Reduction of Staphylococcus aureus and quality changes in apple juice processed by ultraviolet irradiation, pre-heating and pulsed electric fields

Apple juice inoculated with Staphylococcus aureus (SST 2.4) was processed by a hurdle sequence including ultraviolet irradiation (UV; 30 min, 20 °C), pre-heating and pulsed electric fields (PEF). Four different levels of pre-heating temperature (35-50 °C), electric field strength (28-40 kV cm^-1) and total treatment time (25-100 μs), were used in an orthogonal design, evaluating their impact on S. aureus. Selected physical and chemical quality attributes were measured and compared to conventional pasteurisation (26 s, 94 °C). A higher reduction (P < 0.05) of S. aureus was achieved with a hurdle approach (UV; 46 °C (PEF inlet) and 58 °C (PEF outlet); PEF 40 kV cm^-1 and 100 μs) in comparison to conventional pasteurisation (9.5 vs. 8.2 log₁₀, respectively). Bacterial inactivation was not affected by pre-heating temperature (P ≥ 0.05), while increasing electric field strength and treatment time both induced a reduction in bacterial numbers (P < 0.05). An analysis of the normalised energy densities for all UV/PEF processing conditions indicated a maximised energy efficiency at higher electric field strengths and shorter treatment times. Treatments showed little effect on the measured quality attributes.