Prediction of the concentrations of ethanol and acetic acid in the culture broth of a rice vinegar fermentation using near-infrared spectroscopy

Abstract

Near-infrared spectroscopy (NIR), which is a nondestructive analytical technique, was employed for the simultaneous prediction of the concentrations of ethanol and acetic acid in the culture broth sampled from a rice vinegar fermentation. The broth was placed in a near-infrared spectrophotometer and the absorbance at wavelengths between 400 and 2,500 nm was measured at 2 nm intervals. To obtain calibration equations, multiple linear regression (MLR) was conducted on the NIR spectral data and on the ethanol and acetic acid concentrations obtained by gas chromatography of a calibration sample set. The value of the multiple correlation coefficient ( R) was 0.999 when using the wavelengths of 1,686 and 1,738 nm for ethanol. The value of R for acetic acid was 0.940 when using the wavelengths of 1,674 and 1,718 nm. To validate the calibration equations obtained, ethanol and acetic acid concentrations in a prediction sample set which was not used for calibration were calculated using the calibration equation, and compared with the concentrations measured by gas chromatography. Excellent agreement between the results of the conventional method and those of NIR was observed for both constituents. The concentrations of ethanol and acetic acid in the culture broth could be analyzed simultaneously by NIR. The procedure of NIR was simple, and the operation time required to predict the concentrations was only 5 min. These results indicate that NIR may be a useful method for the monitoring and control of rice vinegar fermentation.