Effect of extrusion processing on lipid stability of rye bran

The aim of this study was to investigate under which processing conditions the extrusion of rye bran could produce a material with a long shelf life in terms of lipid chemistry and possibilities to link lipid stability with physical properties of the extrudates. In addition to water content (13-30%) and temperature (80-140°C) as variables during extrusion, the effect of rye bran particle size (633 and 15 µm) was studied. Stability of lipids was measured by volatile compounds and the loss of tocopherols and tocotrienols. Of the studied process parameters, the water content had a significant effect both on the physical properties and on lipid stability. Low water content (13 or 16%) in the extrusion of coarse or fine bran led to the highest expansion and porosity and the most stable lipids during storage. The improved oxidative stability when processed at low water contents in extrusion was connected with a higher formation of Maillard reaction products, which could have acted as antioxidants. In spite of the loss of tocols during grinding, the lipids in the fine rye bran extrudates were at least as stable as the lipids in the coarse rye bran extrudates when processed at low water content. However, the high furfural content in the fine rye bran extrudate at 13 % indicated that Maillard reaction was extensive which could lead to undesirable flavour. Thus, the overall best performance was obtained when the coarse rye bran was processed at the lowest water content.