Inactivation of Escherichia coli in a tropical fruit smoothie by a combination of heat and pulsed electric fields

Moderate heat in combination with pulsed electric fields (PEF) was investigated as a potential alternative to thermal pasteurization of a tropical fruit smoothie based on pineapple, banana, and coconut milk, inoculated with Escherichia coli K12. The smoothie was heated from 25°C to either 45 or 55°C over 60 s and subsequently cooled to 10°C. PEF was applied at electric field strengths of 24 and 34 kV/cm with specific energy inputs of 350, 500, and 650 kJ/L. Both processing technologies were combined using heat (45 or 55°C) and the most effective set of PEF conditions. Bacterial inactivation was estimated on standard and NaCl-supplemented tryptone soy agar (TSA) to enumerate sublethally injured cells. By increasing the temperature from 45 to 55°C, a higher reduction in E. coli numbers (1 compared with 1.7 log ₁₀ colony forming units [CFU] per milliliter, P < 0.05) was achieved. Similarly, as the field strength was increased during stand-alone PEF treatment from 24 to 34 kV/cm, a greater number of E. coli cells were inactivated (2.8 compared with 4.2 log₁₀ CFU/mL, P < 0.05). An increase in heating temperature from 45 to 55°C during a combined heat/PEF hurdle approach induced a higher inactivation (5.1 compared with 6.9 log ₁₀ CFU/mL, respectively [P < 0.05]) with the latter value comparable to the bacterial reduction of 6.3 log₁₀ CFU/mL (P ≥ 0.05) achieved by thermal pasteurization (72°C, 15 s). A reversed hurdle processing sequence did not affect bacterial inactivation (P ≥ 0.05). No differences were observed (P ≥ 0.05) between the bacterial counts estimated on nonselective and selective TSA, suggesting that sublethal cell injury did not occur during single PEF treatments or combined heat/PEF treatments.