This study aims to examine the changes of microbial contamination on the beef carcass surface and in the contact environment during cattle slaughtering. A total of 322 microbial samples were collected during the skinning, trimming and weighing, washing and chilling processes. The sampling sites were the hind legs, middle back, chest, front legs and neck as well as the operatives’ hands and knives. The aerobic plate count (APC), and the counts of Lactobacillus, coliform bacteria, Staphylococcus aureus and Pseadomonas were analyzed. The results showed that the level of microbial contamination on the carcass surface increased firstly and then decreased, and the highest level of microbial contamination was observed in the trimming and weighing process, with APC of 2.82 (lg(CFU/cm²)). The chest was the most contaminated part during slaughtering, with average APC of 2.10 (lg(CFU/cm²)). Slaughtering air was most contaminated during rinsing and its bacterial count was 271.33 CFU per plate. The operatives’ hands and knives were also a major source of microbial carcass contamination.

In order to understand the changes of protein structure during the processing of spiced beef, protein turbidity, intermolecular interactions, microstructure, secondary structure and water distribution of samples collected at different stages of processing (raw meat, tumble marination, cooking, and secondary sterilization at 90, 100, 110 or 120 ℃) were detected by a microplate reader, a scanning electron microscope, a Fourier transform infrared spectrometer, and a nuclear magnetic resonance analyzer. Results showed that the turbidity of myofibrillar protein extracted from spiced beef was higher than that of sarcoplasmic protein. Tumble marination reduced the turbidity of sarcoplasmic and myofibrillar protein (P < 0.05), whereas heat treatment had the opposite effect. Tumble marination increased the protein-protein interactions, while heat treatment reduced the electrostatic interaction between protein molecules. Hydrogen bond, hydrophobic interaction and disulfide bond were the main intermolecular interaction forces. Different treatments destroyed the structures of sarcoplasmic and myofibrillar protein, and the degree of damage increased with increasing number of processing steps and increasing secondary sterilization temperature. Tumble marination, cooking and secondary sterilization had significant effects on the secondary structure of proteins, the free water content and total water content were the highest after sterilization at 100 ℃ among the four temperatures, and the effect of different sterilization temperatures on the bound water content was not significant (P > 0.05).