**4. Conclusions**

In this study, we selected Mal-mPEG5000 as the best modifier from six modifiers and optimized the modification parameters by the response surface method. We used column chromatography to isolate and purify Mal-mPEG5000-SPA from the reaction solution between SPA and Mal-mPEG5000. Thereafter, the enzymatic properties were determined and compared in the presence and absence of Mal-mPEG5000. For the results, under the optimal conditions (the molar ratio of Mal-mPEG5000 to SPA, 1:4, modification temperature, 58 ◦C, pH 6.0), the specific activity of Mal-mPEG5000-SPA was (2.150 ± 0.055) × <sup>10</sup><sup>4</sup> U/mg, an increase by 24.06% than that of the unmodified SPA. Mal-mPEG5000-SPA was separated and purified, and a single band was noticed and its molecular weight was about 67 kDa. The enzyme properties of Mal-mPEG5000-SPA were significantly improved as the optimum temperature declined from 55 ◦C to 45 ◦C, the thermal stability and pH stability were obviously enhanced; the Km value of sweet potato starch by Mal-mPEG5000-SPA declined by 12.95%, while Vmax increased by 26.87%, and Ea dropped by 12.63%, which showed that Mal-mPEG5000-SPA had greater affinity for sweet potato starch and higher the speed of hydrolysis than SPA; as for Mal-mPEG5000-SPA, Mn2+, K+, Zn2+ and Ca2+ demonstrated activation effect; Mg2+ showed a slight inhibitory effect; Cu2+, NH4 +, Fe3+, Al3+, Ba2+, EDTA, Hg+ and Ag<sup>+</sup> had a relatively strong inhibitory effect; there was no significant difference of metal ions' effect on Mal-mPEG5000-SPA and SPA. It is concluded that Mal-mPEG5000-SPA will have a wider range of application in beer processing, maltose production and other food industries. Therefore, the studies on SPA application in food industries are theoretically and practically significant.

**Author Contributions:** J.S. and X.L. conceived and designed the experiments; X.L. and W.Z. performed the experiments; J.R. and L.J. analyzed the data; L.F. and B.L. provided the software and formal analysis.

**Funding:** This research was supported by the National Natural Science Foundation of China (No. 31771941), the Program for Science & Technology Innovation Talents in Universities of Henan Province (No. 16IRTSTHN007) and the Major Science and Technology Project in Henan (No. 182102110109).

**Conflicts of Interest:** The authors declare no conflict of interest.
