EGCG combined with pulsed ultrasound inhibited browning of frozen-thawed apples by synergistically stabilizing cell structures, inducing flavonoid accumulation, and inhibiting PPO expression.

Abstract

Fresh apples are prone to browning, and freeze-thawing exacerbates this process, reducing commercial value, limiting processing applications, and causing industry losses. This study showed that EGCG combined with pulsed ultrasound (EU) under slow freezing (SF) effectively inhibited browning of frozen-thawed apples and outperformed quick freezing (QF). EU treatment significantly inhibited browning (up to 61.92%) and PPO activity (up to 61.08%), which is attributed to EGCG-pectin stabilizing the cell structures, thereby limiting interactions between phenolics and browning-enzymes. Furthermore, integrated analysis of proteomics and metabolism (HPLC-MS/MS) revealed that EU induced flavonoid accumulation (e.g., EGCG and epicatechin gallate, increased by over 135-fold) and downregulated PPOs protein expression (up to 95.67%), delaying the browning process. Molecular docking/dynamics simulations also revealed that EU could enhance the binding of EGCG to PPO, thereby changing the PPO conformation and activity. These synergies highlight that the EU is an effective strategy for inhibiting browning in frozen fruit processing.