How does mechanical pressing extract oil from seeds and nuts?

Mechanical pressing extracts oil from seeds and nuts by physically rupturing the cellular structures that contain the oil. The process typically follows a series of preparatory steps:1

• Preparation: Seeds are cleaned to remove debris and metal, and shells or hulls are removed. The kernels or meats are then ground, flaked, or rolled into a coarse meal to break down cell walls.21
• Pressing: The meal is subjected to high pressure in hydraulic presses or modern continuous screw presses. In a screw press, a worm screw moves the material through a slotted barrel, progressively increasing pressure—sometimes up to 30,000 pounds per square inch—to squeeze the oil out through the slots.312
• Temperature Control: Pressing may be performed with or without preliminary heating. Cold-pressed oil is produced without heat, resulting in higher purity and better flavor. Heating the meal before pressing increases the oil yield but also extracts more impurities.21

After pressing, a residual cake remains that still contains 3 to 15 percent oil, which is often further recovered using solvent extraction.24