Oxygen Indicator is a means of detecting the presence of Oxygen. These indicators are of great importance especially to the food packaging industry. Oxygen is considered as one of the main causes of spoiling of food. Oxygen reacts with food causing oxidative rancidity, making the food foul to smell and harmful to eat. Therefore, its removal is of utmost importance.
There are many established methods for the detection of Oxygen, which includes the Clark electrode  and gas chromatography , however, such methods are way too expensive and time consuming to allow 100% quality assurance. Hence there is an increasing interest in the development of a cheap, easy-to-use and efficient oxygen indicator. One of the alternatives is using a redox-indicator, generally methylene blue, which, in the absence of oxygen, maintains its colourless, chemically reduced, leuco form, on adition of a reducing agent. Whereas in the presence of oxygen, leuco methylene blue is oxidised to a coloured form.
An ideal oxygen indicator should possess the following properties:
‘ Very Inexpensive.
‘ Independent of an expensive instrument for its interrogation.
‘ Long shelf-life.
‘ Tuneable w.r.t oxygen sensitivity.
‘ Must exhibit an irreversible response towards oxygen.
‘ Capable of assessment using the human eye.
Types of Oxygen Indicators
The above mentioned parameters for an ideal oxygen indicator are difficult to achieve in an actual indicator, however listing out the requirements helps one to focus better. There are two categories of Oxygen Indicators namely, Lumophoric or Colorimetric. The former involves measurement of Luminescence intensity whereas the latter works on the observation of change in colour either by an oxygen binding reaction, a redox reaction or a light activated redox reaction.
We are here mainly concerned with the Colorimetric type oxygen indicators that can be further classified on the basis of the reaction mechanism involved.
Colorimetric oxygen binding-based Indicator: The idea comes from the well established reaction wherein de-oxy-haemoglobin is converted to oxy-haemoglobin which has an entirely different colour, on reaction with oxygen. This forms the basis of one of the first reported colorimetric oxygen indicators, consisting of a layer of oxy-haemoglobin, immobilised on a cation exchange resin, positioned at the end of a fibre optic bundle . The indicator was reversible, although it was not found to be very stable and did not last more than two days at room temperature. Moreover it required specific storage conditions which were difficult to be maintained.
Almost a decade later, in 1995, other workers developed a more robust dissolved oxygen indicator based on Myoglobin (Mb) encapsulated in a sol-gel glass matrix . However, the absorption maxima of deoxyMb and oxyMb were seen at 432nm and 418nm respectively, which did not show any specific color changes. Hence, it could come nowhere near ideal as oxygen indicator.
Colorimetric redox dye-based Indicator: Ageless Eye’, produced by the Mitsubishi Gas Company, is a colorimetric redox dye-based indicator which is very commonly used in food packaging.
Figure 1 Typical oxygen scavenger sachet in a food package