Cooking oil is a staple in most pantries and is used frequently by the chef(s) of a household to prepare any number of culinary variations. The preservation of cooking oil is often overlooked until one goes to pour a spoonful into a pan and it smells, looks (or tastes) funky. Such rancidity often goes unnoticed for some time because most oils bought at a store are highly refined and deodorized; but aside from the taste factor, rancid oils present significant health risks such as heart disease, atherosclerosis, and cancer. Clearly, this should be avoided at all cost by ensuring that cooking oils are stored properly from the time they leave the store.
The cause of rancidity is oxidization, an inevitable process due to the exposure of a fat or oil to air, heat, and light. The more polyunsaturated a fat is, the faster it will go rancid. Examples of oils with a high polyunsaturated fat content are flaxseed oil, sunflower oil, corn oil, and soybean oil. Butter, palm oil, and olive oil have relatively small amounts, but they are by no means safe from early rancidity if not handled with care.
In general, unless cooking oil has been specially preserved, an unopened bottle has a shelf life of about a year if it has been properly stored. Highly polyunsaturated oils don’t keep as well. Although an active chef may go through a large quantity of cooking oil, the average person should avoid buying in bulk. It may be more expensive to continuously buy smaller bottles rather than an annual large one, but it’s better to eat the cost than allow a large quantity to sit, be intermittently exposed to air and heat, and end up eating half a bottle of rancid cooking oil.
Cooking oil’s shelf life should be accounted for while shopping. Like other food items, products tend to be fresher at busier stores with a higher inventory turnover rate: Mind the expiration date, too. Provided that cooking oil is fresh at purchase, those with more preservatives added by the manufacturer last longer than those without them. Also, while darker colored oils tend to be more flavorful than paler colored oils, it’s important to note that ingredients contributing to the flavoring and coloring of the product also lead to faster rancidity. Thus, for maximum shelf life, a consumer should always buy paler colored oils. The bottling of the product is also important, as some bottles are more gas impermeable than others; the most preserve-friendly package for the product would be an opaque, airtight glass or metal container.
Some people don’t like to buy a product simply based on its bottling; if this is you, the solution to meeting your product of choice with smart storage is to transfer the cooking oil into a different bottle when you get home. If the oil comes in a plastic container, particularly clear plastic, then it’s a good idea to transfer it to a gas impermeable glass or metal container that can be sealed airtight. If possible, vacuum seal the storage container so to remove as much lingering oxygen as possible.
The ideal storage scenario for cooking oil is somewhere cool, dark, and without air. Indeed, left at room temperatures, opened bottles of cooking oils can become rancid in anywhere from a week to a couple of months, though it may take several more months before developing an odor. It’s thus important to refrigerate stored oil, especially after it’s been opened, and to keep transparent glass and plastic containers stored in a dark place such as in a box to avoid speeding up the oxygenation process. A lower temperature may cause the cooking oil to become cloudy or solid, but in fact the fat is still perfectly usable and will return to its normal liquid, clear state once left out before use and warmed back to room temperature.
If these tactics go by the wayside over time, then your best bet is to rotate oil more frequently: Throw it out and get another one. Even a bottle of cooking oil that is yet to display symptoms of rancidity isn’t safe if treated without caution, and no amount of preservatives that can be added to oil will substitute for proper storage and rotation.