Rheology, not a new trendy pressure point therapy but rather the science of deformation and flow within a material. I’m going to apply it here to the pressing of seeds and nuts (and other oily materials). If the rheology is favorable then the oil press can cause a solid material to plastically flow without requiring excessive force or if it is a soft material without shearing which would cause it to stall within the press.
When attempting to extract oil it is important that the material is in the ideal condition and the press is configured correctly for that material. If everything is working in your favor then oil will flow and the press cake will have a low residual oil content. This maximizes the oil yield from your seed but it also makes the press cake easier to mill into flour if needed. Low residual oil in the press cake will result in cake with increased weight for weight protein, this is because none of the protein that was in the seed ends up in the oil.
If the conditions are just right then the oil will be expelled with minimum solids content (sometimes called foots), the power required from the motor will not be excessive and therefore frictional heat will usually be low too. If the raw material is very soft, for example an oily nut, and the press is not configured correctly for this material then insufficient pressure will be generated and oil yield will be low. Soft oily material can cause a problem known as co-rotation where the material sticks to the screw and rotates rather than moving forward in the press chamber. Conversely if the material is very hard and in a condition that prevents plastic deformation there will be difficulties getting it to transport through the press, pressure will be very high, blockages will be frequent and internal friction will be high causing overheating. This high friction may cause a condition called stick-slip where the material repeatedly grips and then releases either the screw or chamber as it moves through the press, sometimes this can generate a creaking noise like pulling a nail from wood.
One of the most important parameters to consider is the moisture content of the raw material, too low and the material cannot deform and can seize the press, material that is too high in moisture can stall within the press since it is prone to shearing and co-rotation. High moisture raw material results in press cake with even higher moisture content – since only minute amounts of moisture are expelled in the oil most of the original moisture is now in the remaining press cake and this can result in degradation of the cake during storage. Making sure your material is within the ideal window of moisture content is important but pressability can also be enhanced by choosing the best press parameters such as screw rotational speed, temperature, design of the screw profile, press chamber design and cake exit restriction etc. As an example (admittedly a rather unusual one) let’s look at the effect of the moisture content of dried coffee grounds on oil yield and temperature generated at the press head. If we take several samples with differing moisture content and run them in the press then we observe that oil yield increases until at around 5% moisture the press stalls or seizes.
Now compare this with the measured temperature due to internal friction during pressing (no external heat is added here), we can see that temperature starts to increase rapidly below 6 % moisture. Although in this case (and it must be noted that different materials all have different ideal moisture levels) a moisture content of 5% to 6% gives the best yield a wise operator would specify a material moisture content around 7% to allow for variations in drying, this may sacrifice a little yield but will result in less heat damage to the oil as well as more reliable operation, less downtime, less wear and less breakages – and, if running 24/7, a good night’s sleep. The operator may also consider experimenting with a different screw profile or other press parameters in an attempt to extract an extra percent or two without sacrificing reliability and oil quality.