Cold-pressed

Most people are surprised to hear that oils labeled 'cold-pressed' are not cold-pressed at all. Store owners and customers alike believe that when oil is labeled cold-pressed (wrongly implying that the oil remained cold while being pressed), it is nutritionally superior; and they also believe (rightly, in part) that heat destroys oils. What does 'cold-pressed' actually mean?

History of cold-pressed

'Cold-pressed' is a term searching for a meaning. Since neither industry nor government has an agreed-upon definition, anyone is free to invent one that suits their particular purpose.

One company calls oils that have been heated to very high temperatures during deodorization 'cold-pressed' because "no external heat was applied to seeds while they were being pressed." This definition belies the fact that external heat is never necessary in modern presses, because the pressing itself produces heat due to pressure and rotational friction. It also belies the heat applied before and after pressing, including deodorization temperatures of over 200°C (400°F) for up to an hour.

'Cold-pressed' is a translation of the German words 'kalt geschlagen', which literally mean 'cold pummeled'. A hundred years ago in Germany, oil was produced at home in very slow, mallet-hammered, manually-operated wedge presses. Seeds were poured into a wedge-shaped container, and a wooden wedge was driven into it. Every hour or so, the housewife would hit the wedge with a wooden mallet, and the oil would drip for an hour. Then she would give it another whack and the oil would drip again. She would carry this on all day, to produce the oil needed for her household. 'Kalt geschlagen' meant that no heat was applied to either seed or oil, and referred to a completely natural, unrefined crude, fresh, high-quality oil.

The term is still used in Germany today for oils made more by more modern methods. It gives the sense of the quality of the old method without actually employing that method. In North America also, the term is widely used for oils that are made by heat-producing methods.

In fact, it is almost impossible today to find oils commercially pressed without heat. Note: Virgin olive oils are one exception; the other exception is a brand of peanut oil available in the natural foods trade, which is made by the old hydraulic pressing method that produces no heat.

Seeds were cooked to increase oil yield even when hydraulic presses were being used 80 years ago. Screw (mechanical, expeller) presses generate heat by friction as seeds and crushed material are simultaneously compressed and rotated into a squeeze. Heat makes oils run out of seed meals faster. The higher the heat, the less oil remains in the pressed seed cake, the more efficient the operation, the better the price and profit, and the less waste.

The lowest temperature at which it is possible to expeller press oils in small presses is around 50°C (122°F), although the temperature inside the press head gets higher than that. Inside, small presses heat up to between 54 and 72°C (130 to 160°F); the next size up, 65 to 85°C (150 to 180°F). Huge presses run even higher temperatures. It is customary in the industry to measure the lower temperature of the oil dripping out of the press and call that its pressing temperature, although it would be better to call that the 'dripping' temperature.

In Switzerland, 'cold-pressed' is defined to mean that oils have reached temperatures not exceeding 50°C (122°F) during their entire journey from seed to bottle. In North America, there is no such agreed-upon definition, so anything goes. The usual temperature of oil that drips out of huge presses may be between 85 and 95°C (185 to 203°F). Inside the press, the temperature is somewhat higher, and some presses generate so much heat under the tremendous pressure and friction at which they operate that the oil dripping out of the machine has a slightly burned taste. Some people prefer this taste, and some oils on the market contain added burned flavoring to cater to this taste preference.

Pressing oil with minimum heat

Oils should be pressed with minimum heat for two reasons.

First, as temperature increases, chemical reactions speed up. For every 10°C (18°F), the speed of chemical reactions more than doubles. The higher the temperature of the oil, the faster it is destroyed by light, oxygen, and other chemical reactions. This can be minimized by excluding light and air from the pressing process. Pressing facilities usually run without this protection.

Second, internal changes take place in oil molecules at high temperatures. Unsaturated fatty acids may twist into unnatural trans- configurations, or fatty acids may crosslink, oxidize, dimerize, or polymerize, changing the shape and properties of the fatty acid molecules, destroying their nutritional and biological value, and making them toxic.

These processes begin to take place measurably when oil temperature reaches about 160°C (320°F), and become really serious above 200°C (392°F). Oil pressing temperatures rarely exceed 100°C (212°F). Thus, the heat produced during pressing is not a major problem if light and air are excluded from contact with oils. 'Cold-pressing', in this sense, is based on fiction and ignorance. It offers no quality advantage.

The term is meaningless. Its use by manufacturers is unethical, to cater to uninformed consumers who still believe that 'cold-pressed' means high quality. For quality, it is more important that the oil was protected from light and oxygen during pressing, and was also sheltered during bottling, storage, and shipping.

Too hot

Deodorization, carried out for about an hour at high temperature (245°C, 473°F), destroys the nutritive value of the oils, and produces trans- fatty acids and chemical changes.

Hydrogenation, used to turn liquid oils into semi-solid or solid fats, is carried out at a temperature of 250°C (482°F) for several hours. Hydrogenation purposely creates trans- fatty acids, because trans- fatty acids have higher melting points and are more solid than cis- fatty acids, and give products made from oils (such as margarines and shortenings) body, consistency, texture, and shelf life.

Frying and deep-frying with oils, especially if the oil is allowed to sizzle or boil for hours or even days, occurs at temperatures between 160 and 220°C (320 to 428°F), depending on the kind of oil used, and produces trans- fatty acids, as well as light-, oxygen-, and heat-induced chemical destruction of fatty acids.

While the pressing temperature should be kept as low as possible, the major heat problem in oil manufacture is not the pressing temperature if we exclude light and air from the oil, but temperatures reached during deodorizing, hydrogenating, and frying.

The term 'cold-pressed' is based on misunderstanding, and has no value whatsoever as a term denoting oil quality.