Effect of high-pressure processing on bacterial inactivation in acai juices with varying pH and soluble solids content

This study evaluated microbial inactivation effects of high-pressure processing (HPP) applied to acai juices formulated with varying pH and soluble solids content (SSC). Acai juice with pH 4.3 and 2.9 degrees Brix was initially inoculated with cocktails of 5 strains of E. coli O157:H7, Listeria monocytogenes or Salmonella spp. and processed at varying pressures (300, 400 and 600 MPa) and dwelling times (1 and 3 min) at 5 degrees C. The lethality at 400 MPa for 3 min was > 6-log CFU/mL. Further inactivation was observed during storage under refrigeration in the populations of Salmonella spp. and L. monocytogenes. In order to study the influence of pH and SSC on inactivation of Salmonella spp. by HPP, pH of acai juice samples was adjusted to a range between 4.0 and 5.5 and SSC was adjusted between 2.9 and 14.9 degrees Brix. The ability of HPP to provide a 5-log reduction in the population of Salmonella spp. was reduced with increasing pH and SSC. Immediately after HPP the juices with pH 4.0 and 2.9 degrees Brix presented > 6-log reduction while the one with 8.9 degrees Brix resulted in 5-log reduction. In one week of refrigerated storage (7 degrees C), the juices (pH 4.0-14.9 degrees Brix and pH 4.5-2.9 degrees Brix) also presented > 6-log reduction in Salmonella spp. concentration. These results indicated that a less intense process (below commonly recommended commercial conditions 600 MPa/3 min) could be applied for acai juice thus assuring required safety, in addition to an extra microbial inactivation verified during refrigerated storage. Industrial relevance: Our results showed that viability of cells of pathogenic strains continues to fall after processing and that this effect can be reversed in higher pH and higher concentration of soluble solids. This observation can help the design of more efficient protocols for validation of HPP processing, leading to milder processing conditions that will allow reduction of maintenance and energy costs associated with HPP. In addition, our results help to clarify the criteria to be adopted in the microbiological validation of juices processed by high hydrostatic pressure.