Effects of lipids and processing conditions on lipid oxidation of extruded dry pet food during storage1

Effects of fat type (beef tallow and poultry fat), fat content (0–0.75 g kg−1) and processing conditions (initial moisture: 1.6–2 g kg−1; screw speed: 200–400 rpm) on lipid oxidation of extruded dry pet food were studied. The results showed that the lipid oxidation rate constant of the extruded dry pet food was a function of fat types, added fat content and feed moisture content. Control (no fat) samples had the highest lipid oxidation rate, as compared to the samples with fat addition. Samples with poultry fat had a higher lipid oxidation rate than those with beef tallow addition. Samples containing a higher fat content resulted in a lower rate of lipid oxidation during storage. Feed moisture content had the same effect as the fat content; the extrudates with a higher moisture content resulted in a lower lipid oxidative rate. Pet foods extruded at 300 rpm had a significantly higher lipid oxidation rate than the ones produced at 200 and 400 rpm. Lipid oxidation of the extrudates appeared to be affected mainly by the degree of extrudate expansion. Products with a higher degree of expansion, which had larger cells and thinner cell walls, were more susceptible to oxidation.