Pressing and rendering recover oils and fats without any solvent: pressing squeezes oil from seeds and nuts with a screw or lever press, and rendering melts animal fat slowly away from connective tissue to give tallow or lard. Oil recovered is the product of seed mass, oil fraction and press recovery, so a better press is worth more than more seed. Never heat fat fast or leave it unattended, because a fat fire cannot be put out with water.
Part of Organic Chemistry
Mechanical and thermal separation of oils and fats from seeds and animal tissue, the feedstocks of soap, candles, fuel and food.
Two Mechanical Routes To Oils And Fats
The carrier splits the job in two.
Cold pressing. Mechanical pressure extracts oil from seeds and nuts, using a screw press or a lever press. The carrier lists olive, sunflower and linseed oil as products and notes linseed as the basis of paint and wood finish. For the food-processing treatment see Oil Extraction, and for press design see Seed Pressing.
Rendering. Heat animal fat slowly to melt it and separate it from connective tissue. The products are tallow (beef and sheep) and lard (pig). The carrier calls these essential for soap, candles, waterproofing and cooking. The wet and dry methods and their yields are in Animal Fat Rendering.
What You Need
- A press, or a heavy weight and a perforated container, to squeeze seed paste.
- A kettle, heavy and deep, for rendering, with a lid large enough to smother a fire.
- Clean cloth to strain.
- Cool, dry storage in clean jars. Oil and fat go rancid when they meet air, light and heat; this is a fact of ordinary food storage, not a corpus figure.
Method: Pressing And Rendering
Pressing. Crack or crush the seed; the carrier’s “cold pressing” means no added heat. Press slowly and let the oil drain, then settle it and decant the clear layer from the sediment.
Rendering. Cut the fat small, add a little water to the kettle if rendering wet, and heat gently, stirring. The fat melts out and the connective tissue shrinks into cracklings. Strain through cloth into clean jars. The fat is ready when it is clear and no longer spits, which signals the water has gone.
Worked Example: What A Press Is Worth
Assume 100 kg of seed that is 40% oil by weight (an illustrative figure, not a measured one). The seed contains 100 × 0.40 = 40 kg of oil.
- A press recovering 50% of it gives 40 × 0.50 = 20 kg.
- A press recovering 30% gives 40 × 0.30 = 12 kg.
The better press gives 20 − 12 = 8 kg more oil from the same harvest, equal to 8 ÷ 12 = 67% more. Doubling the seed would be far more work than building a better press.
Common Mistakes
- Rendering too fast or too hot. Fat that smokes is burning and tastes acrid, and at higher heat it ignites.
- Letting water meet hot fat. A splash of water in fat above 100 °C flashes to steam and throws burning fat. Never pour water on a grease fire; smother it with the lid.
- Storing wet fat. Moisture left in a render spoils the batch; render until the sizzle stops.
- Burning the cracklings and then using the fat anyway. It carries a scorched taste into soap and food.
Safety
A fat fire is a chemical and a burn hazard
Heat fat slowly, keep a fitting lid beside the kettle, and never leave it unattended. If it ignites, cut the heat and smother with the lid. Do not use water. Fat that has spilled onto the ground near the fire can carry flame back to the kettle. Hot fat causes deep burns: handle with leather gloves and keep children and animals away.
What’s Next
- Learn next: Biodiesel — one use of the oil you pressed
- Combine with: Saponification — turning fat into soap