Compositional changes in the “red heart” of Chinese water chestnuts during thermal processing and sterilization

• Residual bud and basal plate of CWC show red heart after thermal processing. • The red heart degree correlated with heating time, heating temperature and oxygen. • After vacuum packaging and heating at 70 °C, a "blue heart" appeared in the CWC. • Some compounds, including monoterpenes, likely contributed to red heart. "Red heart" often occur in canned Chinese water chestnuts (CWC, Eleocharis tuberosa ) after thermal processing and sterilization, severely affecting their commercial value. This study revealed that CWC develop a "red heart" not only under autoclaving conditions (121 °C) but also at temperatures above 60 °C, both the temperature and time of thermal processing positively correlated with the degree of reddening which was found only in the residual tissue of the sprout and base after thermal processing and sterilization. The present study analyzed the differentially metabolites in five different parts of CWC (sprout tissue, central tissue, edge tissue, central tissue after autoclaving and edge tissue after autoclaving), using ultra-high performance liquid chromatography coupled with triple quadrupole mass spectrometry (UPLC-QQQ-MS/MS). A total of 12 categories and 1193 compounds were detected from all the samples, including 293 flavonoids (24 %), 182 amino acids and their derivatives (15 %), 57 terpenoids (5 %), and 12 tannins (1 %). Among them, the levels of 70 amino acids (Ser-Phe-His, Tyr-Glu-Cys, etc.), 19 monoterpenoids, especially geniposide, hastatoside, 8 proanthocyanidins, especially procyanidin B3, procyanidin B1, and 9 anthocyanins, especially pelargonidin, delphinidin, petunidin significantly decreased after autoclaving. It is hypothesized that the differential distribution of these substances between the central and edge tissues of the CWC tuber, as well as their transformation under autoclaving, may be related to the "red heart" phenomenon. This study was the first comprehensive analysis of compound changes during the process of "red heart" development in CWC during thermal processing.