Identification and stability evaluation of polyphenol oxidase substrates of pineapple fruit

Pineapple as an important tropical fruit in the world is prone to internal browning (IB) caused by the oxidation of phenolic substances. To address the occurrence of IB, endogenous polyphenol oxidase (PPO) substrates were extracted, separated, and purified from pineapple fruit, and then four compounds were obtained and identified as catechin, epicatechin, chlorogenic acid and pyrocatechol. The contents of these substrates increased, reached the maximum value in the middle storage period, decreased gradually, and then maintained a relatively stable level by the end of storage. Meanwhile, pineapple PPO substrates were unstable in structure and susceptible to be degraded under the condition of light, high temperature (>40 °C), alkalinity (pH > 9) or metal ion (Ca²⁺, Cu²⁺, Fe²⁺, Pb²⁺ and Zn²⁺). Additionally, the presences of O₂ and H₂O₂ increased the enzymatic oxidation of the pineapple PPO substrates, whereas CO₂, N₂, Vc (Vitamin C), l-cysteine, Na₂SO₃ and Na₂S₂O₅ inhibited this oxidation.