Why Do Cooking Oils Go Rancid

Are you worried that your cooking oil might expire soon? Do you want to know how to preserve the freshness of your cooking oils? We have taken the time to put together this post so that we can answer some of the major questions people have about the expiration of cooking oils.

Cooking oil does expire. The expiration date of oil depends on the type of cooking oil. They can last six months to a couple of years, and the way they are stored can determine how long they can last before becoming rancid.

Below, we will discuss why cooking oils become rancid and what you can do to preserve the freshness of cooking oils.

Why Do Cooking Oils Go Rancid?

Cooking oils have been used for thousands of years as a kitchen ingredient. Our ancestors intuitively understood that a bad smell means it is probably a bad idea to eat or drink something.

The reasons that cooking oils become rancid and expire include exposure to heat, light, water, air, and particular microorganisms.

Fortunately, there are many ways to prevent cooking oils from going bad especially since we know why oils become rancid.

Basically, the breaking of chemical bonds in cooking oils is what makes oils rancid. Any oil that includes polyunsaturated fats or unsaturated fats can go rancid.

These bonds can be broken by air through oxidation which leads to the creation of carbon-oxygen bonds.

Carbon-oxygen bonds lead to the production of ketone, aldehyde, or carboxylic acid. These products have rancid odors and tastes.

H2O has an oxygen atom in it which leads rancid smells too. H2O molecules will insert itself into the carbon-carbon double bond. Rates of hydration and oxidation are increased in the presence of light too.

Heat can make oil go rancid more quickly since it increases the rate of these chemical processes. This is because the molecules are move around faster when exposed to heat.

Storing cooking oil in the refrigerator will lead to a cloudy look since the cold temperature will slow down the movement of molecules. You will not get sick from consuming cooking oil in this state, but most people like cooking oil that is clear.

Many different microbes can also make cooking oils go bad. Cooking oils contain a chemical compound called triglycerides that have one glycerol molecule and three fatty acids.

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Some microbes can break off the fatty acids from the triglyceride backbone. This leads to a rancid cooking oil solution.

How To Tell if Oil is Bad, Rotten or Spoiled

Practicing proper food safety techniques and hygiene will help prevent cooking oils from becoming rancid.

The texture, color, and clearness of cooking oil may change with time. However, when the cooking oil goes bad, an awful odor and taste will be present.

If a bad, wine smell or taste is developed, then the cooking oil container was probably not sealed completely. If you feel like you need to throw up, then you can be sure that the cooking oil is rancid.

How to Store Oil to Extend Its Shelf Life

Storing oils is in their original airtight containers in a dark, cool place like a pantry is the best way to store oils.

An opened container of cooking oil will probably be fine in the pantry. However, many cooking oil containers will live a longer shelf life if it is refrigerated.

As previously stated, cooking oil may become cloudy in the fridge, but if left alone in room temperature for a short time before use, it will liquify quickly.

Olive and other cooking oils are best left in the pantry. However, hazelnut, avocado, sesame and walnut oils should be stored in the fridge.

There are many benefits of proper food storage such as: cutting food costs, eating healthier, and helping the environment by avoiding waste.

Also, avoid using plastic bottles. Plastic tends to strain chemicals over time which can affect the overall taste of cooking oil. If your oil came in a plastic bottle, then consider putting it in a glass jar or bottle with a tight-fitting lid.

Avoid preserving cooking oils in containers made from copper or iron since these metals can create a chemical reaction when they come in contact with oil.

Which Types of Oils Are Best Stored at Room Temperature

It is important to know which types of oils can be stored at room temperature. The following types of oils can be stored at room temperature:

  • Palm oil can last for many months.
  • Ghee oil can last for several months.
  • Vegetable oils can last for around a year.
  • Peanut oil (refined) will last for around two years.
  • Olive oil may be kept in the cupboard for up to fifteen months.
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Which Types of Oils Are Best Stored in a Refrigerator

Know which types of oils are best saved in a refrigerator. Some oils will spoil if they are not kept in a cool place. These oils should be kept in the fridge:

  1. Safflower oil will last for six months.
  2. Mustard oil will last five to six months.
  3. Avocado oil will last for nine to twelve months.
  4. Corn oil will last up to six months.
  5. Sesame oil will last for six months.
  6. Truffle oil will last for six months.

Where To Avoid Storing Oil

Avoid storing oil where it can be degraded. As previously stated, sunlight and constant temperature fluctuations can cause oil to go rancid. Avoid keeping your cooking oil in the areas listed below:

  • Windowsills
  • Next to the stove or oven
  • On the counter
  • Near cooking appliances such as kettles, waffle makers and toasters.
  • The back edge of a stove
  • In a cabinet above the stove
  • Next to the refrigerator (the external side of a fridge can get quite hot through a cupboard partition)

Refined Oil Versus Unrefined Oil and How Long Each Type of Oil Lasts

It is important to understand that oil stays fresh for a short period of time. Cooking oil comes in two different types: refined and unrefined. The label on the jar or bottle will reveal if the oil is unrefined or refined.

Refined oils are processed. They typically have a little to no flavor or nutritional value. They will normally last six to twelve months if stored properly.

Unrefined oils tend to be more nutritious and pure. They will normally last for three to six months if stored properly.

Here are some details to look for when dealing with rancid oil:

  • Check the expiration date. If the oil has become rancid because you did not use it in time, then get a bottle that is smaller the next time you go shopping.
  • Was the cooking oil stored in a plastic container? Some kinds of plastic leach into cooking oil and can cause it to taste bad.
  • Remember, if the cooking oil tastes metallic, a little bit like a glass of bad wine, or just plain awful, then it probably has turned rancid and oxidized. So, always check the smell of cooking oil.
  • Do not pour cooking oil down your drain. This is especially important if your cooking oil is normally solid at room temperature. Getting rid of used oil by pouring it down a drain may seem convenient, but it will only lead to a clogged drain. The best way to dispose of used, cooking oil is to drain it into a leak-proof container. Put the cooking oil into a jar or zippered plastic bag, and then dispose of the container into a trashcan.
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Final Thoughts

The answer to the main question “can cooking oil go bad?” is a big “yes.” Some cooking oils will last six months and some types will last up two years.

However, in the majority of cases, becoming rancid is not a problem because cooking oil will be fresh for a lot longer than it’s expiration date. This is true assuming it’s stored properly.

The biggest issue is that oil deteriorates and after some time its taste changes and it becomes rancid. Exposure to heat, air, microorganisms, and water can speed up the process of becoming rancid.

Some cooking oils are best stored in refrigerators while others are best stored at room temperature. However, all oils are best stored in dark places because light can lead to oil becoming rancid at a faster rate.

After a couple of months after the “Use by” date, you will probably decide to throw it out because of the bad smell. Remember, a bad wine smell indicates that cooking oil is rancid.

Related Questions

Is rancid oil safe to eat?

Rancid oil does not normally make people sick in the short term even though it smells terrible. However, rancid oil does contain free radicals that could increase your risk of developing diseases such as heart disease or cancer in the future.

Is vegetable oil and olive oil the same thing?

Olive oil is a type of vegetable oil. Different vegetable oils have slightly different properties. Many olive oils have a particularly strong flavor when compared to canola or soy.

How long is cooking oil good for when served in a dish?

That is dependent on many different factors. It will soak into most ingredients. Typically, it will be one of the very last expiring ingredients in any dish.