On the difficulty of pressing hulled sunflower seeds: challenges in mechanical oil extraction

This study investigated the cold pressing of dehulled oilseeds, specifically sunflower kernels, using an instrumented continuous press at two rotational speeds (3.5 and 7 rpm). The research combined physical measurements (pressure, temperature) with oil content analysis of the cake inside the press. At 3.5 rpm, the extraction yield reached 91% with oil flow throughout the entire cage. However, at 7 rpm, the yield drastically decreased to 43%, with oil flow limited to the first half of the press. These variations corresponded to distinct pressure profiles within the press. At high speed, cake filtration resistance became limiting, leading to oil blockage due to capillary network collapse. Unidirectional compression tests revealed that increased compression speed resulted in rapid pressure rise followed by solid expulsion, which is an indicator of resistance to filtration by the dehulled product. Contrary to the most widespread hypothesis in the literature, the poor performance of screw presses with dehulled seeds is not due to lack of friction, but to increased fragility of the oil-conducting capillary network. These findings explain the poor performance of presses operating on low-fibre matrices and suggest potential improvements through adapted compression profile designs. The study also provides new insights into solid matter evolution, proposing the concept of a dominant screw segment whose flow rate determines upstream matter reflux and restricts downstream segment feeding.