Does Rancidity, As Measured by Peroxide Value, Affect Animal Performance?
C. R. Hamilton, Ph. D.1 and D. Kirstein, M.S.2
Darling international Inc.
Rancidity refers to the oxidative state of fats, which is a characteristic that may have nutritional relevance and, in extreme cases, affect the well being of poultry and livestock. However, these effects are poorly understood. Most tolerances and standards used to evaluate animal feeds and ingredients for rancidity are derived from human thresholds for detecting off-flavors associated with rancidity. Therefore, it is assumed that feeding fats that have undergone severe oxidation may reduce feed consumption and growth rate in animals.
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Peroxides are measured at time intervals. The number of hours required to reach 100 milliequivalents (meq) of peroxide per kg of fat is reported as the AOM for the fat. This method is time consuming and seldom used any more.
The thiobarbaturic acid (TBA) test measures aldehyde formation as an indicator of the stability or rancidity of fats. The test was originally developed to detect malonaldehyde, but other aldehydes also react with 2-thiobarbituric acid. However, aldehydes are volatile - low molecular weight compounds and may be lost when fats are heated for extended periods of time as is necessary to facilitate handling and processing in a feed mill. The TBA test is most often used to determine the stability or antioxidant status of animal tissues, such as plasma, serum, organs and other tissues.
p-Anisidine Value is another indicator method. Anisidine reacts with the non-volatile portion of fatty acids left behind when hydroperoxides break down to aldehydes. The aldehydes are volatile and follow the same formation-degradation curve as other oxidation products. Even if aldehydes are no longer detectable with the TBA test, the Anisidine value may still indicate that the fat has been oxidized because the residues detected are not volatile. While this method may provide some indication of the oxidative history of a fat, it is not suitable for the detection of fat oxidation.
Oxidative Stability is a predictive test that measures resistance of a fat to oxidation and gives some indication of shelf-life. Heat, oxygen and catalytic metals accelerate the rate of oxidation. Saturated (harder) fats are more resistant to oxidation than unsaturated (soft) fats. The cost of this analysis is approximately two times that of the AOM test.
Peroxide Value (PV) is the most widely used indicator of fat oxidation. A peroxide value is required only for USDA certified edible animal fats, such as tallow or lard. However, the feed industry also uses PV to assess the stability or rancidity of fats used as feed ingredients, by measuring lipid peroxides and hydroperoxides formed during the initial stages of oxidation. Values are reported as meq of peroxide per kg of fat. It should be noted that difficulties in the titration end point are common for low PV levels, which may account for a portion of the high variability generally associated with the method.
Peroxides (O2) are intermediate products of fat oxidation and breakdown rapidly to aldehydes, ketones and other products. An example of a peroxide production curve is shown in Figure 2 using choice white grease exposed to heat and oxygen for 11 days (DeRouchey et al., 2000). The peroxide values peaked on day 7 at about 105 meq/kg of fat and subsequently declined to baseline values.
Determining Peroxide Value
Peroxide value is most often determined using the AOCS Official Method Cd 8-53 (same as AOAC Official Method 965.33). An alternative procedure that is...