Isolation of cellulose from soybean hulls: efficiency and sustainability of different strategies

Recently, biopolymer extraction from (waste) biomasses has been considered a valuable approach to obtain sustainable compounds usable for different purposes, avoiding the indiscriminate dispersion of putrescible matter in the environment or the costs of waste disposal. Indeed, agro-industrial residues are rich in components exploitable as new raw materials in several application fields, such as food packaging, bioplastics, and in the biomedical and water treatment sectors [1,2]. Such wastes are mainly composed of cellulose, hemicellulose, lignin, polyphenols, starch, etc., depending on the starting crop [2]. Traditional extraction routes of these components often employ multistep processes in acidic/alkaline conditions to promote hydrolysis and their consequent isolation from the matrixes [3], which are pretty far from the green chemistry fundamentals, which suggest avoiding polluting, corrosive and hazardous reactants. Cellulose, in particular, is one of the most abundant carbohydrates in nature and a versatile compound, easily modifiable to create a plethora of innovative materials. In this research work, we focused our attention on the optimization of its green isolation from soybean hulls already deprived of peroxidase enzyme. Soybean hulls are one of the by-products of soybean (Glycine max) crushing and, to give an idea of the quantity involved, soybean global production is around hundreds million tons [4]. The classical cellulose isolation through acid-basic hydrolysis was compared with that performed by Microwave-assisted Subcritical Water Extraction (MASWE). MASWE technique has (i) fast heating, (ii) increased selectivity and yields and (iii) high thermal efficiency as the main advantages [5]. Both the obtained cellulosic samples and the liquid extracts have been characterized (SEM, TGA, DSC, FT-IR, NREL, phenolic content) to select the best extraction conditions. Overall, the outcomes from cellulose isolation ability and final material characterization revealed that the coupling of MASWE operated at 180 °C with an alkaline treatment was the most efficient procedure.