Rancidity considerations

Figure 1. Peroxide values for products tested. (Source for CODEX maximum levels for edible oils and fats: Codex Standard for Named Animal Fats (CODEX-STAN 211-1999) and Codex Standard for Named Vegetable Oils (CODEX-STAN 210-1999.)

Rancidity is a general term referring to food containing fats that have ‘gone bad’ or spoiled. Oils 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 in a manner that can change 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. Those able to choose between various foods strongly avoid eating rancid products. Very young mammals being fed a milk replacer formula as a single food source may not have a choice to avoid a rancid formula. While there are various reasons that animals may resist or refuse to eat a previously successful diet (e.g. formula), it is possible that the food or product could be rancid (either initially out a newly opened package or previously opened and used but has received excessive exposure to oxygen).

Consuming a few meals of rancid products may cause gastrointestinal upset, including diarrhea, nausea and loss of appetite. Consumption of rancid products over a longer period can result in much more serious problems, including 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.

During research on milk replacer powders, WildAgain arranged for Midwest Laboratories to conduct peroxide value tests to assess rancidity on various milk replacer powders. Most of the cans that were tested by the certified independent lab did not show elevated peroxide value levels which can indicate spoilage.

The peroxide value tests revealed that two cans of Esbilac® powder lots 1639E and 0759E had elevated peroxide value levels shortly after opening. These two cans were from Esbilac® lots were produced in 2009 with the single-step spray-dry process that was intended to help improve product stability. An elevated peroxide value was also noted in a can of KMR® powder lot K1469K. Since then, wildlife rehabilitators and others have found and reported a few packages of Esbilac® and KMR® powder from other lots that also had ‘off odors,' potentially indicating spoilage.

PetAg placed a statement on its website that while the milk replacer powders met standards when leaving PetAg, a small number of packages had been reported spoiled during transit, presumably after exposure to excessive heat. Information was posted on Petag.com about susceptibility of milk powders to spoilage, especially those without preservatives. Effective storage of KMR® and Esbilac® powders was emphasized. PetAg.com provides tips to identify signs of and prevent spoilage. PetAg switched to producing KMR® powder with the single-step spray-dry method in autumn 2009 to help improve its stability, just as they had with Esbilac® powder in December 2008.

It should be noted that elevated peroxide value levels may occur in any milk powder produced by any manufacturer, including products with 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.

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 feeding milk replacers to very young animals take precautions to prevent product spoilage.

It is a good idea to examine any food product, including milk replacer powders, before using them:

1. Examine the powder for any off colors. If the color looks unusual (bright yellow, pale green, etc.), do not use and immediately contact the manufacturer.

2. Sniff the powder. Milk replacement powders should have a bland, somewhat milky odor. If the aroma is strong, smells old or smells like something other than milk, do not use it.

3. Add hot water to the powder. Stir. If it does not mix well, separates, or it looks different from what was expected, contact the manufacturer.

4. Solubility tests have shown that the mixed formula that has been allowed to ‘rest’ in the refrigerator for at least 4 hours to be better rehydrated and uniform. Stir it again to ensure it is well mixed. Examine visually. Sniff.

Milk replacer products tend to have high levels of fats. Such fats may spoil if the powder is exposed to high heat 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. Store unopened milk replacer powders in a cool room, refrigerator or freezer. Packages of opened milk replacer powders are stored in refrigerators or freezers. Some rehabilitators prefer to order milk replacer powders during cooler seasons to reduce the risk of exposure to high temperatures during shipping.

Oxidation may cause of rancidity in milk products. This could occur if the milk replacer powder is exposed to considerable air, such as if a package of milk replacer powder is left open on a counter. Some rehabilitators purchase milk replacer powders in bulk and then place it in smaller packages for distribution to other rehabilitators or for storage in freezers. Since milk replacer powder in larger packages may settle and compact more which can affect the distribution of nutrients and measurement accuracy, it is essential for the packages to be shaken and/or turned to distribute the powder. If milk replacer powders are ‘repackaged’ into smaller containers, vacuum sealers may reduce the potential for oxidation and lengthen storage.

Milk powders 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.