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Air Classification of Pin-Milled Soybean Hulls¹

¹Presented in part at the AACC Meeting, Charlotte, NC, October 2001. Contribution from USDA-ARS, National Center for Agricultural Utilization Research, Peoria IL 61604. ²Retired. ³Plant Polymer Research Unit, USDA-ARS, National Center for Agricultural Utilization Research. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others that may also be suitable. ⁴Corresponding author. Fax: 309-681-6686. E-mail: sessadj@ncaur.usda.gov ⁵Fermentation Biotechnology Research Unit, USDA-ARS, National Center for Agricultural Utilization Research. ⁶Analytical Chemistry Support Unit, USDA-ARS, National Center for Agricultural Utilization Research.

Commercial soybean hulls (14.6% crude protein) were pin milled and then air classified into five fractions to determine whether the hull structure can be disrupted and the protein constituents concentrated. The number of pin millings had only a small effect on the weight distribution of the five fractions. After one grinding, the sum of fractions 1 and 2 (<15–18 μm) represented only 3% of total hulls, and on three grindings, they amounted to only 6% of the hulls. Fraction 3 (19–24 μm) shifted from 16 to 20% with three grindings, while fraction 4 (25–30 μm) remained unchanged at 5%. Fraction 5 (>30 μm) shifted from 75 to 69% on three pin millings to compensate for the shifts noted in fractions 1–3. Scanning electron microscopy revealed that fractions 1 and 2 consisted of the parenchymal cell layers (innermost portion) of the hulls; these fractions had three times the protein content and more than twice the amount of lipids found in the ground starting material. Fraction 3 contained many of the hourglass cells typically found in the middle layer of the hulls plus parenchyma cell material and exhibited about twice the amount of protein and lipid found in the starting hulls. Fraction 4 included large hourglass cells plus globular material and contained about twice the amount of protein and about one and one-half times the lipid of the starting material. Fraction 5 consisted primarily of clumps of palisade cells (outer cellular layer) adhering to each other and had a lower protein and lipid content than the starting hulls. Thus, pin milling causes some selective disruption of the hulls where parenchyma cell layers and hourglass cells are partially released. The palisade cells, however, are the most difficult to disrupt and constitute the fraction most resistant to pin milling. Our findings suggest that pin milling soybean hulls in combination with air classification can be used to concentrate the proteins and lipids in the fines fractions.