Survival of Listeria monocytogenes on Stainless Steel Exposed to Dry Heat

Introduction:  Listeria monocytogenes, the causative agent of listeriosis and a major concern for the food industry, is widespread in the environment and may be introduced into food processing facilities resulting in contaminated ready-to-eat (RTE) foods. While most RTE dry foods are generally considered not to support the growth of L. monocytogenes they may, in some cases, be used as ingredients for RTE foods that do support Listeria growth. Many dry food manufacturing facilities maintain a dry environment for lengthy periods followed by a wet cleaning and sanitation cycle. Very limited data is available regarding survival of Listeriaon non-porous dry surfaces exposed to dry and moist heat.

Purpose:  The purpose of this study was to investigate the ability of L. monocytogenes to survive on stainless steel in dry and moist heat conditions.

Methods:  L. monocytogenesATCC 49594 was inoculated onto stainless steel carriers and exposed to several different dry and moist heat conditions. After the indicated exposure times the organisms were recovered and log reductions were calculated.

Results:  A 2-log reduction was achieved after 4 h at 90°C dry heat while over a 3-log reduction was achieved after 1 h at 100°C dry heat. When coupons were soiled with 10% whole milk or 5% lactose and exposed to 100°C dry heat, a > 3-log reduction was achieved after 30 min. When a 5% glucose solution was used the same reduction was achieved after 15 min. In the presence of steam (80-90°C) a > 3 log-reduction was observed after 5 min with or without the whole milk or carbohydrate soils.

Significance:  This study indicates dry heat can eliminate Listeria monocytogenes from non-porous surfaces, such as stainless steel, but will require extended time periods and/or high temperatures.