Oil that has been used for frying should not be stored for extended periods of time. The quality and safety of used frying oil deteriorates quickly and it’s best to discard it after 1-2 uses. However, there are some guidelines you can follow to safely store and reuse oil for a limited time.
Quick Summary
Here are some quick tips on storing used frying oil:
- Store oil in a cool, dark place like the pantry – avoid heat and light
- Use within 1-2 weeks for best quality and food safety
- For short term storage (1-2 days), strain oil through a fine mesh strainer to remove food particles
- For longer storage, strain oil first and then store in an airtight container
- Avoid reusing oil that smells rancid or looks overly cloudy or dark
- The longer oil is stored, the more degradation occurs leading to higher levels of free fatty acids (FFAs)
- Test strips or meters can measure FFA levels that indicate lower oil quality
- When in doubt, discard old frying oil to be safe
What Happens to Oil During Frying
Frying causes physical and chemical changes to oil that affect the shelf life.
When heated to high temperatures (>300°F), frying oils undergo oxidation, hydrolysis and polymerization:
- Oxidation: Oxidation occurs when hot oil is exposed to oxygen. Oil molecules react with oxygen to form hydroperoxides and free radicals. This leads to breakdown of fatty acids, decreased nutritional value, and formation of off-flavors and odors.
- Hydrolysis: Moisture from foods causes some of the oil’s triglycerides to hydrolyze (breakdown) into free fatty acids (FFAs) and glycerol. FFAs cause off-flavors and decrease smoke point.
- Polymerization: Unsaturated fatty acids (like those in vegetable oils) polymerize and combine into complex molecules with higher molecular weights. This increases oil viscosity and darkens color.
Food particles, water, and minerals from fried foods also get incorporated into the oil during frying. All of these changes decrease the shelf life of frying oils.
How Long is Used Frying Oil Safe to Store?
There are no set guidelines on maximum storage times for used frying oils. However, for food safety and quality, oils should only be stored for short periods:
- Less than 1 week: Best flavor and nutrition retention
- 1-2 weeks: Maximum recommended storage time
- Over 2 weeks: Higher risk of oil degradation and rancidity
Here are some general rules of thumb for oil storage times:
| Storage Time | Guidelines |
|---|---|
| Less than 1 day | Oil can be safely strained and reused for one more fry |
| 1-2 days | Strain oil through mesh strainer, store in airtight container in cool, dark place |
| 3-7 days | Maximum recommended storage time. Strain oil first. Discard sooner if smells rancid or looks very dark. |
| 1-2 weeks | Do not recommend storing this long for food safety reasons. If reused, strain thoroughly first. |
| Over 2 weeks | Discard oil – high risk of oxidation, rancidity, and off-flavors. |
The key is minimizing time and temperature exposure. Storing oils for longer periods allows more degradation to occur.
How to Store Used Frying Oil
Follow these tips for safe, short term storage of used frying oils:
1. Let Oil Cool Completely
After frying, let the hot oil cool down to room temperature before straining and storing. Hot oil is more likely to oxidize and degrade during storage.
2. Strain Thoroughly to Remove Food Particles
Food particles provide surface area for oil oxidation. Straining removes these particles. Pour oil through a fine mesh strainer lined with cheesecloth or paper filters.
3. Store Oil in an Airtight, Opaque Container
Light, heat, and air all accelerate oil degradation. Store oil in an airtight, opaque container like a jar with a tight lid.
4. Keep Oil in a Cool, Dark Place
The ideal storage spot is a dark pantry or cupboard away from heat sources like the stove. Refrigeration can help slow rancidity but may also cause condensation.
How to Tell if Stored Frying Oil Has Gone Bad
Here are signs that stored frying oil should be discarded:
- Bad odor – If oil smells rancid, burnt, or “off” it has degraded.
- Change in appearance – Oil darkens with use. Discard if it looks dark brown or has thick black specks.
