Identification and characterization of extracellular enzymes secreted by Aspergillus spp. involved in lipolysis and lipid-antioxidation during katsuobushi fermentation and ripening

Characterization the microbial diversity and metabolites of four varieties of Dry-Cured ham in western Yunnan of China

In this study, high-throughput sequencing and metabolomics analysis were conducted to analyze the microbial and metabolites of dry-cured Sanchuan ham, Laowo ham, Nuodeng ham, and Heqing ham that have fermented for two years produced from western Yunnan China. Results showed that at the genus level, the dominant bacteria in the four types of ham were Halomonas and Staphylococcus, while the dominant fungi were Aspergillus and Yamadazyma. A total 422 different metabolites were identified in four types of ham, mainly amino acids, peptides, fatty acids, and their structural analogs, which were involved in pantothenate and coenzyme A biosynthesis, caffeine, and tyrosine metabolism. The dominant microorganisms of the four types of ham were mainly related to the metabolism of fatty acids and amino acids. This research enhances the identification degree of these four types of dry-cured ham and provides a theoretical basis for developing innovative and distinctive ham products.

Elucidating the formation of the uniform "glass-like" texture in dried-bonito during processing based on microstructure and protein properties

Dried-bonito (Katsuobushi) exhibits a unique uniform "glass-like" texture after traditional smoke-drying. Herein, we developed a novel processing method for dried-bonito and elucidated the mechanism of transformation of loose muscle into a "glass-like" texture in terms of texture, microstructure, and protein properties. Our findings showed that the unfolding and aggregation of proteins after thermal induction was a key factor in shaping the "glass-like" texture in bonito muscle. During processing, myofibrils aggregated, the originally alternating thick and thin filaments contracted laterally and aligned into a straight line, and protein cross-linking increased. Secondary structural analysis revealed a reduction in unstable β-turn content from 26.28% to 15.06%. Additionally, an increase in the content of SS bonds was observed, and the conformation changed from g-g-t to a stable g-g-g conformation, enhanced protein conformational stability. Taken together, our findings provide a theoretical basis for understanding the mechanism of formation of the uniform "glass-like" texture in dried-bonito.

Microbial communities, functional, and flavor differences among three different-colored high-temperature Daqu: A comprehensive metagenomic, physicochemical, and electronic sensory analysis

High-temperature Daqu (HTD) is the starter for producing sauce-flavor Baijiu, with different-colored Daqu (white, yellow, and black) reflecting variations in fermentation chamber conditions, chemical reactions, and associated microbiota. Understanding the relationship between Daqu characteristics and flavor/taste is challenging yet vital for improving Baijiu fermentation. This study utilized metagenomic sequencing, physicochemical analysis, and electronic sensory evaluation to compare three different-colored HTD and their roles in fermentation. Fungi and bacteria dominated the HTD-associated microbiota, with fungi increasing as the fermentation temperature rose. The major fungal genera were Aspergillus (40.17%) and Kroppenstedtia (21.16%), with Aspergillus chevalieri (25.65%) and Kroppenstedtia eburnean (21.07%) as prevalent species. Microbial communities, functionality, and physicochemical properties, particularly taste and flavor, were color-specific in HTD. Interestingly, the microbial communities in different-colored HTDs demonstrated robust functional complementarity. White Daqu exhibited non-significantly higher α-diversity compared to the other two Daqu. It played a crucial role in breaking down substrates such as starch, proteins, hyaluronic acid, and glucan, contributing to flavor precursor synthesis. Yellow Daqu, which experienced intermediate temperature and humidity, demonstrated good esterification capacity and a milder taste profile. Black Daqu efficiently broke down raw materials, especially complex polysaccharides, but had inferior flavor and taste. Notably, large within-group variations in physicochemical quality and microbial composition were observed, highlighting limitations in color-based HTD quality assessment. Water content in HTD was associated with Daqu flavor, implicating its crucial role. This study revealed the complementary roles of the three HTD types in sauce-flavor Baijiu fermentation, providing valuable insights for product enhancement.

