Internal versus external fat in extrusion of dry expanded dog kibbles containing soy – Impact on process stability and product quality

Abstract

Pet food does not exceed 200 g/kg fat due to lubrication and related processing and product quality issues; however, increased fat is needed for premium pet foods. Use of whole soybeans (WSB) containing internal fat may alleviate this problem. The objective of this study was to determine the effects of fat levels and type (internal from WSB vs external by pumping soybean oil into the preconditioner) on process stability and product physicochemical characteristics during extrusion of dry dog food. Using a 2 × 3 factorial arrangement of treatments with 2 levels of fat (high fat, 80 g/kg as-is basis; HF vs. low fat, 72 g/kg as-is basis; LF) and 3 fat insertion sites (no fat; NO vs. internal fat; IN, from WSB vs. external fat; EX, from soybean oil; SBO), six dog diets were produced by a single screw extruder. Physical properties and physicochemical characteristics of kibbles were analyzed. Data were statistically analyzed using a GLM procedure for mixed models with fat content and fat insertion site as fixed effects. Results were considered significant at P < 0.05. The EX required the highest extruder screw speed (average; 404 rpm), to achieve the target bulk density, followed by IN and NO (351 and 309 rpm, respectively). The EX led to process instability including extruder surging, and higher variability in kibble dimensions as indicated by the calculated variance. The EX had higher sectional expansion index (SEI) and lower specific length (SL; P < 0.05), leading to similar bulk density to IN and NO. The fat levels did not change the kibbles dimensions or bulk density (P > 0.05), but affected texture and viscosity of the kibbles (P < 0.05). In conclusion, SBO adversely affected extruder stability and product expansion compared to WSB. By utilizing WSB in the formula in exchange for liquid fat, the pet food industry might be able to increase the energy density and palatability of kibbles, while keeping cost of processing low.