## 3 Results and Discussion  
### LAB Screening and Selection  
Screening tests were carried out in order to select the best lactic acid producers and investigate the effect of the presence of oxygen on microbial growth.  
It is well known that LAB in general are facultatively anaerobic, and in presence of glucose they are able to produce lactic acid [34]. Thinking in terms of low-cost lactic acid production, the economic impact of air supply or air removal devices could be relevant, the best choice being to invest on microaerofilic LAB, which tolerate the presence of oxygen in the environment, reaching a sustainable combination of yield and productivity. For this reason, LAB were grown in aerobic, microaerophilic and strictly anaerobic conditions in order to compare cell growth and lactic acid production, as well as glucose depletion and glucose depletion rate. The results of the fermentation tests are shown in Table 2. As expected, anaerobic conditions appeared to favour lactic acid production and substrate consumption, whereas biomass growth rate was higher overall in the presence of oxygen [35].  
_Lactobacillus alimentarius, Lactobacillus curvatus_ and _Lactobacillus reuteri 20015_ were promptly excluded because they partially consumed glucose without producing lactic acid in any of the tested conditions. In anaerobic and microaerophilic conditions the majority of the strains completely depleted glucose in 72 h, whereas only four strains (_Lactobacillus casei_, _Lactobacillus furciminis_, _Lactobacillus ferrantum 20052_ and _Lactobacillus kadleri_) consumed glucose in an aerobic environment although with a lower \(q_{s}\). Generally, except for a few cases, lactic acid yield factors (\(Y_{\text{LA}}\)) and specific productivity (\(q_{\text{LA}}\)) were low, but comparable to those obtained in other batch fermentations on glucose without pH control [36]. The best lactic acid producers were _L. casei_ and _L. farciminis_ with \(Y_{\text{LA}}\) > 90% in all conditions, although \(Y_{\text{LA}}\) went down to 79% and 71%, respectively, with the proportional increase in oxygen availability.  
_L. casei_ is reported to be a facultative anaerobic microorganism with a homofermentative nature [37]. Consequently, the microorganism grows better in a static culture where the fermentation conditions favour low oxygen concentrations. Furthermore, _L. casei_ is known to be acidotolerant with an optimum pH of 5.5 and it is relatively insensitive to product inhibition by lactic acid. In microaerophilic conditions, _L. casei_ achieved a final lactic acid concentration of 15.89 \(\pm\) 0.56 g/L compared to 20.00 \(\pm\) 0.20 g/L of glucose, versus 19.50 \(\pm\) 0.79 g/L obtained in anaerobic conditions. _L. farciminis_, known as a homofermentant microorganism isolated from meat and meat products [38], in our experimental conditions produced 18.37 \(\pm\) 0.94 g/L of lactic acid in anaerbiosis and 15.71 \(\pm\) 0.54 g/L in microaerophilia.  
Several tested LAB were reported to produce both l-lactic and d-lactic acid stereoisomers in the different experimental conditions (Table 3). In particular, as shown in Figure 1, while _L. casei_, with increasing dissolved oxygen, proportionally increased the amount of d-lactic acid at the expense of l-lactic acid, _L. farciminis_ only produced l-lactic acid in microaerophilic conditions. This could be an interesting point, because the stereochemistry of the monomer lactic acid has a great influence on the properties of the polylactide. In fact, the stereoregularity is the key factor that makes polylactic acid (PLA) a highly crystalline (pure l-lactic acid), fully amorphous or semicristalline polymer (by the inclusion of different percentages of the d-isomer) [39]. After recovery from the culture broth and cell biomass, the possibility to directly use the l-lactic acid produced without any further steps with respect to optical purification, could be of some interest.