The purpose of this study was to investigate the effect of high hydrostatic pressure with a mild heat treatment on Staphylococcus aureus 485, Escherichia coli O157:H7 933 and Salmonella Enteritidis FDA in apple, orange, apricot and sour cherry juices. The effectiveness of the treatment on polyphenol oxidase activity in apple juice and pectinesterase activity in orange juice were also determined. An inoculum of microorganisms was completely inactivated at 350 MPa and 40 degrees C in 5min. The residual polyphenol oxidase activity in apple juice after treatment at 450 MPa and 50 degrees C for 60 min was obtained as 9 +/- 2.2%. The residual pectinesterase activity in orange juice after treatment at 450 MPa and 50 degrees C for 30min was determined as approximately 7 +/- 1.6%. It compares with 12 +/- 0.2% at a treatment of 40 degrees C and 450 MPa for 60 min. Pressure resistant isoenzymes were thought to be responsible for the final residual activity. The inactivation is irreversible and the enzyme is not reactivated upon storage. High pressure processing constitutes an effective technology to inactivate the enzymes in fruit juices. Pressures higher than 400 MPa can be combined with mild heat (<50 degrees C) to accelerate enzyme inactivation. (c) 2004 Elsevier Ltd. All rights reserved.