Comparative Growth of Spoilage and Pathogenic Organisms on Modified Atmosphere-Packaged Cooked Beef

The influence of CO₂ atmospheres on the growth of spoilage and pathogenic microorganisms on cooked, sliced roast beef was investigated. An atmosphere containing 75% CO₂, 15% N₂ and 10% O₂ was identified as the most effective in the simultaneous inhibition of Pseudomonas fragi, Salmonella typhimurium, Staphylococcus aureus, and Clostridium perfringens. Inoculated roast beef was exposed to two temperature abuse regimes to observe the resultant microbiological changes in air and three modified atmospheres containing 75% CO₂ and O, 5 or 10% O₂ (balance N₂). The modified atmospheres inhibited the growth of S. aureus during abusive storage, and atmospheres containing 5 or 10% O₂ inhibited the outgrowth of C. perfringens. The modified atmospheres were less effective in inhibiting the growth of S. typhimurium. However, high Salmonella counts were accompanied by high Pseudomonas counts. The concept of a Safety Index, which compares numbers of spoilage and pathogenic organisms, was developed to aid in the evaluation of modified atmospheres.