Reexamination of Aspergillus cristatus phylogeny in dark tea: Characteristics of the mitochondrial genome

To enhance our understanding of Aspergillus cristatus, an important functional microorganism, the characteristics of its mitochondrial genome were analyzed and compared with related species. The mitochondrial genome of A. cristatus was determined to be 77,649 bp in length, with 15 protein-coding regions. Notably, its length surpassed that of the other species, primarily attributable to the intron length. Gene order exhibited significant variations, with greater conservation observed in the genus Penicillium compared to Aspergillus. Phylogenetic tree analyses indicated that the genera Aspergillus and Penicillium are closely related but monophyletic. Furthermore, the phylogenetic tree constructed based on protein-coding genes effectively distinguished all strains with high branching confidence. This approach provides a robust reflection of the evolutionary relationship between A. cristatus and its related species, offering potential for the development of molecular markers suitable for Aspergillus and Penicillium.

Rational and random mutagenesis of GDEst-95 carboxylesterase: New functionality insights

Lipolytic enzymes are important contributors in industrial processes from lipid hydrolysis to biofuel production or even polyester biodegradation. While these enzymes can be used in numerous applications, the genotype-phenotype space of certain promising enzymes is still poorly explored. This limits the effective application of such biocatalysts. In this work the genotype space of a 55 kDa carboxylesterase GDEst-95 from Geobacillus sp. 95 was explored using site-directed mutagenesis and directed evolution methods. In this study four site-directed mutants (Gly108Arg, Ala410Arg, Leu226Arg, Leu411Ala) were created based on previous analysis of GDEst-95 carboxylesterase. Error-prone PCR resulted three mutants: two of them with distal mutations: GDEst-RM1 (Arg75Gln), GDEst-RM2 (Gly20Ser Arg75Gln) and the third, GDEst-RM3, with a distal (Ser210Gly) and Tyr317Ala (amino acid position near to the active site) mutation. Mutants with Ala substitution displayed approximately twofold higher specific activity. Arg mutations lead a reduced specific activity, retaining 2.86 % (Gly108Arg), 10.95 % (Ala410Arg), and 44.23 % (Leu226Arg) of lipolytic activity. All three random mutants displayed increased specific activity as well as improved catalytic properties. This research provides the first deeper insights into the functionality of understudied Geobacillus spp. carboxylesterases with 55 kDa in size.

Sensomics-Assisted Characterization of Fungal-Flowery Aroma Components in Fermented Tea Using Eurotium cristatum

Fermented tea (FT) using a single Eurotium cristatum strain can produce a pleasant fungal-flowery aroma, which is similar to the composite aroma characteristic of minty, flowery, and woody aromas, but its molecular basis is not yet clear. In this study, solvent-assisted flavor evaporation and gas chromatography-mass spectrometry/olfactometry were applied to isolate and identify volatiles from the FT by E. cristatum. The application of an aroma extract dilution analysis screened out 43 aroma-active compounds. Quantification revealed that there were 11 odorants with high odor threshold concentrations. Recombination and omission tests revealed that nonanal, methyl salicylate, decanoic acid, 4-methoxybenzaldehyde, α-terpineol, phenylacetaldehyde, and coumarin were the major odorants in the FT. Addition tests further verified that methyl salicylate, 4-methoxybenzaldehyde, and coumarin were the key odorants for fungal-flowery aroma, each corresponding to minty, woody, and flowery aromas, respectively. 4-Methoxybenzaldehyde and coumarin were newly found odorants for fungal-flowery aroma in FT, and 4-methoxybenzaldehyde had not been reported as a tea volatile compound before. This finding may guide future industrial production optimization of FT with improved flavor.

The potentials and challenges of using fermentation to improve the sensory quality of plant-based meat analogs

Despite the advancements made in improving the quality of plant-based meat substitutes, more work needs to be done to match the texture, appearance, and flavor of real meat. This review aims to cover the sensory quality constraints of plant-based meat analogs and provides fermentation as a sustainable approach to push these boundaries. Plant-based meat analogs have been observed to have weak and soft textural quality, poor mouth feel, an unstable color, and unpleasant and beany flavors in some cases, necessitating the search for efficient novel technologies. A wide range of microorganisms, including bacteria such as Lactobacillus acidophilus and Lactiplantibacillus plantarum, as well as fungi like Fusarium venenatum and Neurospora intermedia, have improved the product texture to mimic fibrous meat structures. Additionally, the chewiness and hardness of the resulting meat analogs have been further improved through the use of Bacillus subtilis. However, excessive fermentation may result in a decrease in the final product's firmness and produce a slimy texture. Similarly, several microbial metabolites can mimic the color and flavor of meat, with some concerns. It appears that fermentation is a promising approach to modulating the sensory profiles of plant-derived meat ingredients without adverse consequences. In addition, the technology of starter cultures can be optimized and introduced as a new strategy to enhance the organoleptic properties of plant-based meat while still meeting the needs of an expanding and sustainable economy.

