Impact of a combined processing technology involving ultrasound and pulsed light on structural and physiological changes of Saccharomyces cerevisiae KE 162 in apple juice

- The heat build-up generated by PL might be used to enhance microbial inactivation.
- Certain doses of PL led to the presence of viable but not culturable yeasts cells.
- Single or combined US+PL provoked membrane damage and progressive loss of esterase activity.
- TEM images complemented FC by revealing additional structural changes involved in yeast inactivation.

This study analyzed the effect of single ultrasound (US) (600 W, 20 kHz and 95.2 μm wave amplitude, 10 or 30 min at 20 or 44 ± 1 °C), or combined with pulsed light technology (PL) with controlled heat build-up (Xenon lamp, 3 pulses/s, 71.6 J/cm2, temperature ranges: 2-20 ± 1 °C and 44-56 ± 1 °C) on the inactivation of Saccharomyces cerevisiae KE 162 cells in commercial (pH: 3.5 ± 0.1; 12.5 ± 0.1 °Brix) and freshly pressed (pH: 3.4 ± 0.1; 12.6 ± 0.1 °Brix) apple juices. Structural damages were analyzed by transmission electronic microscopy (TEM) and induced damage by flow cytometry (FC). Cells were labeled with fluorescein diacetate (FDA) and propidium iodide (PI) for monitoring membrane integrity and esterase activity. US+PL treatment at the highest heat build-up led up to 6.4 and 5.8 log-cycles of yeast reduction in commercial and freshly apple juices, respectively. TEM images of treated cells revealed severe damage, encompassing loss and coagulated inner content and cell debris. In addition, FC revealed a shift of yeasts cells with esterase activity and intact membrane to cells with permeabilized membrane. This effect was more notorious after single 30-min US and all combined US+PL treatments, as 91.6-99.0% of treated cells showed compromised membrane. Additionally, heat build-up enhanced this shift when applying 10 min US (20 °C) in both juices.