Key Takeaways & Executive Findings
- •• • Optimal VFA production at 35 °C: 344.22 mg/g COD, with peak VFA concentration of 14,267 mg/L after 5 days, significantly outperforming 55 °C (57.16 mg/g COD), indicating mesophilic fermentation is economically favorable for industrial acid production. • • Acetic acid constitutes 95.6%–98.9% of VFAs across all temperatures, simplifying downstream purification and making the product suitable as a carbon source for biological nutrient removal. • • Protein and carbohydrate degradation rates are highest at 35–45 °C, while lipid degradation peaks at 55 °C, suggesting temperature-specific substrate targeting for optimized co-digestion strategies. • • Fungal community at peak acid production is dominated by cellulose-degrading genera (e.g., Oligophagozyma at 90.07% at 35 °C), with saprotrophic fungi prevailing, indicating a key role in lignocellulose breakdown.
Abstract
Brewer's spent grain (BSG), a major byproduct of the brewing industry, is produced in large quantities globally, yet its high-value utilization remains limited. This study investigated the effects of fermentation temperature (35, 40, 45, 50, and 55 °C) on volatile fatty acid (VFA) production and fungal community structure during anaerobic fermentation of BSG. Results showed that mesophilic temperatures significantly enhanced VFA yields, with the highest production at 35 °C, reaching 344.22 mg/g COD converted, and a peak VFA concentration of 14,267 mg/L after 5 days. Acetic acid dominated the VFA profile (95.6%–98.9%) across all temperatures. Protein and carbohydrate degradation rates were highest under mesophilic conditions, while lipid degradation peaked at 55 °C. Fungal community analysis revealed that at peak acid production, cellulose-degrading fungi were predominant, with Oligophagozyma being the dominant genus at 35 °C (90.07%) and 40 °C (55.31%). Higher temperatures increased fungal diversity and evenness. Mantel tests indicated that carbohydrates and lipids promoted fungal growth, whereas total dissolved solids, nitrate, and phosphate inhibited it. These findings provide insights into the role of fungi in VFA production from BSG and support its resource utilization.
1. Introduction
Brewer's spent grain (BSG) constitutes approximately 85% of brewing byproducts, with global annual production reaching 38.6 million tonnes. Despite its high lignocellulosic content (70% dry basis) and protein (20%), current utilization is largely limited to low-value animal feed, and regulatory pressures (e.g., FDA guidelines) are restricting even this outlet. The need for high-value valorization is urgent, particularly as a low-cost feedstock for bioproducts. However, direct fermentation of BSG is hindered by its low soluble organic matter content, necessitating pretreatment to release fermentable substrates. Previous studies have shown that acid pretreatment can enhance VFA yields, yet the role of fungi in cellulose degradation during fermentation remains underexplored.
This study addresses the bottleneck by systematically evaluating the impact of fermentation temperature on VFA production and fungal community dynamics from acid-pretreated BSG. By identifying optimal thermal conditions and the associated microbial ecology, we provide a scientific basis for process optimization. The findings demonstrate that mesophilic fermentation at 35 °C maximizes VFA yield and rate, with acetic acid as the dominant product, offering a viable route for producing carbon sources for wastewater treatment. Furthermore, the elucidation of fungal community structure and its correlation with substrate degradation provides mechanistic insights that can guide bioaugmentation strategies to enhance process efficiency.
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HONG Wenfei, XU Yufeng, TONG Juan, WEI Yuansong, CHEN Hexiang (2026). Effect of Fermentation Temperature on Volatile Fatty Acid Production and Fungal Community Structure from Brewer's Spent Grain. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202511094
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Frequently Asked Questions
What is the optimal fermentation temperature for maximizing VFA yield from brewer's spent grain, and what are the corresponding yields?
The optimal temperature is 35 °C, yielding 344.22 mg/g COD (as acetic acid equivalents), with a peak VFA concentration of 14,267 mg/L after 5 days. This is significantly higher than at 55 °C (57.16 mg/g COD), indicating a strong temperature dependence.
How does temperature affect the degradation of major organic components (proteins, carbohydrates, lipids) in brewer's spent grain?
Protein and carbohydrate degradation rates are highest under mesophilic conditions (35–45 °C), while lipid degradation is maximal at 55 °C. This suggests that temperature can be tailored to target specific substrate fractions, but overall VFA production is favored at lower temperatures.
What is the dominant VFA species produced, and what implications does this have for downstream use?
Acetic acid dominates the VFA profile, comprising 95.6%–98.9% of total VFAs across all temperatures. This high purity simplifies recovery and makes the product directly suitable as a carbon source for biological nutrient removal in wastewater treatment.
What are the key fungal genera involved in fermentation, and how does temperature influence fungal community structure?
At peak acid production, cellulose-degrading fungi such as Oligophagozyma dominate, with relative abundances of 90.07% at 35 °C and 55.31% at 40 °C. Higher temperatures increase fungal diversity and evenness, but mesophilic conditions favor a more specialized community that enhances VFA production.
What physicochemical factors influence fungal growth during fermentation, and how can this knowledge be applied?
Carbohydrates and lipids promote fungal growth, while high concentrations of total dissolved solids, nitrate, and phosphate inhibit it. This suggests that controlling these parameters could optimize fungal activity and, consequently, VFA production.
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