High pressure inactivation kinetics of amylase in apple juice

High pressure (HP) inactivation kinetics of commercial amylase in apple juice was evaluated under various test conditions (100400 MPa, 0-60 min and 6-40 degreesC) using a central composite design of experiments. The enzyme sample was dissolved in clarified apple juice, sealed in dual peel sterilizable plastic bags and subjected to different HP treatments. Residual enzyme activity was evaluated (spectro-photometrically at 513 nm and 25 degreesC) by adding a starch substrate to the test sample and monitoring the kinetics of color formed from the reducing action of hemiacetal groups (librated from starch hydrolysis) on 3,5-dinitrososalicylic acid. Inactivation kinetics at three pH conditions (3.0, 3.5 and 4.0) was modeled using a response surface methodology. Inactivation of amylase was more pronounced at higher pressure levels, higher temperature, longer treatment times and lower pH of juice. The sudden initial drop in the enzyme activity with the application of pressure pulse (with no-hold time) was designated as the pressure pulse effect (PE). The magnitude of PE was dependent on pressure level, with higher PE achieved at higher pressure levels. During the pressure hold, as expected, the associated decimal reduction times (D values) decreased with an increase in pressure. Pressure dependency of D values was well described by the conventional death time model and the associated z(p) values (pressure range to result in a decimal change in D values) at pH 3.0, 3.5 and 4.0 were 518, 646 and 705 MPa, respectively (at 20 degreesC). Almost complete inactivation of the enzyme was possible with the combination of lower pH, higher pressure and higher temperatures. (C) 2003 Elsevier Ltd. All rights reserved.