Oxidation of fats and oils (rancidity) is a natural process: a reaction between unsaturated fatty acids and free oxygen. Certain oils, such as soy and corn oil that are used frequently in animal feeds, are especially rich in unsaturated fatty acids. For example, the ratio between saturated and unsaturated fatty acids in coconut oil (a prime source of lipid) is over 11, whereas in corn oil it is only 0.15, indicating the richness of corn oil in unsaturated, and thus vulnerable to oxidation, fatty acids. To reduce lipid oxidation the following measures are usually employed:
1. Storage of lipid sources
Lipid oxidation is enhanced by exposure to air, light, high temperatures, and certain inorganic minerals such as iron and copper. The reaction is autocatalytic and, once started, oxidized fatty acids continue to form. The end result is peroxides, the end product of oxidation, that accumulate.
Antioxidants are frequently added to sources of lipids and complete feeds that contain high levels of fats and oils. Antioxidants can only delay the process of oxidation by stabilizing reactive fatty acids, but given enough time, fatty acids will eventually react with available oxygen. Common antioxidants include ethoxyquin, butylated hydroxytoluene, citric acid, and vitamins C and E. The last three antioxidants are usually too expensive to be used as antioxidants in feed, yet when they are included in relatively high amounts for other reasons they contribute to the antioxidant status of the feed.
3. Feed formulation
Oxidized fatty acid can react with certain amino acids, rendering them unavailable to the animal. Methionine and tryptophan are particularly susceptible to oxidation by free radicals. Crystalline amino acids are even more vulnerable (and expensive). Lipid oxidation can be caused also during steam flaking of cereals. This has been shown to reduce bioavailability of methionine and tryptophan in wheat, rye, barley and oats by as much as 26 percent.
4. Quality control
Peroxide analysis is a valid index of rancidity, but it does not provide all information required to assess the oxidative status of feed and lipids. Other common tests include analyses for malonaldehyde and thiobarbituric acid (TBA). A combination of all three tests is usually required to provide a complete picture of oxidation state.
5. Feed storage
At the farm level, to prevent feed from oxidizing rapidly, it pays to keep feed bags sealed and outside the animal rooms, preferably in a shaded area. And certainly, during the summer months, feeding little and often will ensure that feeders are not overfilled with feed that quickly turns rancid and unpalatable. It is also a good idea to limit feed orders to what can be consumed in a month or even less.