Amino Acid Nitrogen Research Articles

Efficient ammonia oxidation by Pseudomonas citronellolis strain YN-21 under strongly acidic conditions: Performance and mechanism

Ammonia oxidation microorganisms generally tend to have low rates of ammonia oxidation under acidic conditions, as the protonated ammonia is not a substrate for ammonia monooxygenase. In this work, heterotrophic ammonia oxidation bacteria (HAOB) Pseudomonas citronellolis strain YN-21 showed high efficiency in removing NH4+ (12.7 mg/L/h) even at initial pH 4.5. The potential acid resistance mechanisms (H+ efflux, H+ consumption, and production of alkaline substances) maintained intracellular pH neutrality. Transcriptome analysis showed that genes involved in amino acid metabolism, carbohydrate metabolism, ABC transporter and nitrogen metabolism were significantly up-regulated, which facilitated the rapid removal of NH4+ in an acidic environment. Moreover, urea could be used as an alternative nitrogen source for YN-21 in a strongly acidic environment, and the production of NH3 from urea hydrolysis provided a substrate for ammonia oxidation. These results provide new insights into efficient ammonia oxidation in acidic environments.

Characterization and quantification of the taste profiles of black garlic via a novel multi-channel colorimetric sensor array and chemometrics

Taste profiles are crucial for the market success of black garlic, yet investigations into its physicochemical constituents closely related to taste quality remain limited. Additionally, the development of prediction models for taste sensory quality has not been extensively explored. This study aims to develop a cost-effective colorimetric sensor array (CSA) capable of simultaneously and quantitatively predicting the key physicochemical constituents and taste sensory quality of black garlic, integrated with chemometric algorithms. A multi-channel taste visualization sensor array was designed based on pH indicator color changes, indicator displacement assay (IDA), and silver nanoparticles. To establish quantitative prediction models, chemometric algorithms including partial least squares regression (PLSR) and support vector machine regression (SVR) were employed. The results revealed that nonlinear SVR models outperformed linear PLSR models in predicting reducing sugars, amino acid nitrogen, total acid, and the taste sensory attribute, achieving correlation coefficients for prediction (Rp) of 0.9863, 0.9232, 0.9666, and 0.9170, respectively. These findings highlighted the potential of integrating CSA with appropriate chemometric strategies as a reliable and promising approach for monitoring dynamic changes in key physicochemical constituents during black garlic processing and assessing the taste sensory quality of black garlic and similar food matrices.

Root morphology, nitrogen metabolism and amino acid metabolism in soybean under low phosphorus stress.

Phosphorus deficiency is a major influence on growth and development of soybean. Therefore, improving phosphorus utilization efficiency in soybean is a research priority for the soybean community. In this experiment, Liaodou 13 (high phosphorus utilization: HPE) and Tiefeng 3 (low phosphorus utilization: LPE) were used as test varieties. We investigated changes in root morphology, amino acid content, and content of key substances of the nitrogen metabolic pathway with normal phosphorus (0.5 mM) and low phosphorus (0.005 mM) treatments. The results showed that the root length, root surface area and number of lateral roots of HPE roots were higher than those of LPE roots under normal and low phosphorus conditions. The contents of different types of amino acids showed different trends in two varieties. The HPE showed small change in the content of total hydrolyzed amino acids under the low phosphorus condition when compared to the normal phosphorus treatment by a 6.67% decrease, on the contrary LPE showed a drastic decrease by 20.36%. However, HPE exhibited similar decreasing trends in the contents of hydrolyzed and free aspartic acid with the low phosphorus treatment. Moreover, the contents of free histidine and valine in LPE were significantly increased by 657.84% and 149.29% respectively, in contrast to significant decreases in HPE. In aspects of major nutrient elements, the contents of phosphorus, total nitrogen and ammonia nitrogen in both HPE and LPE varieties decreased to dramatic levels. However, the nitrate nitrogen content significantly increased 78.51% for HPE and 65.12% for LPE. Compare to the normal condition, the GOGAT activity in HPE decreased by 5.18% but increased by 33.10% in LPE. Compare to the normal condition, the GS activity in HPE increased by 7.26% but decreased by 21.72% in LPE under phosphorus deficiency. In summary, the phosphorus-efficient soybean variety HPE exhibited superior tolerance to low phosphorus deficiency through advantageous root morphology, phosphorus uptake and transfer capability, and balanced amino acid metabolism and nitrogen metabolism pathways.

