Using a Flatbed Scanner and Automated Digital Image Analysis To Determine the Total Phenolic Content in Beer

Abstract

Here, students determine the total phenolic content in beers using the Folin–Ciocalteu assay. The Folin–Ciocalteu reagent is a yellow complex, in an alkaline medium; it reacts with phenols and non-phenolic reducing substances to form a blue complex. Quantitative analysis was carried out using absorbance measured at 765 nm (standard method) and digital images (proposed method). In the proposed method, samples were placed in a 96 microwell plate; then, a plate image was obtained with a flatbed scanner. The ImageJ plugin ReadPlate extracts red (R)-values from all wells of the scanned image at the same time; then, R-values were exported to a spreadsheet that converted these values to absorbances. Using absorbances obtained with R-values, the spreadsheet also provides standard calibration plots, limits of detection (LODs), limits of quantification (LOQs), and beer sample total phenolic contents. The proposed method and the standard method were compared using two-tailed significance tests; students observed that both methods provide equivalent results, that both methods were linear in a 2–10 mg L–1 gallic acid concentration range, and that both methods had close LODs and LOQs, 0.55 and 1.70 gallic acid mg L–1, respectively. Beer sample total phenolic contents were compared using one-tailed significance tests. This laboratory experiment provides an effective instrumental alternative to spectrophotometric methods, which can be especially beneficial, where purchasing and maintaining a spectrophotometer is a challenge, and it also presents to the students one-tailed and two-tailed significant tests.