Heat inactivation of foodborne pathogens in whey concentrate

Introduction: Pasteurization aims to achieve at least a 5 log10 reduction of heat-resistant microorganisms in raw whole milk. In Europe it is done by high temperature short time at 72°C for at least 15 s, according to regulation 853/2002. While heat inactivation of pathogens has been well studied in raw milk, their inactivation in whey concentrate has not received attention.

Goals: The study aimed to determine the inactivation of heat-resistant strains of the pathogens Escherichia coli and Salmonella enterica in whey concentrate with 30% dry matter by applying of HTST-treatment comparable to industrial practice.

Methods: Whey concentrate was inoculated with E. coli AW1.7 or S. enterica serovar Senftenberg 775W at an initial concentration of ca. 5 x 105 CFU/ml and then inactivated in a pilot plant pasteurizer. All experiments were set to a constant product flow rate of approx. 25 L/h and nominal heating temperatures were set to 72.0, 68.0, 64.0, 60.0, and 56.0°C. Heating was done at each temperature for an average of 17.5 s. Survivors were enumerated by plating aliquots of suitable dilutions on agar media.

Results: At the lower heating temperatures of 56°C and 60°C only S. Senftenberg showed a minor reduction in counts of 0.4 ± 0.4 log10 CFU/mL and 0.6 ± 0.7 log10 CFU/mL, respectively. At 64°C, the numbers of S. Senftenberg 775W were slightly reduced by 1.1 ± 0.6 log10 CFU/mL, while E. coli AW1.7 showed a minor reduction of 0.7 ± 0.5 log10 CFU/mL at this temperature. Starting from 68°C, the strains S. Senftenberg 775W and E. coli AW1.7 showed a reduction in cell counts of 3.1 – 4.2 log10 CFU/mL. At 72°C, viable cells of both strains tested were reduced by over 5 log10 CFU/mL.

Summary: Results showed that a 5 log10 reduction of both the highly heat-tolerant strains of Salmonella and E. coli in whey concentrate can be achieved when heating at 72°C for an average of 17.5 s in a continuous pasteurization unit close to industrial practice. Thus, products made from whey concentrate, when used as an ingredient in foods, should not lead to contamination with these bacterial pathogens, provided that the whey concentrate was pasteurized at 72°C for 17.5 s.