Comprehensive analysis of bacterial succession and flavor components in Xiguajiang with different watermelon juice supplementation during fermentation

This study investigated the physicochemical factors, bacterial communities, and volatile aromatic components in watermelon soybean pastes (WSP) prepared with different ratios of different watermelon juice to koji during the fermentation process. The results indicated that high concentration of watermelon juice could significantly increase the contents of amino acid nitrogen, reducing sugar, and free amino acids, but decreased bacterial diversity. Enterococcus, Bacillus, Enterobacter, Staphylococcus, Pediococcus, Lactobacillus, and Weissella were identified as the dominant genus bacteria in WSP samples, while Enterococcus and Pediococcus were found to account for more than 50% in WSP-4 samples. In WSP samples, this study identified 142 volatile components, the concentration of volatile components was more abundant in WSP-4 samples compared to WSP-2 and WSP-3 samples, especially in the pyrazine family. Furthermore, E-nose results showed that the influence of watermelon juice on the aroma components of WSP samples was mainly focused on the late fermentation periods, but aroma profiles were relatively similar in all WSP samples regardless of watermelon juice concentration throughout the fermentation period. Therefore, this study discovered that controlling the amount of watermelon juice and fermentation time can affect the aroma profiles of WSP samples to a certain extent, and improve the industrial production and food quality.

Perturbations in a pelagic food web during the NE pacific large marine heatwave and persistent harmful diatom blooms

Unprecedented warm ocean conditions, driven by the Large Marine Heatwave (LMH) and the 2015–16 El Niño in the Northeast Pacific favored pervasive toxigenic Pseudo-nitzschia spp. blooms that caused widespread ecological impacts, but little is known about the magnitude to which marine food webs were altered. Here, we assessed the trophic transfer of domoic acid (DA; a neurotoxin) and changes in trophic position from multiple key species during the peak of the LMH and El Niño in 2015 in comparison with 2018, a reference non-anomalous warm year. DA and amino acid nitrogen isotopes (δ15N AAs) were quantified using liquid and gas chromatography mass spectrometry, respectively. Our integrative approach revealed extremely high levels of DA in anchovy viscera (>3000 μg/g) with contrasting baseline values (δ15N Phe) for southern California fish. These results together with data from northern CA revealed an unforeseen latitudinal isotopic variation in key DA vectors along California, possibly driven by anomalous restructuring of water masses. At the regional level, the observed cross-shore differences in baseline isotope values and DA toxicity suggest distinct pathways for DA trophic transfer for nearshore vs. offshore sites. Given the high levels of environmental disturbance during the LMH and persistence of toxigenic P. australis blooms, our resultant higher trophic position proxies in 2015 compared to 2018 were particularly unexpected. Such results highlight complex trophic interactions, where the trophic status of some species increased while others decreased in response to changes in net primary productivity, and biodiversity, and abundance of forage species. Our study demonstrates the use of δ15N AAs to identify pathways of N and DA trophic transfer and to quantify shifts in animal trophic position, a critical facet of understanding the response of food webs to climate change and DA production.

Effects of exogenous nitrogen addition on soil organic nitrogen fractions in different fertility soils: Result from a 15N cross-labeling experiment

