Inactivation of Escherichia coli in a Tropical Fruit Smoothie by a Combination of Heat and Pulsed Electric Fields

ABSTRACT: 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 log₁₀ 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 log₁₀ 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.