Insight into the impact of pre-cooking and freezing on the colour stability of pakchoi during thermal processing: A study on chlorophyll and organic acid changes

Abstract

This study investigated the effects of pre-cooking and freezing on the colour stability, changes of chlorophylls and organic acids during re-cooking and 4-h heating of pakchoi. Pre-cooked and stored pakchoi exhibited yellowish-brown discolouration upon cooking, especially in the stalks, with green values (−a) 42.53 %, 34.50 %, and 49.81 % lower than the unpre-cooked, unstored, or neither counterparts, respectively. Pre-cooking expedited the production of pheophytins and pyropheophytin a, which contribute to poor appearance, during brief cooking. Throughout processing, pre-cooking potentially facilitated leaf chlorophyll a demagnesiation by promoting oxalic, citric, and malic acid dissociation, while enhancing the correlations of stalk fumaric and oxalic acids with pheophytins. Additionally, pre-cooking accelerated stalk pyropheophytin a accumulation, leading to faster discolouration. Storage primarily affected stalk colour during brief cooking by accelerating pheophytin a formation. The findings offer the possibility of improving the colour quality of frozen leafy green vegetable products by focusing on key chlorophyll derivatives.