Ultrasonic degradation of sweet potato pectin and its antioxidant activity

The effect of ultrasound factors (time, power, and duty cycle) on sweet potato pectin molecular weight, neutral sugar composition, pectin structure, and antioxidant activity was investigated. Sweet potato pectin dispersions (0.0025, 0.005 and 0.01 g/mL) in deionized water were sonolyzed for 5, 10 and 20 min to assess effect of sonication time and pectin concentration on sonolysis. For further experiments 0.0025 g/mL was sonicated under varying ultrasonic power and duty cycle levels, subsequently the molecular weight, galacturonic acid content, degree of methoxylation and antioxidant activity of sonicated pectin products were investigated. Results showed that ultrasound treatment reduced pectin molecular weight, while polydispersity did not show clear trend which characterized random pectin scission, increasing duty cycle from 20% to 80% resulted in approximately threefold reduction in pectin molecular weight, increased sonication power from 100 W to 400 W led to significant increase in galacturonic acid content from 72.0 ± 1.2% in native pectin to between 85.0 ± 3.2% and 92.0 ± 2.7%, the degree of methoxylation significantly reduced from 12.0 ± 3.0% to between 5.25% and 6.28%, sonication led to increase in galactose and decrease in rhamnose consistent with debranching of pectin. Moreover, sonication lead to increased antioxidant capacity, both 200 W and 400 W sonicated pectin having higher ORAC and FRAP values, with highest pectin concentration 4 mg/mL in ORAC and 0.8 mg/ml in FRAP giving substantially high antioxidant activity than native and 100 W treated pectin. The ORAC value of 400 W sonicated pectin increased five hold above the native pectin, while it's FRAP value was almost three fold higher than native pectin. However, ultrasound did not alter pectin primary structure as showed by FTIR and HPAEC results. The results indicated that ultrasound offers effective and green process for pectin transformation and creation of antioxidant potent pectin products.