## 4 Conclusions  
Two strains were isolated from cheese whey, one that corresponded to the LAB _Enterococcus faecalis_, which has an optimal growth temperature of 32 \({}^{\circ}\)C, is neutrophil and alkalophilic, and has a tolerance to salinity up to 4% NaCl. The second strain belonged to the yeast _Kluyveromyces lactis_, which has an optimal growth temperature of 32 \({}^{\circ}\)C, tolerance to acidic pH and mainly to neutral and alkaline pH, and a high resistance to salinity, reaching a tolerance of up to 12%, which gives it an advantage against extreme environments. The _Enterococcus faecalis_ had a greater generation of lactic acid in the deproteinized cheese whey, both in suspension and immobilized. The highest generation of lactic acid was achieved with the immobilized strains, achieving up to 50.61 \(\pm\) 34.94 g/L of lactic acid with a fermentation pH that varied between 3.5 and 5 using the deproteinized cheese whey as culture medium without any nutritional supplements. The _Enterococcus faecalis_ suspension strain produced 13.09 \(\pm\) 1.54 g/L of lactic acid at pH 4.90, while when the pH increased, a decline in the generation of lactic acid was observed. The immobilized _Kluyveromyces lactis_ showed a greater tolerance to acidic pH and the highest production of lactic acid (35.70 \(\pm\) 0.15 g/L) at 168 h and pH 3.64 using the proteinized cheese whey. The lactic acid production when the strain was suspended was favorable at an acidic pH and when using the proteinized cheese whey, reaching a value of up to 12.45 \(\pm\) 0.91 g/L at 47 h.  
The optimal purification conditions were determined, and the highest yield of lactic acid after the purification process was 16.92%, which was obtained with _Enterococcus faecalis_ in suspension using the deproteinized cheese whey (purified at 40 \({}^{\circ}\)C for 1 h). Meanwhile, for _Kluyveromyces lactis_, the highest yield was 8.44% using the deproteinized cheese whey and the immobilized strains and when it was concentrated at 40 \({}^{\circ}\)C for 5 h. It was verified through FTIR analysis that all the samples obtained corresponded to lactic  
Figure 10: FTIR spectra of lactic acid obtained by _Kluyveromyces lactis_ in suspension (KS) and immobilized (KI). KS2_P1: proteinized cheese whey, concentrated at 40 \({}^{\circ}\)C for 5 h; KS2_P2: proteinized cheese whey, concentrated at 45 \({}^{\circ}\)C for 5 h; KI1_P1: deproteinized cheese whey, concentrated at 40 \({}^{\circ}\)C for 5 h; and KI1_P2: deproteinized cheese whey, concentrated at 45 \({}^{\circ}\)C for 5 h.  
acid, with some variation in the peaks being observed at the different fermentation and purification conditions. In conclusion, although _Kluyveromyces lactis_ showed a greater resistance to acidic pH, _Enterococcus faecalis_ had the highest performance in obtaining and purifying lactic acid. It was determined there is a difference when using deproteinized and proteinized cheese whey, in the strain used (_Enterococcus faecalis_ and _Kluyveromyces lactis_), in the disposition of the strain (in suspension and immobilized), in the yield, and in the physicochemical characteristics of the lactic acid, which could probably affect their probable uses. Conceptualization, J.A. and C.G.-A. ; methodology, J.E.B.-D.-C. and M.V. ; validation, D.T.-Q. and F.R. ; formal analysis, J.A. ; investigation, M.V. and J.-A. ; resources, J.E.B.-D.-C. and C.G.-A. ; data curation, F.R. ; writing--original draft preparation, M.V. ; writing--review and editing, F.R. and J.A. ; visualization, C.G.-A. ; supervision, C.G.-A. ; project administration, D.T.-Q. ; funding acquisition, C.G.-A. All authors have read and agreed to the published version of the manuscript. This research was funded by the Catholic University of Saint Mary, grant number 26617-R-2019. Not applicable.