Ultrasound-assisted extraction of pectin from guanabana (Annona muricata) for development of hydrogels with antimicrobial activity

Abstract

Annona muricata, better known as soursop, guanabana or graviola, is an exotic conical shaped fruit native from central America. Only in Mexico, the production rounds 30,000 tons, but defective fruits with no commercial value and its shelf-life of only five days post-harvest, increases the loss up to 30%. To add value to these waste residuals, the extraction of several bioactive compounds present in soursop fruit composition, such as phenols, flavonoids, and tannins, in addition of fiber, including pectin, has been proposed. Pectin is a major component of plant cell wall and is a polysaccharide composed of galacturonic acid (GalA) that can be methyl esterified. Recently, alternative pectin sources from citrus peels and apple pomace have been evaluated looking for pectin with specific technological properties, which depends on the source and the extraction process that can lead to large variation in pectin chemical structure depending on conditions such as temperature, time, and ph. Ultrasound assisted extraction has been used for bioactive compound obtention due to its shorter period, higher yield, and solvent consumption, enhancing contact between solvent and targeted compounds by disrupting plant cell walls. Pectin extracted from natural sources can be used for applications in food industry, wound healing and drug delivery by the development of gel films constructed of a network of crosslinked polymer chains with a 3D structure named hydrogels due to its antimicrobial activity and its high cell compatibility. Therefore, the main objective of this thesis was to optimize the ultrasound-assisted extraction of pectin from Annona muricata pulp for development of hydrogels with antimicrobial activity. Sample characterization was performed in which a three-day post harvest ripening stage was determined. Then, the alcohol-insoluble material (AIM) was obtained for later proximate composition evaluation, where 33.42% (g/100 g dry weight) was for total dietary fiber, from which, 20.5% corresponded to insoluble fiber, 0.99% to low molecular weight soluble dietary fiber, and 11.94% to soluble fiber, where pectin is included. For pectin extraction, a response surface method combined with a central composite design was used to evaluate three process conditions: time, temperature, and amplitude, to determine their effects on yield, degree of esterification (DE), and galacturonic acid (GalA) content in extracted pectin, which ranged from 4.72 to 12.01% (w/w), 62.92 to 71.8%, and from 285.34 to 568.35 mg GalA/g AIM. FTIR assay was performed for evaluation of pectin structural features. Hydrogel films were prepared by dissolving the extracted pectin (2.5 and 5%), glutaraldehyde (0.8%) and glycerol (20%) in water. SEM and FTIR analysis for structural features and functional characteristics were also performed. Finally, antimicrobial and cytotoxicity assays were carried out where both PeH 2.5% and PeH 5% showed a big zone of inhibition when tested against S. aureus, and a smaller one against E. coli. Also, both hydrogels with different concentrations presented a cell viability above the 80% the first 24 h, and around the 75% at 48 h. However, at 72 h, PeH 5% presented a cell viability of 71% but PeH 2.5% exhibited a cell viability of only 63%, which can be considered with a cytotoxic effect. Overall, pectin extracted from soursop fruit pulp showed to be a good candidate for hydrogels development with potential biomedical applications.