## 4 Conclusions  
The present study showed the alteration of the conformational, microstructural, and antioxidant properties of KGM caused by power US treatment. The original KGM presented a semi-flexible coil conformation and a linear morphology in water. Power US treatment of KGM led to a reduction in the average MW and apparent viscosity, as well as in the size of gel aggregates within a short period of time (\(<\)10 min). Meanwhile, in the time range of 0-10 min, the polydispersity index \(M_{\rm w}/M_{\rm n}\) value of KGM decreased from 1.5 to 1.2, showing a narrower or more uniform MW distribution. The reduction in MW with time during the US treatment of KGM followed the kinetic model for random chain scission. Compared with the original KGM, the US-degraded KGM had more flexible chain conformation, as well as fragmentary and particulate aggregates with a lower viscosity and Newtonian flow behavior. The US degradation of KGM in most cases resulted in higher radical-scavenging activities. Additionally, the effect of US treatment on the protection of KGM against H\({}_{2}\)O\({}_{2}\)-induced GES-1 cell apoptosis (data not shown) was evaluated, but no significant effect was observed. Overall, this study indicates that power US is a simple and effective means for rapid and partial degradation of high-MW KGM and for modification of the conformation and rheological properties, and enhancement of antioxidant activity.  
The following are available online at [[http://www.mdpi.com/2076-3417/9/3/461/s1](http://www.mdpi.com/2076-3417/9/3/461/s1)]([http://www.mdpi.com/2076-3417/9/3/461/s1](http://www.mdpi.com/2076-3417/9/3/461/s1)): Figure S1: Mark-Houwink plots of KGM following different ultrasonic times. The \(L_{\rm P}\) value was calculated according to the Bohdanecky method by plotting \((M^{2}/[\eta])^{1/3}\) against \(M^{1/2}\) (not shown) for each molecular-weight distribution, which was the same as Mark-Houwink plot consideration. Molecular weight distribution selected for the above calculations covered 5-95% of determined mass for each KGM. KGM treated with ultrasonication is labeled as KGM-US. For example, KGM-US-2 means KGM was subjected to ultrasonication for 2 min at an amplitude of 60%; Figure S2: SEM images of native KGM without US treatment; Table S1: Physicochemical properties and monosaccharide composition of KGM.  
Conceptualization and supervision, J.-Y.W.; data curation, L.-Y.M.; formal analysis, investigation, and writing--original draft, J.-Y.Y.; formal analysis and data curation, L.-Y.M.; methodology, K.-C.S.  
This research received no external funding.  
This work was financially supported by the National Key R&D Program of China [2017 YFD0400104], the Hong Kong Scholars Program [X]2016058], Hong Kong Polytechnic University, the Shenzhen Basic Research Project [JCY]20160531184200806], and the Postdoctoral Science Foundation of Jiangxi Province [2017 KY03].  
Figure 6: In vitro antioxidant activities of original (KGM) and US-treated KGM (KGM-US-2, KGM-US-20, and KGM-US-60): (**a**) 2,2\({}^{\prime}\)–diphenyl-1-picrylhydrazyl (DPPH) radical-scavenging activity; (**b**) 2,2\({}^{\prime}\)–azino-bis-3-ethyl-benzothazoline-6-sulphonate (ABTS) radical scavenging activity; (**c**) ferric-reducing antioxidant power. All values are expressed as means \(\pm\) SD (\(n\) = 2). Original KGM and US-treated KGM fractions were prepared with distilled water to various concentrations of 0.125, 0.25, 0.5, 1.0, and 2.0 mg/mL.