Yard-stick of detection in Snyder's test is:
Snyder's test uses Bromocresol green as an indicator, which is blue-green at pH 5.4 or above and turns yellow at pH 3.8 or less. 
The "critical pH" for enamel dissolution is 5.4-5.5.
Therefore, it would be more appropriate to use an indicator such as Bromocresol purple, which is purple at pH 6.8 and yellow at pH 5.2.

Soben Peter 5th edition
Page number  434
Acid formation
pH change
Count of microorganisms
Enamel solubility