title: 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 degrees C to either 45 or 55 degrees C over 60 s and subsequently cooled to 10 degrees 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 degrees 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 degrees 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 log(10) CFU/mL, P < 0.05). An increase in heating temperature from 45 to 55 degrees 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 log(10) CFU/mL (P> or = 0.05) achieved by thermal pasteurization (72 degrees C, 15 s). A reversed hurdle processing sequence did not affect bacterial inactivation (P> or = 0.05). No differences were observed (P> or = 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.
authors.affiliation: School of Agriculture, Food Science and Veterinary Medicine, College of Life Sciences, UCD Dublin, Dublin 4, Ireland.
authors.forename: M
authors.initials: M
authors.lastname: Walkling-Ribeiro
authors.forename: F
authors.initials: F
authors.lastname: Noci
authors.forename: D A
authors.initials: DA
authors.lastname: Cronin
authors.forename: J G
authors.initials: JG
authors.lastname: Lyng
authors.forename: D J
authors.initials: DJ
authors.lastname: Morgan
mesh_terms: D031230:Ananas / Q000382:microbiology; D001628:Beverages / Q000382:microbiology*; D032084:Citrus sinensis / Q000382:microbiology; D003059:Cocos / Q000382:microbiology; D015169:Colony Count, Microbial; D004560:Electricity; D048168:Escherichia coli K12 / Q000254:growth & development / Q000302:isolation & purification*; D005511:Food Handling / Q000379:methods; D005519:Food Preservation / Q000379:methods*; D005638:Fruit / Q000382:microbiology*; D006358:Hot Temperature; D027845:Malus / Q000382:microbiology; D028521:Musa / Q000378:metabolism