Development and validation of a 96‑well microplate assay for the measurement of total phenolic content in ginger extracts

A microplate-based assay was developed for the measurement of total phenolic content in polar ginger extracts, using the Folin-Ciocalteu method. The method optimisation process included investigating the efects of wavelength, incubation time and incubation temperature. The method was relatively robust, with most of these parameters having minimal impact on the results obtained. The optimised method could be conducted at room temperature and required only 20 min of combined incubation time. The repeatability of the microplate reader was 0.57%, with the gallic acid standards curve showing good linearity (R2>0.999). The mean intra- and inter-day reproducibility of the entire assay, which included operator error, was 4.4% and 7.8%, respectively. The optimised method was applied to analyse the total phenolic content of 200 ginger extracts in triplicate within approximately 8 h of labour time, fnding a mean total phenolic content of 1713±235 mg GAE 100 g−1 in the dried ginger powder samples. The total phenolic contents of the ginger samples showed a moderate positive correlation (R2=0.46) with the sum of gingerol and 6-shogaol contents, as measured by high-performance liquid chromatography (HPLC). This method could be useful to aid in screening large numbers of samples for total phenolic content in order to identify high-phenolic genotypes or for use in quality assurance settings.