Smartphone application-based colorimetric fish freshness monitoring using an indicator prepared by rub-coating of red cabbage on paper substrates

This work developed a simple, easy-to-prepare, and inexpensive indicator (R-Paper) based on rub-coating of red cabbage on readily available paper substrates to detect the freshness of fish in out-of-lab settings. A single-step rub-coating method was used to incorporate anthocyanins on paper substrates, without the extraction of the compound from red cabbage. The preparation of the indicator and its usage is quick and does not require additional chemicals, personnel expertise, or laboratory facilities. The functional colorimetric interface created by directly rubbing red cabbage on paper was used for fish quality monitoring, which displayed a naked-eye detectable color change from purple (for fresh fish) to blue (for spoiling fish) and then to blue-green (for spoiled fish), corresponding to total volatile basic nitrogen (TVB-N) and pH changes. To provide a user-friendly quantitative analysis of this color change, we used the free Android software Color Grab to quantify the color using L* , a* , b* , and RGB indices. To the best of our knowledge, this work is the first report on an indicator prepared by rub-coating red cabbage onto paper-based substrates for fish spoilage monitoring through smartphone-based analysis. To verify that the R-Paper indicator performs on a par with the indicators using extracted anthocyanins, its performance was compared with indicators prepared using anthocyanins extracted from red cabbage (AR-Paper). Naked-eye analysis, simple preparation, ease of use, low cost, and free smartphone-based analysis make the R-Paper indicator an appropriate food quality indicator in resource-limited areas.