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Rancidity and 5 Tests You Need to Know

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Rancidity testing is used widely in the food, feed and pet food industries as an indication of product quality and stability. Anyone who has opened an old bag of potato chips or vegetable oil knows the plastic-like smell that can result from a rancid product. The last thing you need as a manufacturer is another call about how your product “doesn’t smell right.” With this in mind, product-appropriate testing is an essential component in determining the shelf life of a product and the quality of your raw materials.

Fats are highly susceptible to degradation due to their chemical nature. Their long carbon chains and variable degree of unsaturation leave them vulnerable to a variety of breakdown pathways. Aside from microbial degradation, the two chemical pathways for fat rancidification are:

Oxidative rancidity results from the breakdown of unsaturated fatty acids in the presence of oxygen. Light and heat promote this reaction which results in the generation of aldehydes and ketones – compounds which impart off-odors and flavors to food products

Hydrolytic rancidity can occur during the breakdown of triglycerides in the presence of water. As the fatty acids are separated from the triglyceride structure, they become more volatile due to their lighter molecular weight. Fatty acids such as caprioc and capric acids derive their name from the latin word for goat – which should give you an idea of what these fatty acids smell like. Unfortunately most foods contain triglycerides and water, so this pathway is common for the development off-odors.

Rancidity testing is common, although not all tests are equally appropriate.  Below are helpful hints regarding how each test is performed, what it reveals, limitations  and which may be more important for a given scenario:

  • Peroxide Value (PV) testing determines the amount of peroxides in the lipid portion of a sample through an iodine titration reaction targeting peroxide formations  Peroxides are the initial indicators of lipid oxidation and react further to produce secondary products such as aldehydes. Because peroxide formation increases rapidly during the early stages of rancidification but subsequently diminishes over time, it is best to pair PV testing with p-Anisidine Value to obtain a fuller picture of product quality.
  • p-Anisidine (p-AV) is a determination of the amount of reactive aldehydes and ketones  in the lipid portion of a sample.  Both compounds can produce strong objectionable flavors and odors at relatively low levels. The compound used for this analysis (p-Anisidine) reacts readily with aldehydes and ketones and the reaction product can be measured using a colorimeter. Samples that are particularly dark may not be the most applicable for this analysis as the colorimeter may not be able to adequately measure the wavelength required.
  • TBA Rancidity (TBAR) also measures aldehydes (primarily malondialdhyde) created during the oxidation of lipids. This analysis is primarily useful for low-fat samples, as the whole sample can be analyzed rather than just the extracted lipids.
  • Free Fatty Acids (FFA) testing determines the amount of fatty acids that have been liberated from their triglyceride structure.  A titration is performed on the extracted fat from a particular sample and the FFA content is determined through calculation of the amount of titrant used to reach the endpoint. Knowing what type of fat or fat containing product is being tested is important for this analysis to ensure that the appropriate calculation is applied. Because the test does not differentiate between fatty acid types, samples with high palmitic or lauric fatty acid composition should have a different calculation factor applied in order to accurately represent the free fatty acid result.   
  • Oxidative Stability Index (OSI) indicates how resistant a sample is to oxidation.  Samples are subjected to heat while air is injected – a process which accelerates oxidation reactions.  The samples are monitored and the time required for the sample to reach an inflection point is determined.  This  particular test is useful when testing the efficacy of an antioxidant added to a product. Antioxidants should inhibit free radical propagation and thus increase a samples ability to hold up under the stressing conditions imposed by the OSI analysis.

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