Power ultrasound and pulsed light in combination: microorganisms response in apple juice

FERRARIO, M.; GUERRERO, S.N.; ALZAMORA, STELLA MARIS

Madrid

Congreso; BioMicroWorld 2013; 2013

The industry is faced with an ever-increasing demand for fresh-likeness, convenience and ?health? in foods. To harmonize these trends without compromising safety, a number of milder preservation factors had been intensely investigated.  This trend for mildly preserved foods comes with the combined preservation/hurdle technology as the principle in designing the overall treatment. In this research, the effect of  pulsed light (PL) and  ultrasound (US), applied as single factors or in sequential mode, on the inactivation of Alyciclobacillus acidoterrestris ATCC 49025 spores and Saccharomyces cerevisiae KE162  in commercial (pH: 3.5; 10 °Brix) and natural squeezed (pH: 3.4; 11 ºBrix) apple  (Pyrus malus L, var. Granny Smith) juices was examined . US treatment was performed in a 150 mL-double wall cylindrical vessel connected to a water bath whose temperature was fixed to attain 20, 34 or 44 °C in the juice. In each experiment, 5 mL of inoculum (~ 1x106 UFC/mL) was added to 95 mL of juice and sonicated during 10 or 30 min at 20 kHz and 95.2 (80%) µm of wave amplitude. Sonicated juice was immediately transferred to a glass container and treated during 20 or 60 s (23.88 or 71.6 J/cm2) with PL (Xenon lamp; 3 pulses/s; 5 mL; 0.1 m distance ) reaching 20, 44 or 56 °C in the juice, depending on initial temperature and the use of refrigeration during the PL treatment. Survivors were counted along treatments using the standard plate count counting method (Potato Dextrose Agar or Bacillus acidoterrestris medium for S. cerevisiae or A. acidoterrestris respectively). A. acidoterrestris samples were heated (80 °C, 10 min) before plating. According to inactivation results, the behavior of the microorganisms subjected to selected US-PL combined treatments was evaluated along storage (15 days, 6±1 ºC for the yeast and 40±1 ºC for the bacteria). Experiences were done in triplicate. ANOVA and Tukey tests were performed to find significant differences among treatments. Reduction of the population depended on type of treatment; exposure time, temperature, microorganism and juice matrix. PL treatment led up to 3.0 log-cycles of spore reduction in commercial apple juice and 2.0 log-cycles in natural juice, but the spores were not sensible to US.  On contrary, S. cerevisiae counts decreased with US or PL aplication, and up to 6.4 and 5.8 log-cycles of yeast reduction were achieved with combined treatments in commercial and natural apple juices, respectively. In commercial juice, S. cerevisiae treated by 10 min US and 20 s PL (final temperature 56 ºC) in combination did not recuperate during storage. However, control and 20 s PL treated yeasts grew @ 2.0 log-cycles after 15 days at 6 ºC. A. acidoterrestris treated by 60 s LP (final temperature 56 ºC) showed a much reduced growth during storage as compared to untreated bacteria. It can be concluded that using US and PL stressors in sequence significantly reduced S. cerevisiae load and prevented proliferation of remaining refractory or sub-lethally damaged yeast cells during refrigerated storage.