Polysaccharides from wastes of vegetable industrial processing: new opportunities for their eco-friendly re-use

Natural polysaccharides constitute a major group of biopolymers widespread in the whole vegetable kingdom. Plant polysaccharides comprise highly heterogeneous biopolymers that play diverse biological roles as structural elements, energy reserve and biological signalling. Polysaccharides from natural sources have attracted significant interest as biotechnological products, due to their commercial uses in a wide range of industrial applications. Some of them, for example, possess strong antigenic and antipathogenic activities and are successfully employed by the pharmaceutical industry for the formulation of vaccines or as a matrix for drug-delivery applications; others are utilised as food additives taking advantage of their physical-chemical properties (emulsifying power, viscoelasticity, polyelectrolyte, adherence, bio-compatibility, stabilizer, etc). Production of these biopolymers for industrial applications has both economic and environmental costs, depending on the starting materials used as feedstocks and on the chemical/thermal treatments required for their extraction. Recently, wastes from industrial processing of vegetables for food production have been proposed as potential sources of useful polysaccharides. Indeed, transformation and packaging of fruits and vegetables generates huge amounts of wastes, since only a fraction of the incoming biomass is effectively used. The management of such waste biomasses is a problem worldwide, from both economic and environmental standpoints, and therefore research is encouraged to implement new strategies for their re-use. An advantageous option is the extraction of value added chemicals, including different kinds of polysaccharides, besides chemically different species such as polyunsaturated fatty acids, natural pigments, tannins, carotenoids, antioxidants etc. Treatment of vegetable residues by means of newly and environmentally sustainable extraction techniques represents at the moment a fascinating challenge for the valorisation of agro-industrial wastes. Remarkable examples in this framework are represented by polysaccharides isolated from wastes of tomato industrial processing and of tropical fruit juices production based on granadilla, a fruit species of Passiflora, distributed mainly in the warm temperate and tropical regions of America and Africa. These polysaccharides resulted to possess useful biotechnological properties and interesting biological activities. An accurate chemical characterization demonstrated that the main biopolymer obtained from tomato wastes was an heteropolymer with a molecular weight higher than 1 ×106 Da constituted by glucose/ xylose/ galactose/ galactosamine/ glucosamine/ fucose in a relative molar ratio of 1: 0.9: 0.5: 0.4: 0.2: trace, whereas the polysaccharide isolated from waste peels of granadilla (Passiflora liguralis) fruits (with a molecular weight higher than 1 ×106 Da as well) was characterised by the presence of six different sugar residues: xylose/ glucose/ galactose/ galactosamine/ unknown component/ fucose in the relative ratios of 1:0.5:0.2:0.06:0.05:trace. The study of rheological properties showed for both biopolymers an high thermal resistance and an high viscosity, depending, in particular for granadilla waste polysaccharide, on concentration and pH, with a maximum value of 1.4  at a concentration of 3% in distilled water and a maximum value of 7.0  in citrate buffer solution. The main point of interest was represented by their suitability to produce elastic and biodegradable films potentially useful in agriculture for mulching applications in fields protection. Further studies were performed to evaluate the biological activity of these biopolymers. Tomato waste polysaccharides resulted to inhibit NF-κB activation and iNOS gene expression in J774 macrophages by preventing the reactive species production, thus suggesting a key role of these compounds in controlling oxidative stress and/or inflammation. Biological activity of polysaccharides from granadilla peels was assessed by means of brine shrimp bioassay: the isolated compound was able to strongly inhibit the cytotoxic effects produced by avarol, whose LD50 was increased of about 10-fold with respect to the control in the absence of polysaccharide. More recent studies have been focused on lemon wastes. Citrus processing produces a considerable amount of waste biomass called “lemon pomace” consisting of peels (flavedo and albedo), pulp and seeds, mainly composed by water, soluble sugars and fibres, together with other compounds such as oils, flavonoids and vitamins. Lemon wastes, generated in huge amounts by Italian industries for “Limoncello” liquor production, have been treated in order to separate useful polysaccharides. A major polysaccharide fraction was isolated and purified: analysis by means of gel filtration allowed to determine a molecular weight higher than 1 ×106 Da. The sugar components were identified after acid hydrolysis, by means of TLC and HPAE-PAD chromatography: the main components resulted to be galactose, galacturonic acid and arabinose. Further studies are now being implemented to study both rheological properties and biological activity of lemon polysaccharides. In conclusion, polysaccharides are among the most interesting biopolymers that can be recovered from vegetables: the use of wastes from industrial processing of different vegetables together with eco-friendly extraction techniques allowed the isolation of remarkable polysaccharides. Some of them showed to possess interesting rheological properties, potential biotechnological applications and finally promising bioactivities as potential anti-inflammatory agents, as tested in different biological model systems.