Inhibition of microbial growth in chub-packed ground beef by refrigeration (2°C) and medium-dose (2.2 to 2.4 kGy) irradiation

Two studies were conducted to monitor microbial growth in chub-packed ground beef. In the first study, the effect of storage temperatures (2 and 7°C) on chubs was examined for up to 18 days. Storage of chubs at 2°C slowed microbial growth such that total counts exceeded 7.5 log 10 cfu/g within 10 days compared to within 4 days for chubs stored at 7°C. Microbial counts ≥7.5 log 10 cfu/g were associated with off-odors and samples were considered spoiled. The predominant microbes were homofermentative lactococci, although a gas-producing bacterium, Hafnia alvei, was prevalent in one of three trials. In the second study, medium dose (2.2 to 2.4 kGy) X-ray irradiation extended the shelf-life of chub-packed ground beef at 2°C by approximately 14 days. Microbial counts on irradiated ground beef did not exceed 7.5 log 10 cfu/g during 34 days of storage, while this level was reached in non-irradiated chubs by day 13. The predominant microbes on both non-irradiated and irradiated chubs were homofermentative lactococci; H. alvei was not isolated in the second study. These data indicated that refrigerated (2°C) storage significantly delayed microbial spoilage in chub-packed ground beef, and that spoilage was further delayed when refrigerated (2°C) ground beef chubs were exposed to medium dose irradiation.