shows the change in CO2 level monitored by colorimetric
mixed-dye-based indicator labels in golden drop at 25 and 10 ◦C.
These findings showed that the minimal spoilage level of golden
drop occurred on days 6 and 28 during storage at 25 and 10 ◦C,
respectively. Indicator labels showed a clear spectrum from bright
light green to orange-redwhenexposed toCO2 during storage time.
Carbon dioxide from headspace dissolved in the filter layer, which
is hydrophilic material, forms carbonic acid in the presence of moisture.
Carbonic acid is diprotic, having two hydrogen atoms which
may dissociate from the parent molecule, forming hydrogen ions
(H+) and bicarbonate ions (HCO3−) with pKa of 6.36 at 25 ◦C [24].
Then, as a proton, a hydrogen ion combines with a water molecule
to form a hydronium ion, H3O+. Hydronium ions react with the
basic form (In−) of the indicator label, resulting in an acid form
(HIn) which in turn produces a color change of the indicator label