Rheological and structural characteristics of peach tree gum exudate

The peach tree (Prunus persica), after mechanical injury or infection by microorganisms, produces a copious gum exudate on its trunk and branches. After aqueous extraction, peach gum (cv. ‘chimarrita’) yielded a soluble aqueous extract (AE) and a particle-rich hydrogel fraction (RG). After freeze-drying, AE was solubilized in water and added to excess ethanol to give a polysaccharide (POLY) precipitate. RG, AE, and POLY were composed of Ara, Xyl, Man, Gal, and uronic acids in 46:14:2:32:6, 54:12:1:27:6, and 47:13:1:33:6 molar ratios respectively, with traces of Rha. The 13C NMR spectra of all fractions were similar, with C-1 signals of α-l-Araf (δ 109.6–107.4) and main ones from β-d-Galp units (δ 103.7, 103.2), suggesting the presence of arabinogalactan-type polysaccharides. Rheological analyses were carried out with AE and POLY. Solutions of both fractions at 4% (w/v) had a non-Newtonian shear-thinning behavior, however that of AE showed higher apparent viscosity than that of POLY. The presence of salts decreased the apparent viscosity of AE dispersion. Dynamic oscillatory analyses showed that 4% (w/v) AE and POLY had a gel-like structure, being AE hydrogel a stronger gel when compared with POLY. This suggests that other compounds, in addition to polysaccharide, are involved in the gel network in aqueous dispersions of AE. Weaker gels were formed with 2% (w/v) AE in water and 4% (w/v) AE in various salt solutions. These results showed that the peach gum had an interesting rheological behavior and that it could have a future application.