Synergistic action of ⁶⁰Co- γ irradiation on quality preservation of fresh-cut lotus root: microbial control and targeted polyphenol oxidase suppression

Abstract

Fresh-cut lotus root is highly susceptible to quality deterioration, primarily due to enzymatic browning and microbial growth. This study investigated the efficacy and mechanism of⁶⁰Co-γ irradiation in preserving the quality of fresh-cut lotus root. The results demonstrated that irradiation effectively controlled total bacterial numbers within 5 log₁₀ CFU/g throughout the storage period. Irradiation significantly inhibited browning, reducing the browning index by 19.6%, 25.3%, and 28.6% at 1, 2, and 3 kGy, respectively, compared to the control. This inhibitory effect was attributed to the delayed degradation of total phenolics and the suppressed accumulation of soluble quinones, the direct browning precursors. Irradiation induced a dose-dependent inhibition of polyphenol oxidase (PPO) activity, with the 3 kGy treatment exerting the strongest suppressive effect; however, the activities of phenylalanine ammonia-lyase (PAL) and peroxidase (POD) remained unaffected. Circular dichroism (CD) and fluorescence spectroscopy revealed three kGy irradiation induced rearrangements in the secondary structure, and disruption of the tertiary structure of PPO. Specifically, CD analysis definitively characterized conformational changes in PPO, showing a dramatic reduction in the α-helix content from 35.4% to 18.4%. Meanwhile, fluorescence spectra of PPO exhibited a 12 nm redshift accompanied by significant fluorescence quenching. This phenomenon provides direct evidence that the enzyme's tertiary structure was disrupted, which in turn lead to the exposure of its hydrophobic core and ultimately resulted in functional inactivation.⁶⁰Co-γ irradiation is a highly effective technology for maintaining the quality of fresh-cut lotus root, primarily through direct microbial inactivation and the suppression of the PPO-mediated browning pathway.

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