Valorisation of Citrus Fruit Peel Wastes and Blackcurrant Pomace via Acid-free Microwave Hydrothermal Processes

The chemical and allied-industries have been for too long reliant on crude oil and fossil fuels for their chemicals, materials and energy needs. These industries are significant contributors (approximately 4% of global CO2 emissions) to anthropogenic-induced global warming. Future thinking requires consideration of alternative, carbon-neutral renewable feedstocks, such as biomass, that develop biorefineries in place of traditional petroleum refineries. Unavoidable food supply chain wastes, such as citrus peels and blackcurrant pomace, are exemplars of large volume, renewable feedstock, which can be exploited (valorised) for the production of biobased chemicals, materials and bioenergy.
Herein, the valorisation of citrus peels (orange and lemon) and blackcurrant pomace (BCP) via acid-free hydrothermal microwave processing, as opposed to conventional heating in acidic media, is reported. This valorisation approach formed two fractions: a hydrolysate which was rich in pectin (citrus) and antioxidants (BCP), and a solid fraction, giving microfibrillated cellulose (MFC) (citrus) and residues (BCP).
Citrus pectin is linear polysaccharide with smooth region (homogalacturonan, HG) and hairy region (rhamno-galacturonan, RG). Orange pectin processed at 160 oC resulting in an RG-I pectin rich in galactan, which proved that hydrothermal microwave processing of orange pectin at 160 oC is a selective degradation. MFC was successfully characterised as a nanostructured material with properties highly dependent on the treatment temperature.
BCP microwave hydrolysates (MHs) produced at low temperatures (<120 oC) were characterised as a complex mixture of variety compounds presenting antioxidant activity.
In conclusion, the presented valorisation of citrus peels and BCP confirmed its potential as a valuable bioresource for the production of pectin, MFC and antioxidants with numerous potential applications.