TCA–NSI
This was conducted according to the method by Chen et al. (2021). Briefly, TCA was added into samples to a final concentration of 15%. The mixtures were mixed well, then stood for 1 h at room temperature. Subsequently, the samples were centrifuged (7000g , 15 min) to obtain transparent supernatants, whose nitrogen contents were determined by the micro-Kjeldahl method. TCA–NSI was calculated as nitrogen content in supernatant/nitrogen content in original mixture *100%, which could be used to quantitatively explain the combined activity of endopeptidases and exopeptidases.