Heat resistance of Salmonella typhimurium and Listeria monocytogenes in suspension and in a biofilm matrix

Abstract

The heat resistance was determined for Salmonella typhimurium and Listeria monocytogenes Scott A suspended in 2% UHT processed milk and in a biofilm matrix. Pure cultures at an initial concentration of 10⁵ / ml were used. Heat resistance was determined by two methods. One method was sealed borosilicate glass TDT tubes that were completely submerged in the heating menstrum. Biofilms were grown on Buna-n rubber o-rings (4.46 mm O.D. x 1.41 mm]. D.) for 36 hours. All other cultures used were in stationary phase of growth. The three treatments tested were: inoculated milk, sterile milk and a biofilm on an o-ring, and inoculated milk with a sterile o-ring. At the three temperatures tested (60, 63, 67°C), there was no significant difference (p>0.05) in D-values between treatments. There was a significant difference (p<0.001) between the D-values for Salmonella and Listeria. The second method used a laboratory scale HTST pasteurizer to determine the difference in heat resistance of the same organisms suspended in 2% milk vs. sloughed off pieces of biofilm in milk. Pure cultures of the organisms at an initial inoculum of 10⁵ / ml were used. Flow rates of the pasteurizer were adjusted to achieve two different F-values for each organism at a reference temperature of 71.7°C. Neither S. typhimurium nor L. monocytogenes Scott A was recovered from pasteurized samples of either treatment. The heating involved in come up and cool down of the transit lines was considered in determining F-value. Under commercial HTST processing, concentrations of 10⁵ / ml of S. typhimurium and L. monocytogenes Scott A would not survive pasteurization. The results also show that if pieces of biofilms (3.8 x 10⁻⁴ mm²- 8.8 x 10⁻³ mm²) were sloughed off gaskets in the processing lines they would not survive pasteurization. The heating characteristics of these two systems were so dissimilar they could not be compared. It should however be noted that in the TDT tubes it was necessary to obtain a slightly higher F-value before no growth was seen as compared with the pasteurizer. In the pasteurizer the laminar flow properties would contribute to a more uniform heating. The TDT tube experiences convection heating which can produce cold spots in the tubes and could explain the need for an increased F-value.