Oxygen indicators are used in intelligent food packaging, to monitor the condition of packaged food and give information on its quality and any possible deterioration.
Ultra violet (UV)-activated colorimetric oxygen indicators are not activated until they are irradiated with UV light, which allows in-pack activation and a longer shelf-life.
Alginate is a linear anionic polysaccharide containing blocks of (1,4)-linked β-d-mannuronate (M) and α-l-guluronate (G) residues from brown seaweeds and bacterial species, said the authors.
They added that the high water-resistance is attributed to insoluble complex of the dye with alginate and it has fast colour recovery, unlike using sulfonated polystyrene an encapsulating polymer which was effective but took five days at ambient room temperature to recover its original colour.
Water immersion
When this zein-coated film was immersed in water for 24 hours, the dye leakage was as high as 80.80 ± 0.45%. However, introduction of alginate concentration in 1.25% from 0.25% as the coating polymer considerably diminished the dye leaching to only 5.80 ± 0.06%.
When the zein concentration was increased from 5% to 20%, the thionine leaching into water after 24 hours was hardly diminished.
The oxygen level in the package headspace can increase with time due to poor sealing, air permeation through the package materials, and the package tampered with or damaged during storage and/or transportation.
The researchers said visual oxygen indicators are cheap and enable consumers to detect the presence of oxygen in the food package with naked eyes.
Typical indicators
Typically UV-activated colorimetric oxygen indicators comprise a redox dye (D), a sacrificial electron donor (SED), a UV-absorbing semiconducting photocatalyst (SC), and a coating polymer.
The UV-activated oxygen indicator ink is coated onto the inner side of food package films, and indicates oxygen inside; the ink-coated film loses its colour upon exposure to UV, remains colourless in the absence of O2, and regains original colour in the presence of oxygen in the package.
However, the redox dyes encapsulated in the water-insoluble polymer film (e.g. zein) leaches out when in contact with water which the food may contain.
Indicator ink comprised thionine acetate (40 mg), glycerol (0.6g), and P25 TiO2 (0.6g) as D, SED, and SC, respectively.
Source: Food Chemistry Volume 140, Issues 1–2, 1–15 September 2013, Pages 52–56
Online ahead of print, DOI: 10.1016/j.foodchem.2013.02.056
“Novel water-resistant UV-activated oxygen indicator for intelligent food packaging”
Authors: Chau Hai Thai Vu, Keehoon Won