Exogenous nitrogen (N) addition serves as a pivotal nutrient management strategy, significantly enhancing agricultural production by regulating soil N availability and retention. However, the dynamics of soil organic nitrogen (SON) fractions in response to various forms of exogenous N addition across differing soil fertility levels remain inadequately understood. This study utilized data from a 25-year fertilization experiment and a 15N cross-labeling experiment in Northeast China to assess and quantify the effects of mineral N fertilizers and organic materials (manure and straw) on SON fractions in NPK (mineral fertilizer addition) and NPKM (NPK combined with composted pig manure) treatments. Our findings indicate that long-term incorporation of manure substantially elevates soil fertility compared to the exclusive use of mineral fertilizers. Notably, exogenous N primarily boosts soil N availability by enhancing acid-soluble organic N fractions, particularly ammonium nitrogen (AN) and amino acid nitrogen (AAN). Organic materials, particularly straw, significantly enhanced the retention of mineral fertilizer N in both NPK and NPKM treatments (9.54 % vs 10.70 %). Moreover, over 70 % of the N from straw or manure remained in the soil as stable SON fractions. While straw rapidly improves low-fertility soils, manure contributes to enhanced soil N reserves and increased crop yields. Therefore, incorporating organic matter may bolster soil N sequestration in Northeast China, which is contingent upon soil fertility and tailored fertilizer management strategies. This research elucidates the distribution and conversion of exogenous N within SON pools, facilitating optimized N management, sustaining yields, reducing farmland N pollution, and promoting agricultural sustainability.

Analysis on microbial communities and characteristic flavor metabolic of PXDB-meju by partially substituting wheat flour with soybean flour and gluten flour

Pixian Douban (PXDB) is one of the most popular condiments in China due to its unique flavor. Wheat flour that contains abundant nutrients is an important raw material in producing flavors during PXDB fermentation. In this study, wheat flour was substituted with soybean flour and gluten flour that have higher proteins in portions of 10.4% and 4.2% (F1), 8.9% and 7.2% (F2), 9.6% and 5.8% (F3). The results indicated that the substitutions increased the amino acid nitrogen content and improved flavor quality compared with traditional group (CT). Especially, the key amino acids including spartate, glutamic, arginine and lysine, and the phenylacetaldehyde as one of the most important volatile compounds exhibited preferable higher contents in F2 group than those in CT group. Metagenomic analysis showed that the abundances of predominant bacteria, including Kosakonia_cowanii, unclassified_f__Enterobacteriaceae and unclassified_g__Enterobacter, were higher in the F2 compared to the CT. Lupinus_albus and Plutella_xylostella were the top two fungi in relative abundance, with higher growth rates in F2 than in CT. Furthermore, metabolism pathway analysis revealed higher relative abundance of enzymes producing key amino acids and phenethylaldehyde in the F2 compared to the CT. Meanwhile, these enzymes were exclusively annotated to the Kosakonia_cowanii, Bacillus_velezensis and Escherichia_coli in F2. This study provided a theoretical foundation for improving PXDB flavor quality in industry production.

Combined effect of molybdenum and nitrogen fertilization on nitrogen metabolism and amino acid content in tobacco leaves

IntroductionThis study investigated the combined effects of molybdenum (Mo) and nitrogen (N) fertilization on N metabolism and amino acid content in the leaves of flue-cured tobacco (Nicotiana tabacum L.) during its mature stage through a pot experiment.MethodsDifferent application levels of Mo (0, 0.15, 0.30 mg/kg soil) and N (0.06, 0.24, 0.42 g/kg soil) were set to observe and analyze changes in leaf quality, N, and amino acid content in the tobacco plants.ResultsThe results revealed that the N fertilizer application level exhibited a primary effect on regulating the total nitrogen, nitrate nitrogen, soluble protein, and amino acid nitrogen concentrations within tobacco leaves, while the effectiveness of Mo fertilization was influenced by the level of N applied. Specifically, under the conditions of 0.24 g/kg soil N and 0.30 mg/kg soil Mo application, the N content, N accumulation, and dry matter mass of the tobacco plants increased significantly by 110%, 204%, and 48%, respectively. Concurrently, nitrate reductase activity increased by 107-fold, and the nitrate nitrogen content was relatively low, contributing to enhanced tobacco yield and safety. Moreover, this treatment led to a notable (170%) increase in free amino acid nitrogen content, with minimal impact on total amino acids and soluble proteins. Notably, it effectively increased the content of free amino acids beneficial to the sensory quality of tobacco (such as histidine, arginine, aspartic acid, isoleucine, and glutamic acid) without reducing the total amino acid content, while simultaneously reducing other amino acids that might affect quality. Therefore, the combined application of 0.30 mg/kg soil Mo and 0.24 g/kg soil N specifically optimized the amino acid composition in tobacco leaves, positively impacting overall quality and market competitiveness.DiscussionThis study provides a theoretical basis for the rational application of Mo fertilizer in Mo-deficient areas to improve the yield and quality of flue-cured tobacco.

