This paper reports numerically a high-performance phononic crystal sensor to sense the acidity of natural and industrial vinegar. The designed sensor is proposed based on a fluid-fluid phononic crystal structure in which the Mercury is selected as the host matrix and water-filled rods are the scatterers. The defect mode is introduced to the band gap by applying a cavity into the perfect phononic crystal, which, in turn, is created by eliminating one rod and modifying the radii of the two-sided rods. The resonant cavity has been constructed in the waveguide in order to introduce a super flat mode in the audible frequency range. It has been shown that by utilizing the finite element method, this sensor is capable of sensing the acidity of the natural and industrial vinegar with high accuracy. The sensitivity of the sensor is super remarkable and reached up to the value of 1275 Hz and consequently, the figure of merit could reach a value of 910. Finally, the sensor has been evolved to a tunable four-channel demultiplexer that could separate neighboring frequencies, each is relevant to a vinegar concentration. Based on the attained results, the proposed sensor can be used for evaluating the quality of many industrial foods and aqueous solutions.