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Tyrosol is an important component of pharmaceuticals, nutraceuticals, and cosmetics, and their biosynthetic pathways are currently a hot research topic. d-Erythrose 4-phosphate is a key precursor for the biosynthesis of tyrosol in Saccharomyces cerevisiae. Hence, the flux of d-Erythrose 4-phosphate determined the yield of tyrosol synthesis. In this study, we first obtained an S. cerevisiae strain S19 with a tyrosol yield of 247.66 mg/L by metabolic engineering strategy. To increase the production of d-Erythrose 4-phosphate, highly active phosphoketolase BA-C was obtained by bioinformatics combined with tyrosol yield assay. The key residue sites 183, 217, and 320 were obtained by molecular docking, kinetic simulation, and tyrosol yield verification. After mutation, the highly efficient phosphoketolase BA-CHis320Met was obtained, with a 37.32 % increase in enzyme activity. The tyrosol production of strain S26 with BA-CHis320Arg increased by 43.05 % than strain S25 with BA-C and increased by 151.19 % compared with the strain S19 without phosphoketolase in a 20 L fermenter. The mining and modification of phosphoketolase will provide strong support for the de novo synthesis of aromatic compounds.

Abstract Background The 2-phenylethanol (2-PE) tolerance phenotype is crucial to the production of 2-PE, and Pdr1p mutation can significantly increase the tolerance of 2-PE in Saccharomyces cerevisiae. However, its underlying molecular mechanisms are still unclear, hindering the rational design of superior 2-PE tolerance performance. Results Here, the physiology and biochemistry of the PDR1_862 and 5D strains were analyzed. At 3.5 g/L 2-PE, the ethanol concentration of PDR1_862 decreased by 21%, and the 2-PE production of PDR1_862 increased by 16% than those of 5D strain. Transcriptome analysis showed that at 2-PE stress, Pdr1p mutation increased the expression of genes involved in the Ehrlich pathway. In addition, Pdr1p mutation attenuated sulfur metabolism and enhanced the one-carbon pool by folate to resist 2-PE stress. These metabolic pathways were closely associated with amino acids metabolism. Furthermore, at 3.5 g/L 2-PE, the free amino acids content of PDR1_862 decreased by 31% than that of 5D strain, among the free amino acids, cysteine was key amino acid for the enhancement of 2-PE stress tolerance conferred by Pdr1p mutation. Conclusions The above results indicated that Pdr1p mutation enhanced the Ehrlich pathway to improve 2-PE production of S. cerevisiae, and Pdr1p mutation altered the intracellular amino acids contents, in which cysteine might be a biomarker in response to Pdr1p mutation under 2-PE stress. The findings help to elucidate the molecular mechanisms for 2-PE stress tolerance by Pdr1p mutation in S. cerevisiae, identify key metabolic pathway responsible for 2-PE stress tolerance.

Selenium is an essential trace element for human health. To date, a hotspot of functional foods is strengthening the content of organic Se in food using biological Se enrichment. Herein, Se-enriched rice pancakes were produced by directly adding different sodium selenite concentrations into the fermentation process. The effects of sodium selenite addition on the texture properties, structure, and Se species of rice pancakes were investigated. Meanwhile, the bioaccessibility of Se and the changes of Se species in Se-enriched rice pancakes were determined by digestion experiments in vitro. The results showed significant differences in hardness, adhesiveness, chewiness, porosity, and flavor substances of Se-enriched rice pancakes after adding sodium selenite (p

This study focused on improving the flavor quality of seasonings, and enzymatic hydrolysis of soybean protein isolate (SPI) seasoning via traditional technology may lead to undesirable flavors. Herein, we aimed to develop a new type of SPI seasoning through microbial fermentation to improve its flavor quality. The effect of Corynebacterium glutamicum fermentation on the flavoring compounds of seasonings in SPI enzymatic hydrolysate was examined. Sensory evaluation showed that the SPI seasoning had mainly aromatic and roasted flavor, and the response signals of S18 (aromatic compounds), S24 (alcohols and aldehydes), and S25 (esters and ketones) sensors of the electronic nose differed significantly. Overall, 91 volatile compounds were identified via gas chromatography–mass spectrometry. SPI seasonings contained a higher number of alcohols, ketones, aromatics, and heterocyclic compounds than traditional seasonings, which had stronger cheese, fatty, and roasted aromas. According to the relative odor activity value (ROAV) analysis, n-pentylpyrzine, 2,6-dimethylpyrazine, and tetramethylpyrazine are the key flavoring compounds (ROAV ≥ 1) of SPI seasoning, which may impart a unique roasted and meaty aroma. Therefore, the fermentation of SPI enzymatic hydrolysate with C. glutamicum may improve the flavor quality of its products, providing a new method for the development and production of new seasoning products.

