Jenni Järvinen completed her Master Thesis titled "Extrusion of rye bran: Effect on physico-chemical and mechanical properties of extrudates" under supervision of Dr Nesli Sozer (VTT) and Prof Simo Laakso (Aalto University). The aim of the Master's Thesis was to identify the basis of rye bran extrusion and study how extrusion processing and raw material particle size affect physico-chemical and mechanical properties of extrudates. Another objective was to study a bran-enriched starchy model snack food and evaluate the effects of bran concentration and particle size on product quality.
Extrusion cooking is a short time, high temperature-high shear process, which effectively modifies functional properties of food ingredients and texturizes them. It is a versatile technique used to produce a wide range of products, such as puffed snacks and breakfast cereals. Rye bran may be used as an ingredient in extruded foods in order to increase their dietary fibre content and to improve the nutritional value.
Rye bran of three median particle sizes (coarse: 450 µm, medium: 150 µm and fine: 28 µm) was processed by a co-rotating twin-screw extruder under different conditions of screw speed (300 or 500 rpm), barrel temperature profile (40-70-75-75-90-110-110 °C or 40-70-75-90-95-130-130 °C), feed moisture (17 % or 19 %) and hydration regimen (in barrel or preconditioning). The effects of extrusion and raw material particle size on dietary fibre profile, starch gelatinization, water absorption and solubility, pasting properties, macro- and microstructure and mechanical properties of extrudates were studied. In addition, a bran-enriched (15/30 %) model snack food based on rye flour and corn starch was designed and the effects of bran particle size (coarse-fine) and concentration on product quality were evaluated.
Extrusion had no significant effect on dietary fibre composition of coarse rye bran. For medium and fine rye bran insoluble dietary fibre content increased by 7.1-9.5 % and 11.3-12.3 % compared to raw bran. The total dietary fibre contents for medium and fine particle sized bran increased by 6.2-9.9 % and 9.0-10.5 %, respectively. Soluble dietary fibre content was not significantly changed by extrusion processing. Initial and peak viscosities of rye bran increased upon extrusion due to gelatinization and swelling of starch, whereas final viscosity decreased as a result of molecular breakdown of starch granules and molecules. Particle size reduction of raw material had a distinct positive effect on quality of extruded products. Micronization of bran particles to 28 µm improved crispiness of rye bran extrudates by promoting expansion, increasing air cell size and porosity. This study also showed that it is possible to produce directly puffed extrudates based on rye flour and corn starch with 30 % fine rye bran concentration without interfering with textural and structural properties.