Fractionation of pea flour with pilot scale sieving. I. Physical and chemical characteristics of pea seed fractions

Facing a lack of methods and criteria to characterize feed particle properties, a grinding-fractionation process was applied to pea seeds to obtain different particle size fractions, physically and chemically defined. Seeds were first ground with a hammermill on a 4 mm screen. The flour obtained was divided into nine granulometric fractions with a sieving classifier using a large range of mesh openings from 2.5 to 0.122 mm. To interpret the results, hulls and kernels were separated with an air classification device and ground on a 0.5 mm screen. Classification efficiency of the granulometric fractions was checked. Each fraction was characterized by chemical composition (crude protein - CP, starch, crude fiber - CF, water-insoluble cell walls - WICW and ash) and also physically by particle size distribution, median diameter (d(50)), a dispersion parameter (logarithmic standard deviation), specific surface area (SPSA), total porous volume (TPV) using a nitrogen adsorption apparatus, bulk density and apparent density (DENap. - helium multipycnometer device). The fractionation process led to well-differentiated fractions. Great differences in chemical composition of the granulometric fractions were noted. The smaller the size, the higher the contents of both CP (from 21.8 and 25.7% DM) and starch (from 47.9 to 62.2% DM), and the lower the cell wall contents (CF from 0.9 to 11.8% DM and WICW from 5.9 to 22.4% DM). Within a range of d50 (from 59 to 2720 mu m), the SPSA (from 0.102 to 0.418 m(2) g(-1)) increased with particle fineness, and seemed to be an interesting criterion to characterize specifically each fraction. The TPV values were low (from 0.1 to 0.8 mm(3) g(-1)) and the densities values showed irregular evolution on the granulometric distribution (bulk density from 530 to 666 g dm(-3) and DENap, from 1.427 to 1.465 g cm(-3)). A general significant evolution (p < 0.05) of chemical composition according to the geometry of the particles (d(50) and SPSA) appeared. The chemical and physical characteristics of the size fractions seemed to reflect the different comminution laws followed by the hulls and the kernels, while grinding pea seeds. This phenomenon would have led to a physical separation of the botanical constituents of the pea seed, with coarse fractions in which hulls are accumulated together with a majority of parietal constituents, finer fractions enriched with kernels and cellular constituents and the smallest fraction mainly composed of starch granules. (C) 2000 Elsevier Science B.V. All rights reserved.