Ultraviolet light assisted by mild heat treatment for microbial inactivation in carrot-orange juice blend

GARCÍA CARRILLO, MERCEDES; FERRARIO, MARIANA; GUERRERO, SANDRA

Cesena

Conferencia; 2017: International Conference on Food Innovation (Food Innova 2017); 2017

University of Bologna

Ultraviolet light (UV-C) has considerable promise to reduce the levels of microbial contamination for a wide range of liquid foods and beverages. Although, due to the presence of color and suspended aggregated particles, many fresh juices transmit relatively little UV-C light, and consequently, this treatment has been mainly limited to clear beverages. Its use in a hurdle strategy could represent a good alternative for the treatment of turbid liquid foods. The aim of this research was to study the inactivation of Escherichia coli ATCC 35218, Saccharomyces cerevisiae KE 162 and Pseudomonas fluorescens ATCC 49838 in fresh carrot-orange juice blend processed by a combination of UV-C and mild heat treatment. Additionally, flow cytometry (FC) was used to analyze the microbial physiology and induced damage on a single-cell basis. The indigenous flora of uninoculated treated juice blend was analyzed along storage.Materials and Methods. Inoculated carrot-orange juice blend (1:1 ratio (v/v), pHadj: 5.0, 10.6 °Brix, Ɛabsorptivity: 0.32 cm-1, turbidity: 7667±15 NTU, particle size= D[3,2]: 5.1±0.1 µm, D[4,3]: 60.81±8.68 µm) was recirculated through an annular reactor (750 mL) consisting of two serially connected UV germicidal lamps (253.7 nm, 100 w), each one inside a 0,87 m-long glass tube leaving an annular flow channel (1.6 L/min, 15 min, 0-11.4 kJ/m2, T: 20, 40, 45 and 50 °C). Single and combined treatments (UV,; T,; UV-T) were evaluated. Samples were taken at preset intervals and analyzed for survivors by the CFU method. Response Surface Methodology (RSM) was used to study the relationship between treatment temperature (T) and time (t) with microbial log reductions (MLR). For FC study, single and combined treated microbial cells were labeled with fluorescein diacetate (FDA) for detecting esterase activity, and with propidium iodide (PI) for monitoring membrane integrity at different UV doses and 50 ºC. Additionally, surviving indigenous flora was monitored in single and combined treated juice samples along 13-day refrigerated storage (4 °C). Results. After single UV exposure (20 ºC), 2.0-2.9 MLR were obtained in the juice, being E. coli and P. fluorescens more sensitive than S. cerevisiae. Combined UV-T treatments were significantly more efficient achieving between 2.7-6.0 MLR, with additive and synergistic effects compared to the single treatments. RSM revealed that, t was the most influential parameter, followed by t2, T y T2 in the estimation of MLR (R2adj: 89.6-98.9), evidenced by the curvature of the surface plots, with an exponential increase of MLR with t. This rate of change increased with T and was higher for P. fluorescens and E. coli. FC study revealed the same damage pattern (although at different UV doses): a gradual shift from FDA+-PI- gate (esterase active cells without membrane damage), to FDA--PI+ gate (cells with non-detectable esterase activity and compromised membrane), indicating that all treatments, with different degree of intensity, provoked rupture of cytoplasmic membrane allowing PI to penetrate cells and progressive loss of esterase activity. The shift was markedly more pronounced for the UV-T treated juice sample. Double stained cells F+PI+ (5.3-8.5 %) were found at high doses only for single treatments, indicating that cytoplasmic membrane integrity was affected, but cells retained esterase activity; whilst the combined treatment ensure the absence of sub-lethally damaged cells. UV-T treatment induced 0.5-2.5 MLR of aerobic mesophilic, moulds and yeasts, and coliforms, respectively, and prevented from its recovery along 7 days of refrigerated storage, while in control system, native flora began to grow since the first day of storage.Conclusions. UV assisted by mild heat treatment significantly improved microbial inactivation in this turbid juice observed for single treatments, specially at 50ºC. FC provided additional information about the microbial inactivation mechanism, detecting the presence of sub-lethally damaged microbial cells which could seriously affect the shelf-life of this juice. It was determined that membrane permeabilization would not be the sole inactivation mechanism, as much more log reductions of viable cells by the CFU method were obtained in comparison with the log increase in permeabilized cells at a given UV dose. UV-T treatment inhibited the growth of native flora during 7-day refrigerated storage, in contrast to single treatments.