Introduction

Beer freshness is a quality attribute of great concern to consumers and a key indicator for major breweries. Because Chinese beer brewing widely adopts high-gravity and high-adjunct technology, the beer body becomes increasingly light and refreshing, which weakens the masking effect on aging flavors and makes freshness issues more prominent.

Systematic research on beer aging can be traced back to the 1960s. Currently recognized aging mechanisms mainly include:

• Lipid oxidation
• Amino acid degradation
• Oxidation of higher alcohols
• Maillard reaction
• Esterification and lactonization

Generally, aging substances exist below their sensory thresholds, but the combined action of multiple compounds can lead to the perception of aging flavor.

In this study, detectable aging substances are defined as aging indicators, whose concentration changes correlate with the intensity of aging flavor and can indicate the degree of beer aging. According to aging mechanisms and indicative roles, these compounds can be further classified into:

• Oxygen-load indicators
• Thermal-load indicators
• Lipid-oxidation indicators

These are used to evaluate oxygen exposure, heat stress, and raw-material freshness during brewing.

Previous research mainly focused on:

• Detection methods of aging compounds
• Changes during finished beer storage

Studies on changes during the brewing process itself remain limited. With deeper research, shifting the investigation earlier into the brewing process has become a major direction in freshness control research worldwide. However, most reported studies are based on lager beer, while investigations on light-flavor beer brewed with domestic yeast are still lacking.

Therefore, this work selected 18 representative aging compounds with different indicative roles as research targets. Based on established detection methods, their variation trends during the brewing process were explored.

Materials and Methods

1. Materials and Instruments

Aging-related compounds analyzed included:

• Aldehydes: 2-methylpropanal, 2-methylbutanal, 3-methylbutanal, pentanal, hexanal, furfural, methional, phenylacetaldehyde, trans-2-nonenal
• Other aging indicators: furfuryl ethyl ether, acetylfuran, methylfurfural, benzaldehyde, ethyl nicotinate, phenethyl acetate, phenethyl acetate (duplicate OCR), γ-nonalactone, β-damascenone

Reagents and equipment included:

• Ethanol (chromatographic grade, Merck, Germany)
• Ultrapure water system (Millipore, Germany)
• Automatic solid-phase microextraction sampler (CTC, Switzerland)
• SPME fibers (Supelco, USA)
• GC–MS system (PerkinElmer, USA)
• DB-5MS capillary column (Agilent, USA)

2. Determination Methods

2.1 Detection of Aging Aldehydes

Sampling points during brewing:

• Cold wort
• Full tank after yeast pitching
• Main fermentation
• Post-fermentation
• Cold storage
• Finished beer

Detection employed on-fiber derivatization SPME–GC–MS.

Key conditions:

• Helium carrier gas
• Splitless injection
• Programmed temperature rise
• Electron ionization (70 eV)
• Full-scan qualitative analysis + selected-ion quantitative analysis
• External standard calibration using beer matrix

2.2 Integrated Detection of Non-Aldehyde Aging Indicators

Compounds included:

• Furfuryl ethyl ether
• Acetylfuran
• Methylfurfural
• Benzaldehyde
• Ethyl nicotinate
• Phenethyl acetate
• γ-Nonalactone
• β-Damascenone

These represent different aging reaction pathways. Detection also used SPME–GC–MS headspace extraction, with similar chromatographic and mass-spectrometric conditions. Quantification applied external standard calibration with compound-specific ions.

2 Results and Analysis

2.1 Variation Trend of Aging Aldehydes During Brewing

By tracking the industrial brewing process, the contents of aging aldehydes in:

• cold wort
• full tank after yeast pitching
• main fermentation
• post-fermentation
• cold storage
• finished beer

were analyzed.

The brewing conditions were:

• Wort gravity: 13 °P
• Yeast pitching rate: 1.0 × 10⁷ cells/mL
• Main fermentation: 10 days
• Post-fermentation: 12 days
• Cold storage: 13 days
• Finished beer obtained after dilution of cold-stored beer to 8 °P

The comparison of aldehydes between cold wort and full-tank samples is shown in Figure 1, while the variation during fermentation is shown in Figure 2. The chart on page 3 illustrates that aldehydes drop sharply immediately after yeast pitching and remain at very low levels throughout fermentation.

Aldehydes are degradation products of amino acids and are usually regarded as oxygen-load indicators during storage. They may form during:

• malt kilning
• mashing
• wort boiling
• sterilization
• storage

They can also be generated in finished beer via oxidation of higher alcohols.

Compared with wort, all aldehydes decreased to extremely low levels at the full-tank stage. For example:

• 2-methylpropanal
• 2-methylbutanal
• 3-methylbutanal
• methional
• phenylacetaldehyde

all dropped markedly after yeast pitching.

Unsaturated aldehydes such as:

• trans-2-nonenal
• hexanal
• pentanal

originate from unsaturated fatty-acid degradation and are commonly used as raw-material freshness indicators. These compounds may increase during beer storage due to enzymatic and non-enzymatic lipid oxidation.

Furfural, formed via reactions between amino acids and reducing sugars at high temperature, is typically a thermal-load indicator during brewing. Its content decreased significantly from cold wort to the full-tank stage, and total aldehydes also declined sharply.

The sharp decline of aldehydes after yeast pitching is mainly due to yeast reduction of aldehydes to corresponding alcohols during early fermentation. Throughout the remaining fermentation stages, aldehyde levels stayed low, with only slight increases caused by packaging and sterilization before final beer formation.

