That said, many essential oil companies have said in their experience they've had oils on the shelf for years without issue. The best way is to just use your nose and if one smells or tastes off, discard it. If they become discolored or cloudy that is also an indication that there is an issue.
genuina as mentioned above, oxidation degrades essential oils' aromatic and therapeutic properties. Spoilage can also result in irritation or sensitization, which can cause skin rashes, burns, peeling skin, or other unpleasant side effects.
Smell it. Rancid oil has a harsh, bitter smell, very similar to that of rancid nuts (not coincidentally). If you're not sure, taste a little, and really roll it around your tongue. If you don't get a bitter taste or a kind of astringent, slightly painful sensation on the tongue, the oil isn't actually rancid.
Peroxide Value Test
Any detection of peroxide suggests rancidity in unsaturated fats and oils. This test also measures to what extent an oil sample has undergone primary oxidation, but not it's stability. The test works by having a fat or oil sample dissolved. Then an excess amount of potassium iodide is added.
More often oil rancidity occurs when oil is exposed to oxygen resulting in the formation of free radicals1. These free radicals contribute to the change in taste and smell of cooking oils. Despite the decrease in quality, rancid oil does not generally make a person sick if consumed1.
As an essential oil oxidizes its smell changes a little, some brightness/freshness disappears, though you may not realize this unless you compare with a new, fresh oil. Citrus oils sometimes go cloudy at the same time as oxidizing.
Avoid essential oils that contain filler ingredients or synthetic additives. Look for the phrase “100% pure essential oils” to be certain that you're getting the real deal.
This could be because the essential oil you're using contains harmful additives or is impure. On the flip side, the essential oil could be pure and high quality, but you may have a health condition or are taking medication that doesn't interact well with the oil.
Citrus oils, oils high in phenols, monoterpenes, ketones, esters, oxides, aldehydes, alcohols, sesquiterpenes, and lactones should not be mixed. Mixing incompatible essential oils can lead to skin irritation, respiratory problems, headaches, nausea, vomiting, and allergic reactions.
Methods of Essential Oil Testing
Description: Gas chromatography is used to separate the components of an essential oil, enabling precise identification of its chemical profile. GC is often paired with mass spectrometry (MS) for more detailed analysis.
The longest-lasting oils, which improve as they age, tend to be the thick resins such as frankincense and myrrh, woods such as sandlewood, roots like vetiver, as well as other oils, including spikenard and patchouli. Essential oils need to be stored away from heat and light to preserve their freshness and potency.
Essential oils do oxidize eventually, but this happens more quickly with increased exposure to high temperatures, light and oxygen (air). To prolong their lifespan, keep essential oils in a cool, dark place, such as a refrigerator.
Rancid oils can cause skin irritations, whiteheads, blackheads, and blemishes, so never use expired oil on your skin. A rancid oil is packed with skin-damaging free radicals and will smell 'off'.
Quality is Worth Every Penny
The price of the essential oil reflects the complex nature of the plant it came from- where that plant is able to grow, how easy or challenging it is to grow and harvest, and how much essential oil can be produced from the plant.
Safety Precautions
It's generally advised to avoid strong essential oils like Clary Sage or Rosemary. However, Lavender and Sweet Orange are often considered safe when used sparingly and diffused in a well-ventilated area.
One common method is the peroxide value test, which measures the amount of peroxides (indicators of rancidity) in the product. Another method is the p-anisidine test, which measures the level of aldehydes (another indicator of rancidity) in the product.
According to Miller, one indicator of rancidity would be oils in clear bottles that are going brown. These have likely been sitting on the shelf for a long time. Before you make your purchase, be sure to check the “best before” date on the bottle.
Rancid oil smells like crayons, wet cardboard, sawdust or putty.
(Rancid = oxidized. When oxidation occurs, free radicals [cancer-causing agents] are formed.) These oils -- sunflower, safflower, corn, soy, peanut, canola oil and margarine -- are high in PUFAs and omega-6 fatty acids. Furthermore, these oils quickly and easily turn rancid when exposed to heat, air, and light.
Rancidity and antioxidants
Oxidation leads to the formation of smaller molecules such as ketones, aldehydes and fatty acids that give rancid foods a characteristic rank, strong and metallic scent.