INTELLIGENT PH INDICATORS BASED ON BLACK BEAN HUSKS

Anthocyanins are natural pigments found in various fruits, vegetables and leaves.  Bean husks can be a by-product of bean processing and they have a high anthocyanin content. These pigments change color when exposed to different pH values ​​and may have potential use as real-time freshness indicators. This work aims to evaluate the potential of using anthocyanins extracted from black bean husks as intelligent pH indicators. Husks from commercial samples of black beans were ground in a blender. The samples were mixed with 85% ethanol solvent in a proportion of 1:10 (w/v) in a vortex for 5 minutes. The solution was centrifuged at 4000 rpm for 30 minutes at 20 °C,  the supernatant was transferred to a flask and kept under agitation at 40ºC for 2 hours. The extract solution was filtered and subjected to a UV-vis spectrophotometer in pH values from 2.0– 10.0 and measured in the range of 400– 700 nm. Anthocyanin pigments undergo reversible structural transformations with a change in pH manifested by different absorption spectra.  Between pH 2 and 7, the maximum absorption wavelength showed little change, but the absorption intensity decreased significantly with changing pH,  from 4.054 to 0.963. The highest absorbance values ​​were found in the wavelength range between 510 -532.5 nm at very acidic pHs (pH 2-3), indicating the presence of the flavylium cation, great stability and tending to an intense red color. At basic pH (8-10), the quinoid form is predominant, with peaks between 581-627 nm and a gradual increase in absorbance. The values ​​are typical of anthocyanins of the delphinidin-3-glucoside type, common in black beans. Anthocyanins extracted from black bean husks have great potential as a low-cost, naturally-derived alternative for intelligent pH indicators, covering the range of 2 to 10.