Carbon dioxide colorimetric indicators for food packaging application: Applicability of anthocyanin and poly-lysine mixtures

Microbial spoilage of food is accompanied by a pronounced production of carbon dioxide (CO2). Sensors that visually highlight the presence of CO2 inside the food packages can be thereby used as labels for indicating the freshness of the marketed products. Moreover, the monitoring of the optimal CO2 level is also required under modified-atmosphere packaging (MAP) conditions, being this gas prone to permeate food and package materials. In this study we investigated the CO2 sensing performance of a food grade mixture constituted of an aminoacid (L-Lysine) a polypeptide (ε-poly-l-lysine, EPL) and natural occurring dyes (anthocyanins). Test carried in distilled water showed that the mixture at 0.01% w/v concentration level was very reactive, turning from azure to purple colour in less than five minutes, even when exposed to 2.5% gaseous CO2. A reverse direction transition was also observed for the 0.1% w/v solution when the CO2 levels fall down below 20%. The system was then dissolved in a biopolymeric matrix giving a coating totally constituted of food grade ingredients and usable as colorimetric CO2 indicator for cold preserved food.