“…Although colorimetric pH-sensitive sensors are typically not convenient for selective analysis, it is often sufficient to evaluate the food quality based on the change of the pH, as deteriorating proteins produce alkaline volatile nitrogen compounds (cadaverine, putrescine, histamine, and ammonia) (Bulushi et al, 2009;Prester, 2011). Exploiting this indirect sensing mechanism, curcumin based indicator films, e.g., in gelatin (Musso et al, 2017) and bacterial cellulose membranes (Kuswandi et al, 2012), blueberry and red grape skin pomace in chitosan and carboxymethyl cellulose matrix (Kurek et al, 2019), chitosan-corn starch film with red cabbage extract (Silva-Pereira et al, 2015), alginate beads with red cabbage extract (Majdinasab et al, 2018) as well as red cabbage extract in pectin films (Dudnyk et al, 2018) have been shown as feasible indicators of meat, shrimp and fish spoilage by detecting amines and cyclic N-containing compounds. In a similar way, acidic CO 2 evolves during the metabolism of pathogens in the food thus lowering pH, which may be detected, e.g., by anthocyanin/polylysine in cellulose matrix in a reversible manner as demonstrated by Saliu and Pergola (2018).…”