Starch gelatinization implications for nutrient digestibility and fermentation products in the faeces of Beagle dogs

A combination of raw material particle size and extrusion parameters promotes starch gelatinization (SG). However, the extent of processing, represented by the SG degree, needed for a proper digestibility of dog diets is not completely understood. The effects of processing conditions and SG on the coefficients of total tract apparent digestibility (CTTAD) of nutrients for dogs were evaluated in two studies. First, a formulation with maize as starch source was ground in a hammer mill using screen sieve sizes of 0.5, 1.8 or 2.0 mm and extruded with different specific mechanical energy (SME) applications to obtain treatments with: 99.9% SG (raw material mean geometric diameter [MGD]=224 mm; SME=22 kW-h/t); 76% SG (MGD=283 mm; SME=12 kW-h/t); 63% SG (MGD=312 mm; SME=11 kW-h/t). A fourth treatment used raw material with an MGD= 312 mm, which was pelletized to obtain 26% SG. Twenty-four Beagle dogs were used (six animals per treatment) to determine CTTAD and to assess fermentation products in faeces. The results were evaluated by polynomial contrast according to SG degree (P < 0.05). In the second study, data from a scientific publication (n = 9) and two databases of commercial extruded diets (UNESP, Jaboticabal, Brazil [n = 18] and University of Zaragoza, Zaragoza, Spain [n = 33)]) were used to construct polynomial regressions between SG degree and CTTAD of nutrients. In Study 1, feed intake was similar (P > 0.05) and faeces production was adequate for all dogs. A linear increase in crude protein (P < 0.002) and a quadratic increase in starch CTTAD (P < 0.001) followed the increase in SG of the diets. No differences were detected for the CTTAD of the other nutrients (P > 0.05). Fermentation products in faeces decreased linearly as the SG level of the diets increased (P < 0.01). Lactic acid in the faeces of dogs fed the pelletized diet was 6.8 times greater than the observed for 99.9% SG diet (P < 0.001). In Study 2, the CTTAD of organic matter (R-2 varying from 0.63 to 0.26, depending on the database), crude protein (R2 from 0.91 to 0.26), nitrogen-free extract (R-2 from 0.95 to 0.23), and fat (R-2 from 0.84 to 0.46) all increased with increasing SG (P < 0.05). The CTTAD of crude protein was the most positively influenced by SG. Overall, the SG degree has an impact on nutrient digestibility, and although a linear response was found for some databases, values from 83% to 87% maximized the CTTAD of nutrients. SG also influences fermentative activity and is a potential nutritional intervention for promoting gut health by modulating the formation of butyrate.