Transforming the fermented fish landscape: Microbiota enable novel, safe, flavorful, and healthy products for modern consumers

Regular consumption of fish promotes sustainable health while reducing negative environmental impacts. Fermentation has long been used for preserving perishable foods, including fish. Fermented fish products are popular consumer foods of historical and cultural significance owing to their abundant essential nutrients and distinct flavor. This review discusses the recent scientific progress on fermented fish, especially the involved flavor formation processes, microbial metabolic activities, and interconnected biochemical pathways (e.g., enzymatic/non-enzymatic reactions associated with lipids, proteins, and their interactions). The multiple roles of fermentation in preservation of fish, development of desirable flavors, and production of health-promoting nutrients and bioactive substances are also discussed. Finally, prospects for further studies on fermented fish are proposed, including the need of monitoring microorganisms, along with the precise control of a fermentation process to transform the traditional fermented fish to novel, flavorful, healthy, and affordable products for modern consumers. Microbial-enabled innovative fermented fish products that consider both flavor and health benefits are expected to become a significant segment in global food markets. The integration of multi-omics technologies, biotechnology-based approaches (including synthetic biology and metabolic engineering) and sensory and consumer sciences, is crucial for technological innovations related to fermented fish. The findings of this review will provide guidance on future development of new or improved fermented fish products through regulating microbial metabolic processes and enzymatic activities.

Heterologous expression and characterization of salt-tolerant β-glucosidase from xerophilic Aspergillus chevalieri for hydrolysis of marine biomass

A salt-tolerant exo-β-1,3-glucosidase (BGL_MK86) was cloned from the xerophilic mold Aspergillus chevalieri MK86 and heterologously expressed in A. oryzae. Phylogenetic analysis suggests that BGL_MK86 belongs to glycoside hydrolase family 5 (aryl-phospho-β-D-glucosidase, BglC), and exhibits D-glucose tolerance. Recombinant BGL_MK86 (rBGL_MK86) exhibited 100-fold higher expression than native BGL_MK86. rBGL_MK86 was active over a wide range of NaCl concentrations [0%-18% (w/v)] and showed increased substrate affinity for p-nitrophenyl-β-D-glucopyranoside (pNPBG) and turnover number (kcat) in the presence of NaCl. The enzyme was stable over a broad pH range (5.5-9.5). The optimum reaction pH and temperature for hydrolysis of pNPBG were 5.5 and 45 °C, respectively. rBGL_MK86 acted on the β-1,3-linked glucose dimer laminaribiose, but not β-1,4-linked or β-1,6-linked glucose dimers (cellobiose or gentiobiose). It showed tenfold higher activity toward laminarin (a linear polymer of β-1,3 glucan) from Laminaria digitata than laminarin (β-1,3/β-1,6 glucan) from Eisenia bicyclis, likely due to its inability to act on β-1,6-linked glucose residues. The β-glucosidase retained hydrolytic activity toward crude laminarin preparations from marine biomass in moderately high salt concentrations. These properties indicate wide potential applications of this enzyme in saccharification of salt-bearing marine biomass.

