Thermal resistance of intracellular Listeria monocytogenes cells suspended in raw bovine milk

Abstract

The thermal resistance of Listeria monocytogenes associated with a milk-borne outbreak of listeriosis was determined in parallel experiments by using freely suspended bacteria and bacteria internalized by phagocytes. The latter inoculum was generated by an in vitro phagocytosis reaction with immune-antigen-elicited murine peritoneal phagocytes. The heat suspension medium was raw whole bovine milk. Both suspensions were heated at temperatures ranging from 52.2 to 71.7°C for various periods of time. Mean D values for each temperature and condition of heated suspension revealed no significant differences. The extrapolated D71.7rc (161F) value for bacteria internalized by phagocytes was 1.9 s. Combined tube and slug-flow heat exchanger results yielded an estimated D71.7'c value of 1.6 s for freely suspended bacteria. The intracellular position did not protect L. monocytogenes from thermal inactivation.

Thermotolerance of Listeria monocytogenes and Salmonella typhimurium after sublethal heat shock

“…A No endpoint reflects a relatively safe reduction process and is directly comparable to the data and interpretations of Mackey and Derrick (17,18). 11.3c (2) 11.7c (2) 11.1d(2) 10.3d (2) lo.ge (2) 9.2d.e (2) 9.3c (3) 7.8c (2) 7.Oc (2) 6.9c (2) S. typhimurium 30 0.r7-(2) 0.9f (2) 2.1k (2) 0.8f (1) a Inactivation temperature after heat shock was 57.8'C. The number of observations is given in parentheses.…”

“…As in our previous reports (1)(2)(3)(4), estimates of heat resistance are expressed as D values, thereby assuming that the log1o reduction in bacterial count is linear with respect to time at a constant temperature (first-order kinetics). This estimate of heat resistance was used even when deviations from linearity, e.g., initial short lag (12,17), were observed, because estimates of D are routinely used in designs of processing plants for milk and other foods.…”

“…Several equations will yield an ?I 1, i.e., a polynomial. As part of the comparative exercise for methods of endpoint estimation, the equations log1o N = bo + b1t2 (1) (log1o No -log1o N)bl = bot (2) were used (6,17,21). The latter equation was used by Mackey and Derrick (17,18).…”

Inactivation and Injury of Listeria monocytogenes in a Minimal Medium as Affected by Benzoic Acid and Incubation Temperature

“…2), calculation of the z-value for inactivation of L. monocytogenes is justified. The average z-value for L. monocytogenes in the minimal medium (lacking benzoic acid) and plated on TA medium, was 16.6"C. z-Values of 4.3-6.7"C were observed at temperatures commonly used to inactivate milk-borne L. monocytogenes by heat (Bradshaw et al, 1985;Bunning et al, 1986;Donnelly and Briggs, 1986;El-Shenawy et al, 1989). The z-value obtained in this study cannot be compared directly with those reported earlier since different media and higher temperatures were used in those investigations.…”

Living with Listeria

“…However, in 1960 Gray demonstrated its presence in poorly made silage. Since then there have been many isolations: (Bearns& Girard, 1958) Destroyed by German HTST pasteurization (Obiger, 1976) 5 X 10*/ml survive 74OC for 42 s (Stajner et al, 1979) D value 0.9 s at 71.7"C (Bradshaw et al, 1985) D value 1.9 s at 71.7OC (Bunning et al, 1986) Destroyed by USA HTST pateurization (Donnelly & Briggs, Survive USA HTST pasteurization (Doyle et al, 1987) Up to 10*/ml destroyed by Spanish HTSTpasteurization (G S However, it is now generally agreed that pasteurization destroys Listeria monocytogenes .…”