Standardized amino acid digestibility and nitrogen-corrected true metabolizable energy of frozen raw, freeze-dried raw, mildly cooked, and retorted dog foods using the precision-fed cecectomized and conventional rooster assays

The application of heat in dietary processing is known to influence nutrient digestibility. Novel pet food formats with differing processing methods are gaining popularity, but few studies have examined their digestibility. Most research evaluating dietary processing type on nutrient digestibility has tested commercial foods that were vastly different regarding ingredient inclusion and macronutrient content, making it difficult to determine the processing influences. To address this research question, the current study aimed to determine amino acid (AA) digestibility and nitrogen-corrected true metabolizable energy (TMEn) of diets having the same ingredient formulations and nutrient concentrations but manufactured using different processing methods. Five diets were manufactured using the following processing methods: retort (RT), mildly cooked [sous vide (SV) and steamed (ST)], and raw [high-pressure processing (HPP) and freeze-drying (FD)]. Those diets were compared against the raw ingredient batch (RAW) that served as a control. Two precision-fed rooster assays utilizing Single Comb White Leghorn (1.5 to 2.5 y old, 2.5 to 3 kg body weight) were conducted to determine the standardized AA digestibility (30 cecectomized roosters; n = 5) and TMEn content (30 conventional roosters; n = 5) of the 6 pet foods. Prior to feeding, wet diets (RT, SV, ST, HPP, and RAW) were freeze-dried, and all diets were ground. Following crop intubation, excreta were collected for 48 h and analyzed, and then AA digestibility and TMEn calculations were performed. Data were analyzed using the Mixed Models procedure of SAS with P < 0.05 accepted as statistically significant and P < 0.10 a trend. The digestibility of 6 indispensable AA were affected by processing. The SV and ST diets had greater (P < 0.05) histidine digestibilities than all other diets. For valine, methionine, leucine, phenylalanine, and isoleucine, the RAW diet tended to have greater (P < 0.10) digestibility than the RT diet. The RT diet had lower (P < 0.05) aspartic acid digestibility than ST, HPP, FD, and RAW diets. Dietary TMEn was higher (P < 0.05) for the SV and ST diets than the RT, HPP, and FD diets, suggesting that those cooking methods are less damaging to macronutrients. Overall, the RT diet had lower indispensable digestible AA concentrations than RAW, likely due to the high heat of processing. Future research should test differences in these diet types in the target species (i.e., dog) to evaluate how they perform.