What it is and how to detect it
“Rancidity” is a general term referring to a food containing fats that has ‘gone bad’ or spoiled. Oils/fats can be extremely susceptible to rancidity because their chemistry can make them exceptionally vulnerable to oxygen damage. When food scientists discuss rancidity, they are often talking about "oxidative rancidity," which is oxygen damage to foods. During the process of oxidative rancidity, oxygen molecules interact with the structure of the oil and harm its natural structure. This damage can cause a change in its odor, palatability, nutritional quality and value, and even safety for consumption.
Human taste buds are very sensitive to the unpleasant odors or unpalatable flavors of rancid foods, and thus we choose to avoid eating them. Taste is directly influenced by the sense of smell, and the human olfactory is comprised of about 5-6 million receptors. For reference, rabbits have about 100 million and dogs a whopping 220 million. Young mammals also have a keen sense of smell. However, when they are being fed a milk replacer formula as a single food source, their hunger may transcend their instinct to avoid a rancid formula. There are various reasons that animals may resist or refuse to eat a previously successful diet or formula. One possibility could be that the food or product is rancid. This can occur either in a newly opened package or an opened container that has had excessive exposure to oxygen. With animals being tube fed, taste is not a factor, but rancidity can cause adverse medical issues.
Consuming just a few meals of a rancid product may cause gastrointestinal upset, including diarrhea, nausea and loss of appetite. However, consumption of rancid products over a longer period can result in much more serious problems. These can include severe digestive disturbances, poor growth rate, weight loss, skin problems, hair loss, anorexia, emaciation, intestinal hemorrhage, degeneration of the heart, enlarged liver, death, and more.
While conducting research on milk replacer powders, WildAgain consistently examines the powder colors, texture, agglomeration, and odors. If off-odors are detected, arrangements are made for the independent lab to conduct peroxide value tests (described below) to assess rancidity. Most of the cans/packages that were tested for rancidity by the lab during the 2008-2010 time frame did not show elevated peroxide values that could indicate spoilage. During the testing of late 2019 and early 2020, cause did not arise to test for rancidity - until three recent and unopened cans of GME®. Those lots, upon testing, did show elevated peroxide levels, some significantly elevated. Plans are underway to conduct more peroxide value tests on various products, as well as more lots of GME® - both those suspected of off-odors as well as those that are not. This will help provide data needed to compare rancidity levels for more products.
It should be noted that elevated peroxide value levels may occur in any milk powder produced by any manufacturer, including products that contain preservatives. Since spoiled food can cause a variety of health problems, wildlife rehabilitators are encouraged to use effective storage practices and closely examine the milk replacer powders as part of their general practice.
Storing milk replacer powders
Milk replacer powders, like other foods, are perishable. While gastrointestinal upset may develop for people and other mammals that eat spoiled food, the consequences of eating spoiled food may be more serious for the very young. Thus it is essential that wildlife rehabilitators using milk replacers take the following precautions to prevent product spoilage:
1.) Always check the storage recommendations for milk replacer powders or formulas. These can be found on product label or on the manufacturer’s website.
2.) Store unopened milk replacer powders in a cool room, refrigerator or freezer. Packages of opened milk replacer powders should be stored in refrigerators or freezers. Milk replacer products tend to have high levels of fats. Such fats may spoil if the powder is exposed to high heat. This can occur in warehouses, transit vehicles (trucks, cars, ships, planes, cars, etc.), stores, storage facilities, homes or even on a kitchen counter during a hot summer day. Some rehabilitators prefer to order milk replacer powders during cooler seasons to reduce the risk of exposure to high temperatures during shipping.
3.) Oxidation is another cause of rancidity in milk products. This could occur if the milk replacer powder is exposed to considerable air, such as if the package is left open. Some rehabilitators purchase milk replacer powders in bulk and then divide it into smaller packages, either for distribution to other rehabilitators or for easier storage in freezers. If milk replacer powders are ‘repackaged’ into smaller containers, vacuum sealers may reduce the potential for oxidation and lengthen storage.
4.) Keep milk replacer powders in a dry environment. They may also spoil if they become wet or are exposed to excessive dampness. This could cause excessive clumping, discoloration, mold, or other signs of spoilage.
Rancidity tests (peroxide values)
The following chart shows the product lots that have been tested by the lab for the peroxide value. The dotted blue line on the chart indicates the maximum levels established for edible oils and fats by Codex. The specific reference for this value is included in the Codex Standard for Named Animal Fats (CODEX-STAN 211-1999) and the Codex Standard for Named Vegetable Oils (CODEX-STAN 210-1999.)