Preparation and flavor characteristics of plant-based meat flavoring by mixed fermentation of multiple strains

In this study, plant-based meat flavoring was prepared by mixed fermentation and thermal reaction. Based on the changes of amino acid nitrogen, reducing sugar, and total acid during the fermentation process and combined with sensory evaluation, the optimal fermentation conditions were determined as 4 % total inoculation amount of Monascus purpureus, Actinomucor elegans, and Bacillus subtilis with an inoculation ratio of 1:1:1, and 36 h of fermentation time. The odor characterization of meat flavoring was identified by solid-phase microextraction–gas chromatography–mass spectrometry (SPME–GC–MS), aroma extract dilution analysis (AEDA), quantitative measurements, odor activity value (OAV) and aroma recombination and omission experiments. 64 volatile compounds and 28 odorants were identified. The compounds with FD ≥ 27 were precisely quantified by five-point standard calibration curve and OAVs were calculated, and those with OAV ≥ 1 were subjected to aroma recombination and omission experiments. Finally, furfuryl mercaptan, difurfuryl disulfide, 4-methyl-5-ethylthiazole, 1-octanal, and 2-acetylthiophene were determined to be the key aroma-active compounds. These compounds have been recognized as key aroma-active compounds or detected in other meats or meat flavorings. In this study, we prepared plant-based meat flavoring through mixed fermentation, and provide new ideas for the development of plant-based thermal reaction meat flavoring process.

The effect of adding Gracilaria on flavor and quality of low-salt fermented soy sauce

With the increasing awareness of health, there are higher expectations of customers for the unique flavor, umami, and nutritional value of soy sauce. To explore the potential application of edible seaweed in soy sauce, this study utilized high nutritional value seaweeds to ferment soy sauce and investigated the effects of the addition of Gracilaria on the physicochemical properties and flavor substances of soy sauce. The contents of amino acid nitrogen and total nitrogen in Gracilaria soy sauce were 0.8 g/100g and 1.4 g/100g, respectively, which were higher than those in traditional soy sauce. The free amino acid content of Gracilaria soy sauce (17.82 mg/g) was 1.35 times higher than that of traditional soy sauce, the organic acids content of Gracilaria soy sauce (15.78 mg/mL) was 1.12 times higher than that of traditional soy sauce. Compared with traditional soy sauce, the addition of Gracilaria promoted the accumulation of pyrazines and acids in soy sauce, which contributed to the seafood-like flavor. In general, the addition of Gracilaria to ferment soy sauce can improve the quality and give it a unique flavor. This study can provide a reference for improving the quality and flavor diversity of soy sauce by using seaweed as a fermentation substrate.

Mechanistic insights into the pyrolysis of pine resin sludge based on experiments and ReaxFF-MD simulation: High-value bio-oil production, nitrogen conversion, kinetics and environmental risk assessment

Forestry and agro-industrial waste conversion into high-value chemicals and fuels is considered one of the hottest ongoing research and industrial topics toward sustainable development. As one of the forestry and agro-industrial waste, pine resin sludge (PRS) with amino acids, pine resin residues and their derivatives such as abietic acid and 4-Epidehydroabietal, have ecological toxicity and require environmentally friendly treatment. Herein, a thermal process of PRS that generates high-value biofuels while reducing environmental pollution was reported. The results of the experiment and simulation reveal that the N of bio-oil is the major evolution direction of N, and the nitrogenous gas product is mainly NH3, a useful synthesis gas. ReaxFF-MD simulations elucidated the migration and conversion mechanism of nitrogen in amino acids. The main products of amino acid pyrolysis are 3-oxo-2-Propenenitrile, 2,4-Cyclopentadien-1-ylidenemethanone and 4R)-4-amino-5,5-dihydroxypentanoic acid. PRS's thermal weightlessness was divided into three phases, and the average activation under non-isothermal conditions was calculated at 78.17, 85.68, 206.42 kJ/mol (thermal decomposition) and final 117.78, 149.31, 195.54 kJ/mol (thermal oxidation). Thermal decomposition and oxidation process followed Avrami-Erofeev model (n=2), three-dimensional diffusion model (n=3) and mampel power (n=1). PY-GC/MS analysis shows that pyrolysis at different temperatures recovered mainly 4-Epidehydroabietal (37.15–89.67 %) for potential use as the biosynthesis of dehydrofolate. The bio-oil from PRS pyrolysis shows an excellent yield (31.43 %) and a high calorific value (37.21 MJ/kg), which is comparable to biodiesel. This work focused on offering an appealing and instructive guide for the logical treatment of sludge from pine resin production.

