Fats and Oils

All oils are fats, but not all fats are oils.  They are similar to 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 culinary purposes are actually 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 particular compositions of fatty acids in the fats.


There is a problem with storing oils and fats for the long term and that is they want to go 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 in water and it is the oxidation resulting from this exposure that is the primary cause of rancidity.  The less saturated a fat is, the faster it will go off.  This may not, at first, be readily apparent because vegetable oils have to become several times more rancid than animal fats before our noses can easily detect it.   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.

Because of this difficulty in storing fats and oils for any long period of time many books and articles on the subject of food storage make only passing mention of them, if they say anything at all.  This is unfortunate because 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.  For small children, infants, nursing mothers, and the elderly, they may not be able to consume the volume of food that would be necessary in the course of a day to get all of the calories they would need to avoid weight loss and possible malnutrition. Additionally, fats play an important role in our perception of taste and texture and their absence would make many foods more difficult to prepare and consume. 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 problematical, but it is not impossible.  There are some general rules you can follow to get the most life out of your stored cooking oils and fats.

#1 – 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.  If you purchase it in plastic, particularly clear plastic, then transfer it to a gas impermeable glass or metal container that can be sealed airtight.  If you have a means of doing so, vacuum sealing the storage container is an excellent idea as it removes most of the air remaining inside, taking much of the oxygen with it.  Transparent glass and plastic containers should be stored in the dark, such as in a box or cabinet.  Regardless of the storage container, it should be stored at as cool a temperature as possible and rotated as fast as is practical.  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.

#2 – 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, though it may take several more months to reach such a point of rancidity that it can be noticeably smelled.

#3 – 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.


I take no position on doing this, but if obtaining the maximum possible shelf life in your cooking fats is important to you, it is possible to add anti-oxidant preservatives to the fat you have purchased.  Used in conjunction with a gas impermeable container, either opaque in color or stored in a dark place, and cool storage temperatures (70º F 21ºC or less) then shelf life can be extended to about five years, possibly longer.

The anti-oxidant in question is Butylated HydroxyToluene (BHT).  It is often used in the food industry to slow the development of off-flavors, odors and color changes caused by oxidation, mostly in foods with significant fat contents.  BHT is on the U.S. Food and Drug Administration’s Generally Recognized As Safe (GRAS) list as a common preservative.  The FDA limits the use of BHT to 0.02% or 200 parts per million (ppm) of the oil or fat content of a food product.  The directions that I give below will be for the FDA limit.

BHT is available over the counter in the retail trade, typically found in health or natural foods stores or vitamin and nutritional supplement suppliers.  It may also be found from various suppliers on the Internet.

To get the best results you will need the freshest oil you can find.  Purchasing from a large, busy supermarket will usually suffice.  You’ll also need containers that are gas impermeable such as glass jars, or metal cans.  There may be plastic containers with high gas barrier properties that will also serve, but I cannot knowledgeably say about this.   It is important that your containers be food grade, clean, dry and dust-free.

In keeping with the FDA’s GRAS guidelines you want to add 5.3mg of BHT crystals per fluid ounce of oil or fat.  If you’re using a scale calibrated in grains, such as a reloading powder scale, you may use the following table.

HT in grains OIL BHT in milligrams
0.1 grain 1 fl oz 5.3 mg
0.7 grain 8 fl oz (1 cup) 42.4 mg
1.3 grain 16 fl oz (1 pint) 84.8 mg
2.6 grain 32 fl oz (1 quart) 169.6 mg
5.2 grain 64 fl oz (1/2 gal) 339.2 mg
10.3 grain 128 fl oz (1 gallon) 678.4 mg
NOTE: The grain weight measurements have been rounded up to the nearest tenth grain since most powder scales will not accurately measure less than one-tenth of a grain.

IMPORTANT NOTE: If you are using a reloading powder scale, be sure the balance pan is clean and the balance has been calibrated recently with a reliable set of check weights.

Remove the BHT crystals from their gelatin capsules and weigh them, if you’re going to.  Once you have the appropriate amount, add the crystals to a pint or so of the oil, shaking vigorously.  It may take several hours for the preservative to dissolve completely.  Bringing the oil up to a warm, NOT HOT, temperature will speed the process.   Once completely dissolved, pour the anti-oxidant laden oil into the rest of the oil and mix thoroughly.  Once mixed, the oil can then be poured into its storage containers leaving approximately 1/2 inch of headspace.  If you have a vacuum sealer the jars or cans may be vacuum sealed to remove most of the oxygen laden air from the container, otherwise just seal the lid. Store in a cool place and if using transparent jars, be certain to put them in a larger container such as a box to keep the contents in the dark.  Don’t forget to label and date the jars.

Before I close out this section on fats and oils, please allow me to reemphasize that no amount of preservatives that can be added to your stored fats will substitute for 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.