UV-activated oxygen indicators have the advantages of in-pack activation and irreversibility; however, these dye-based oxygen indicator films suffer from dye leaching upon contact with water.
Typically, UV-activated oxygen indicator ink is spread onto the food contact side of the package films and then encapsulated in a polymer film.
Researchers introduced carrageenans to develop UV-activated colorimetric oxygen indicator films resistant to dye leakage.
Carrageenans are natural sulfated polysaccharides extracted from edible red seaweed used in the food industry as gelling and thickening agents.
Carrageenan-based indicator films were fabricated using redox dyes (methylene blue (MB), azure A, and thionine), a sacrificial electron donor (glycerol), a UV-absorbing photocatalyst (Titanium Dioxide, TiO2), and an encapsulation polymer (carrageenan).
The dyebinding ability of carrageenans lowered the leakage into water of all the redox dyes tested.
Using it as an encapsulating polymer at a concentration of 0.2% only 2.1% of MB on the film leached into the water, found the study.
The MB/TiO2/glycerol/carrageenan oxygen indicator film was bleached upon UV irradiation, and it regained colour rapidly in the presence of oxygen compared to other waterproof oxygen indicator films.
“We prepared UV-activated oxygen indicator films using carrageenan as the coating polymer; the packaging film was spin-coated with ink composed of redox dyes, glycerol, and TiO2 and then dip-coated with carrageenan solutions,” said the researchers.
As a comparison a zein-based film was fabricated in the same way. However, little dye remained on the film because they had dissolved into the aqueous zein solution during the dip-coating step.
So the indicator films using zein as the encapsulation polymer were prepared by spin-coating the packaging film with the ink comprising redox dyes, glycerol, TiO2, and zein.
Prevention of other dyes
Researchers examined whether carrageenan can also prevent dyes other than MB from leaching from the film into water.
The carrageenan (0.2%)-based oxygen indicator films were prepared with azure A (AA) and thionine (Th) in the same way and diminished the leakage to a greater degree than the zein-based comparison due to the electrostatic interaction of the anionic carrageenan with the cationic dyes.
The oxygen indicator film was recovered within about eight hours, he colorless reduced form of MB on the film was re-oxidized by oxygen, and the original blue color of the indicator film was restored.
Recovery time may not be satisfactory compared to conventional water susceptible indicator films (under 6 min), but it is shorter than other waterproof films of five days using sulfonated polystyrene and 2.5 days using low-density polyethylene.
“This leaching-resistant colorimetric oxygen indicator film with in-pack activation and irreversibility can be applied to verify that all oxygen is removed using oxygen absorbers/ oxygen scavengers, and it can also be used as a seal and leak detector with modified atmosphere packaging; it will be essential for intelligent food packaging,” said the researchers.
Source: Journal of Agricultural and Food Chemistry
Online ahead of print, DOI: 10.1021/jf5014764
“Leaching-Resistant Carrageenan-Based Colorimetric Oxygen Indicator Films for Intelligent Food Packaging”
Authors: Chau Hai Thai Vu and Keehoon Won