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

Applejuice inoculated with Staphylococcus aureus (SST 2.4) was processed by a hurdle sequence including ultraviolet irradiation (UV; 30 min, 20 degrees C) pre-heating and pulsed electric fields (PEF). Four different levels of pre-heating temperature (35-50 degrees C) electric field strength (28-40 kV cm(-1)) and total treatment time (25-100 mu s), 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 degrees C). A higher reduction (P < 0.05) of S. aureus was achieved with a hurdle approach (UV; 46 degrees C (PEF inlet) and 58 degrees C (PEF outlet); PEF 40 kV cm-1 and 100 Vs) in comparison to conventional pasteurisation (9.5 vs. 8.2 loglo, 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. (C) 2008 Elsevier Ltd. All rights reserved.