Towards a “dry” bio-refinery without solvents or added water using microwaves and ultrasound for total valorization of fruit and vegetable by-products

This study aims at total valorization of fruit and vegetable by-products, moving towards developing an original concept of a “dry” bio-refinery (DBR). Indeed, all valuable products were recovered from food by-products without addition of solvents or water and using green processes. Ginger was chosen as a reference matrix since its juice processing generates a large amount of press cake currently considered as waste. Therefore, in this study, after juice processing, ginger press cake (GP) was firstly treated by microwave hydrodiffusion and gravity processing (MHG) to recover essential oil (EO) and constituent water present in ginger by-products. Gingerols and 6-shogaol remaining in the ginger presscake residue after MHG (GPMHG) were then extracted by ultrasound assisted extraction (UAE) at different ultrasonic intensities (UI) using constituent water as solvent. The assessment of microwave (MW) power enabled us to determine that a power of 1.6 W g−1 was optimal to recover constituent water and EO, preserving extract quality in a reduced time. The mass extraction yield was enhanced by UAE (16.7 W cm−2; 0.303 W cm−3) with an increase of 126% compared to conventional maceration (CM). Total valorization of ginger by-products was achieved since juice, essential oil, extract rich in phenolics, and solid residue rich in fibers and phenolic acids were obtained from ginger rhizomes (GR) using “dry” bio-refinery without solvent and added water. Finally, the performances of “dry” bio-refinery and conventional bio-refinery (CBR) were compared in terms of processing time, energy consumption, quantity of waste and quantity of solvent.