Majority of colorimetric CO2 optical indicators are solventbased
(i.e. non-aqueous) systems, incorporating a phase transfer
agent (PTA) to solubilize the usually lipophilic dye and base
(Mills et al., 1992). However, poor shelf life stability of such
systems is a major drawback when stored under dark and/or
ambient conditions (Mills & Monaf, 1996; Weigl & Wolfbeis,
1995). Thus, the dye in such indicators changes irreversibly to
its acidified form, rendering the indicator ineffective in short
time. Therefore, usually an inert atmosphere is used in packaging,
but with additional cost to production. Interestingly,
water-based CO2 indicators have shown markedly greater lifetimes,
than their solvent-based counterparts. A water-based ink
containing base-sodium hydrogen carbonate and plasticizerglycerol,
having improved operational lifetimes, quick response
and recovery times (2 min), reversible color change on all
tested surfaces, and works under ambient condition has been
reported (Mills & Skinner, 2010). However, sensitivity to
humidity and temperature is needed to be addressed before
realizing potential usage for intelligent food packaging applications.
But, luckily, a fixed temperature (chilled, 4 C) and
unchanged % RH is usually maintained for MAP food packages
and, therefore, water-based CO2 sensors could be used.