**5.1 Sulphuric acid-Cysteine-Tryptophan method (Messineo & Edward Musarra,1972)**

Fructose is dehydrated in acid medium for formation of furfural derivative, which complexs with cysteine hydrochloride to produce a chromophore at starting of green color that is unstable. Immediately, this first chromophore formed after react with tryptophan hydrochloride forming now a chromophore of pink colour that has greater stability chemical than the first and stability of 48 hours.

The reaction is also sensitive because can detect a low quantities like 1µg of fructose. The reaction also is specific because aldohexoses, aldopentoses, and ketopentoses do not interfere, however these compounds do not react even if its concentrations are higher, as 5mg.mL-1. The sensitivity of the method is from 1 to 50µg of fructose. A standard solution of fructose is used to build the calibration curve and to get the equation of the linear regression to quantity samples.
