Survival, growth, and thermal resistance of Listeria monocytogenes in products containing peanut and chocolate

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. 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, wholefat (4.0%) and reduced-fat (1.0%) chocolate milk, a chocolate-peanut spread (39% fat), and peanut butter (53% fat). The D-60degreesC value (decimal reduction time at 60degreesC) in peanut beverage (3.2 min) 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 min). The pathogen was significantly more resistant to heat when enmeshed in chocolate-peanut spread (water activity [a(w)] of 0.46; D-60degreesC = 37.5 min) and peanut butter (a(w) of 0.32; D-60degreesC = 26.0 min) than in liquid products. At 10degreesC, the pathogen grew most rapidly in whole-fat chocolate milk and slowest in peanut beverage. At 22degreesC, populations increased significantly within 12 and 16 h in whole-fat milk and reduced-fat chocolate milk, respectively, and within 8 h 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 a(w) of 0.33 and 0.65 declined, but the pathogen was not eliminated during a 24-week period at 20degreesC. Survival was enhanced at reduced a(w). 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 a(w) range that encompasses that found in these products.