Differences in chemical constituents between vascular bundles and nonvascular bundles of cacao (Theobroma cacao L.) hull

Abstract

Cacao (Theobroma cacao L.) hulls were physically separated into vascular bundles (VBs) and nonvascular bundles (NVBs) to investigate their chemical compositions and the structural features of abundant polyphenolic compounds. Glucose content was determined to be 21.4% for VBs and 17.5% for NVBs, together with xylose content as 13.1% for VBs and 2.8% for NVBs. In addition, uronic acid in NVBs (12.9%) was much higher than that in VBs (5.2%). The content of total (poly)phenolic compounds (35.9%-39.1%) quantified as Klason residues (KRs) and acid-soluble phenolic compounds (ASPs) were similar in both cell types, although there were great differences in the structural characteristics of polyphenolic compounds. The pyrogram of VBs clearly showed high intensities of guaiacol and 4-vinylguaiacol together with low intensities of catechol and 4-methylcatechol. On the other hand, that of the NVBs showed opposite trends. These results were confirmed by alkaline nitrobenzene oxidation based on total yields of vanillin and syringaldehyde. Therefore, the accumulation of various polyphenolic compounds in cacao hulls relies strongly on the cell type and is correlated with the development of a secondary wall.