Hydroxycinnamoyl conjugates in potato tubers : diversity and reactivity upon processing

Despite the large amount of reports on the content of the hydroxycinnamic acid (HCA) caffeic acid and its conjugates (HCAcs; chlorogenic acid (ChA, 5-O-caffeoyl quinic acid), crypto-ChA (4-O-caffeoyl quinic acid) and neo-ChA (3-O-caffeoyl quinic acid)), other HCAcs/dihydroxicinnamic acid conjugates (DHCAcs) are rarely described in potato tubers. Furthermore, scarce information has been provided on the fate of phenolic acids when no anti-browning agents, or when the anti-browning agents ascorbic acid and NaHSO₃, are used during the preparation of potato juice. This thesis presents a method for the analysis of HCAs/HCAcs/DHCAcs based on RP-UHPLC-DAD-(H)ESI-MSⁿ. This analytical method revealed a total of 78 compounds tentatively identified as HCAs/HCAcs/DHCAcs when several Colombian potato cultivars and a single Dutch cultivar were analysed. The contribution of the less documented HCAcs/DHCAcs ranged from 7.1 to 20.1 % (w/w) of the total HCAs/HCAcs/DHCAcs in whole tuber. We provide for the first time molecular evidence on the formation of sulfonic acid derivatives when potato extracts are prepared in the presence of NaHSO₃. In that scenario, no ChA isomers were found but their sulfonic acid conjugates, with 2’-sulfo-ChA being found as the most abundant, followed by 2’-sulfo-crypto-ChA. Based on model experiments it was found that NaHSO₃ has a dual anti-browning effect: it reacts with ChA quinone, generated upon oxidation catalysed by polyphenol oxidase (PPO), to produce sulfo-ChA, and it inactivates PPO in a time-dependant way. When no anti-browning agent was used during the preparation of potato juice, the majority of the free ChA present in potato tubers was found to be associated with proteins. These results might explain why potato proteins, used as food ingredient, can turn brown in applications, when the protein is not isolated from potato tubers employing appropriate processing methodology.