Investigating the temporal evolution of physicochemical attributes and flavorome profiles in Sichuan shai vinegar utilizing diverse aging techniques

Sichuan shai vinegar (SSV) is an acidic condiment with a long history and unique flavor. Aging is an important production process that determines the unique flavor of shai vinegar. This study compared the differences in physicochemical properties, volatile flavor, and non-volatile flavor substances between the constant temperature aging and natural aging methods of SSV Cupei. The results showed that the total acid and amino acid nitrogen content in naturally aged vinegar was higher than that in constant temperature aged vinegar. Furthermore, 20 different volatile flavor substances and 65 different non-volatile flavor substances were obtained after 21 and 25 days of constant temperature aging and 1, 3, and 5 years of natural aging SSV samples. This indicates that the temperature environment during natural aging was more moderate, allowing acidic compounds to form and stably exist in the aging environment. Correlation analysis further demonstrated that there were more differentiated non-volatile substances significantly related to the Maillard reaction under constant temperature aging condition and related to antioxidant activity under natural aging condition. The results of this study provide a comprehensive understanding of the characteristics and applications of the two aging methods of SSV and guide strategy for factories in the selection of aging methods.

Comparative analysis of the microbiota succession and flavor formation in spontaneously fermented fish sauces made with silvery pomfret (Pampus argenteus) and little yellow croaker (Larimichthys polyactis)

Our study delved into exploring the bacterial profiles and volatile compounds during the fermentation of two distinct fish sauces, i.e., Larimichthys polyactis (LP) and Pampus argenteus (PA), and a co-occurrence network was employed to investigate the underlying relationship between flavor and microorganisms. The pH (6.97–5.66) remarkably declined while the total volatile basic nitrogen (TVB-N, 92.36–116.55 mg/100 mL) and amino acid nitrogen (AAN, 104.49–638.68 mg/100 mL) contents gradually rose with the fermentation of LP and PA samples. Bacterial community analysis revealed that Psychrobacter was the dominant genus before LP fermentation, and its relative abundance was gradually occupied by Staphylococcus after 7M. The most dominant genus before PA fermentation was Photobacterium, while Virgibacillus emerged as the predominant genera of PA samples after 7M. Seven main volatile compounds made salient contributions (VIP ≥1.0, ROAV ≥1.0) to the flavor of LP- and PA-fermented samples, respectively. Among them, three typical genera (Tetragenococcus, Streptococcus and Staphylococcus) and four specific genera (Tetragenococcus, Psychrobacter, Vagococcus and Lactobacillus) were the core microorganisms contributing to flavor compounds in LP and PA samples, respectively. Therefore, acquiring insight into the microbes responsible for key flavors in multi-variety fermented fish sauces could provide essential indicators for optimizing commercial fermentation processes.

Bioturbation effect of high - Yield pyrazine strain on the microbial community and flavour metabolites of fortified Daqu

In order to comprehensively explore the regulatory effect of bioturbation on the assembly and overall metabolic characteristics of Daqu microbial community. This study used metatranscriptomics and metabolomics to investigate the composition of the microbial community, physicochemical properties, and overall metabolic profile of Daqu before and after bioturbation by Bacillus velezensis. The results demonstrated that the acidity, moisture, amino acid nitrogen, and starch content of Daqu exhibited a significant increase following bioturbation. The symbol microorganisms of Daqu have also changed significantly, from the original Aspergillus chevalieri, Rasamsonia emersonii, Aspergillus ruber, Aspergillus glaucus, Aspergillus wentii and Aspergillus cristatus to Paecilomyces varioti, Trichomonascus ciferrii and Staphylococcus gallinarum. Correlation analyses and integration pathways indicated that altered microbial community abundance also promoted the metabolism of starch, ethanol, fatty acids, and pyrazine, resulting in a significant enhancement of Daqu's saccharification and liquefaction power, as well as the content of some volatile and non-volatile metabolites, with pyrazines increased by 24.50%. The application of fortified Daqu to Baijiu fermentation can significantly enhance the content of various pyrazines in Baijiu. These results are helpful to elucidate the mechanism of biochemical changes, microbial assembly and flavor production of Daqu after biodisturbance.

