Histological and cytological comparison of seed structures of Theobroma cacao, T. grandiflorum and T. bicolor relevant to post-harvest processing and product quality

Abstract

For the present study, anatomical details of seeds of Theobroma cacao L., Theobroma grandiflorum (Willd. ex Spreng.) Schum. and Theobroma bicolor Humb. & Bonpl. were investigated using light and transmission electron microscopy (LM and TEM). The focus was on features with possible significance for the course of fermentation and drying to generate cocoa-like aroma. It was found that the seed coat of T. cacao has the smallest diameter and contains only a thin sclereid layer, whereas T. grandiflorum and especially T. bicolor exhibit much more massive palisade layers. The thicker seed coats of T. grandiflorum and T. bicolor could delay the passage of liquids during fermentation and seed deshelling may require more effort than with T. cacao. In addition, existing differences in the amount of plant mucilages and phenolic substances in the testa may affect the composition and activity of the microflora during fermentation. Differences were also evident in the cotyledon tissue, which is processed to obtain cocoa-like products. Besides a higher degree of maceration of the parenchyma of T. grandiflorum and T.bicolor, the main distinction was the different number and distribution of polyphenol cells. There were also differences in the species-specific fat and protein content.