###### Abstract  
Cheese whey is a byproduct of the cheese industry that causes high levels of pollution in the environment, but its high lactose content means that it can be used as a source to obtain lactic acid. In this study, two strains, one belonging to a yeast and the other one to a bacteria (_Kluyveromyces lactis_ and _Enterococcus faecalis_), were isolated from cheese whey and molecularly characterized, and the optimal growth conditions were determined. Then, using proteinized and deproteinized cheese whey, batch fermentation was carried out with the strains arranged in suspension and immobilized. The consumption of lactose and the production of lactic acid were measured through Brix degrees and acidity analysis. Afterwards, the lactic acid was purified, and its yield and physical and chemical characteristics were determined. It was proven that there were differences between each of the strains; arranged in free or encapsulated cells, the proteinized and deproteinized cheese whey, under the same purification conditions, achieved different yields, colors, and densities of lactic acid. Immobilized _Enterococcus faecalis_ had the highest yield (50.61 \(\pm\) 34.94 g/L) using the deproteinized cheese whey compared to the immobilized _Kluyveromyces lactis_ (35.70 \(\pm\) 0.15 g/L) using the proteinized cheese whey. Kluyveromyces lactis; Enterococcus faecalis; cheese whey; lactic acid; immobilized cell; suspended cell +
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due to its high lactose composition (44-46 g L\({}^{-1}\)) [1], can be used for other applications, such as the biotechnological production of lactic acid. Lactic acid is a product of high importance due to its wide applications, mainly in the pharmaceutical, cosmetic, chemical, and food industries [3]; in addition, there is currently a high demand to produce polylactic acid (PLA), which is a biopolymer that is classified as environmentally friendly. So, lactic acid has increased in demand as a raw material to produce polylactic acid (PLA) [4]. Lactic acid is an organic acid that can be acquired naturally by extraction from milk sugar or synthetically by extraction from cane, grape, or starch sugar [5]. Its price in the market varies according to its application and is estimated to increase from 1220 kilotons in 2016 to 1960.1 kilotons by 2025; it is used most widely in medicine and cosmetics in Latin America and Asia [6]. Lactic acid production can be synthetically obtained either by fermentation or chemically. The fermentation production process has gained more interest because it has the advantage of obtaining lactic acid with pure isomers of L (+) or D (-); additionally, it is cheaper because of the low cost of renewable raw materials, its low energy consumption, and its easy operating conditions [4]. For the synthesis of lactic acid through fermentation, lactic acid bacteria (LAB) are used, which include _Lactobacillus_, _Enterococcus_, and _Streptococcus_, which are capable of consuming cheese whey to produce lactic acid as a result of lactose fermentation. Among the _Lactobacillus_ strains are _Lactobacillus__lus__delbrueckii subsp.__bulgaricus_, _Lactobacillus__casei_, and _Lactobacillus__acidophilus_, among others, and within of the genus _Enterococcus_ there are _Enterococcus faecalis_, _Enterococcus lactis_, and _Enterococcus__amelliae_, among others. Depending on the species of microorganisms used, a D (-), L (+), or DL lactic acid configuration can be generated. However, some species of yeast that ferment lactose have also been seen, including _E. coli_, _K. lactis_, and _S. cerevisiae_, as well as fungi such as _Rhizopus__orgaze_[6]. Regarding their environmental conditions, it has been seen that the majority of microorganisms grow at low fermentation temperatures (less than 45 \({}^{\circ}\)C) that favor contamination by other species. In addition, bacteria, yeasts, and fungi have distinct advantages and disadvantages when used in the production of lactic acid [6].