Improvement in salt-tolerance of Aspergillus oryzae γ-glutamyl transpeptidase via protein chimerization with Aspergillus sydowii homolog

γ-Glutamyl transpeptidase is one of the key enzymes involved in glutamate production during high-salt fermentation of soy sauce and miso by koji mold, Aspergillus oryzae. However, the activity of γ-glutamyl transpeptidase from A. oryzae (AOggtA) is markedly reduced in the presence of NaCl, thus classifying it as a non-salt-tolerant enzyme. In contrast, the homologous protein from the xerophilic mold, A. sydowii (ASggtA) maintains its activity under high-salt conditions. Therefore, in this study, a chimeric enzyme, ASAOggtA, was designed and engineered to improve salt-tolerance in AOggtA by swapping the N-terminal region, based on sequence and structure comparisons between salt-tolerant ASggtA and non-salt-tolerant AOggtA. The parental AOggtA and ASggtA and their chimera, ASAOggtA, were heterologously expressed in A. oryzae and purified. The chimeric enzyme inherited the superior activity and stability from each of the two parent enzymes. ASAOggtA showed > 2-fold greater tolerance than AOggtA in the presence of 18% NaCl. In addition, the chimera showed a broader range of pH stability and greater thermostability than ASggtA. AOggtA and ASAOggtA were sy over the range pH 3.0 to pH 10.5. Thermal stability was found to be in the order AOggtA (57.5 °C, t_1/2 = 32.5 min) > ASAOggtA (55 °C, t_1/2 = 20.5 min) > ASggtA (50 °C, t_1/2 = 12.5 min). The catalytic and structural characteristics indicated that non-salt-tolerant AOggtA would not undergo irreversible structural changes in the presence of NaCl, but rather a temporary conformational change, which might result in reducing the substrate binding and catalytic activity, on the basis of kinetic properties. In addition, the chimeric enzyme showed hydrolytic activity toward L-glutamine that was as high as that of AOggtA. The newly-designed chimeric ASAOggtA might have potential applications in high-salt fermentation, such as miso and shoyu, to increase the content of the umami-flavor amino acid, L-glutamate.

Integrated the embedding delivery system and targeted oxygen scavenger enhances free radical scavenging capacity

World trends in oil crop growing area, yield, and production over the last 10 years exhibited an increase of 48 %, 82 %, and 240 %, respectively. Concerning reduced shelf-life of oil-containing food products caused by oil oxidation and the demand for sensory quality of oil, the development of methods the improvement oil quality is urgently required. This critical review presented a concise overview of the recent literature related to the inhibition ways of oil oxidation. The mechanism of different antioxidants and nanoparticle delivery systems on oil oxidation was also explored. The current review provides scientific findings on control strategies: (i) design oxidation quality assessment model; (ii) packaging by antioxidant coatings and eco-friendly film nanocomposite: ameliorate physicochemical properties; (iii) molecular investigations on inhibitory effects of selected antioxidants and underlying mechanisms; (iv) explore the interrelationship between the cysteine/citric acid and lipoxygenase pathway in the progression of oxidative/fragmentation degradation of unsaturated fatty acid chains.

Isolation and characterization of a salt-tolerant γ-glutamyl transpeptidase from xerophilic Aspergillus sydowii

Xerophilic Aspergillus molds isolated from halo-alkaliphilic and dry environments are attractive genetic resources for obtaining salt- and osmo-adaptive enzymes. A. sydowii MA0196 secreted the largest amount of γ-glutamyl transpeptidase (GGT) during solid-state fermentation at a low initial water activity (a w = 0.85). Gel filtration analysis revealed that the molecular mass of the purified native enzyme (MA0196 GGT) was 120 kDa. SDS-PAGE analysis showed that MA0196 GGT consists of two subunits with molecular masses of 56.4 and 33 kDa, indicating production from a proenzyme via autoproteolysis. Deglycosylation of the subunits by N-glycosidase F yielded 40.9 and 19.6 kDa species. MA0196 GGT retained transpeptidase and hydrolysis activities and their catalytic efficiency (k cat/K m) under high salt and low water activity. The enzyme displayed broad substrate specificity toward γ-glutamyl acceptors such as amino acids and the imidazole dipeptides, carnosine and anserine. Carnosine and L-glutamine were converted into γ-glutamyl-β-alanyl-L-histidine by MA0196 GGT with a 32.9% yield in the presence of 2% (v/v) dimethyl sulfoxide. Phylogenetic analysis indicated that MA0196 GGT forms a distinct lineage from A. oryzae and A. sojae GGTs. These excellent properties indicate that MA0196 GGT can be used in salted fermentation and for producing bioactive peptides.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03259-3.