Rapeseed hull oil as a source for phytosterols and their separation by organic solvent nanofiltration

Abstract

Hulling of rapeseed leads to hulls as an additional valuable by‐product of rapeseed oil production, containing remarkable concentrations of phytosterols and tocopherols compared to the concentrations in the entire seed and double‐pressed kernel. It was shown that the content of these substances in hull oil is significantly higher than in the entire seed. In particular, the sterol concentration with 3% is remarkably high. Another characteristic of hull oil is the high concentration of alpha‐tocopherol. Therefore, the hull oil can be an interesting source for sterols and tocopherols, which are used as health promoting additives in food industry. The nanofiltration with organic solvent resistant membranes was tested for the enrichment of these substances. The experiments were performed in a dead‐end cell at pressures up to 20 bar at ambient temperature using n‐hexane and n‐heptane as solvents. Fluxes of pure solvent and solutions of triglycerides and sterols were determined. Rejections of sterols and triglycerides were measured in binary (solvent‐sterol or solvent‐oil) and ternary (solvent‐sterol‐oil) solute solvent systems. A five‐fold increase in sterol concentration in the solvent free permeate was possible with a single nanofiltration step. After removal of the solvent from the permeate, sterol crystallization was observed. Further investigations have to be done to optimize membrane separation and coupling with a crystallization of sterols.Practical applications: The hull fraction of rapeseed can be used as a source for phytosterols and tocopherols which are both food additives. Thus, in addition to the advantages with respect to the quality of the oil and protein, hulling leads to additional valuable products, which are oil fractions rich in sterols and tocopherols. The feasibility of an organic solvent nanofiltration process for the separation between unsaponifiables and oil is demonstrated.Rapeseed hull oil has a remarkably high content of alpha‐tocopherol and sterols. It was shown that organic solvent nanofiltration is a process, which can be used for a partial separation of sterols and triglycerides. An enrichment by a factor of five is possible, leading to oils with such a high sterol concentration that crystallization occurred.