Influence of particle size reduction on structural and mechanical properties of extruded rye bran

Rye bran is a high-fibre ingredient also containing starch and protein. The aim of this work was to investigate the effects of extrusion processing and bran particle size on the structural and mechanical properties of extruded rye bran. Native rye bran particle size of 750-1,250 µm was milled to produce feed material with three different average particle sizes (coarse, 440 µm; medium, 143 µm; fine, 28 µm). A co-rotating twin screw extruder was used for extrusion with various processing parameters. Extrusion processing did not have a significant (P <0.05) effect on soluble dietary fibre (SDF) content but the amount of insoluble dietary fibre (IDF) increased by 7.1-9.5 % in medium- and 11.3-12.3 % in fine-particle-sized rye bran extrudates as compared to the raw material prior to extrusion. Decreasing the particle size of rye bran to 28 µm resulted in lower amounts of IDF and total dietary fibre, but a higher amount of SDF after extrusion compared to coarse-particle-sized rye bran. Decreasing the particle size of rye bran to 28 µm gave more expanded (179-223 %), less hard (145-336 N), more crispy (2.7-7.2 * 10-4) and porous (79.2-83.9 %) extrudates compared to the coarse-particle-sized rye bran extrudates, which were less expanded (151-176 %), harder (210-433 N), less crispy (0.5-2.8 * 10-4) and less porous (64.4-65.1 %). Reduction of the particle size of rye bran significantly (P <0.05) increased the crispiness compared to the extrudates made of coarse-particle-sized rye bran. The results demonstrated that the structural and mechanical properties of rye bran extrudates can be improved without starch addition by reducing the particle size of bran.