Detection of Hydrogen Nucleus Chemical Shift and Quantum Chemical Calculation of Major Components in the Aging Process of Nongxiangxing Base Baijiu

Gas chromatography-mass spectrometry (GC-MS) and X-ray photoelectron spectroscopy (XPS) were employed to qualitatively and quantitatively analyze trace aroma compounds in Nongxiangxing Baijiu samples with different aging periods. In addition, nuclear magnetic resonance (NMR) spectroscopy was used to investigate the correlation between the chemical shift of hydroxyl hydrogen nuclei and the aging time of Nongxiangxing base Baijiu in order to verify the feasibility of using chemical shifts to evaluate the aging period. The results showed that the flavor harmony of the Chinse liquor improved as the aging time increased from 5 to 8 years. As the aging time further increased, the system shifted from a polar to a non-polar nature. The electronic shielding of hydrogen nucleus in the hydroxyl group of ethanol was enhanced, leading to a shift in chemical shift from down field to high field. Meanwhile, the de-shielding effect of hydroxyl hydrogen nuclei in the alcohol-water system increased. At this stage, the Baijiu reached its most stable state and was more suitable for storage. Quantum chemical methods were further applied to study the weak intermolecular interactions between ethanol-water clusters and trace aroma compounds, aiming to explore the causal relationships of electronic density interactions among various functional groups during aging. The calculation results revealed that the order of reaction occurrence was lactic acid > hexanoic acid > butyric acid > acetic acid, and the stability of the resulting esters followed the order of ethyl lactate > ethyl acetate > ethyl hexanoate > ethyl butyrate. These findings indicate that ethyl lactate is more stable than other esters during the aging process of Nongxiangxing Baijiu. Therefore, achieving the goal of increasing hexanoate and reducing lactate is difficult under natural storage conditions and requires intervention in the reaction pathway to promote the formation of ethyl hexanoate and inhibit the accumulation of ethyl lactate.