Fractionation of rapeseed meal by milling, sieving and air classification-Effect on crude protein, amino acids and fiber content and digestibility

Rapeseed meal (RSM), obtained as solvent extracted or expeller meal, is a feed commodity that is highly available. The high levels of fiber is a bottleneck for high inclusion in feed for monogastric farmed animals. In the present study, sieving and air classification were used to reduce fiber content in rapeseed products. The two first experiments unveiled the possibility to air classify rapeseed products with lipid content ranging from 20 to 160 g/kg, and to obtain fractions where crude protein (CP) content was increased from 325 to 376 g/kg and neutral detergent fiber (aNDFom) was reduced from 185 to 78 g/kg. Experiment 3 showed that ball milling of RSM in combination with sieving gave high separation of hulls and kernel. In the finest sieved fraction (0-150 mu m), CP was increased from 336 (parent meal) to 394 g/kg with a fraction yield of 423 g/kg. Air classification of pre-sieved RSM had minor effect on CP and fiber levels, indicating a limited potential to further increase CP content when the hulls have partly been removed. Coefficient of total tract apparent digestibility (CTTAD) of CP, amino acids and lipids in RSM fractions obtained with ball milling and sieving was determined in mink (Neovison vison). The average CTTAD for CP was higher (P < 0.05) in the high CP fine fraction (0.748) compared to the parent meal (0.702) and the coarse RSM fraction (0.635). In general, the CTTAD for amino acids followed the same trends as for CP, with significantly lowest digestibility for the coarse RSM fraction containing most hulls. The CTTAD of threonine and lysine was lowest among the essential amino acids, while cysteine had the lowest CTTAD among non-essential amino acids. To conclude, ball milling and sieving showed higher potential for fiber removal from RSM than ball milling and air classification. The reduced fiber content and increased CP content resulted in a higher digestibility of CP and amino acids.