Promotion of aromatic amino acids of extracellular polymeric substance targeted transformation via sulfite mediated iron redox cycling in sludge solid-liquid separation

Highly hydrophilic extracellular polymeric substance (EPS) with gel-like structure seriously plagues the development of sludge deep dewatering. Oxysulfur radicals-based oxidation driven by iron-bearing mineral proposes a promising strategy for effective EPS decomposition. However, the transformation and involved interaction mechanisms of aromatic proteins are still controversial due to the complex EPS structure. Herein, sulfite mediated siderite (denoted as Fe(II)/S(IV)) was developed for targeted transformation aromatic amino acids in EPS oxidation to strengthen sludge solid-liquid separation. The enhanced sludge dewaterability were benefited from the Fe(II)/S(IV) bonded interaction assisted by Fe3+/Fe2+ as redox interface that facilitating the release of intracellular bound water via diminish the hydrophily and bind strength with solid protons. The amide region nitrogen of aromatic amino acids (especially tyrosine and tryptophan) originating from EPS presented looser structure and lower spatial site resistance, which were attributed to the exposure of hydrophobic sites in amino groups after Fe(II)/S(IV) treatment. Furthermore, the effective decline of aromatic amino acids in inner layer-EPS (loosely bound EPS and tightly bound EPS) was directed from Fe-N targeted interaction by triggering a series of sulfate-based radical chain reactions. The good correlation between electron transfer amount (R2 = 0.926) and Fe-N (R2 = 0.925) with bonding interaction demonstrated that the complexation of aromatic amino acids with Fe sites on siderite/sulfite via Fe-N bonds, accounting for efficient sludge solid-liquid separation. This study deepens the understanding of sludge organic matter targeted transformation and provides a tactic for iron-based conditioning of sludge.

A long-term study of stable isotope ratios of fingernail keratin and amino acids in a mother-infant dyad.

To evaluate the potential of compound-specific isotope analysis of amino acids (CSIA-AA) for investigating infant feeding practices, we conducted a long-term study that compared infant and maternal amino acid (AA) nitrogen isotope ratios. Fingernail samples were collected from a single mother-infant dyad over 19 months postpartum. Carbon and nitrogen stable isotope ratios were measured in the bulk keratin of the fingernail samples. Selected samples were then hydrolyzed and derivatized for compound-specific nitrogen isotope analysis of keratin AAs. As in previous studies, infant bulk keratin nitrogen isotope values increased during exclusive breastfeeding and fell with the introduction of complementary foods and eventual cessation of breastfeeding. Infant trophic AAs had elevated nitrogen isotope values relative to the mother, while the source AAs were similar between the mother and infant. Proline and threonine appeared to track the presence of human milk in the infant's diet as the isotopic composition of these AAs remained offset from maternal isotope values until the cessation of breastfeeding. Although CSIA-AA is costly and labor intensive, it appears to hold potential for estimating the duration of breastfeeding, even after the introduction of complementary foods. Through the analysis of a full suite of AAs, it may also yield insights into infant physiology and AA synthesis.

Elevated levels of branched chain fatty acids in low-salt fish sauce by co-fermentation: flavor improvement and metabolism analysis.

