Microwave-Assisted Production of Defibrillated Lignocelluloses from Blackcurrant Pomace via Citric Acid and Acid-Free Conditions

Blackcurrant pomace (BCP) is an example of an annual, high-volume, under-utilized renewable resource with potential to generate chemicals, materials and bioenergy within the context of a zero-waste biorefinery. Herein, the microwave-assisted isolation, characterization and potential application of defibrillated lignocelluloses from depectinated blackcurrant pomace are reported. Depectination was achieved using citric acid (0.2–0.8 M, 80 °C, 2 h, conventional heating) and compared with acid-free hydrothermal microwave-assisted processing (1500 W, 100–160 °C, 30 min). The resultant depectinated residues were subjected to microwave-assisted hydrothermal defibrillation to afford two classes of materials: namely, (i) hydrothermal acid-free microwave-assisted (1500 W, 160 °C, 30 min; DFC-M1-M4), and (ii) hydrothermal citric acid microwave-assisted (1500 W, 160 °C, 30 min; DFC-C1–C4). Thermogravimetric analysis (TGA) revealed that the thermal stability with respect to native BCP (Td = 330 °C) was higher for DFC-M1-M4 (Td = 345–348 °C) and lower for DFC-C1–C4 (322–325 °C). Both classes of material showed good propensity to hold water but failed to form stable hydrogels (5–7.5 wt% in water) unless they underwent bleaching which removed residual lignin and hemicellulosic matter, as evidenced by 13C solid-state NMR spectroscopy. The hydrogels made from bleached DFC-C1–C4 (7.5 wt%) and bleached DFC-M1-M4 (5 wt%) exhibited rheological viscoelastic, shear thinning, and time-dependent behaviour, which highlights the potential opportunity afforded by microwave-assisted defibrillation of BCP for food applications.