With the aim of improving the processing added value of soybean isolate protein, the single factor and response surface testing method were used to optimize the enzymatic conditions of soybean isolate protein using hydrolysis degree and protein recovery as evaluation indexes. The enzymatic digest of soybean isolate protein fermented by Corynebacterium glutamicum was used to prepare the seasoning, which was evaluated by glutamic acid content and sensory evaluation. The results showed that the optimal enzyme digestion conditions were 2.0 g/100 g of enzyme, pH7.0, 15 g/100 mL of substrate, 54.0 ℃, and 13.0 h. The hydrolysis degree (37.1%) and protein recovery (70.3%) of the optimized enzyme solution were 1.7 times and 0.8 times higher, respectively, than those before the optimization. In the shake flask fermentation conditions, the glutamic acid content of the enzymatic digest fermentation (32.1 g/L) was increased by 32.1% compared to that before optimization. Under the batch replenishment condition of 20 L tank, the glutamic acid yield was (83.6 g/L) 10.0% higher than before optimization. The results of sensory evaluation of the seasonings prepared by fermentation with batch replenishment showed that the freshness of the fermented seasonings was significantly improved (P

In this study, corn meal was fermented by Corynebacterium glutamicum to produce a new natural flavoring materia. Firstly, the enzymatic hydrolysis process of corn meal was optimized by single factor experiment and orthogonal experiment. Then, the fermentation conditions of C. glutamicum GL-6 were optimized at shaking flask level to determine the best nitrogen source and content and the best addition amount of corn meal enzymolysis solution. Then verification was carried out at the level of 20 L fermenter. 5% Maltodextrin was added into the supernatant of fermentation broth and spray dried to obtain the umami sample. Finally, the composition of the sample was detected and evaluated. The results showed that the optimal enzymatic hydrolysis conditions were solid-liquid ratio 1:4, enzyme dosage 1%, temperature 90 ℃, time 3 h, under which the enzymatic hydrolysis degree DE value reached 60.84%. The optimal nitrogen source for glutamate production by C. glutamicum GL-6 fermentation was yeast extract FIG12LS 30 g/L, and the optimal addition amount of corn meal hydrolysate was 50%（v/v）. The content of MJ glutamate was 43.9 g/100 g, and the total content of amino acid was 44.8 g/100 g, which increased by 24.0% and 23.4%, respectively, compared with that of CK without corn meal enzymolysis fermentation. In addition, sample MJ was rich in nutty and coffee-flavored pyrazines. Compared with sample CK, its umami content and nutritional value were improved, and its flavor was richer. This study provides a new idea for the production of umami flavoring by fermentation of natural plant materials and improves the market competitiveness of the product.

Glutamic acid is an important amino acid that is used widely in the fields of food, medicine, and agriculture. One of the methods of glutamic acid production is direct microbial fermentation, so the genetic stability and glutamic-acid-producing capacity of the producing strain are the keys to improving glutamic acid concentration. Experiments were carried out using Corynebacterium glutamicum GL−6 as the parental strain, with two iterations of mutagenesis by atmospheric and room temperature plasma (ARTP) and screening with agar plates tolerant to high sugar and malonic acid, and the best strains with stable phenotypes were verified by fermentation in 20 L tanks. The results show that the optimal mutagenesis time of ARTP was 140 s, with lethality and positive mutation rates of 93.0% and 15.6%, respectively. The concentrations of the high-sugar and malonic acid agar plates were 240 g/L and 35 g/L, respectively. A mutant strain, P−45, with improved glutamic acid production capacity and genetic stability, was obtained through two rounds of iterative mutagenesis screening. The concentration of this strain in the Erlenmeyer flasks was 17.7 g/L, which was 18.8% higher than that of the parental strain, GL−6, and could be inherited stably for 10 generations. In the glutamic acid synthesis pathway, the upregulation of the gene encoding citrate synthase (cs), gene encoding isocitrate dehydrogenase (icdh), and gene encoding glutamate dehydrogenase (gdh), and the downregulation of the gene encoding oxoglutarate dehydrogenase complex (odhc) increased the carbon flows of the TCA cycle and its branch metabolic flow to glutamic acid synthesis. P−45 showed a glutamic acid concentration of 147.0 g/L under fed-batch fermentation conditions in 20 L tanks, which was 81.5% higher than the starting strain, GL−6. This study provides a new technical solution for improving microbial metabolites and genetic stability.