High pressure processing of heat and pressure resistant fungi as affected by pH, water activity, sulfites, and dimethyl dicarbonate in a diluted apple juice concentrate

Abstract

High pressure processing (HPP) is a popular method of processing juices to retain “fresh” properties. However, heat and pressure resistant fungi, Byssochlamys spp. and Aspergillus spp., can survive processing and cause spoilage. The goal of this study was to evaluate the pressure and time necessary to cause a reduction of heat and pressure resistant spoilage fungi common in juice and determine the effect of water activity (0.94–1.0 aw) and pH (3.5–7.0) on HPP inactivation kinetics. The effect of sulfites and dimethyl dicarbonate (DMDC) was also assessed as a proof of concept to determine if either aided in fungal inactivation. Refrigerated storage up to 26 weeks after treatment was performed to assess fungal growth. Processing A. pseudoglaucus in pH 3.5 diluted apple juice concentrate (DAJC) at 600 MPa for 15 min at 5 °C was more effective than in pH 4.6, 7.0, or 4.6 with sulfites. Processing A. fischeri at pH 3.5 or 7.0 for 15 min at 600 MPa, 5 °C or at pH 4.6 for 1.5 min at 450 MPa resulted in a 1-log reduction. The addition of DMDC to pH 7.0 DAJC inoculated with A. fischeri and processed at 600 MPa for 15 min resulted in 1-log activation rather than a reduction. The addition of 8 ppm potassium metabisulfite at pH 4.6 had varying effects between species and did not significantly increase the shelf life. Sulfites aided in reducing A. pseudoglaucus but increased the activation of A. fischeri and P. niveus (B. nivea Westling). Processing at 600 MPa for 15 min at 5 °C resulted in the activation of P. niveus at pH 3.5, 4.6 and 7.0 DAJC at 0.94–1.0 aw. Generally, a greater reduction of all fungi tested was observed at 1.0 aw than at 0.94 aw. A 17–26 week storage period could be achieved, but only at refrigeration.