Fig 1.
Schematic illustration of the chemical structures of o-toluidine and vanillin and the condensation reaction between the two compounds.
(a) A light-green color is visible before heating, and (b) a reddish-brown color is visible after heating.
Fig 2.
Absorbance spectra obtained by the UV spectrophotometer.
(A) Spectra of four kinds of acid buffer, (B) spectra of all reagents. (C, E) spectra of standard samples from 1 μg mL−1 to 100 μg mL−1 before heating, (D, F) the absorption spectra of standard samples from 1 μg mL−1 to 100 μg mL−1 after heating.
Table 1.
The absorbance and pH of 50 μg mL−1 vanillin standard samples.
Fig 3.
Absorption spectra of standard vanillin samples obtained by the UV spectrophotometer.
The vanillin concentration ranged from 1 μg mL−1 to 500 μg mL−1. The absorption spectra of the vanillin samples showed that as the vanillin concentration decreased, the absorbance declined.
Fig 4.
Correlation analysis between vanillin concentration and absorbance.
(A) Correlation between vanillin concentration and absorbance from 1 μg mL-1 to 100 μg mL-1 (R2 = 0.9908). (B) R2 = 0.92814 from blank to 107 pg mL-1.
Table 2.
Absorbance of standard vanillin samples.
Fig 5.
Milk powder samples compared with vanillin standard sample (50 μg mL-1).
Table 3.
Measurements of vanillin spiked in milk powder samples (n = 6).