Effect of Dietary Lipid Oxidation on Measurement of Protein Efficiency Ratios

Abstract

Abstract Casein and lactalbumin were used to test the effect on protein efficiency ratio (PER) of variations in procedure allowable within the official AOAC method. A split plot factorial experiment was used to measure the relative effects of (1) the stage at which oil was mixed into the diets, (2) the initial peroxide value (PV) of the oil, and (3) the presence of sucrose on the PV of the dietary lipids and on PER. The effects of storage time and temperature of the experimental diets and of the addition of antioxidants on the PV of dietary lipids and on PER were also measured. Development of rancidity, measured by the PV of the extracted lipids, was shown to be about twice as rapid in casein as in lactalbumin diets. Maximum PVs of approximately 1100 and 600 were obtained for lipids from casein and lactalbumin diets, respectively. The mean PER of casein was high (3.65) when oil containing antioxidant (PV 1.2) was added last in dietary preparation, but it was significantly lower (3.10) when premixes containing antioxidant-free oils (PV 23 and 38) and sucrose were used. The lactalbumin PER was much less affected by these factors—comparable figures were 3.79 and 3.57. Under conditions favoring oxidation, lipids in the diets oxidized slowly at 5°C but rapidly at 22°C; additions of antioxidants retarded oxidation and significantly increased the PER of casein. We suggest that consistency in the PER method will be improved if dietary oxidation is controlled, and that clusters of phosphoserine in the casein molecule, chelating with iron and copper ions of the diet to form potent oxidative catalysts, may be the reason for accelerated oxidation in casein diets.