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Thermal resistance of Listeria monocytogenes
The heat resistance of L. monocytogenes is influenced by many factors such as strain variation, previous growth conditions, exposure to heat shock, acid, as well as other stresses, and composition of the heating menstruum .
D-value is used to describe the heat resistance of a certain strain at a certain temperature as it is the time needed to destroy 90% of cells at that temperature. z-value is the temperature difference required to destroy 90% of the bacteria with a 10-fold change in heating time.
The strain variation of 21 L. monocytogenes strains at 55°C in BHI broth at pH 6 was 4.7 fold (23.8 to 111 min) shown by the D55°C-values. L. monocytogenes isolate from brine cheese had 1.5 to 2.8 fold higher D56-value than the three listeriosis outbreak strains tested.
Cells in stationary phase of growth appear to be the most resistant to thermal stress . Stationary phase cells of L. monocytogenes had 2.8 to 5.6 times higher D60°C-values (0.45 to 12.5 min) than the logarithmic phase cells, each tested in three different media (minced beef, tryptic phosphate broth TPB and TPB supplemented with 8 g/l lactic acid) at four different pH from 5.4 to 7. L. monocytogenes Scott A strain had 7.6 times higher D56°C-value at the early stationary phase than at the exponential phase.
The composition of the growth medium, whether a food or laboratory culture broth, affect rates of growth and the synthesis of cellular constituents that determine the thermal tolerance of bacterial cells. The D60°C-values for L. monocytogenes is 2 to 6 fold higher in minced beef than in TPB . In half cream, double cream and butter the D-values of two L. monocytogenes strains were 1.1 to 8 fold higher than in TSB also indicative of notable differences in D-values between food products .
The environment in which cells are grown can be a major determinant of their heat resistance.
L. monocytogenes –strain Scott A growing in tryptic phosphate broth containing 0.09, 0.5, 1.0 or 1.5 M NaCl was heated in media with the same salt concentrations resulting in 4 log reduction at 60 °C in 1.6, 2.5, 7.4 and 38.1 min, respectively .
Increased heat resistance is also induced by starvation, low pH, and addition of antimicrobial compounds like ethanol, or hydrogen peroxide to the growth media.
The growth temperature affects the heat resistance and in general the cells grown at higher temperatures are more heat resistant than those grown at lower temperatures.
The rate at which cells are heated during testing also influences their survival. When cells are heated slowly, they exhibit a greater heat resistance than when heated rapidly
Heat-shock, a short-term exposure of cells to temperatures above the optimum growth, results in increased heat resistance. The degree of enhanced thermal resistance is strain dependent and also varies with the length of the heat shock, the pH of the medium, and the growth phase of the cells. The maximum increase in thermotolerance was 4 and 7 fold for a L. monocytogenes strain previously grown at 37 °C and 4 °C and heat-shocked at 45 °C and at 47.5 °C, respectively. On the other hand cold-shock decreases the heat
resistance of L. monocytogenes Increased heat tolerance can also be induced by short-term exposure to high salt or solute levels . Decreasing aw values and increasing solute concentrations result in greater heat resistance in L. monocytogenes.
The presence of 10% salt or aw<0.92 resulted in a high heat resistance and it became the most heat resistant vegetative pathogen .
Hence validation tests are to be carried out for all food products
Attached Heat Resistance of listeria doc