Air Classification of Pin‐Milled Soybean Hulls

Abstract

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.

Processing of soybean hulls to enhance the distribution and extraction of value‐added proteins

“…Nitrogenous components and lipids are mainly distributed in the finer fractions. In the investigation by Wolf et al, 16 scanning electron microscopy revealed that fractions 1 and 2 consisted mainly of parenchymal cells, which represent the innermost portion of the hull; fraction 3 contained mainly hour-glass cells, typically found in the middle layer of the hulls, plus parenchyma cell materials; and fractions 4 and 5 included hour-glass cells and clumps of palisade cells (outer cellular layer) adhering to each other.…”

“…After two additional regrindings the hulls were fractionated by particle size in a laboratory-scale air classifier (Pillsbury, Minneapolis, MN, USA) as described by Wolf et al 16 Fractions of size <15, 15-18, 19-24, 25-30 and >30 µm were collected. Each fraction was defatted with diethyl ether and extracted with water as described previously.…”

“…Soy hulls from source B were pin-milled and airclassified into five fractions-1, <15 µm; 2, 15-18 µm; 3, 19-24 µm; 4, 25-30 µm; 5, >30 µm -as reported by Wolf et al 16 The proximate analysis of the milled hulls and the five air-classified fractions is given in Table 3. Nitrogenous components and lipids are mainly distributed in the finer fractions.…”

“…Expedient measures that ensure volume conversions of seed components to carbohydrate, protein and oil, for example, waste otherwise useful structural elements and bioactivities that should be considered part of the inherent value of the commodity. The approach to be used incorporates two principal thrusts: (1) technology for the systematic and economical disassembly of soy hulls using selective air classification; 16 (2) use of processing techniques to concentrate the proteins free of the coextracted carbohydrate components.…”

Structure and physicochemical properties of defatted and pin‐milled oat bran concentrate fractions separated by air‐classification4

“…, 1997). Protein enrichment in soybeans has been achieved by removing hulls during air‐classified fractionation (Wolf et al. , 2002).…”