<i>Escherichia coli</i> and <i>Listeria innocua</i> stability in carrot juice preserved by high hydrostatic pressure

Abstract

The effect of High Hydrostatic Pressure (HHP) on Escherichia coli and Listeria innocua in carrot juice was evaluated just after pressurization and during refrigerated storage for 14 days. Samples were processed with different variants of pressure (300,400,500) and time (1 min, 5 min, and 10 min). The number of bacteria in the populations was analyzed using plating count methods. Required 5 log reduction was achieved only for L. innocua strains starting from 400 MPa for 5 min. E. coli strains displayed resistance to pressure, and the maximum reduction achieved was 2 log CFU/mL for the harshest process parameters. Sublethal injuries in the bacterial population were observed for all tested strains. According to two standardized ISO methods, selective conventional agars, TBX, and ALOA were used in the storage test. Additionally, the Thin Agar Layer (TAL) method was applied. In both used methods, the possibilities of recovery were provided. The regeneration was observed exclusively for L. innocua strains. The recovery of sublethally injured cells on ALOA and TAL did not differ statistically in every strain. In turn, results obtained for E. coli suggest that TBX may underestimate the number of HHP-injured bacteria.

Keywords

• carrot juice
• high hydrostatic pressure
• microbial stability
• refrigerated storage