**3.2 Dry extraction**

Dry fractionation can produce protein-enriched products which possess native functional properties. This is mainly because the process is mild [5]. Dry processing involves physical separation of starch and protein. It relies on the principle that milling can separate protein bodies from other seed components to give flour streams that is fractionated into different components. This has applications in starch rich legumes such as pea and faba bean. The method is however not applicable to grains that are rich in oil. In general, size of starch granules partly determines suitability of a particular legume for dry fractionation [5]. Milling is considered effective when it removes the protein bodies from starch granules, the latter being bigger in size. For pea and faba bean, starch and protein can be separated by milling followed by air classification. During air classification, protein separate as fine particles while starch are the coarse particles. Protein are separated as the light fraction, and starch as the heavy fraction based on their different shape, density, and size. This results in protein and starch enriched fractions.

Dry process involves milling and air classification. Starch granules are separated from protein bodies during fine milling usually by pin milling. This enables their separation during fractionation by air classification. Two processes are therefore involved in dry processing, namely, reducing particle size by milling and separation of the particles using their shape, density, size, and electrostatic properties [42]. The starch rich fraction is re-milled and fractionated. Particle size and shape are the main properties which are manipulated during dry extraction. The particle size can be varied by size reduction in the range, coarse, >500 μm; fine, 50–500 μm and ultrafine, <50 μm. Particles sizes selection enhance protein and starch enrichment and also reduce content of undesirable components such as anti-nutrients. Impact mill can produce the above range of particle size, while hammer milling can be used to produce only coarse and fine particle size. Ultrafine milling is used for protein and starch enrichment with pulses such as peas and faba beans. Air classification is applied after fine milling to obtain protein enriched fraction. The resulting end product has its protein content doubled compared to the raw material. Dry milling of faba bean and Lima bean have been reported to give protein yields of 63–75% and 43–50%, respectively [5, 42]. Air classification has not been found to be successful with oilseeds due to generation of free lipids [5].
