Cold pressing

The fruit is washed and fed into a machine with a rasping surface or spiked rollers. The surface scrapes and presses the outermost layer of peel, bursting the oil sacs just below it.

The mixture of oil, juice and peel fragments is collected and then centrifuged to separate the three phases. The oil is the lightest phase and is drawn off from the top.

There is no heat at any stage. That is the defining feature of the method, and the reason the product keeps molecules that distillation would burn off.

It keeps almost the whole smell of fresh peel, including components that distillation would burn off. That is why cold-pressed orange oil smells like a real orange, while distilled orange oil smells like the memory of one.

It works only for citrus peel, where the oil lies right at the surface. The product carries non-volatile compounds with it, so it is photosensitising and has a much shorter shelf life than distilled oil.

Cold-pressed oils are usually coloured — yellow, green or orange depending on the species — while the distilled product of the same species is almost colourless. The colour comes from non-volatile pigments that only a mechanical method can carry over.

They are also light-sensitive and have a much shorter shelf life. If a bottle of citrus oil states a three-year shelf life, either it is not cold-pressed or the date cannot be trusted.

Cold pressing is older than distillation in principle — people peeled oranges and smelled the oil spraying out long before any equipment existed. Industrialisation took place in Sicily and southern Italy from the eighteenth century, where citrus peel was an abundant by-product of the food industry.

The traditional method pressed sponges against the peel to soak up the oil, called spugna. Today machines have replaced hands, but the mechanical principle is intact.