Identification and quantification of free and bound phenolic compounds contained in the high-molecular weight melanoidin fractions derived from two different types of cocoa beans by UHPLC-DAD-ESI-HR-MSn

Abstract

The aim of the present study was to establish the profiles of soluble free phenolics (SFPs) and bound phenolics (BPs) in high molecular weight (HMW) melanoidin fractions isolated from raw and roasted beans of two Theobroma cacao L. varieties. Samples were prepared using three methods (saline treatment and acidic and alkaline hydrolysis) to obtain different forms of phenolic compounds. A total of fifteen phenolics, including three flavan-3-ols, seven phenolic acids, one phenolic aldehyde, and four N-phenylpropenoyl-L-amino acids (NPAs), were identified using ultra-high-performance liquid chromatography coupled to diode array detection and electrospray ionization high-resolution mass spectrometry (UHPLC-DAD-ESI-HR-MSn). In HMW fractions from both studied cocoa types, the main SFPs were N-caffeoyl-L-Asp and procyanidin B2, whereas the main BPs were catechin, epicatechin, ellagic acid, protocatechualdehyde, and N-caffeoyl-L-Asp. The concentrations of individual BPs were much higher than the content of total SFPs. It was also found that, as compared to alkaline hydrolysis, acid hydrolysis released a significantly higher amount of BPs from HMW melanoidin fractions. A comprehensive quantitative analysis indicated significant variation in the investigated phenolic compounds depending on the cocoa type and roasting conditions. An increase in treatment temperature from 110 to 150 °C led to a decline in SFPs and an increment in BPs. The HMW fractions of unroasted Criollo beans exhibited the highest content of SFPs and the lowest content of BPs. The highest BP concentrations were obtained for both cocoa bean varieties roasted at 150 °C. The present study revealed that HMW melanoidin fractions from cocoa beans of different varieties roasted at higher temperatures are a good source of phenolic compounds that can be released under both acidic and alkaline conditions.