Simultaneous Analysis of the Stable Carbon Isotope Ratios of Acetoin and Acetic Acid by GC-C-IRMS for Adulteration Detection in Brewed Rice Vinegar Products

In this study, we developed a method to simultaneously measure the stable carbon isotope ratio for acetic acid (δ ¹³C_acetic acid) and acetoin (δ¹³C_acetoin) in rice vinegar by gas chromatography-combustion-isotope ratio mass spectrometry. The method showed good precision and accuracy. With this method, data from 16 brewed rice vinegars and 10 acetic acid samples were used to evaluate the feasibility of adulteration detection. On the basis that all δ¹³C_acetoin values of brewed rice vinegars are nearly constant, a characteristic pattern of the stable carbon isotope in rice vinegar was built with the 95% confidence intervals for δ¹³C_acetic acid (−26.97 to −25.38‰), δ¹³C_acetoin (−28.14 to −27.09‰), and Δδ¹³C (0.61 to 2.27‰). An adulteration detection curve of Δδ¹³C was proposed based on the results of vinegar and acetic acid samples and confirmed by vinegar spiked with different amounts of acetic acid. This method could be useful in estimating the blending ratio of adulterated rice vinegar products. Products containing more than 10% of synthetic acetic acid could be possibly identified.