Abstract
Polyphenol oxidase (PPO) causes the browning reaction in fruits and vegetables and deteriorates the quality. Thermal treatment for enzyme inactivation may result in defects as opposed to high pressure CO2 (HPCD) processing. In this study, the changes in activity, dissociation, aggregation and conformation of purified PPO from thermal and HPCD treated juice were investigated. HPCD exhibited inactivation of PPO at 55–65 °C whereas thermal processing alone at the same temperature resulted in PPO still showing activity. Under thermal treatment at 25 and 65 °C, the browning degree was higher (0.39 and 0.24) than for HPCD-treated juice (0.23 and 0.12). Fluorescence and circular dichroism spectral results indicated that HPCD induced large decreases in intensities, revealing a rearrangement of the secondary structure and destruction of the native configuration of the PPO molecule. The particle size distribution (PSD) pattern revealed structural modification leading to initial dissociation and subsequent aggregation of PPO after HPCD treatment. Polyacrylamide gel electrophoresis (PAGE) analysis exhibited that molecular size of protein was 40 kDa. In conclusion, the HPCD method was found to be more effective than thermal treatment to inactivate PPO. Structural modifications provided better insights into the phenomena of activation and inactivation of PPO.
Highlights
Quince (Cydonia oblonga Miller) is the fruit of the Maloideae, a sub-family of the Rosaceae family [1].Commercially significant fruits including apples and pears belong to this family [2]
high pressure carbon dioxide (HPCD) treatment was more effective in lowering the PPO activity and structural
HPCD treatment was more effective in lowering the PPO activity and structural modification of quince PPO in comparison to thermal treatment
Summary
Quince (Cydonia oblonga Miller) is the fruit of the Maloideae, a sub-family of the Rosaceae family [1]. The active site of the PPO enzyme contains two copper ions, which are surrounded by 6 histidine residues and a single cysteine residue It serves as structural support for enzyme molecule, thereby affecting the activity of protein. HPCD is considered an innovative and emerging non-thermal technology, which is an alternate to traditional heating or high-pressure processing of fruit juices and is deemed comparatively safe for those compounds which are heat-liable [10,11]. The enzymatic inactivation of PPO, quality parameters and structural analysis of purified PPO from quince juice treated with thermal and non-thermal technology (HPCD) are addressed in the present paper. No work has been done on fruit quality attributes, enzymatic inactivation and structural modification of PPO purified from juice after thermal and HPCD treatments. Structural analysis was conducted to explore the mechanism of thermal and HPCD technology on the aggregation and deformation of the protein after purification with the activity of the polyphenol oxidase enzyme
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