Processing humidity levels influence survival of bacteria.

For the thermal destruction of pathogens we assume that all microbial cells in a population have the same sensitivity to heat, and that heat strikes the target within cells, causing the death of microbial cells. The thermally processed food market has grown within the last few years. To extend the shelf life of these foods, they are often stored at a chilled temperature in a package under vacuum or modified low-oxygen atmospheres.

A knowledge of the thermal inactivation of potential pathogens during processing and the growth of surviving pathogens during product storage is essential in evaluating the microbial safety of thermally processed products. However, little information is available regarding the growth of heat-injured Salmonella and Listeria in cooked chicken products.

The growth of heat-treated pathogens is likely to differ from that of unheated pathogens, since a portion of the heated pathogens may be inactivated, and others may be sublethally injured, affecting or delaying their ability to grow. Researchers at the University of Arkansas (Department of Biological and Agricultural Engineering, Fayetteville, AR 72701) evaluated the thermal inactivation of Salmonella and Listeria in ground chicken patties processed in a pilot-scale air convection oven. They also investigated the growth of the heat-survived or heat-injured Salmonella and Listeria in the thermally processed patties under different gas environments.

The investigators chose S. Senftenberg, S. Typhimurium, S. Heidelberg, S. Mission, S. Montevideo and S. California as the target microorganisms. L. innocua Ml was used in this research. Chicken breast patties were inoculated with a mixture of these microbes. The initial inoculation of bacteria was approximately 107 log10 CPU/g. The inoculated patties were processed in a pilot-scale air convection oven at an air temperature of 177 C, an air velocity of 9.9 m3/minute, and in low or high humidity. The patties were processed to a final center temperature of 65 C to 75 C. The survivors of Salmonella and Listeria in the processed patties were evaluated.

Researchers found that processing humidity affected the survivors of the bacteria. More survivors of Salmonella and Listeria were obtained for the patties cooked at low humidity than at high humidity levels. After thermal processing, the patties were stored under air, vacuum or carbon dioxide at refrigerated or thermally abused (8 C to 15 C) temperatures. Storage temperature, time and gas environment affected bacterial growth.

Higher storage temperature and longer storage time correlated to an increased growth of bacteria in the cooked chicken patties. Fewer Salmonella and Listeria cells were obtained in the patties stored under vacuum conditions than in air. Storing the patties in 30% carbon dioxide reduced the growth of Salmonella. At a carbon dioxide level of 15%, a 1 log10 CFU/g reduction of Listeria occurred in cooked chicken patties.

Further information. Rong Murphy; phone: [phone]; fax: [phone]; email: [e-mail].