Microbial evaluation of automated sorting systems in stone fruit packinghouses during peach packing

Automated fruit sorting systems with individual fruit carriers are utilized in modern fruit packing facilities. This study evaluated the levels of naturally occurring microflora on the surfaces of peaches and fruit carriers during automated sorting operations at stone fruit packinghouses in California. The study also assessed the growth potential of Salmonella enterica and Listeria monocytogenes on fruit carriers under various environmental conditions. No difference of microbial loads was found on peaches (n = 420) before, during, and after fruit sorting at seven packinghouses. The average surface total microbial, coliform, and yeast and mold levels of peaches during sorting were 3.6, 2.7, and 1.9 log CFU/cm², respectively. Environmental swab testing indicated routine cleaning of fruit carriers (n = 192) reduced total microbes from 3.9 to 3.2 log CFU/cm² (P = 0.003) and coliforms from 1.5 to 0.9 log CFU/cm² (P = 0.001) on carriers' fruit contact surfaces. Laboratory exposures to temperature (22, 28, 34 or 40 °C) and humidity (65, 75, 85 or 95%) conditions significantly reduced inoculated Salmonella and Listeria on clean and commercially used (deposited with wax, fuzz, dirt, etc.) fruit carriers within 24 h (P < 0.001). The observed Salmonella reduction was greater on clean carriers (P < 0.001). On used carriers, Salmonella was persistent at 95% humidity and Listeria was persistent at 22 °C. The results showed the levels of surface microflora on peaches during fruit sorting, the reduction of microbial loads on fruit carriers due to packinghouses' cleaning, and the reduction, rather than growth, of Salmonella and Listeria under tested conditions on fruit carriers.