Polyphenol oxidase inactivation from apple juice by Al-based metal-organic frameworks: New anti-browning strategy in fruits and vegetables.

Polyphenol oxidase (PPO) is critical due to enzymatic browning in fruits and vegetables, developing economic impact in fruits industry. Metal-Organic Frameworks (MOF) have shown interesting characteristics such as water stability, low toxicity, and good adsorption yield, making them good candidates for PPO inactivation. Al-based-MOFs, MIL-53(Al), DUT-5, and MIL-110 were tested as PPO inactivators in apple juice by enzyme-MOF interactions at r.t. through two possible mechanisms, i) substrate scavengers (substrates:catechol and 4-methylcatechol) or ii) enzyme activity modifiers. The scavenging behavior of Al-based-MOFs was moderate, in the same magnitude, being catechol adsorption better than 4-methylcatechol. PPO activity was reduced by at least 70% by MIL-53(Al)/DUT-5 in 10/30 min respectively, and MIL-110 inactivated PPO in 50 min with some structural modifications. Enzyme-MOF interactions are major responsible for PPO inactivation. This could be a new applicability of MOFs, as an alternate PPO inactivation process, easily included in juice processing, retaining sensorial/nutritional properties, developed at r.t thus energy-cost-effective.

Purification and biochemical characterization of polyphenol oxidase from soursop (Annona muricata L.) and its inactivation by microwave and ultrasound treatments.

The soursop (Annona muricata L.) is a climacteric fruit that may undergo enzymatic browning during ripening, mainly by the activity of polyphenol oxidase (PPO). Soursop PPO was purified 160-fold by hydrophobic interaction and ion-exchange chromatography. The native structure has a molecular weight of 112 kDa corresponding to a dimeric structure. The protein has an optimum pH and temperature of 6.5 and 25°C, respectively; and activation energy of 40.97 kJ·mol-1 . The lowest Km value was observed for caffeic acid (0.47 mM); the best substrate was 4-methyl-catechol (1,067 U·mM-1  min-1 ). Inactivation assays showed that PPO was completely inactivated by tropolone, Na2 S2 O5 and ascorbic acid, and thermally at 55°C for <5 min, microwave exposure reduced activity to 57% at 70 W in 30 s and ultrasound treatment diminished activity to 43% at 120 W in 220 s. This study allows a better understanding of soursop PPO behavior and provides inactivation information. PRACTICAL APPLICATIONS: The conservation of fresh fruits is complicated due to the enzymatic reactions that are present in fruits, such as enzymatic browning. The enzymes responsible for these reactions can be inactivated by, different chemical compounds as well as by the use of emerging technologies, such as microwaves and sonication, which seek to satisfy the consumer needs to obtain fresh products with good nutritional characteristics and adequate safety.

Purification and partial biochemical characterization of polyphenol oxidase from mango (Mangifera indica cv. Manila).

Polyphenol oxidase (PPO) is an enzyme widely distributed in the plant kingdom that has been detected in most fruits and vegetables. PPO was extracted and purified from Manila mango (Mangifera indica), and its biochemical properties were studied. PPO was purified 216-fold by hydrophobic interaction and ion exchange chromatography. PPO was purified to homogeneity, and the estimated PPO molecular weight (MW) by SDS-PAGE was ≈31.5 kDa. However, a MW of 65 kDa was determined by gel filtration, indicating a dimeric structure for the native PPO. The isolated PPO showed the highest affinity to pyrogallol (Km = 2.77 mM) followed by 4-methylcatechol (Km = 3.14 mM) and catechol (Km = 15.14 mM). The optimum pH for activity was 6.0. PPO was stable in the temperature range of 20-70 °C. PPO activity was completely inhibited by tropolone, ascorbic acid, sodium metabisulfite, and kojic acid at 0.1 mM.