Structure and function of the seed coat of <em>Theobroma cacao</em> L. and its possible impact on flavour precursor development during fermentation

Abstract

Seed coats are known to regulate the transfer of substances between the embryo and both the mother plant and the environment. The permeability of the seed coat of the fermenting seeds of Theobroma cacao may significantly affect its final flavour quality. Many studies suggest that the flavour quality is genetically determined and uniform within one clone. This implies that the flavour quality can only be inherited exclusively from the mother plant. Consequently, factors affecting flavour quality should be located in the maternal tissues surrounding the seed. As only the fruit pulp and the seed coat are exclusively maternal in origin, whereas the embryo and the endosperm are the progeny of both parents, the flavour quality may be associated with transport characteristics of the seed coat. In this context the influence of the seed coat on the transport of acetic acid, the main product of fermentation is of high relevance. We investigated the transport characteristics of the seed coat of T. cacao with modern light- and fluoroscence microscopic methods. Tracers were used in order to evaluate the potential impact of structures such as barriers and entry sites for the transport of water and solutes. Our morphological, histochemical and microspectrophotometrical data and the interpretation of the tracer distributions demonstrate that certain structures of the seed coat strongly influence the course of transport processes in the mature seed coat of T. cacao. These include the inner contact zone of fruit pulp and seed coat, hilum, sclereid layer, hypostase, micropyle and endosperm cuticle. Under natural conditions these structures may prevent desiccation and loss of nutrients of the recalcitrant seed. During fermentation the mentioned structures appear to affect the influx of acetic acid, in analogy to water and solutes. Insufficient or excessive acidification impedes sufficient formation of flavour precursors resulting in a flat or acid taste. Asynchronous and uneven infiltration of acetic acid into the cotyledon cells will lead to inconsistent and suboptimal flavour quality. In conclusion we suggest that flavour quality of the fermented seeds is predominantly due to transport kinetics of water and solutes during the fermentation process rather than a reflection of genetically coded differences in storage proteins.