Inactivation of Listeria innocua on blueberries by novel ultrasound washing processes and their impact on quality during storage

We explored the efficacy of ultrasound in combination with carvacrol and carbonated water as a treatment to decontaminate fresh produce using blueberries as a model system. We also evaluated its impact on the quality of blueberries during refrigerated storage. Blueberries inoculated with Listeria innocua, a model pathogen, were washed in presence of individual treatments by either 20 kHz ultrasound at (20 kHz), 1 MHz ultrasound at (1 MHz), 2 mM carvacrol (CR), or carbonated water (CW) as well as their combinations at 20 degrees C for 10 min. Individual treatments resulted in around 2 log CFU/g L innocua reductions on the blueberries' surface with 3-4 log CFU/mL viable bacteria remaining in the wash water. Comparatively, both 20 kHz + CR and 1 MHz + CR + CW led to significantly lower (P < 0.05) bacterial counts on blueberries' surface (similar to 1 log additional reduction) with 2.6 and 2.9 log CFU/mL in the washed water. 20 kHz + CR and 1 MHz + CR + CW treatments were selected to further investigate their effects on blueberry quality compared to washing with sodium hypochlorite (SH) containing 100 ppm free chlorine, or water during subsequent 4 degrees C storage for up to 20 days. Unwashed blueberries were used as control. Blueberries washed by selected treatments had equal or lower (P < 0.05) total aerobic and total mold and yeast counts compared to those treated by water, SH and the control. There was no influence of ultrasound washing processes on the total phenolic content and total anthocyanin content of blueberries, however, a significant impact on color and texture was observed. The findings showed that washing blueberries with novel combined ultrasound washing processes have the potential to enhance the safety and shelf-life of ready-to-eat blueberries.

Automated summary

The study demonstrated that ultrasound combined with carvacrol and carbonated water can effectively remove bacteria from fresh produce, while maintaining the quality of blueberries. The washing processes did not affect the total phenolic and anthocyanin content of blueberries, but had a significant impact on color and texture.