Application of Low-Field Nuclear Magnetic Resonance (LF-NMR) Technology in the Study of Flavor Compounds in Aged Yellow Wine

Overview of Flavor Compounds in Aged Huangjiu

Figure 1: Aged Huangjiu

Chinese Huangjiu has a history of over 5,000 years. Renowned for its unique flavour and delicate mouthfeel, it remains one of the most popular alcoholic beverages in East Asia.

The production of Huangjiu generally includes raw material preparation, fermentation and pasteurisation, and ageing. Ageing is an essential step to refine the flavour profile. Fresh Huangjiu often tastes astringent, bland, or bitter. During ageing, a series of physical and chemical reactions occur, resulting in a rich composition of chemical compounds in the matured wine, including volatile flavour compounds, free amino acids (FAA), and organic acids. These changes improve sensory characteristics, enhance flavour and mouthfeel, and increase market value. Both consumers and winemakers desire high-quality Huangjiu within a shorter ageing period.

Understanding the formation mechanisms of flavour compounds that influence the aroma of aged wine is crucial for improving quality and reducing ageing time. Among these, volatile flavour compounds are the most critical to sensory perception.

Research shows that the odour activity values (OAV) of volatile flavour compounds in aged Huangjiu exceed 1, with medium-chain fatty acid esters and aromatic compounds predominating as characteristic flavour compounds.

Research on Flavor Compounds in Aged Huangjiu:

Currently, researchers have studied the key volatile flavour compounds and potential “ageing markers” in Huangjiu. However, the changes in these compounds and the mechanisms of marker formation remain unclear. Volatile flavour compounds exist in trace amounts in Huangjiu, while ethanol and water make up approximately 95% of the composition.

This highlights that the molecular interactions between water and ethanol significantly affect the concentration and formation of dissolved flavour compounds. Thus, monitoring water content in Huangjiu can characterise the molecular interactions between water and dissolved flavour compounds, enabling the study of key flavour changes during the ageing process.

Figure 2: Aged Huangjiu

Time-Domain NMR Principle

Time-domain nuclear magnetic resonance (TD-NMR) relaxation measurement is a rapid, non-destructive method to evaluate physical and chemical changes in food systems.

TD-NMR typically uses hydrogen nuclei as probes, based on measuring relaxation times and signal amplitudes of relaxation spectra under different pulse sequences.

Relaxation times depend on the state of hydrogen protons in the sample. Different states produce different relaxation times. The stronger the molecular mobility, the longer the transverse relaxation time; signal amplitude is proportional to the number of hydrogen protons in the sample. Therefore, relaxation times and signal amplitudes can differentiate components in the sample and quantify their relative proportions.

Figure 3: T₂ Relaxation Spectra of Three Different Water States in a Sample

Application of TD-NMR in Studying Aged Huangjiu Flavor Compounds

Based on TD-NMR principles, hydrogen protons in different states exhibit distinct relaxation times, and the amplitude of relaxation spectra is proportional to the number of stable protons.

In aged Huangjiu, the state and number of stable protons are influenced by molecular interactions between water and dissolved flavour compounds. As ageing progresses, the interaction strength between water and macromolecules changes, affecting proton mobility.

TD-NMR can rapidly measure single-component relaxation times (T2W) to reflect overall distribution in the sample. Multi-exponential fitting provides NMR relaxation behaviour of different components (distribution of transverse relaxation times T2 and relaxation amplitudes), characterising molecular interactions between water and dissolved compounds, thus elucidating how different flavour compounds influence Huangjiu aroma and their mechanisms.

Figure 4: T₂ Relaxation Spectra of Huangjiu Samples of Different Ages

Experimental Results

The single-component relaxation time (T2W) of Huangjiu decreases significantly with longer ageing periods. As ageing progresses, molecular interactions between water and macromolecules strengthen, restricting hydrogen proton mobility and reducing T2W, as shown in Figure 4a.

Three water types were detected across different Huangjiu samples. Relaxation times T21 and amplitudes of bound water showed significant differences among samples (Figure 4b).

With increasing Huangjiu age, the amplitude of semi-mobile water peaks fluctuates slightly. Relaxation time T22 shifts right for wines aged 1–5 years and shifts left for wines older than 5 years (Figure 4c). Free water relaxation behaviour exhibits similar trends (Figure 4d).

Clearly, LF-NMR sensitively reflects differences in hydrogen states caused by Huangjiu ageing. LF-NMR analysis shows promise as a rapid method to determine Huangjiu age.