Thermal inactivation of salmonella senftenberg and micrococcus freudenreichii in retentates from ultrafiltered milks

Abstract

Heat resistance of Salmonella senftenberg 775W and Micrococcus freudenreichii MS66 in unconcentrated milks and in retentates from ultrafiltration (and sometimes diafiltration) of the same lots of milk was determined. Age (<28-h-old) of either culture grown in skim milk had little effect on heat resistance of the bacterium when tested in skim milk at 66 and 62°C (D-values of 0.35 ± 0.05 for S. senftenberg and 2.1 ± 0.4 min for M. freudenreichii, respectively). The z-values for S. senftenberg were 8.0, 7.9, 6.7 and 7.3°C, when heating in skim milk, a 4X retentate made from skim milk, whole milk and a 4X retentate (diafiltered) made from whole milk, respectively. The z-values for M. freudenreichii were 6.3 and 6.8°C when heating was in skim milk and a 5.7X diafiltered retentate made from skim milk, respectively. The D-values for M. freudenreichii were 1.8, 0.28, and 0.059 min at 63.5, 67.5, and 73°C, respectively, when heating was in whole milk. Comparable values were 1.9, 0.23, and 0.59 min when heating was in a 3X retentate made from whole milk. The D-values for M. freudenreichii were 3.5 and 5.6 min, when heating at 62°C was in whole milk and in evaporated whole milk, respectively. Osmolality of retentates changed very little as total solids increased. The osmolality, in milliosmol/kg, was 140 for a diafiltered 5.7X retentate (24.7 g/100 g total solids) made from skim milk, 258 for skim milk (9.0 g/100 g total solids), 229 for a 4X retentate (25.7 g/100 g total solids) made from whole milk, 242 for whole milk (11.3 g/100 g total solids) and 676 for evaporated whole milk (24.9 g/100 g total solids). Thermal protection of bacteria in milks and concentrated milks appears to be influenced more by their content of low molecular weight solutes than by total solids. © 1993 Academic Press. All rights reserved.