Assessment of microbiological safety in the complex processing of secondary raw materials and food industry by-products into finished products

This study analyses the qualitative and quantitative composition of the microflora in secondary raw materials of animal and plant origin used to produce extruded complete dry feed for service dogs. A total of 42 pooled samples were analysed — 18 samples of raw materials (chicken meat-and-bone meal, chicken fat, wheat, wheat bran, wheat gluten, corn gluten) and 24 samples of finished feed (four formulations). Microbial counts were determined by standard plate-count methods on meat-peptone agar, wort agar, MRS agar and Czapek agar, and pathogens were screened by membrane filtration on selective media (Endo, chromogenic TTC), following recognised national standards for food microbiology. Plant-derived components showed the highest initial bacterial load (wheat bran, 4.1×10⁵ CFU/g). Immediately after extrusion (up to 130 °C, 7–9 MPa), no mesophilic aerobic and facultatively anaerobic microorganisms, coliforms, Salmonella sp. or Clostridium botulinum were detected in any of the four feed formulations under the applied methods and their detection limits. After 6 months of storage at ambient warehouse temperature (not exceeding 25 °C) and relative humidity not exceeding 75 %, low levels of spore-forming Bacillus subtilis and B. mesentericus (1×10¹–4×10¹ CFU/g) persisted in all four formulations, and Mucor sp. (2×10¹ CFU/g) was additionally detected in one economy-class formulation. These findings indicate that extrusion substantially reduces microbial load but does not eliminate resistant spore forms; the absence of colony growth immediately after processing is reported here as an operational result under the stated detection limits, not as validated commercial sterility.