Chlorophenol red

All reagents were Analytical Reagents grade. m-cresol purple (1), o-cresol red (2), bromothymol blue (3), thymol blue (4), chlorophenol red (5), Ellman’s reagent (6) were purchased from Carlo Erba or Sigma Aldrich.
Dylon Colour Catcher^® was bought in a local supermarket.
Beast poultry meat in slices, sliced beef, pork and cod fillets were bought in a local supermarket (UNES Supermarkets, via Fratelli Cervi, 11 27100 Pavia, Italy), the same day of the delivery from the central slaughterhouse or fish market, a few moments after the food was put on the shelf.
A Smartphone Samsung Galaxy S7 was used to acquire pictures of the array; a portable led light box was used to guarantee the reproducibility of the photos. We also used a Puluz Photography Lightbox, Shenzhen Puluz Technology Limited, already shown elsewhere [12 ].
In this study we employed the sensor array already proposed in a previous work for chicken meat degradation monitoring [12 ]; the sensitive part, the solid support and the experimental setup were kept unchanged, but we extended its applicability towards different food samples.

CPR assay media was prepared by adding filter sterilized 1000 mg/l (w/v) Chlorophenol red (Sigma–Aldrich, St. Louis, MO, United States) and 15 mg/l hygromycin-B in liquid half-strength MS (salts and vitamins with 15% (w/v) sucrose, pH 5.6). Surface sterilized leaves (25 mg) were submerged with 5 ml CPR assay media in a 35 mm petri plate, sealed with PARAFILM^TM, kept in culture room conditions for 72 h (Wright et al., 1996 (link)). Finally leaf samples were scored as positive (yellow and orange color) or negative (red and purple color) according to change in color observed and media pH was measured. For each plant 3 biological replications were taken and pH was recorded by 5 reads by the pH meter.