- Foaming – Excess foaming when heated signals the presence of moisture and FFAs.
- Thick, greasy film – Oil with a thick coating on cooked foods has higher viscosity from polymerization.
- Smoke point decreases – Lower smoke points indicate oxidation.
You can also use oil quality measuring tools:
- Oil test strips – Measure free fatty acid (FFA) levels and peroxide values.
- Tintometers – Assess color changes in oil.
- Refractometers – Measure changes in oil refractive index.
When in doubt, play it safe and discard used oil. The risks are not worth potential food poisoning.
Health Risks of Consuming Rancid Frying Oils
Rancid frying oils present the following health risks:
- Foodborne illness – Bacteria and mold growth produce harmful toxins.
- Digestive problems – Rancid fats can irritate the stomach and intestines.
- Diminished nutritional value – Vitamin and antioxidant loss during oxidation.
- Cancer risk – Potential carcinogens form during oxidation and polymerization.
Eating rancid oil likely will not make you severely ill right away. But over time the oxidative compounds and free radicals can damage cells and potentially increase disease risk.
Tips for Safely Reusing Frying Oil
To safely reuse frying oil:
- Fry similar foods – avoid mixing high moisture foods like fish with oils used for dry foods.
- Strain after each use – removes food particles.
- Do not top off or mix old and new oil – only add fresh oil.
- Monitor smoke point – heated oil should not smoke before ~400°F.
- Watch for foaming – excess bubbles when heated signals degradation.
- Limit reuses – 1-2 times max, then discard oil.
- Check color – if oil darkens significantly, replace it.
- Store properly between uses.
- Sniff oils before reusing – rancid oil smells off.
- Use quality test strips if needed to verify.
With good practices and limited reuse, frying oils can be safely stored and reused for a short time.
Frying Oil Alternatives to Minimize Waste
To reduce oil waste and storage challenges, consider these frying oil alternatives:
1. Use High Oleic Oils
High oleic oils, such as high oleic sunflower or canola oil, contain more monounsaturated fat and less polyunsaturated fat. This makes them more stable at high fry temperatures with a longer shelf life.
2. Try Alternative Frying Methods
Methods like air frying, baking, roasting and broiling use little to no oil for crispy results.
3. Supplement with a Portion of Saturated Fat
Adding a small amount of saturated fat like lard, tallow or palm oil makes vegetable oils more stable for frying.
4. Filter Oil Between Uses
Special commercial fryers filter and re-circulate oil through filters to remove food particles and extend oil life.
Frequently Asked Questions
1. Does heating oil cause it to go rancid faster?
Yes, exposure to heat speeds up the oxidation process that causes oils to go rancid. Oils also degrade faster at frying temperatures.
2. How should you dispose of rancid oil?
Let the oil cool completely and pour it into an empty can or jar. Seal the container and dispose in your regular household trash. Rancid oil can also be composted in some areas, but check local guidelines first.
3. Are there differences between oils in how long they last after frying?
Yes, oil composition affects stability and shelf life. In general, oils higher in polyunsaturated fats like corn, soybean, and canola oil degrade faster than oils higher in monounsaturated fats like olive and high oleic oils.
4. Does refrigerating used oil make it last longer?
Refrigeration can help slow the oxidation process in oils, potentially extending freshness by a few extra days. However, moisture buildup in the fridge can negate benefits. Proper storage conditions are most important.
5. How many times can you reuse frying oil?
Ideally, frying oil should only be reused 1-2 times before discarding. With adequate filtering and proper storage between uses, oil can potentially be reused up to 6-10 times before quality degrades too far.
The Bottom Line
Used frying oil should not be stored for extended periods due to safety and quality concerns. For best flavor and nutrition, reuse oil within 1-2 weeks maximum, stored properly between uses. Discard oil that appears dark, smells rancid, foams excessively when heated, or leaves a thick greasy film on foods. With good practices, frying oil can be safely reused for a limited time.
