Characterization of an Aspergillus niger for Efficient Fatty Acid Ethyl Ester Synthesis in Aqueous Phase and the Molecular Mechanism.

Youqiang Xu,Chengnan Zhang,Chunsheng Zhang,Huiqin Huang,Hongyun Lu,Weiwei Li,Zhigang Zhao,Mengwei Lin,Mengqin Wu,Baoguo Sun,Xiuting Li

doi:10.3389/fmicb.2021.820380

Abstract

Fatty acid ethyl esters are important flavor chemicals in strong-flavor baijiu. Microorganisms are the main contributors to ester synthesis during baijiu manufacture. However, the ester synthesis was unstable between batches. This was owing to a limited knowledge of the mechanisms for ester synthesis by microorganisms. In this work, a fatty acid ethyl ester synthesizing Aspergillus niger strain CGMCC (China General Microbiological Culture Collection) 3.4309 was identified. The conversion ratios of ethyl valerate, ethyl caproate, ethyl caprylate, and ethyl caprate were 7.87, 29.20, 94.80, and 85.20%, respectively, under the optimized conditions. A comparison of transcriptomes under the initial and optimized ester synthetic conditions indicated that 23 genes were upregulated in transcription level and encoded enzymes with potential abilities for ester synthesis. Eleven of the enzymes were expressed, and three of them, numbered An605, An1097, and An3131, showed the ability to catalyze fatty acid ethyl ester synthesis under aqueous phase, with capric acid as the preferred substrate. The possible enzymatic catalytic mechanism was proposed based on homology modeling and molecular docking. This study reported for the first time that A. niger showed the ability to efficiently catalyze the synthesis of short- and medium-chain fatty acid ethyl esters in aqueous phase, identified the key enzymes, and analyzed the basic enzymatic properties. This is helpful to promote the application of related microorganisms and enzyme resources in the baijiu industry.

Highlights

Chinese traditional fermented baijiu is an important composition of Chinese food (Xu et al, 2022b)
Aspergillus niger for Ester Synthesis processes are still based on an empirical mode, with low efficiency and poor stability between batches (Xu et al, 2022b)
A. niger was generally recognized as an important functional microorganism in baijiu manufacture (Wu et al, 2015)

Summary

Introduction

Chinese traditional fermented baijiu is an important composition of Chinese food (Xu et al, 2022b). Strong-flavor baijiu accounts for more than 70% of the baijiu market share and is a representative of traditional fermented alcoholic beverages (Xu et al, 2021b). With a history of thousands of years, strong-flavor baijiu faces problems in the process of modernization. It is urgent to clarify the mechanism of synthesizing flavor substances in traditional strong-flavor baijiu, so as to ensure the quality of products. This is an inevitable requirement for the modernization of the baijiu industry (Ye et al, 2021)

Methods

Results

Conclusion