Yeast vitality and strain characteristics therefore play a decisive role in aldehyde levels and overall beer freshness. Besides reduction ability, yeast also contributes endogenous antioxidants such as:

• sulfur dioxide
• glutathione
• thiol proteins

and can absorb pro-oxidative metal ions like iron and copper.

2.2 Variation Trend of Aging Indicators Detected by the Integrated Method

The variation of non-aldehyde aging indicators during the same brewing process is shown in Figure 3 on page 3, which presents concentration curves across fermentation stages. The diagram indicates that some flavor-related aging indicators increase markedly during fermentation.

Phenethyl acetate increased sharply during fermentation due to rising levels of:

• acetic acid
• phenethyl alcohol

Its fruity aroma represents a positive freshness flavor in beer. After reaching a peak in cold storage, it decreased slightly in finished beer.

γ-Nonalactone, a lactone compound with coconut-like aroma, also rose significantly during fermentation as a result of yeast fatty-acid metabolism. Although not unpleasant itself, its sweetness can reduce the perception of freshness and thus serves as an aging indicator. Synergistic effects between γ-nonalactone and trans-2-nonenal may intensify aging flavor.

β-Damascenone, a carotenoid-derived carbonyl compound with fruity aroma, increased slightly during fermentation and is known to rise further during beer storage.

Benzaldehyde and methylfurfural decreased initially due to yeast reduction but increased again during high-temperature sterilization in packaging.

Other compounds—including:

• furfuryl ethyl ether
• acetylfuran
• phenethyl acetate esters (trace)

showed only minor fluctuations without clear trends. Ethyl nicotinate increased slightly. Yeast cannot metabolize furfuryl ethyl ether; therefore, yeast addition may reduce cardboard-like aging flavors but cannot eliminate solvent-like notes from this compound.

Acetylfuran, a Maillard-reaction product, generally increases during storage and reflects thermal load, though its concentration change during brewing is not obvious.

Further Changes of Aging-Related Compounds During Fermentation

During fermentation, β-damascenone showed a slight increase, rising from a lower initial concentration to a higher level in finished beer. This compound belongs to carotenoid-derived carbonyl compounds and exhibits a fruity aroma; its content is known to increase further during beer storage.

Benzaldehyde and methylfurfural are also aldehyde compounds. Similar to other aldehydes, their concentrations decreased at the early fermentation stage due to yeast reduction. However, during the packaging stage leading to finished beer, high-temperature sterilization caused a significant rise in methylfurfural.

Furfuryl ethyl ether, acetylfuran, and phenethyl acetate esters showed only slight fluctuations during fermentation, with no clear variation trend. Ethyl nicotinate displayed a slight increase. Furfuryl ethyl ether itself has a solvent-like odor, and experiments confirmed that yeast has no transformation ability toward this compound. Therefore, although yeast addition can greatly improve cardboard-like aging flavors (such as trans-2-nonenal), it cannot eliminate the solvent-like note caused by furfuryl ethyl ether.

Ethyl nicotinate and ethyl acetate both belong to ethyl esters. Their concentrations are relatively low during fermentation but may increase significantly during storage due to the presence of abundant ethanol. Acetylfuran, a Maillard-reaction product, gradually increases during beer storage and is directly related to thermal load; however, because of its low concentration, its change trend during brewing is not obvious.

Overall Interpretation of Fermentation Effects on Aging Indicators

In summary, the beer fermentation process is characterized by:

• Extensive yeast reduction of aldehydes, and
• Simultaneous formation of flavor-active aging indicators such as
  • phenethyl acetate
  • β-damascenone
  • γ-nonalactone

The ability of yeast to reduce aldehydes depends on strain characteristics. Under fixed industrial strains, yeast vitality and physiological state directly determine:

• aldehyde reduction capacity
• redox potential of finished beer

In freshness control of beer, besides reducing:

• oxygen load
• thermal load
• transition-metal ion content

yeast management is another core regulatory factor. Any operation affecting yeast vitality will influence:

• aldehyde reduction ability
• production of endogenous antioxidants
• formation of alcohol-ester flavor compounds

Improper yeast management may even lead to yeast autolysis, releasing:

• iron and copper ions (pro-oxidative factors)
• fatty acids (aging precursors)

which accelerate beer aging.

Conclusion

This study employed a GC–MS detection method developed in our laboratory to track the variation of 18 aging-related compounds during industrial brewing and clarified their change trends throughout the process.

Results demonstrated that:

1. Yeast exhibits strong reducing capacity
   • Aldehydes in wort are greatly reduced at the early fermentation stage
   • Their levels remain nearly unchanged during subsequent fermentation
   • Yeast strain vitality decisively affects aldehyde distribution in finished beer
2. Phenethyl acetate, a representative freshness flavor compound,
   • Is produced in large quantities during fermentation
3. γ-Nonalactone and β-damascenone,
   • As yeast metabolic products, increase significantly during fermentation
4. Other detected compounds such as furfuryl ethyl ether
   • Show no clear variation pattern

Overall, the study identifies the critical influence of yeast on beer aging indicators and clarifies the importance of strengthening yeast management, thereby providing strong support for freshness control in industrial beer production.

References

Yin, H., Tian, Y., Hao, J., & Dong, J. (2013). Changes of 18 aging indicator compounds during beer fermentation. Beer Science and Technology, 34(1), xx–xx.