The effect of extrusion on the nutritional value of canine diets as assessed by in vitro indicators

A 3 × 2 × 2 factorial trial was designed to investigate the effect of different extrusion conditions and product parameters on the nutritional quality as determined by a number of in vitro measurements (e.g. reactive lysine, and starch gelatinization degree) as well as physical quality of the kibble (durability and hardness) of a canine diet. The parameters investigated were mass temperature (110, 130 or 150 °C), moisture content (200 or 300 g/kg) of the diets prior to extrusion and number of times (once or twice) extruded. Total lysine and other amino acids were unaffected by the extrusion conditions employed. Extrusion conditions had a clear effect on the reactive lysine content with the ratio of reactive to total lysine increasing from 0.71 to 0.80 and higher as a result of extrusion and temperature. After a second extrusion, a decrease was observed from a ratio reaching 1.0 to about 0.9. Initial moisture content affected lysine reactivity. Protein digestibility as measured in vitro was not affected by different extruding conditions. There were no obvious differences in protein dispersibility index (PDI) of all the extrudates. In vitro glucose digestibility coefficients as well as starch gelatinization degree (SGD) showed a tendency to increase with an increase in each individual parameter tested. The increase in temperature from 110 to 150 °C as well as extrusion for a second time decreased kibble durability while increasing moisture content increased durability. Optimisation of extrusion conditions during commercial pet food production should include measurement of the reactive to total lysine ratio.