Thermal Inactivation of Staphylococcus aureus in Liquid Whey

Introduction: Whey is a useful byproduct from cheesemaking. In some production facilities, the whey is transferred to large storage tanks before further processing. The excessive handling predisposes the whey to recontamination by Staphylococcus aureus, which produces a heat-stable enterotoxin if the whey was not sufficiently cooled. However, additional heat treatment or use of hydrogen peroxide to control pathogen growth may result in lower quality whey products. Therefore, data is needed to determine the minimum temperature and time required for pasteurization and safety.

Purpose: The objective of this study was to determine the D- and z-values of Staphylococcus aureus in whey.

Methods: Six lots of whey (three lots each from cheese made with or without mesophilic starter culture) were standardized to pH 6.5+0.1 with NaOH and inoculated with a three-strain mixture of S. aureus to yield nine log CFU/ml. Samples (1ml aliquots) were vacuum-sealed in moisture-impermeable pouches and heated to 60.0, 62.8, 65.6, or 68.3°C (140, 145, 150, or 155°F) by submersion in a water bath. For each treatment, duplicate samples were removed from heating at appropriate time intervals and enumerated for surviving S. aureusby plating on Baird-Parker agar overlaid with Tryptic Soy agar to aid in the recovery of heat-injured cells.

Results: There was no statistical differences in thermal inactivation rates for whey, regardless of whether it was produced with starter culture (P>0.05). Pooled data was used to calculate thermal inactivation rates. D-values were 1.32, 0.38, 0.12, and 0.07 minutes for samples heated at 60.0, 62.8, 65.6, or 68.3°C, respectively. The z-value was calculated to be 6.1°C (11.56°F) with a best-fit reference temperature of 62.8°C.

Significance: These data can be used by the dairy industry to develop pasteurization procedures to kill vegetative pathogens such as S. aureus.