Isolation, Purification, Structural Characterization And Antioxidant Activity Of Apple Peel And Pulp Polysaccharides

Apple is the fruit of Rosaceous apple genus. It is nutritious and convenient to eat, ranking first in the world’s top four fruits. China is rich in apples and apple pomace has become a new kind of resource. The cold-extracting technique can separate fruit peel, fruit seeds and fruit flesh before pressing, thus making it possible to investigate fruit peel and flesh respectively. There is no systematic comparison between apple peel and apple flesh polysaccharides among the researches on apple polysaccharides up to now. Therefore, choosing apple peel pomace and apple flesh pomace obtained from cold-extracting as the materials, this research optimized apple polysaccharide extracting process, isolated and purified apple peel polysaccharide (APP) and apple flesh polysaccharide (AFP) for the first time and compared the structure and antioxidant activities of the purifuird polysaccharides fractions obtained in the hope of providing new ways to exploite apple pomace and laying theoretical foundation for the distinct application of apple peel polysaccharide and apple flesh polysaccharide. The main conclusions of the study were as follows:1 X-5 macroporous resin was used to optimize the extracting process of apple polysaccharide and the optimum extracting parameters were gained according to single factor experiment and response surface methodology (RSM) with apple polysaccharide yield as the target. The regression model equation was:Y=-163.8244+4.8373A+ 23.0560B+2.5436C+0.1475AB-0.0398AC+0.01315BC-1.1610A~2-2.9135B~2-0.01289C~2. The three factors affected apple polysaccharide yield in the order of sample concentration (B)> alcohol concentration (C)> flow rate (A).The optimized parameters were:flow rate:0.64 mL/min, sample concentration:concentrating 4 fold, alcohol concentration:95%, under this condition, apple polysaccharide yield was 12.67%. Besides, the purity of the apple polysaccharide obtained using X-5 macroporous resin increased to 71.58% from 61.35% with its color changing to white from yellow, that is to say, the optimized extracting process had the apple polysaccharide quality improved significantly.2 DEAE-52 and Sephadex G-200 column chromatography were employed to isolate and purify APP and AFP, eventually getting two purified fractions:apple peel polysaccharide fraction one (APP-2) and apple flesh polysaccharide fraction one (AFP-2). UV、FT-IR、 HPAE、 HPGPC、 Congo-red experiment and ESEM were combined to character APP-2 and AFP-2’s structure.UV spectra showed no absorption peaks at 260 nm and 280 nm, which indicated that APP-2 and AFP-2 contained no nucleic acid and protein. Both APP-2 and AFP-2’s FT-IR spectra contained the characteristic absorption peaks of polysaccharides in the fingerprint region of 1200-950 cm-1 and pyranoid rings. HPAEC results indicated that APP-2 was acidic heteropolysaccharide being composed of Rha, Ara, Gal, Glu, Xyl, Man and GalUA in the mole percentage of 6.47%:28.56%:31.79%:2.34%:3.96%:1.55%: 25.33%, while AFP-2 was acidic heteropolysaccharide being composed of Rha, Ara, Gal, Glu, Xyl and GalUA in the mole percentage of 7.80%:30.82%:22.05%:2.6 0%:6.54%:30.20%. According to HPGPC results, both APP-2 and AFP-2 contained singal and symmetic peak proving that they were homogeneous polysaccharide fractions and the molecular weights of APP-2 and AFP-2 were 906 kDa and 762 kDa, respectively. APP-2 turned out to have triple-helix structure while AFP-2 had no triple-helix structure basing on Congo-red results. ESEM showed that both APP-2 and AFP-2 were flake morphologies.3 The antioxidant activities of APP-2 and AFP-2 were evaluated by antioxidant evaluation system in vitro. The antioxidant results showed that both APP-2 and AFP-2 exhibited significant reducing power and relatively strong DPPH·and ·OH scavenging capacity in the concentration tested, the higher the apple polysaccharide concentration was, the stronger the antioxidant activity was. APP-2’s antioxidant activity was superior to AFP-2 under the same concentration, which might be due to the structure differences between APP-2 and AFP-2.