Evaluation of ethyl carbamate formation in Luzhou-flavor spirit during distillation and storage processes

Formation and hazard of ethyl carbamate and construction of genetically engineered Saccharomyces cerevisiae strains in Huangjiu (Chinese grain wine)

Huangjiu, a well-known conventional fermented Chinese grain wine, is widely consumed in Asia for its distinct flavor. Trace amounts of ethyl carbamate (EC) may be generated during the fermentation or storage process. The International Agency for Research on Cancer elevated EC to a Class 2A carcinogen, so it is necessary to regulate EC content in Huangjiu. The risk of intake of dietary EC is mainly assessed through the margin of exposure (MOE) recommended by the European Food Safety Authority, with a smaller MOE indicating a higher risk. Interventions are necessary to reduce EC formation. As urea, one of the main precursors of EC formation in Huangjiu, is primarily produced by Saccharomyces cerevisiae through the catabolism of arginine, the construction of dominant engineered fermentation strains is a favorable trend for the future production and application of Huangjiu. This review summarized the formation and carcinogenic mechanism of EC from the perspectives of precursor substances, metabolic pathways after ingestion, and risk assessment. The methods of constructing dominant S. cerevisiae strains in Huangjiu by genetic engineering technology were reviewed, which provided an important theoretical basis for reducing EC content and strengthening practical control of Huangjiu safety, and the future research direction was prospected.

Molecular mechanism of Mare Nectaris and magnetic field on the formation of ethyl carbamate during 19 years aging of Feng-flavor Baijiu

Ethyl carbamate (EC) is carcinogen occurring naturally in fermented foods, while the EC formation pattern in Feng-flavor Baijiu during Mare Nectaris storage and magnetic field treatment remains controversial. In this work, variation of EC in Mare Nectaris and magnetic field were investigated for the first time through ultra high performance liquid chromatography quadrupole-orbitrap high resolution mass spectrometry (UHPLC-Q-Orbitrap). Quantification results revealed that EC decreased significantly in the stage of 3-9 years and kept at 12.4 μg L-1 after 10 years of aging. Arginine succinate synthase (ASS) and urease were deemed as vital factors for EC decomposition. Degradation effetc of EC in 250 mT is simillar to that of EC in Baijiu stored in Mare Nectaris for 8 years. This is due to that aging process was accelerated by magnetic field and the content of total acid in Baijiu was increased, creating a favorable environment for decomposition of EC and urea.

Sequence and Structure-Guided Engineering of Urethanase from Agrobacterium tumefaciens d3 for Improved Catalytic Activity

The amidase from Agrobacterium tumefaciens d3 (AmdA) degrades the carcinogenic ethyl carbamate (EC) in alcoholic beverages. However, its limited catalytic activity hinders practical applications. Here, multiple sequence alignment was first used to predict single variants with improved activity. Afterward, AlphaFold 2 was applied to predict the three-dimensional structure of AmdA and 21 amino acids near the catalytic triad were randomized by saturation mutagenesis. Each of the mutation libraries was then screened, and the improved single variants were combined to obtain the best double variant I97L/G195A that showed a 3.1-fold increase in the urethanase activity and a 1.5-fold increase in ethanol tolerance. MD simulations revealed that the mutations shortened the distance between catalytic residues and the substrate and enhanced the occurrence of a critical hydrogen bond in the catalytic pocket. This study displayed a useful strategy to engineer an amidase for the improvement of urethanase activity, and the variant obtained provided a good candidate for applications in the food industry.

Analysis of Key Genes Responsible for Low Urea Production in Saccharomyces cerevisiae JH301

There is a potential safety risk with ethyl carbamate (EC) in Hongqu Huangjiu production; 90% of the EC in rice wine is produced by the reaction of the urea with the alcohol of Saccharomyces cerevisiae. In our previous experiments, we screened and obtained a S. cerevisiae strain JH301 that offered low urea production. However, the key genes responsible for low urea production of strain JH301 remain unclear. Here, the whole genome sequencing of S. cerevisiae strain JH301 was accomplished via a next-generation high-throughput sequencing and long-read sequencing technology. There are six main pathways related to the urea metabolism of strain JH301 based on KEGG pathway mapping. Three species-specific genes are related to the urea metabolism pathways and were found in comparative genome analysis between strains JH301 and S288c during Hongqu Huangjiu production for the first time. Finally, the ARG80 gene was found to be likely a key gene responsible for low urea production of S. cerevisiae strain JH301, as determined by PCR and qRT-PCR check analyses from DNA and RNA levers. In conclusion, the results are useful for a scientific understanding of the mechanism of low urea production by Saccharomyces cerevisiae during Hongqu Huangjiu fermentation. It also is important to control the urea and EC contents in Hongqu Huangjiu production.

Occurrence of Ethyl Carbamate in Foods and Beverages: Review of the Formation Mechanisms, Advances in Analytical Methods, and Mitigation Strategies

ABSTRACT

Ethyl carbamate (EC) is a process contaminant that can be formed as a by-product during fermentation and processing of foods and beverages. Elevated EC concentrations are primarily associated with distilled spirits, but this compound has also been found at lower concentrations in foods and beverages, including breads, soy sauce, and wine. Evidence from animal studies suggests that EC is a probable human carcinogen. Consequently, several governmental institutions have established allowable limits for EC in the food supply. This review includes EC formation mechanisms, occurrence of EC in the food supply, and EC dietary exposure assessments. Current analytical methods used to detect EC will be covered, in addition to emerging technologies, such as nanosensors and surface-enhanced Raman spectroscopy. Various mitigation methods have been used to maintain EC concentrations below allowable limits, including distillation, enzymatic treatments, and genetic engineering of yeast. More research in this field is needed to refine mitigation strategies and develop methods to rapidly detect EC in the food supply.

