Nondestructive Determination of Nitrate Ion Content of Leaf Stalk of Pak-choi (Brassica campestris L. chinensis group) by Ultraviolet (UV) Spectroscopy

Abstract

The nitrate ion content of vegetables is important from the viewpoint of food safety. There is a weak absorption band (π*←n electric transition) of nitrate ions at 302 nm spanning from 272 nm to 332 nm and this absorption band may be useful for the nondestructive determination of the nitrate ion content of leafy vegetables that have high nitrate ions concentration. Therefore, the objective of this study is to evaluate the potential of nondestructive determination method for the nitrate ion content of the leaf stalk of pak-choi (Brassica campestris L. chinensis group) by ultraviolet (UV) spectroscopy that measures the π*←n electric transition. Partial least squares (PLS) regression analyses of spectra (301-399 nm) of the leaf stalks of pak-choi (number of samples (n) = 108) showed a correlation coefficient of calibration (R cal.) of 0.90 and a standard error of calibration (SEC) of 775 ppm. The calibration was cross-validated and showed a correlation coefficient of validation (R val.) of 0.87 and a standard error of cross-validation (SECV) of 874 ppm. No large difference of more than 3000 ppm between the reference and nondestructively UV-estimated values of nitrate ions was observed in pak-choi samples even at lowest nitrate ion content, those harvested in autumn, as compared with the visible (VIS) - near infrared (NIR) method. The variable importance for projection (VIP) value which indicates the importance of independent variables was highest at 301 nm. The powder of KNO₃ was also measured to confirm its absorption band. On the other hand, no obvious peaks were found in the VIS to short NIR region. Thus, the absorbance at around 301 nm shows that the absorption of nitrate ions contributed markedly to the nondestructive determination.