Acid-Adapted Polyphenol Oxidases from Agricultural Wastes: Extraction, Characterization, and Application in Plant Protein Crosslinking

Polyphenol oxidases (PPOs) are enzymes that oxidize mono- and diphenolic compounds to o-quinones, facilitating pigment formation and protein crosslinking in food systems, thereby improving their techno-functional properties. However, most PPOs function optimally near neutral pH, limiting their application in acidic food products. This study aimed to extract acid-adapted PPOs from various fruit by-products, including Hass avocado seeds (pH 5.9), Anjou pears (pH 4.0), Bartlett pears (pH 4.0), Red Delicious apples (pH 4.0), and McIntosh apples (pH 3.3), and characterize PPO properties and its substrate specificity using colorimetric assay. SDS-PAGE was used to assess PPOs’ molecular weight and PPOs’ capacity for plant protein crosslinking. The results showed that PPOs from Anjou and Bartlett pear pomace exhibited the most robust acid-adapted activity, with effective catalytic performance in the pH ranges of 4.0–5.0 and 5.0–8.0, respectively, and an optimal temperature of 20 °C. SDS-PAGE analysis revealed bands at ~44 kDa and ~25.6 kDa, consistent with previously found pear PPO isoforms. Both pear pomace PPO oxidized L-DOPA and EGCG efficiently, but showed minimal activity toward L-tyrosine, gallic acid, caffeic acid, tannic acid, and ferulic acid. In the presence of EGCG, both pear pomace PPOs are capable of crosslinking plant proteins at pH 4.0. These findings provide the first evidence that agricultural by-products are a promising but underutilized source of acid-adapted PPO for modifying soy protein hydrolysates.