Acid-free microwave-assisted hydrothermal extraction of pectin and porous cellulose from mango peel waste - towards a zero waste mango biorefinery

Mango is the second most consumed tropical fruit after banana and the by-products of mango processing (peel, kernel and seed) roughly comprise 35-60% of the total fruit weight, thus representing a potentially high volume resource of exploitable biobased chemicals and materials. Herein, conversion and characterisation of waste mango peels from three different cultivars (Alphonso, Honey and Tommy Atkins) into pectin and porous cellulose using low temperature microwave-assisted acid-free hydrolytic conditions is reported. Microwave-assisted acid-free extraction yielded up to 11.63% (dry weight basis) of pectin which was characterised by IR, NMR (both solution and solid phase) and TGA which showed close similarity to commercial (acid extracted) pectin. The degree of esterification of pectin was determined by C-13 NMR (75.6-86.2%) and titrimetry (79.3-87.7%) and the pectin showed excellent gelling ability. The molecular weight as determined by GPC was in the range 14 130 (Honey)-25 540 (Tommy Atkins). Porosity measurements on the depectinated residue, i.e., residual cellulosic matter showed mesoporous characteristics: average pore diameter, 9.3 nm (Alphonso)-10.5 nm (Honey), however with poor surface area 16.3 m(2) g(-1) (Honey)-26.0 m(2) g(-1). Interestingly, a second microwave hydrothermal treatment on these residues retained mesoporosity whilst significantly increasing surface area (88.8 m(2) g(-1) (Honey)-124.0 m(2) g(-1) (Alphonso)) and pore volume by approximately six-fold. This is the first detailed combined study of microwave-assisted extraction to yield pectin and mesoporous cellulose towards a potential zero waste mango biorefinery.