Structural elucidation of a crystalline mannan from açaí (Euterpe oleracea Mart.) seeds

Açaí is an Amazonian "superfruit" whose pulp production generates large amounts of seed waste. As nearly half of the dry mass of this residue is composed of mannan, this study investigated the structure and distribution of mannan in açaí seeds to support its bioconversion into value-added products such as mannose and mannan-oligosaccharides. To this end, the seeds were fractionated to obtain soluble and water-insoluble fractions, which were analyzed to assess their mannan content. In soluble fractions, glycomic profiling revealed the presence of an alkali-soluble mannan together with low amounts of pectin, (hetero)xylan, xyloglucan, and glucan. The water-insoluble fraction was further analyzed by solution-state NMR, where ¹H/¹³C HSQC and HMBC spectra showed signals consistent with a backbone of 1,4-linked β-D-Manp, with chemical shifts similar to those reported for mannan from ivory nut, a material known to contain linear mannan. Gas chromatography analysis indicated a predominantly linear mannan with mannose-to-galactose ratio of 50:1, suggesting only minor galactose substitutions. Furthermore, due to the limited solubility of this fraction, ¹³C CP/MAS solid-state NMR was employed to enable a comprehensive structural assessment. Solid-state NMR revealed the presence of a mannan I polymorph, with its crystalline nature further confirmed by X-ray diffraction. The identification of a highly crystalline linear mannan in açaí seeds provides structural insight relevant to the development of strategies aimed at reducing mannan crystallinity and designing specialized enzymatic cocktails for its efficient hydrolysis, thereby supporting the sustainable valorization of this abundant agro-industrial byproduct.