Oils are liquid fats used to carry flavour, give mouth-feel to food, and facilitate the cooking of food without its sticking to the cooking vessel.
Two good oils to have on hand are an all-purpose vegetable oil, with no flavour, and an olive oil, that does have flavour.
Many recipes will ask for a tablespoon of some specialty oil, such as Truffle Flavoured Olive Oil, or Walnut Oil, etc. Before buying these specialty oils, which don't have a very long storage life, consider using as a substitute another oil that you already have on hand or are likely to be able to easily use again.
Generally, use cold-pressed oils in dishes that don't get cooked, because unless heated very carefully, you will destroy any taste advantages you gained by paying extra for the cold-pressed.
Oil is very efficient at transferring heat into food during cooking.
Cold Oil Pressing
The presses used can be stone or steel. The pressure is applied but because temperature increases with pressing, care must be taken. There is no consistent, world-wide definition of just how high the temperature can get. In some places it is 120 F (49 C), other places it is as high as 140 F (60 C.) Sometimes, temperature control is done by controlling the pressure applied. Othertimes, it is controlled by running cold water through the machinery.
Cold pressing is the best method for preserving flavour. However, it also gives the lowest oil yield. With olives, you will get only about 90% of the potential oil in the olives.
Mechanical Oil Pressing
A machine presses the seed, nut or vegetable. The intense pressure raises the temperature to a range of 185 F to 200 F (85 C to 93.3 C.) Oftentimes, nuts and seeds are heated even higher first, up to 250 F (120 C), so that even more oil will come out during the pressing. The oil retains some of the flavour, but not as much as with cold pressing. However, the oil yield is higher.
This method yields the most oil, so it is preferred commercially. Nuts or seeds are cracked, then soaked in a mix containing a solvent such as hexane. The hexane draws the oil out. The oil is then heated to about 300 F (150 C), which evaporates the solvent away, leaving just the oil. The process yields oils with a high smoke temperature and long shelf life but not much of the original flavour.
Unfortunately, those beautiful clear bottles that oil can look so beautiful in are actually deadly for the oil inside. Keep these bottles in dark cupboards, as tempted as you are to put them on display.
Oil can go rancid. You'll know by the smell; it will smell like motor oil.
Palm oil started to be imported after the Napoleonic wars. At first it was used only industrially, as it has a high acid content of 15 to 20%. By the 1850s, oil producers in Hamburg, Germany had figured out how to reduce its acid content that it could be used as food. It was a Hamburg firm as well that began importing dried coconut for processing into a food oil.
By 1938, Western Europe used 3.5 million tons of vegetable oil a year. Only 1.1 million tons of that was produced in Western Europe, mostly olive oil. The balance was imported oils from the tropics. In terms of food, a good deal of this oil went into making margarine.
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