Traditional fish sauce products rely on relatively long fermentation time and high salt concentration, resulting in inconsistent quality and health risks. Branched-chain fatty acids (BCFAs) are associated with nutritional benefits and health-care effects, mainly derived from food fermentation. This study aimed to screen BCFAs-producing bacteria with high protease and aminotransferase activity as starter cultures for fish sauce fermentation. The low-salt fish sauce products were obtained by co-fermentation with three chosen strains. Trichloroacetic acid (TCA)-soluble peptides and amino acid nitrogen concentrations were higher in the co-fermentation group (FH group). The organoleptic evaluation showed co-fermentation optimized flavor composition and endured with rich taste. The levels of BCFAs and branched-chain amino acids (BCAAs) significantly increased by co-fermentation. Volatile metabolomics analysis indicated that BCFAs, branched-chain esters, and pyrazines were the key flavor compounds in the co-fermented group. The co-fermentation system with selected strains to ferment low-salt fish sauce has the potential to increase BCFA content and improve flavor and nutrition. © 2024 Society of Chemical Industry.

Non-destructive determination of volatile compounds and prediction of amino acid nitrogen during sufu fermentation via electronic nose in combination with machine learning approaches

Electronic nose along with different machine learning approaches was tested as tool to predict degree of sufu fermentation, with amino acid nitrogen content as marker. Consistent increase in E-nose signal values and amino acid nitrogen content were noted with fermentation time. The number of volatile compounds in sufu increased from 19 to 61 upon fermentation, with 46 compounds identified as major aroma contributors. Among them, acetone, ethyl palmitate and indole demonstrated strong correlations with amino acid nitrogen content. Three prediction models, i.e., partial least-squares regression, support vector machine, and artificial neural network, were tested for abilities to predict amino acid nitrogen content. All models were capable of quantitatively predicting the content of amino acid nitrogen, with ANN-based model demonstrating the highest prediction accuracy (RP2 = 0.968, RMSE = 5.49 × 10−8). Electronic nose along with appropriate prediction model is noted as a promising tool for evaluating the degree of sufu fermentation.

Regulatory mechanisms of the microbial starter fortification on the community and flavor characteristics in Doubanjiang

Doubanjiang is a special Chinese seasoning paste produced by mixing broad bean paste with chili paste. Besides the antimicrobial substances, such as capsaicin contained in chili, the microbiota and metabolism are directly developed by the stress effect of abiotic factors that periodically change during the fermentation process. The microbial starter is a promising way to improve the quality of Doubanjiang production. In the present study, the regulatory mechanisms of the microbial starters (Tetragenococcus halophilus, Zygosaccharomyces rouxii, and Candida versatilis) were explored by comparing the microbial community, flavor characteristics and their correlation networks between the fortified and control groups. In comparison to the control group, the volatile compound content increased by 30.77%, with notable increases in phenylethyl alcohol, beta-ionone, etc. Conversely, there was a reduction in non-volatile compounds, including reducing sugar, acidity, amino acid nitrogen, and organic acids. The microbial structure underwent a shift, with an increase in the relative abundance of C. versatilis, while Z. rouxii demonstrated no variance and T. halophilus was not detected. The correlations of the specific microbes and certain flavor compounds were modified by the starter (p < 0.05). Notably, the correlation between Enterobacter/Malassezia and 3-furaldehyde was shifted from a negative to a positive conversion. The observed increase in beta-ionone, ethyl phenylacetate, etc., resulted from the synergistic effects, correlated transformations, and reduction of antagonistic interactions. These findings indicate that the starter powerfully regulates the interactions between microbial communities and metabolic networks, thereby provide valuable insights for the improvement of Doubanjiang quality.

Biochemical and chemosensory characterization of doubanjiang fermented via two-stage controlled temperature

This study aimed to elaborate the biochemical and chemosensory characteristics of doubanjiang fermentation via a two-stage controlled temperature. HL group with variable temperature (40 °C → 30 °C) and NTF group fermented following traditional technique were prepared and their volatile and non-volatile metabolites were compared through multivariate statistical analysis. HL group favored the accumulation of amino acid nitrogen, free amino acids and organic acids in the early stage and maintained adequate total acids and biogenic amines in the mid-late stage. HL group also had preferred jiang and fruity flavor through sensory evaluation. A total of 116 volatile metabolites were identified in HL or NTF groups and 22 marker volatile metabolites were screened through the combinational use of OPLS-DA and Random Forest analysis. Stronger anti-oxidant ability was observed in HL group while adequate number of acidic compounds and biogenic amines were ensured. This indicated that the two-stage controlled temperature fermentation was beneficial for doubanjiang fermentation.