Absorbance measurements were performed with a Spark Cyto or Infinite M200 (Tecan) at 434 and 560 nm with 25 flashes. These two peaks from phenol red are strongly changing during the acidification of the reaction (434 nm absorbance is increased, 560 nm absorbance is decreased). To obtain a good readout of the color change, absorbance at 560 nm was substracted from the one at 434 nm. This difference was denoted ΔOD.