Combine steam and vacuum to kill bacteria.

Steam is a gas, and its mean free path is less than the size of bacteria. Steam should be able to enter a product's pores and kill bacteria. USDA-ARS researchers have developed a patented pilot-plant process that exposes solid food to a vacuum, then steam and then a vacuum again to kill product bacteria.

The technology--the vacuum-steam-vacuum (VSV) process--involves using equipment that includes a mandrel to apply vacuum and steam to a product. The system was first used on chicken, but initially there was no statistically significant bacterial kill. The prototype process did not work for whole carcasses. There was no effective exposure to vacuum and steam in the cavity.

Retrofitting various mandrels to the main chamber of the system exposed the cavity to vacuum and steam, and enabled the concept to work. Researchers applied the process to whole carcasses using three different mandrels. They tested the system using saturated steam at 138 C, a steam time of 0.1 sec and a vacuum time of 0.5. Mandrel one yielded a 0.72-log cfu per mL kill; mandrel two, a 0.70-log cfu per mL kill; and mandrel three a 0.79-log cfu per mL kill, all statistically significant at the 95% confidence level.

The VSV technology has been field-tested. Optimum process conditions consisted of three cycles at 138 C for 0.1 sec per cycle. Vacuum time was 0.1 sec except for the final vacuum, which was 0.5 sec. Bacterial kill was 1.1 log to 1.4 log cfu per mL. For most products, typical VSV process conditions are 0.1 sec for vacuum and steam, except 0.5 sec for the final vacuum. The steam is saturated at 138 C or 143 C, and two or three cycles are preferred. Under these conditions, there is little or no thermal damage.

Since the treatment time is so short, the VSV process can be integrated into existing process lines. The VSV process was originally intended for chicken. The technique could be inserted into the processing line before the cold water chill tank. In addition to chicken, the process may be used to treat hot dogs, fruits and vegetables, and catfish. Fish treatments will require a differently shaped treatment chamber.

Researchers are working with a company under a Cooperative Research and Development Agreement (CRADA) to commercialize the technology. The technology is not available for licensing while the CRADA is in effect. The current agreement extends to the end of the year.

Patent. 6,245,294. Method and apparatus for surface treatment of materials. Issued: June 12, 2001. Inventors: Neil Goldberg, et al. Assigned to USDA. The killing of microorganisms on the surface of porous and non-porous materials involves exposing the material to controlled applications of a biocidal treatment gas in a sub-atmospheric environment. The material is first exposed to a vacuum then, without an intervening flush step, full-treated with an air-free treatment gas prior to being re-exposed to a vacuum.

Further information. Michael Kozempel, USDA-ARS Eastern Regional Research Center, Room 3024B, 600 E. Mermaid Lane, Wyndmoor, PA, 19038; phone: [phone]; fax: [phone]; email: [e-mail].