**5. Conclusions**

In this study, a new GH2 β-galactosidase (Gal) was successfully cloned, purified and characterized from the deep-sea bacterium *Alteromonas* sp. ML52. Recombinant Gal is a cold-adapted enzyme and was able to efficiently hydrolyze lactose in milk at refrigerating temperature. These characteristics suggest that Gal could be a potential cold-active biocatalyst and usefully applied to the production of lactose-free milk in the dairy industry.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1660-3397/16/12/469/s1, Figure S1: HPLC-RID profiles of hydrolysis of lactose in milk at 4 ◦C (A), 10 ◦C (B) and 25 ◦C (C). (1) Glucose; (2) Galactose; (3) Lactose.

**Author Contributions:** Data curation, J.S., C.Y. and Z.Z.; Funding acquisition, J.S., W.W. and J.H.; Methodology, J.S., W.W. and J.H.; Project administration, J.S., C.Y., J.L. and F.D.; Writing-original draft, J.S.; Writing-review and editing, J.S. and J.H.

**Funding:** This research was funded by Central Public-interest Scientific Institution Basal Research Fund, YSFRI, CAFS (No. 20603022018006); the Financial Fund of the Ministry of Agriculture and Rural Affairs, P. R. of China (No. NFZX2013); Central Public-interest Scientific Institution Basal Research Fund (No. 2017HY-ZD1003); the Scientific and Technological Innovation Project Financially Supported by Qingdao National Laboratory for Marine Science and Technology (No. 2016ASKJ14).

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