Phenolics are secondary metabolites found in various fruits and vegetables. To estimate polyphenol concentrations, the Folin-Ciocalteu (F-C) reagent can be used to react with any reducing agent such as polyphenols, producing a color change that can be measured with spectrophotometry. When known concentrations of phenolics (e.g., tannic acid) are used to create a standard curve, F-C reagent enables estimation of total phenolic concentration. In this protocol, a high-throughput 96-well microtitre plate reader is used rather than a single-sample spectrophotometer. Methods are based on two publications: (1) Attard, E., 2013. "A rapid microtitre plate Folin-Ciocalteu method for the assessment of polyphenols". Open Life Sciences 8, 607; and (2) Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture. 2000. "Use of nuclear and related techniques to develop simple tannin assays for predicting and improving the safety and efficiency of feeding ruminants on tanniniferous tree foliage".