**4. Conclusions**

The enzymatic hydrolysis of potato juice in a membrane reactor influenced both the nutritional value as well as the biological activity of this raw material. The use of ultrafiltration systems resulted in a product containing soluble proteins in a concentration several times higher than that of a simple enzymatic hydrolysis. Mineral compounds were also concentrated in this process. The additional use of a nanofiltration module resulted in a further concentration of the solutes. The products of the enzymatic hydrolysis of potato juice were characterized by a significantly higher antioxidant activity and concentration of polyphenolic compounds than the raw materials.

It was also found that the enzymatic hydrolysis of potato juice in the reactor with the ultrafiltration membrane separation system increased the cytotoxic activity of the processed material. IC<sup>50</sup> toxic doses of the hydrolysate for cancer cells were significantly lower than those of fresh potato juice. Moreover, IC<sup>50</sup> toxic doses of the concentrate were lower for cancer cells than for normal cells. Therefore, the additional use of a nanofiltration system to concentrate the obtained hydrolysate further increased the cytotoxicity of the product against cancer cells.

**Author Contributions:** Conceptualization, P.Ł.K. and G.L.; Data curation, P.Ł.K., A.O. and M.Z.-D.; Formal analysis, P.Ł.K.; Funding acquisition, P.Ł.K.; Investigation, P.Ł.K., A.O. and I.R.; Methodology, P.Ł.K., A.O., M.Z.-D., W.B. and G.L.; Supervision, G.L.; Validation, W.B.; Writing—original draft, P.Ł.K., A.O., I.R., M.Z.-D. and G.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** The work was financially supported by the grant of the National Science Centre (Poland) No. 2019/03/X/NZ9/00229.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of the data; in the writing of the manuscript, or in the decision to publish the results.

**Sample Availability:** Samples of PJPH and cPJPH are available from the corresponding author.
