The role of L-lactate is not always beneficial for the yogurt fermentation due to excessive L-lactate can provide an optimized growth condition for yeast and mold. Therefore, it is important to detect the concentration of L-lactate. Acid-base titration is a common method for it, but this method is complicated and time-consuming. In order to monitor L-lactate quickly and conveniently, we designed a biosensor for detecting L-lactate concentration by using the lldPRD L-lactate operon and QS system in E. coli. One of these parts is able to induce the lldPRD genes expression, LuxS protein, in the presence of L-lactate. LuxS protein catalyzes the SAM cycle and produces a small signaling molecule AI-2 that motivates our second part promoter of LsrA&K to promote GFP expression. The optic fiber is able to detect the GFP signal and convert it into current. Simultaneously, the entire device container will be made by 3D printing.