In building a standard response curve, the measurement of
predetermined amounts of free Ca2+ in the aqueous solution by ISE
produced an ideal Nernst response of the electrode to the cation,
yielding nearly a perfect correlation (r = 0.999) in the following
equation: y = 27.25x + 58.78 where y is the EMF and x is the free
calcium concentration in logarithm (Fig. 2). Validation of the ISE
measurement result from the standard aqueous solution was
conducted on six individual samples from the same soymilk
fortified with 1.0 mmol/L CaCl2. The testing showed
0.31 0.01 mmol/L free calcium in unfortified soymilk versus
1.23 0.08 mmol/L in fortified samples. This gave a recovery rate
of 92% which indicated a good repeatability and accuracy. Therefore,
the ISE procedure was considered appropriate for the subsequent
analysis of calcium release by the phytase treatments.