Old Fryer Oil: Food Poisoning Risk?

Old fryer oil can pose a health risk, and it is recommended to use fresh oil each time you fry. However, reusing oil is common, especially for those who deep-fry large amounts of food frequently. To reuse oil safely, it must be strained of food particles, stored properly, and not overheated. If oil is not properly strained and stored, bacteria can grow, and it can become rancid, which may cause food poisoning or other negative health effects.

Botulism and other bacteria

Botulism is a rare and potentially fatal form of food poisoning that can be caused by the growth of Clostridium botulinum bacteria in food. While it is true that this bacterium can grow in oil, it is important to understand that it requires very specific conditions.

Firstly, Clostridium botulinum thrives in anaerobic (oxygen-free) environments. In the case of oil, this means that there must be food particles present for the bacteria to feed on, and the oil must be unrefrigerated. Properly strained and refrigerated oil is therefore much less likely to become a breeding ground for Clostridium botulinum.

Secondly, the bacteria typically grow in plant matter that is cut off from air, such as garlic or spices stored under oil. The oil itself is not a hospitable environment for the bacteria, but rather serves as an airless buffer between the plant matter and the environment.

Even if contaminated oil is consumed, it is important to note that the botulinum toxin can be easily destroyed by heat. The spores will die at temperatures above 80 degrees Celsius, so thorough cooking can inactivate the toxin and prevent food poisoning.

In addition to botulism, there are other types of bacteria that can grow in old fryer oil. These bacteria can cause digestive system distress and lead to symptoms such as stomach aches and diarrhoea. However, it is important to note that these bacteria are typically already present in sinks, sponges, and dishcloths, which are common sources of cross-contamination in home kitchens.

To minimise the risk of bacterial growth and food poisoning, it is important to properly strain and store used fryer oil. This involves straining the oil through cheesecloth, paper towels, or coffee filters to remove food particles, and then storing it in a sealed glass jar in the refrigerator or freezer.

Rancid oil and free radicals

Rancid oil is not only unpleasant to taste and smell but also has harmful effects on health. Rancidity is the oxidation of fats caused by hydration, oxidation, metallic atoms, or microbes. This process results in the formation of free radicals, which are molecules that can cause long-term cell damage and increase the risk of chronic diseases.

Rancidity occurs due to changes in the chemical structure of substances, specifically the breakdown of oils chemically or by lipase into constituent fatty acids. This process is accelerated by heat, moisture, and light, and it can happen during the cooking process or when oil is stored in improper conditions.

Consuming rancid oil may not make you sick immediately, but it can have negative health effects in the long term. Rancid oil contains free radicals that can damage cells and potentially lead to an increased risk of cancer. Additionally, rancid oil can cause digestive system distress and deplete vitamins B and E from the body.

To prevent rancidity, it is essential to choose oils with a high smoke point and store them properly. Oils should be strained to remove any food particles and stored in a clean, sealed container in the refrigerator or freezer. It is also important to use fresh oil for frying and avoid overheating it.

Aldehydes and carcinogens

Aldehydes are produced when the fatty acids in the oil undergo a complex chemical degradation process at high temperatures. This process involves highly reactive free radical species and singlet oxygen. The fatty acids oxidise and break down into lipid hydroperoxides, which further fragment into secondary compounds, including extremely toxic aldehydes. The unsaturated fatty acids in the oil are particularly susceptible to this process, with polyunsaturated fatty acids being the most vulnerable, followed by monounsaturated fatty acids, and then saturated fatty acids.

Aldehydes are toxic because they are aggressive chemical compounds that can cause damage to critical biomolecules in the body, such as DNA. They are powerful electrophilic alkylating agents, which means they can easily bind to and damage DNA, leading to mutations and potentially cancer. Aldehydes can also form covalent bonds with proteins, altering their structure and function.

The health effects of consuming aldehydes include adverse impacts on critical metabolic pathways, such as energy metabolism, the promotion of atherosclerosis and cardiovascular diseases, mutagenic and carcinogenic properties, teratogenic actions (embryo malformations during pregnancy), pro-inflammatory effects, the induction of peptic ulcers, neurotoxic actions, impaired vasorelaxation, and increased blood pressure.

The potential risk of food poisoning from aldehydes and carcinogens in old frying oil depends on various factors, including the type of oil, the temperature and duration of frying, and the frequency of oil reuse. Oils with a higher smoke point, such as refined peanut, soybean, safflower, and extra light olive oil, can be reused if properly strained, stored, and not overheated. However, it is recommended to use fresh oil each time for safety and quality.

Contamination and cleaning

Used cooking oil can be a breeding ground for bacteria, specifically Clostridium botulinum, which causes botulism, a potentially fatal form of food poisoning. This happens when oil is not properly strained and stored after it cools, allowing bacteria to feed on leftover food particles. To prevent this, oil should be strained through layers of cheesecloth, paper towels, or coffee filters to remove food particles, then stored in a sealed jar in the refrigerator or freezer for no longer than a month.

To clean dark cooking oil, first, let it cool, then use a skimmer to remove any large pieces of food from the top or inside the oil. Next, strain the oil using a fine-mesh strainer or cheesecloth to remove smaller particles and sediment. Pour the strained oil back into its original container and store it in a cool, dark location.

It is important to note that reusing oil will cause it to retain some of the flavours and smells of the food previously cooked in it. Therefore, it is recommended to reuse oil for cooking similar types of food. For example, oil used for cooking fish should be reused for frying other proteins rather than something sweet.

Additionally, there are ways to prolong the life of oil before it needs to be cleaned or replaced. Firstly, choosing oils with a higher smoke point means they can be heated to higher temperatures without breaking down. Examples of oils with high smoke points include sunflower, palm, avocado, peanut, canola, and grapeseed oils. Refined oils can also be heated to higher temperatures than raw or unrefined oils.

Another way to keep oil clean is to choose foods that will not leave as much residue in the oil. Battered foods will leave less residue than breaded foods, and floured foods will leave the most residue.

Finally, it is important to control the temperature of the oil to prevent it from breaking down. Oils should be heated below their smoke point, and a frying thermometer can be used to monitor the temperature.

Reheating and chemical changes

Reheating cooking oil causes chemical changes that can be harmful to health.

Heating oil produces harmful products of lipid peroxidation, which are believed to be responsible for the development of many diseases, including cardiovascular disease and cancer. The consumption of repeatedly heated oil may also cause hypertension, atherosclerosis, osteoporosis, and liver and kidney issues.

The chemical composition of the oil itself also changes with each heating cycle. Total saturated fatty acids increase, while unsaturated fatty acids decrease. The smoke point, or the temperature at which oil breaks down and begins to smoke, also drops with each use.

Additionally, reheating cooking oil multiple times can generate compounds such as polycyclic aromatic hydrocarbons (PAH) and aldehydes, which have been linked to cancer in multiple organs, including the lung, colorectal, breast, and prostate.

Therefore, it is important to use fresh cooking oil whenever possible and to properly store and strain oil if reusing it to minimise contamination.

Frequently asked questions