HIGHLIGHTS

Effect of fruit thermal processing on ethyl carbamate content in maesil (Prunus mume) liqueur

The thermal effect of maesil on the content of ethyl carbamate and its precursors during one-year ripening of maesil liqueur was investigated. Fresh maesil (control), fruit blanched for 2 min (blanched), and fruit blanched and dried for 15 h at 50 °C (blanched/dried) were soaked in the liquor containing 25% alcohol at a ratio of 1:2 (w/w) for 100 days at 25 °C and the liquid was further ripened for 260 days. Ethyl carbamate ranged from 13.1 to 204.4 μg/kg with the highest value at 210 day. Thermally treated samples had higher ethyl carbamate concentration than the control, suggesting that thermal treatment increased the formation of ethyl carbamate. A positive correlation between ethyl carbamate content and β-glucosidase activity in all samples indicated that enzymatic hydrolysis of amygdalin by β-glucosidase determined ethyl carbamate concentration during the fermentation of maesil liqueur.

Expression, isolation, and identification of an ethanol-resistant ethyl carbamate-degrading amidase from Agrobacterium tumefaciens d3

Ethyl carbamate (EC), widely found in alcoholic beverages, has been revealed to be a probable carcinogen in humans. Urethanase (EC 3.5.1.75) is an effective enzyme for the degradation of EC; however, the previously identified urethanases exhibited insufficient acid and alcohol resistance. In this study, an enantioselective amidase (AmdA) screened from Agrobacterium tumefaciens d₃ exhibited urethanase activity with excellent alcohol resistance. AmdA was first overexpressed in Escherichia coli; however, the recombinant protein was primarily located in inclusion bodies, and thus, co-expression of molecular chaperones was used. The activity of AmdA increased 3.1 fold to 307 U/L, and the specific activity of urethanase with C-terminal His-tags reached 0.62 U/mg after purification through a Ni-NTA column. Subsequently, the enzymatic properties and kinetic constants of AmdA were investigated. The optimum temperature for AmdA was 55 °C, it showed the highest activity at pH 7.5, and the K_m was 0.964 mM. Moreover, after 1 h of heat treatment at 37 °C in a 5-20% (v/v) ethanol solution, the residual urethanase activity was higher than 91%, considerably more than that reported thus far.

Ethyl carbamate in Chinese liquor (Baijiu): presence, analysis, formation, and control

Ethyl carbamate (EC) is a genotoxic and carcinogenic compound that is also a by-product of fermented foods (bread, sour milk, soy cheese, etc.) and alcoholic beverages (wine, sake, distilled liquor, etc.). Studies have showed that ethyl carbamate is ingested by humans primarily through the consumption of alcoholic beverages. Many countries have thus established EC limits for alcoholic beverages. Chinese liquor (Baijiu) is a traditional and unique distilled liquor, which has a huge consumption in China due to its excellent color, flavor, and taste. Therefore, the control of EC in Chinese liquor is of great significance. This review summarized for the first time the progress in presence level, analysis method, formation mechanism, and elimination strategy of EC of Chinese liquor in recent decades. KEY POINTS: • GC-MS and HPLC are the main methods to quantify EC in Chinese liquor. • EC is formed in the fermentation, distillation, and storage stage. • EC content can be reduced from raw material, microorganism, and production process.

Correlation analysis between aroma components and microbial communities in Wuliangye-flavor raw liquor based on HS-SPME/LLME-GC-MS and PLFA

As the main raw material for commercial Wuliangye-flavor liquor, Wuliangye-flavor raw liquor (WFRL) plays an important role in the formation of flavor components. Aroma components and microbial community diversity of 4 WFRLs produced by the same fermenting cellar were investigated by headspace solid phase microextraction, liquid-liquid microextraction combined gas chromatography-mass spectrometry and phospholipid fatty acid fingerprint method. Correlations between aroma components and microbial community of WFRL were disclosed by multivariate statistical analysis techniques. Results indicated that a total of 75 aroma components were identified, including 39 esters, 11 alcohols, 9 acids, 7 aldehyde, 5 ketones and 4 others, whereas esters were dominant for the samples (67.49%). Meanwhile, Gram-positive bacteria (G+) and aerobe in the fermenting cellar increased gradually from top to bottom, compared with Gram-negative bacteria (G-), anaerobe and fungi showed the opposite trend. Furthermore, 4 WFRLs could be distinguished according to 13 differential aroma components (VIP > 1, P < 0.05). Whereas, the differential aroma components were positively correlated with G-, anaerobe and fungi, but negatively correlated with G+ and aerobe. In particular, 4 key esters including ethyl caproate, ethyl acetate, ethyl butyrate and ethyl lactate that determine the grade of WFRL were positively correlated with anaerobe and fungi. Therefore, the results can be used as a reference to understand the correlation between aroma components and microbial communities in different quality grades of WFRL.