, but with low efficiency. The main barrier to extraction of starch from other plant sources is the difficulty in reducing the solubility of large non-starch plant polymers sufficiently to allow separation from the starch.