Microbial contamination of chilled pork in cold chain logistics and its control chart design

Abstract: The quantity changes of major spoilage microflora (total viable counts, Pseudomonas sp. and Brochothrix thermosphacta) and pathogen microflora (Coliform bacteria and Listeria sp.) during the cold chain logistics were investigated by selective medium, to evaluate the quality and safety of chilled pork by monitoring the microbial growth. The mean-range control chart of microbial contamination of chilled pork during the logistics chain was designed using the statistical process control technology, to analyze their quantity fluctuation and the ratio of eligible quality. The results showed that the microbial quantity (except Coliform bacteria) of pork samples increased significantly (P＜0.05) with the extension of logistics chain, and the quantity of pork samples in supermarket were higher (P＜0.05) than those in slaughterhouse; The microbial contamination of pork samples in the supermarket from external city was higher (P＜0.05) than those from local city. The microbial quantity distribution of chilled pork during the logistics chain was studied by normality test, and it was found that the quantity of Coliform bacteria and Brochothrix thermosphacta showed normal distribution, but Pseudomonas sp. and Listeria sp. did not fit normal distribution. The mean-range control chart of total viable counts and Coliform bacteria of chilled pork during the logistics chain was designed using statistical process control technology, according to the microbial safety limits of chilled pork in the national standard of the People's Republic of China. As showed in the control charts, there were significantly differents in the process capability index (CPk) and the ratio of quality eligibility calculated by different control limits from zero to alerting limit, from zero to maximum limit, and from lower control limit (LUL) to upper control limit (UCL). However, the total viable counts in all pork samples were eligible according to the national microbial quantity limits (1×106cfu/g). Mean-range control chart can be used to show intuitively the abnormal fluctuations and substandard quality of microorganism, and it is not yet or seldom applied to the microbial contamination of pork in the cold chain logistics. The research is meaningful to provide a scientific method in pork quality monitoring and microbial control in the cold chain logistics.