Thermal Inactivation of Staphylococcus aureus in Retentates from Ultrafiltered Milk

Cells of Staphylococcus aureus strains 196E, 481, and 425 were thermally stressed at 56°C for 10 min in milk and enumerated on Plate Count Agar (PCA), Mannitol Salt Agar (MSA), and PCA with an overlay of MSA. PCA recovered more S. aureus 196E and 481 than did PCA/MSA, which recovered more than MSA. PCA/MSA recovered slightly more S. aureus 425 than did PCA, which recovered more than MSA. At 58°C, in order of decreasing heat resistance, the four strains of S. aureus originally isolated from food were 425 > 100 and 481 > 196E. Their D-values were 26,14,13, and 3.0 min, respectively. S. aureus 425 was more heat resistant in the stationary than in the log phase when heated at 58°C in whole milk. Heat resistance at 58°C increased overall during the stationary growth phase, but was fairly stable when the culture was from 17 to 25 h or from 41 to 49 h old. S. aureus 425 exhibited no consistent differences in heat resistance in concentrated (4X by volume) and unconcentrated skim or whole milk. Adjustments of protein (3.5-4.0% to 12.6-16%), milkfat (0.28-1.12% to 10%), and lactose (ca. 4.5-5.0% to ca. 14.5-15%) contents of milk and 4X (volume concentration) UF milk retentates afforded no significant thermal protection to S. aureus 425. Diafiltration of 4X skim milk reduced thermal protection of S. aureus 425 in the retentate over that of unconcentrated skim milk of the same lot when tested at 63 and 74°C. S. aureus 425 had greatest D-values (min) in skim milk (0.36 ± 0.05) and permeate (0.30 ± 0.14) followed by permeate from diafiltration (0.28 ± 0.06) when tested at 63°C.