Inactivate planktonic E. coli O157:H7 strain 87-23 with calcium oxide.

Many of the pathogen intervention methods used for fresh produce involve the use of antimicrobial agents in washes. Although these chemical techniques are popular because of their convenience and economics, many food processing plants, notably in Korea, tend to avoid using them over concerns about toxicity.

So scientists at the University of Illinois and colleagues in Korea investigated the antibacterial effects of calcium oxide sourced from scallop shells on E. coli O157:H7 strain 87-23 at different concentrations and temperatures. They also examined the inactivation kinetics using five linear and non-linear models. Their work indicates that calcium oxide could be used on fresh produce or sprouts as a natural microbial safety intervention technology.

E. coli O157:H7 cells suspended in peptone water were treated with about 0.01% to 0.04% calcium oxide at 5 C, 15 C and 25 C. The time required to achieve a 5-log reduction in survival count was recorded by the researchers at 25 C for the different concentrations. The samples treated for a specific time period were collected and spread on tryptic soy agar. This step was followed by an overnight incubation period at 37 C. Scanning electron microscopy was then used to examine any morphological changes that might have occurred in the cells.

The survival rate of the bacteria decreased with time. An increase in the concentration of calcium oxide enhanced bacterial death. Microscopic images indicated that severe cell damage had occurred, including disruption and leakage of intracellular materials.

A 5-log reduction of bacteria at 25 C was achieved in 40 minutes using a concentration of 0.01% calcium oxide; in 30 minutes using 0.02% calcium oxide; in 20 minutes using 0.03% calcium oxide; and in 12 minutes using 0.04% calcium oxide. The Weibull and biphasic linear modes best described the non-linear inactivation data. The inactivation data at higher concentrations fit the first-order model well. The kinetic models may provide useful information for understanding and predicting the responses of the bacterial cells to the calcium oxide treatments.

Further information. Hao Feng, Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, 382 F Agricultural Engineering Sciences Building, Urbana, IL 61820; phone: [phone]; fax: [phone]; email: [e-mail].