All oils are fats, but not all fats are oils. They are like each other in their chemical makeup, but what makes one an oil and another a fat is the percentage of hydrogen saturation in the fatty acids of which they are composed. The fats which are available to us for cooking purposes are mixtures of differing fatty acids so for practical purposes we’ll say saturated fats are solid at room temperature (70ºF, 21º C) and the unsaturated fats we call oils are liquid at room temperature. For dietary and nutrition purposes fats are generally classified as saturated, monounsaturated and polyunsaturated, which is a further refinement of the amount of saturation of the compositions of fatty acids in the fats.
BUYING AND STORING OILS AND FATS
There is a problem with storing oils and fats for the long term and that is they get rancid. Rancid fats have been implicated in increased rates of heart disease, arteriosclerosis and are carcinogenic (cancer causing) so are best avoided whenever possible.
Oxygen is eight times more soluble in fat than it is in water and it’s the oxidation resulting from this exposure that’s the primary cause of rancidity. The less saturated a fat is, the faster it will go off. An extreme example of rancidity is the linseed oil (flaxseed) that we use as a wood finish and a base for oil paints. In a matter of hours, the oil oxidizes into a solid polymer. This is very desirable for wood and paint, very undesirable for food.
Fat contains nine calories to the gram compared to the four calories contained by either carbohydrates or protein. This makes fat a valuable source of concentrated calories that could be of real importance if faced with a diet consisting largely of unrefined grains and legumes. Additionally, fats play an important role in our perception of taste and texture. Furthermore, a small amount of dietary fat is necessary for our bodies to properly absorb fat soluble vitamins like A,D,E and K.
Long term storage of fats may be difficult, but not impossible. There are some general rules you can follow to get the most life out of your stored cooking oils and fats.
Exposure to oxygen, light and heat are the greatest factors to rancidity. If you can, refrigerate your stored oil, particularly after it’s been opened. If possible, buy your oils in opaque, airtight containers. All other considerations being equal, oils and fats with preservatives will have a greater shelf life than those without, provided they are fresh when purchased.
Unless they have been specially treated, most unopened cooking oils have a shelf life of about a year to a year and a half, depending upon the above conditions. Some specialty oils such as sesame and flax seed have shorter usable lives. If you don’t use a lot, try to not buy your fats in big containers. This way you won’t be exposing a large quantity to the air after opening, to grow old and possibly rancid, before you can use it all up. Once opened, it is an excellent idea to refrigerate cooking fats. If it turns cloudy or solid, the fat is still perfectly usable and will return to its normal liquid, clear state after it has warmed to room temperature. Left at room temperatures, opened bottles of cooking oils can begin to rancid in anywhere from a week to a couple of months. Although darker colored oils have more flavor than paler colored, the agents that contribute to that flavor and color also contribute to faster rancidity. For maximum shelf life buy paler colored oils.
Before I close out this section on fats and oils, please allow me to reemphasize proper storage and rotation. Don’t sit on your oil supply for years without rotating it. A little bit rancid is a little bit poisonous. ‘Nuff said.