Internal Versus External Fat in Extrusion of Dry Expanded Dog Kibbles Containing Soy – Impact on Process Stability and Product Uniformity

Abstract

Pet food does not exceed 20% 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 effect of WSB 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; HF vs. low fat; 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 pilot-scale single screw extruder. Extruder screw speed was adjusted to maintain wet product bulk density at ~350 g/L. Physical properties and physicochemical characteristics were analyzed. Data were statistically analyzed using a GLM procedure for mixed models in statistical software (GLIMMIX, SAS version 9.4) 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 CO. Peak force and compression area for LF were higher (P < 0.05) than those for HF. The HF had lower (P < 0.05) peak viscosity, breakdown, hold viscosity, final viscosity, and setback compared to the LF treatments due to the lower starch content. The total processing energy was greater for LF than HF due to the higher steam retention. There was heat loss from the material to the barrel in the extruder for all treatments due to material temperature being higher than barrel temperature.The 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. Having $42 billion value in 2021 of the pet food market, these findings have the potential for substantial economic impact.

Keywords: extrusion, fat lubrication, oilseed, pet food, soybean oil, whole soybeans