Non-Thermal and Thermal Processing of Fruit Products to Control Enzymatic Browning

Abstract

Browning, an on-going problem of high economy impact to the fruit industry, is caused by polyphenoloxidase (PPO), fruits’ endogenous enzyme. With the increasing demand for high nutrients and ‘fresh-like’ fruit products, there is an interest to study non-thermal preservation technologies such as High Pressure Processing (HPP), ultrasound and pulsed electric fields (PEF). This research investigated the PPO inactivation in fruits by HPP, ultrasound, PEF and thermal processing. PPO inactivation kinetics was modeled for HPP, ultrasound and thermally treated pear, apple and strawberry purees. Additionally, quality assessments after processing and during storage were carried out for strawberry puree and apple juice. Room temperature HPP strawberry puree at 600 MPa for 5 min resulted in 35% PPO inactivation. When combining the same process with mild heat (40-60ºC), a residual activity (RA) of 9-65% as opposed to 44-100% with exclusively thermal processing was obtained. Pear, apple and strawberry purees were HPP combined with mild heat up to 60 min. The pear PPO was found to be resistant even after 60 min at 600 MPa-71°C. HPP-thermal inactivation of apple and strawberry PPOs followed a biphasic first order kinetics exhibiting stable and labile fractions. Ultrasound processing (1.3 W/g, 32°C) for 10 min of pear, apple and strawberry purees significantly reduced PPO activity (25-58% RA). The PPO thermosonication (ultrasound combined with thermal) and thermal inactivation were successfully modeled with a simple first order kinetics. For processed strawberry puree and apple juice yielding ≤18% PPO residual activity, lower processing energy was observed for thermal treatment followed by HPP, ultrasound and PEF. Ultrasound and HPP resulted in better antioxidant activity (70-74%) than thermally treated (60%) strawberry purees. Regarding apple juice, antioxidant activity was improved from 86 to 103% with thermosonication. Ultrasound, HPP and thermal processing produced a 30 days’ shelf-stable strawberry puree and apple juice. However, apple juice processed by PEF required refrigerated storage.