Cellulosic material-based colorimetric films and hydrogels as food freshness indicators

Colorimetric food freshness indicators (CFFIs) are important in food packaging for monitoring the quality of food products during storage and transportation. CFFIs can supplement the conventional “best before date” embedded in food product packages by providing real-time and naked-eye quality information to the stakeholders, thereby ensuring product safety and preventing food waste. Various cellulosic materials have been employed as the matrix/carrier in CFFIs. In addition to their transparency, nontoxicity, high water/dye absorption capacity, unique film-forming property, three-dimensional network formability, superior mechanical properties, easy processability, and widespread availability, cellulosic materials have various functional groups acting as ideal sites for the immobilization of the indicator dye, which play a crucial role in the efficiency of CFFIs. Furthermore, anionic cellulose-derived polymers can act as an electrostatic binding site for positively charged freshness biomarkers, such as total volatile basic nitrogen compounds. This review covers the most-significant peer-reviewed papers on cellulosic material-based CFFIs for meat, fruits, and milk to understand the current research trends and recently developed cellulosic material-based CFFIs. The challenges related to cellulosic material-based CFFIs are presented, including their efficiency and selectivity to other biomarkers and the safety concerns related to certain ingredients, including indicator dyes, crosslinkers, and plasticizers, employed in their fabrication. Solution casting is the most widely used laboratory method for the preparation of cellulosic material-based CFFIs. However, solution casting is not viable for mass production owing to its long drying time. The thickness, dimension, and location of CFFIs inside the food package (their distance from the analytes) affect their colorimetric response. Thus, industrially viable methodologies, such as contact-type CFFIs using innocuous materials such as cellulose and its derivatives doped with food-grade indicator dyes, must be developed. This review shows that cellulosic material-based CFFIs doped with bio-based natural indicator dyes such as anthocyanins (ACNs) offer viable options for intelligent food packaging. Different approaches for improving the sensitivity and reactivity of ACNs are also addressed. Owing to their viability and potential, cellulosic material-based CFFIs will continue to remain an important research topic in the future.