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Extraction and Spectroscopic Characterisation of Biopolymer Cutin from Tomato Peels

Biodegradable packaging films are important food and pharmaceutical research targets, owning to very significant commercial potential. Edible biodegradable films have several advantages: good physical properties, non-toxicity, biodegradability and availability of raw material. In addition, the films can be consumed along with food, may enhance sensory characteristics, can provide nutrients and may include antimicrobials. Polysaccharides, proteins and lipids constitute a group of biomaterials that readily form biofilms. These natural polymers are incorporated in the cuticular membrane of various plants, together with waxes and an insoluble biopolyester cutin. Cutin is a polymeric network of polyhydroxylated C16 and C18 fatty acids cross- linked with ester bonds. This viscoelastic biopolymer present in different fruit and vegetable processing residues (up to 40 % of the raw tomato is typically removed during industrial peeling) has a potential for biofilm production and thus has economic implications for agriculture waste industry. This work reports methodology of cutin polymer and monomer isolation, i.e. extraction and purification from tomato industrial waste (skins and seeds). For that purpose two extraction procedures were used: the exhaustive treatment of dried tomato peels with organic solvents (methanol, chloroform and chloroform/methanol mixture) and treatment of tomato peels with oxalic acid/ammonium oxalate solution at 90 °C for 24 h to remove the pectinaceous glue that attaches the cuticle to the epidermal layer. After that the waxes were removed by Soxlet extraction with chloroform/methanol (1:1, V/V) mixture. To obtain the pure cutin biopolymer, the dewaxed tomato peels were hydrolyzed with 6 M HCl, under reflux. The cutin monomers were isolated from tomato peels after treatments with methanol and potassium hydroxide/methanol mixture. Subsequently, the extraction was performed by chloroform. The extractive-free cuticular polymer fraction and its depolymerised monomers were characterized using FTIR spectroscopy. The obtained spectra indicate two main infrared features that can be found in cutin material: two strong bands located at 2916 and 2848 cm-1 assigned to the asymmetric and symmetric stretching vibrations of the methylene group, the most repeated structural unit in the cutin biopolyester and the strong absorption bands at 1745 and 1162 cm-1, assigned to the C=O and C-O stretching. The results of this preliminary study confirm that both extraction procedures are efficient, but with second extraction better yield of pure product was obtained. FTIR spectroscopy is useful technique for characterisation and validation of extracted polymer and monomer biocompounds from tomato waste.

tomato peel; extraction of cutin; biopolymer; FTIR spectroscopy

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