A portable colorimetric tool using a smartphone camera applied for determining total phenolic contents in coffee products

In the present study, a self-designed colorimetric box equipped with a smartphone as an emerging analytical approach was designed to determine the total phenolic contents (TPCs) in coffee. The formed blue complex between the phenolic compounds and Folin-Ciocalteu reagent was measured its color intensities using RGB (red, green, blue) color system. Different parameters related to the photo capturing were investigated, i.e., the distance between the smartphone and sample holder, constant illumination inside the box, and the reaction time. The method performance based on three color channels (R, G, and B) was evaluated and agreed with Appendix F of AOAC (2016). Different sensitivities among three color channels were achieved, and the linear ranges were wide, from 10.0 mg GAE L -1 up to 120.0 mg GAE L -1 for the B channel, which was compatible with various coffee types (GAE: gallic acid equivalent). No statistical difference between the smartphone-based methods and standard Ultraviolet-visible (UV-Vis) was observed by t-test ( P = 95%). Various advantages were obtained, e.g., simplicity, portability, low-cost operation, and easy accessibility for producers and consumers to test for the raw materials, intermediate and final products. The proposed method was applied to several specific roasted ground coffee products from Lam Dong Province (Vietnam) in the context of the limited scientific data on the internal composition of Vietnamese coffee. The results demonstrate that the TPCs depend on coffee varieties, i.e., Robusta exhibits higher TPCs than Arabica, and roasting levels during the processing, i.e., dark mode generally performs the lowest TPCs, due to the thermal decomposition and formation of different phenolic compounds by Millard reactions.