Pasteurization destroys Listeria monocytogenes in peanut- and chocolate-containing products, according to a published report.

"Outbreaks of listeriosis associated with the consumption of ready-to-eat foods have raised interest in determining growth, survival, and inactivation characteristics of Listeria monocytogenes in a wide range of products,” the report finds.

"A study was undertaken to determine the thermal tolerance of L. monocytogenes in a peanut-based beverage (3.1% fat), whole-fat (3.5%) milk, whole fat (4.0%) and reduced-fat (1.0%) chocolate milk, a chocolate-peanut spread (39% fat), and peanut butter (53% fat)," scientists writing in the Journal of Food Protection report.

"The D-60 degrees C value (decimal reduction time at 60degreesC) in peanut beverage (3.2 minutes) was not significantly different (P>0.05) than the D-60degreesC value in whole-fat milk (3.3 min) or whole-fat chocolate milk (4.5 min) but significantly lower (P less than or equal to 0.05) than the D-60degreesC value in reduced-fat chocolate milk (5.9 minutes).

"The pathogen was significantly more resistant to heat when enmeshed in chocolate-peanut spread (water activity (aw) of 0.46; D-60degreesC=37.5 minutes) and peanut butter (aw of 0.32; D-60degreesC=26.0 minutes) than in liquid products," wrote S.J. Kenney and colleagues.

"At 10degreesC, the pathogen grew most rapidly in whole-fat chocolate milk and slowest in peanut beverage. At 22degreesC," the authors continued, "populations increased significantly within 12 and 16 hours in whole-fat milk and reduced-fat chocolate milk, respectively, and within eight hours in whole-fat chocolate milk and peanut beverage."

"Initial populations (3.37 to 4.42 log CFU/g) of L. monocytogenes in chocolate-peanut spread and peanut butter adjusted to an aw of 0.33 and 0.65 declined," said investigators, "but the pathogen was not eliminated during a 24-week period at 20degreesC. Survival was enhanced at reduced aw."

Kenney concluded, "Results indicate that a pasteurization process similar to that used for full-fat milk would be adequate to ensure the destruction of L. monocytogenes in peanut beverage. The pathogen survives for at least 24 weeks in chocolate-peanut spread and peanut butter at an aw range that encompasses that found in these products."

Kenney and colleagues published their study in the Journal of Food Protection (“Survival, growth, and thermal resistance of Listeria monocytogenes in products containing peanut and chocolate.” J Food Protect, 2004;67(10):2205-2211).

Additional information can be obtained by contacting L.R. Beuchat, University Georgia, Center Food Safety, 1109 Experimental Station, Griffin, GA 30223, USA.