Article Nutritional Value of Extruded Dog Food with Mechanically Separated Chicken Meat or Meat By- products Giorgia Meineri 1 , Pier Giorgio Peiretti 2 *, Sonia Tassone 3 , Alessia Candellone 1 , Erica Longato 1 , Daniele Pattono 1 , Natalia Russo 1 , and Liviana Prola 1 1 Department of Veterinary Sciences, University of Turin, 10095 Grugliasco, Italy. E-Mails: giorgia.meineri@unito.it (G.M.); alessia.candellone@unito.it (A.C.); erica.longato@unito.it (E.C.); natalia.russo@unito.it (N.R.); daniele.pattono@unito.it (D.P.); liviana.prola@unito.it (L.P.) 2 Institute of Sciences of Food Production, National Research Council, 10095 Grugliasco, Italy. E-Mail: piergiorgio.peiretti@ispa.cnr.it (P.G.P.) 3 Department of Agriculture, Forestry, and Food Sciences, University of Turin, 10095 Grugliasco, Italy. E-Mail: sonia.tassone@unito.it (S.T.) * Correspondence: piergiorgio.peiretti@ispa.cnr.it; Tel.: ++39.11.6709233 Simple Summary: Fresh meat, such as mechanically separated chicken meat (MSCM), is usually listed first as an ingredient on pet food labels when selected by industry as a protein component. This commercial strategy may influence consumer choice in the comparison among pet food products. Since most dry pet foods are produced by food extrusion, it is important to compare the nutritional value of a dog diet based on MSCM with one based on meat by-products (MBP) to determine whether the "fresh meat" is really the better choice. Abstract: The objectives of this study were: (a) to evaluate the effect of inclusion of mechanically separated chicken meat (MSCM) in dry dog food on fatty acid profile, in vivo and in vitro digestibility, and palatability as compared with dry dog food containing meat by-products (MBP); (b) to determine, whether or not, the inclusion of the one or the other ingredient changes the microbiology and the storage quality of the two food products; (c) to propose a new system (Daisy II Incubator) to measure the in vitro digestibility of the two products. Their similar chemical composition notwithstanding, the MSCM product had lower palatability but better nutritional quality (with higher polyunsaturated fatty acid [PUFA] content and lower saturated fatty acid [SFA] content) than the MBP product. Microbiological risk assessment showed no microbiological hazards for either product. After 6 months storage, polyamine values were found to be higher in the MSCM than in the MBP. Finally, the Daisy II Incubator proved a valid instrument for the study of in vitro digestibility also for dogs; since it provided data simply, quickly with less variability and cost than obtained with in vivo trials, it could represent the future for pet food digestibility studies. Our results indicate that inclusion of MSCM or MBP as the main protein ingredient in extruded pet food may be used advantageously in product formulations. Keywords: dog food; fatty acid; palatability; digestibility; conservation quality; Daisy II Incubator 1. Introduction Overwhelmingly, pet owners consider their pets to be family members (63% of pet owners in the United States and more than 71% in Italy) [1,2]. Anthropomorphism of dogs and cats has driven pet owner preference for pet foods containing ingredients that they find in their own diet and processed so as to maintain the nutritional integrity of the ingredients and ensure food safety, high palatability, and digestibility. To meet these high expectations, the pet food industry is keen to increasingly include such ingredients, especially fresh and unprocessed meat, in dry dog food Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 18 January 2019 © 2019 by the author(s). Distributed under a Creative Commons CC BY license. Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 18 January 2019 doi:10.20944/preprints201901.0189.v1 © 2019 by the author(s). Distributed under a Creative Commons CC BY license.