Effect of Optical Path Length on Prediction Precision of Calibration Models Developed for Enological Parameters of Rice Wine

Optical path length is one of the most important factors in Beer-Lambert Law which is the basis of nearly all multivariate calibration methods for spectroscopic data. The effect of optical path length on the near infrared (NIR) spectra of rice wine and the consequences on the prediction precision of calibration models developed for measuring total sugar, non-sugar solid, alcohol content, titratable acidity, amino acid nitrogen, and pH was investigated. Samples were scanned in transmission mode using a commercial FT-NIR spectrometer and a demountable liquid cell for versatile path length liquid sampling. Eleven optical path lengths were studied: 0.5 mm, 0.75 mm, 1.0 mm, 1.25 mm, 1.5 mm, 1.75 mm, 2.0 mm, 2.25 mm, 2.5 mm, 2.75 mm, and 3.0 mm. The NIR spectra of rice wine samples in different optical path length show that increasing the optical path length raised the absorption intensity but shortened the wavelength range in which absorbance measurement could be made. And, a linear relationship was observed between the absorption intensity and the optical path length when absorption intensity was lower than 3.0. It could also be observed that optical path length did have influence on the calibration results for the six enological parameters, and the recommended optical path length for total sugar, non-sugar solid alcohol content, titratable acidity, amino acid nitrogen and pH was 0.75 mm, 1 mm, 2 mm, 1.75 mm, 0.5 mm and 2 mm, respectively. The results demonstrated that it was necessary to determine the optimum optical path length to obtain the optimum results for NIR analysis.