If I detect rancidity, what do I do?
During 2009, several users of KMR® reported detection of rancidity in the product. At the time, PetAg® confirmed that there had been some spoilage with a few packages of KMR® powder in 2008-09. PetAg® stated they had changed their manufacturing process in order to reduce the problem of spoilage that could occur as a result of exposure to heat. Concurrently, they posted a statement on their website (since removed) that helped to address the issue as follows, which is probably still relevant advice today:
“High heat stresses these types of products [KMR® and Esbilac®]. These high heat conditions may occur in transit, in warehouses, in in-store storage, in your automobile, or in your house. PetAg®, Inc. tests all of these products before leaving its warehouse to assure that they meet specifications and are suitable for use. Changes occurring after leaving the PetAg® warehouse can only be noted by the consumer.
BEFORE using KMR® or Esbilac® Powders you should always: Examine it visually for any off colors. If the color is extremely yellow, please call. Sniff to determine if the product smells right to you (milk replacement powders should have a bland, somewhat milky aroma). If the aroma is strong, smells old or smells like something other than milk, please call. Reconstitute the product in water and look at and sniff it again. If it does not mix well, it looks or smells different from what you are expecting, there may be a problem so please call.
If the animal you are feeding is hesitant to take the formula or develops diarrhea, please call. It may or may not be a problem with the formula but either way we would like to know so that we may be able to help you with pointers for feeding your newborn or send replacement product if necessary.”
These same suggestions seem applicable to powdered milk replacers from any manufacturer, as any powdered milk replacer, especially those with higher fat content values, are susceptible to the effects of oxidation progressing to elevated levels of rancidity.
What is the Peroxide Value (PV) test?
The Peroxide Value (PV) of an oil or fat is used as a measurement of the extent to which rancidity reactions have occurred during storage (opened or unopened). The peroxide value is the calculated amount of peroxide oxygen per 1 kilogram of fat or oil. Traditionally this was expressed in units of milliequivalents, which has been commonly abbreviated as mequiv or even as meq.
The correlation of rancid taste and peroxide value depends on the type of oil and is best tested with taste panels. The odors and flavors associated with typical oxidative rancidity are mostly due to carbonyl-type compounds. Oils with a high degree of unsaturation are most susceptible to autoxidation. Autoxidation is a reaction involving oxygen that leads to deterioration of fats and oils which form off-flavors and off-odors. The molecular structure of fats and oils play a role in autoxidation. The best test for autoxidation (oxidative rancidity) is the determination of the peroxide value, as peroxides are intermediates in the autoxidation reaction. Other methods are available but peroxide value is the one most widely used.
It should be noted that peroxide values are not static and care must be taken when handling and testing samples. It is difficult to provide a specific guideline relating peroxide value to rancidity. Peroxide values of fresh oils are typically less than 10 milliequivalents /kg. When the peroxide value rises to between 30 and 40 milliequivalents/kg, a rancid taste is noticeable. Higher peroxide values are a definite indication of a rancid fat.
References and further reading (not intended as an exhaustive list)
Aiello, Susan and Asa Mays. 1998. Merck Veterinary Manual, 8th edition. Merck and Company, Inc. Whitehouse Station, N.J.
Allen, J. C. and R. J. Hamilton. 1989. Rancidity in Foods, second edition. Elsevier Applied Science, New York, NY.
Stefania Cesa,Stefania et al. (2015) Infant Milk Formulas: Effect of Storage Conditions on the Stability of Powdered Products towards Autoxidation.
Deshpande, S. S. 2002. Handbook of Food Toxicology. Marcel Dekker, Inc. New York, NY.
Fox, Kate. The Smell Report. Social Issues Research Centre.
Hamilton, C. R. and D. Kirstein. 2008. Does Rancidity, as Measured by Peroxide Value, Affect Animal Performance? Darling International, Internal paper.
Jahn, M. Milk Fat/Rancidity (2010) Handbook of Hydrocarbon and Lipid Microbiology, pp.2377-2391 DOI: 10.1007/978-3-540-77587-4_174
Lai, Po-Han (Leo) and B. Eng (Hons) 2015. Shelf life of Goat Infant Formula Powder, Massey University, Palmerston North, New Zealand.
Vaclavik, Vickie and Elizabeth Christian. 2008. Essentials of Food Science, 3rd Edition. Springer. New York, NY.