Developing a unidirectionally permeable edible film based on ĸ-carrageenan and gelatin for visually detecting the freshness of grass carp fillets.

In this study, a unidirectionally permeable film (UPF, the inner layer is a hydrophilic film and outer layer is a hydrophobic film) was applied to the preservation of grass carp fillets for the first time. The hydrophilic film was made of ĸ-carrageenan, gelatin, and curcumin, which was a smart indicator for aquatic product's freshness detection. The hydrophobic film was made of zein. The optimum dosage of UPF were ĸ-carrageenan (1.5 % w/v), gelatin (1.03 % w/v), curcumin (60 mg/100 mL), zein (10.1 % w/v) and glycerol (0.29 % w/w) according to response surface methodology. The results showed that UPF had better mechanical strength and was more resistant to water than hydrophilic films (p < 0.05). Subsequently, UPF was implemented to pack and store fresh grass carp fillets at 4 ℃. With the increase of storage time, UPF detected the change of freshness and informed consumers through its color changes from yellow to red.