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Sales: (03) 9769 0666   |    Mon to Fri: 8:30am – 5:00pm

Identify Rancid Food by Testing for Peroxide Value

Is Your Food Safe To Eat?

Every food you purchase comes with a use-by and best-before date, and that is usually a good mark to judge on whether the food is still safe for consumption. However, in some cases, food can go bad without you knowing. Rancidity in foods and oils can creep up on you and you might barely notice it. Protect your products from rancidity by testing for Peroxide Value.

What is Rancidity?

Rancidity refers to the complete or partial oxidation or hydrolysis of fats and oils when they are exposed to heat, air, moisture, light, or bacterial activity. It affects taste and odour, causing the rancid food to have a certain off flavour and smell, and will have an impact on the quality of your end product.

Many foods can become rancid during storage, including vegetable oils, olive oils, alternative oils, butter, nuts, grains, flour, baked goods, animal food, animal feeds, and more. For example: have you ever opened up a bag of dog kibble and noticed a stale or sour odour? Odds are the fats and protein in the food have gone rancid, especially if the food wasn’t stored properly or had been exposed to heat or humidity.

Essentially, anything that contains fat or oil can be subjected to rancidity, and this includes a wide variety of processed products such as cookies, chips, and trail mix bars.

Fats that are highly processed face a higher risk of becoming rancid. For example, the fats in potato chips and cookies are all exposed to high heat, and this expedites the oxidation process on the road to rancidity.

However, even fresh foods can go rancid. Frying oils that are heated and reused over and over again will become rancid quickly, creating some sort of a fishy smell that would affect the taste of your food.

How to Identify Rancid Food?

The first thing you’d notice – SMELL

One of the first and most noticeable signs of rancid foods is the smell. Rancid foods tend to have a certain unpleasant or stale odour. The odour can vary, but has often been described as wet cardboard, wood varnish, oil paint, or play dough. So if something smells funky, weird, or just not like how it does usually, it may be time to throw it out.

Does it still TASTE good?

One of the first and most noticeable signs of rancid foods is the smell. Rancid foods tend to have a certain unpleasant or stale odour. The odour can vary, but has often been described as wet cardboard, wood varnish, oil paint, or play dough. So if something smells funky, weird, or just not like how it does usually, it may be time to throw it out.

How does the food FEEL?

In advanced stages of rancidity, certain physical characteristics of the food or product can change as well. If your bottle of oil feels sticky, that could be a sign that your cooking oil is going through polymerisation, which is an advanced stage of the rancidity process.

How does the food LOOK?

How the food looks visually can be a good sign too as to whether it has become rancid. Does the oil look darker than usual? Is there mould on the jar? Does the salad look pale or soggy? Although we do not like to judge based on appearances, this is one of those times when you absolutely are allowed to do so. Throw the food out if it does not look good to you.

Will rancid food harm you?

The dangers of consuming rancid foods are inconclusive. According to the US Centers for Disease Control and Prevention, there are very few documented outbreaks of gastrointestinal illness associated with the consumption of rancid food. However, eating rancid food may contribute to digestive issues.

Rancid foods are also less nutritious, as oxidation breaks down the fatty acids and destroys the good fats as well as some of the vitamin content. This in turn can lead to packaged and shelved food having inaccurate labeling, due to the rancidity that has set in by the time of consumption.

Generally, most experts agree that rancid foods are toxic and should be avoided where possible. Some have also cautioned that the regular consumption of rancid foods may contribute to negative health effects, including inflammatory disease, cardiovascular disease, and even certain cancers.

Should I test my ingredients and raw products for rancidity?

Rancidity testing is an important part of the quality assurance and production process in the food industry. Testing allows you to know the level of oxidation in a sample, and makes it easier to monitor your product’s potential shelf life, stability, as well as flavour or smell consistency.

If you are purchasing ingredients, it is highly recommended that you check oils, fats, nuts and grains, among others, to ensure that they are of the highest quality possible. Testing the rancidity in these foods will allow you to see how far into the oxidation process these foods may be. You can then plan your usage of these ingredients accordingly, to avoid having to dispose of rancid ingredients later on and losing money. It also helps you to adjust and fine-tune your stock purchasing schedule, as well as monitor the quality of your existing ingredients.

As a supplier, it is essential to be able to deliver the best possible product. Testing for rancidity will allow you to back up the freshness and overall quality of your product, and ensure customer satisfaction.

If a consumer purchases your product and notices that the smell or flavour is not up to their expectation, they may return the product and request for a refund. Their opinion of the product and your business may also be negatively affected, which would subsequently damage your brand reputation. Or worse, if your product is rancid and it causes the consumer to be sick, your business may be liable to lawsuits or penalties.

How do I test for rancidity?

There are several ways to test for rancidity, but measuring Peroxide Values (PV) is considered the most common and ‘gold standard’ of rancidity testing. Peroxide Value is defined as the amount of peroxide oxygen per kilogram of oil or fat, where the amount of peroxide is reported in milliequivalents or meq.

This determination of peroxide oxygen is an indicator of lipid oxidation. Measuring the number of peroxides is commonly done on fats and oils to show the stage of oxidation. Generally, any oils with a Peroxide Value of less than 10 meq are considered fresh, while values between 30 and 40 meq are considered rancid.

Peroxides are the first components formed during the oxidation of fat, making Peroxide Value a very useful and effective test to determine how far in the rancidification process your food product is. As Peroxide Value is a good indicator of rancidity due to oxidation (or oxidative rancidity), it is a good idea to pair this with a free fatty acid test, which gives an indication of rancidity due to microbial activity.

Besides Peroxide Values, other tests used to determine rancidity include:

  • Free Fatty Acids (FFAs) – Used to determine hydrolytic rancidity (as opposed to oxidation). Free fatty acids can be the by-product of microbial activity.
  • p-Anisidine (p-AV) – A measure of aldehyde values in fats and oils; particularly unsaturated fats.
  • TBA Rancidity (TBAR) – A measure of aldehydes created from the oxidation of fats.

Oxidative Stability Index (OSI) – Determines the relative resistance of fats or oils to oxidation.

Testing for Peroxide Value (PV)


The photometric method measures the concentration of a substance by determining the absorbance of wavelengths of light. When testing Peroxide Value photometrically, a reagent is added to the sample, where the presence of peroxides will cause a colour change. A light source is then passed through the sample and correlates the amount of light absorbed by the colour to the concentration of peroxide present in the sample. You can test the PV of olive oil and similar edible oils using a photometer– Portable Photometer for Determination of Peroxide Value in Oils.


The titration method is used to determine the concentration of a substance by adding a solution of known concentration (titrant) to a solution of unknown concentration (analyte) until a desired and expected reaction is achieved. The iodometric titration method for peroxide value of fats and oils is referenced by organisations such as AOCS and AOAC. For more complex samples, where the fat and oil is bound to other materials, a hexane extraction is required prior to titration.

Hanna automates the Peroxide Value of refined and unrefined oils using our Automatic Potentiometric Titrator HI932 per AOAC 965.33 or AOCS Cd Sb-90 reference methods. A sample of the fat or oil is dissolved in the Peroxide Value solvent before saturated potassium iodide solution is added to the mixture, where any and all peroxide present will react to form iodine. The volume of sodium thiosulfate (titrant) required to react with the generated iodine is then used to calculate the amount of peroxide present. This iodometric reaction can be monitored with an ORP electrode. With the HI932, all calculations are performed automatically and the Peroxide Value concentration of your sample is listed on the screen.

To find out more about testing for Peroxide Values, or if you have any questions about testing different food sample types, please get in touch with Hanna Instruments Australia.

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