## 3 Results and Discussion
### Batch Fermentation with Strains in Suspension and Immobilized
Therefore, for _Enterococcus faecalis_ isolated from cheese whey, using deproteinized cheese whey provides more favorable conditions for the production of lactic acid; that is, the composition of cheese whey and the pH conditions affect the metabolism of _Enterococcus faecalis_. Figure 3 shows the production of lactic acid with the yeast _Kluyveromyces lactis_ using the deproteinized cheese whey (KS1) and non-deproteinized or proteinized cheese whey (KS2). For this case, it was observed that the highest production of lactic acid was generated with the proteinized cheese whey (KS2), reaching up to 12.45 \(\pm\) 0.91 g/L at 47 h and at pH 5. Meanwhile, with the deproteinized cheese whey (KS1), the production of lactic acid was restricted by the pH value, which in this case tended to neutrality, a range that has been seen to influence the production of lactic acid. Therefore, for _Kluyveromyces lactis_, an  
Figure 1: Lactic acid yield (g/L) of _Enterococcus faecalis_ and its pH variation during the cheese whey fermentation. Figure 2: Lactic acid yield (g/L) of _Enterococcus faecalis_ and its lactose consumption during the cheese whey fermentation. acidic pH favors the production of lactic acid, which is consistent with other works, where _Kluyveromyces lactis_ produced 4.70 g/L of lactic acid at 30 \({}^{\circ}\)C for 36 h of fermentation [18]. So, it has been shown that at the lowest pH, the greatest generation of lactic acid occurred, and when it rose, the production of lactic acid tended to decrease, a behavior also observed by _Enterococcus faecalis_. Figure 4 shows the lactose consumption against the production of lactic acid with _Kluyveromyces lactis_, and it was observed that although the deproteinized cheese whey (KSI) had a higher concentration of lactose, the least amount of lactic acid was generated, while with the proteinized cheese whey (KS2), it had a lower amount of lactose but generated a greater amount of lactic acid, which was due to the effect of the pH. In both strains, when the lactose was most consumed, the highest yield of lactic acid was obtained. Therefore, the use of proteinized and deproteinized cheese whey and the pH medium influenced the lactose metabolism of the yeast _Kluyveromyces lactis_ as well as that of _Enterococcus faecalis_ and, hence, the production of lactic acid. Figure 4: Lactic acid yield (g/L) of _Kluyveromyces lactis_ and its lactose consumption during the cheese whey fermentation. Figure 3: Lactic acid yield (g/L) of _Kluyveromyces lactis_ and its pH variation during the cheese whey fermentation. 3.2 Batch Fermentation with Strains Immobilized Using _Enterococcus faecalis_ and _Kluyveromyces lactis_  
The batch fermentation results using immobilized _Enterococcus faecalis_ and using the deproteinized cheese whey (EI1) and proteinized cheese whey (EI2) are shown in Figure 5.  
As observed in the previous case of the suspended strains, the immobilized strains did not generate a high production of lactic acid because of their pH, which tended toward neutrality. As in the case of the proteinized cheese whey (EI2) with _Enterococcus faecalis_, it reached a value of 6.09 g/L of lactic acid at pH 4.40, with a drastic drop in lactic acid (2.01 g/L) at pH 6.26. Meanwhile, with the deproteinized cheese whey (EI1), a higher production of lactic acid was generated, reaching values of up to 50.61 \(\pm\) 34.94 g/L at a pH ranging from 4.84 to 3.61. Therefore, the optimal pH for growth would be between 3.5 and 5 for _Enterococcus faecalis_, since at that pH range, a greater yield of lactic acid was obtained, unlike when the proteinized cheese whey was used. Hence, the influence of the pH on the generation of lactic acid was confirmed, since it affected two aspects of microbial metabolism, namely, the metabolism of enzymes and the transport of nutrients within cells [19].