The Correlation Mechanism between Dominant Bacteria and Primary Metabolites during Fermentation of Red Sour Soup

Effect of sugar content on characteristic flavour formation of tomato sour soup fermented by Lacticaseibacillus casei H1 based on non-targeted metabolomics analysis

To reveal the formation mechanism of the characteristic flavour of tomato sour soup (TSS), metabolomics based on UHPLC-Q-TOF/MS was used to investigate the effect of sugar addition on TSS metabolomics during fermentation with Lacticaseibacillus casei H1. A total of 254 differentially abundant metabolites were identified in the 10% added-sugar group, which mainly belonged to organic acids and derivatives, fatty acyls, and organic oxygen compounds. Metabolic pathway analysis revealed that alanine aspartate and glutamate metabolism, valine leucine and isoleucine metabolism and butanoate metabolism were the potential pathways for the flavour of TSS formation. Lactic acid, acetic acid, Ala, Glu and Asp significantly contributed to the acidity and umami formation of TSS. This study showed that sugar regulation played an important role in the formation of the characteristic TSS flavour during fermentation, providing important support for understanding the formation mechanism of organic acids as the main characteristic flavour of TSS.

Correlation between microbial communities and key odourants in fermented capsicum inoculated with Pediococcus pentosaceus and Cyberlindnera rhodanensis

BACKGROUND

Fermented capsicum (i.e. pickled pepper) is one of the most popular fermented vegetables. However, the effect of inoculated microbial fermentation on pickled pepper is not yet fully understood.

RESULTS

Cyberlindnera rhodanensis J52 with a rich ester flavour and Pediococcus pentosaceus AL with a strong inhibitory effect on foodborne pathogenic bacteria were selected to prepare single- and double-strain fermented capsicum under low salt (< 10 g L^-1 sodium chloride) conditions. The inhibition zone of P. pentosaceus AL against Escherichia coli was up to 44 mm in diameter. Biochemical indicator analyses found that co-fermentation of P. pentosaceus AL and C. rhodanensis J52 changed the contents of vitamin C and short-chain fatty acids. Analysis of microbial diversity and volatile metabolome showed that 125 microbial species and 72 volatile compounds were detected, and P. pentosaceus was the dominant bacterium that inhibited the growth of other bacteria, while C. rhodanensis was the fungus that contributed the most to flavour. Correlation analysis between microorganisms and flavour compounds showed 725 correlations, and 124 microbial species may have participated in the formation of 69 compounds. Furthermore, 10 and 29 correlations were detected between P. pentosaceus AL or C. rhodanensis J52 and flavour compounds, respectively. Among them, 3-methyl-1-butanol acetate is speculated to be the main substance affecting the flavour of fermented capsicum by inoculation with C. rhodanensis J52.

CONCLUSION

The inoculation of P. pentosaceus and C. rhodanensis had a significant impact on the microbial community and volatile compounds of fermented capsicum and helped to improve its organoleptic qualities. © 2022 Society of Chemical Industry.

Effect of different regions on fermentation profiles, microbial communities, and their metabolomic pathways and properties in Italian ryegrass silage

Introduction

Italian ryegrass is less studied in northern China due to high-quality forage grass has not been fully utilized. Full utilization of high-quality forage grass helps to alleviate the shortage of forage grass in winter and spring season and guarantee stable development of livestock production. Consequently, this study was aimed to evaluate the effects of different regions in northern China on the fermentative products, bacterial community compositions, and metabolic pathways and metabolites of Italian ryegrass silage.

Methods

The Italian ryegrass was harvested from three regions (Ordos-WK; Hohhot-AK; Ulanqab-SYK) and ensiled for 60 days. Single molecule real-time (SMRT) sequencing and ultra-high performance liquid chromatography-mass spectrometry (UHPLC-MS/MS) were used to analyze bacterial communities and metabolites, respectively.

Results

After 60 d of fermentation, the SYK group had the lowest pH (4.67), the highest lactic acid contents (95.02 g/kg DM) and largest lactic acid bacteria populations (6.66 log₁₀ cfu/g FM) among the treatment groups. In addition, the SYK group had the highest abundance of Lactiplantibacillus plantarum (63.98%). In SYK group, isoquinoline alkaloid biosynthesis was the significantly enriched (p < 0.05) and high-impact value (0.0225) metabolic pathway. In AK group, tryptophan metabolism the was the significantly enriched (p < 0.001) and high-impact value (0.1387) metabolic pathway. In WK group, citrate cycle (TCA cycle) was the significantly enriched (p < 0.001) and high-impact value (0.1174) metabolic pathway. Further, Lactiplantibacillus plantarum was positively correlated with cinnamic acid, tetranor 12-HETE, D-Mannitol, (2S)-2-amino-4-methylpentanoic acid L-Leucine, guanine, isoleucyl-aspartate and 3,4-Dihydroxyphenyl propanoate, but negatively correlated with isocitrate and D-mannose.

Discussion

In conclusion, this study can improve our understanding of the ensiling microbiology and metabolomics in different regions to further regulate the fermentation products and promote livestock production.

Analysis of factors related to browning of red sour soup during fermentation

As a traditional fermentation food, red sour soup (RSS) is very popular in China. However, browning is always occurred during the process of fermentation, which influences the sensory quality of RSS and limits its further application. Thus, it is meaningful to elucidate the main factors related to browning during the process of fermentation. Herein, the changes in various factors related to browning from group spontaneous (RSS-SF) and inoculant fermentation (RSS-IF) were determined and analyzed. Firstly, the activity changes of enzymes related to browning indicated that browning of group RSS-SF and RSS-IF during fermentation was not related to enzymatic browning. Secondly, path analysis revealed that the main factors related to non-enzymatic browning of group RSS-SF and RSS-IF were oxidation of polyphenol and degradation of ascorbic acid (Vc). The results of this study not only identifies the main factors associate with browning of RSS, but also provides foundation on how to control the browning of RSS in further study.

Effects of post-fermentation on the flavor compounds formation in red sour soup

Red Sour Soup (RSS) is a traditional fermented food in China. After two rounds of fermentation, sour soup has a mellow flavor. However, the microbial composition and flavor formation processes in post-fermentation in RSS are unclear. This study investigates the bacteria composition of RSS during the post-fermentation stage (0–180 days) using high-throughput sequencing. The results show that lactic acid bacteria (LAB) are dominant during the post-fermentation process, and their abundance gradually increases with fermentation time. Additionally, gas chromatography-mass spectrometry was used to detect volatile flavor compounds in the post-fermentation process. Seventy-seven volatile flavor compounds were identified, including 24 esters, 14 terpenes, 9 aromatic hydrocarbons, 9 alkanes, 6 heterocyclic compounds, 3 alcohols, 3 acids, 3 ketones, 2 phenols, 2 aldehydes, 1 amine, and 1 other. Esters and aromatic hydrocarbons are the main volatile compounds in RSS during the post-fermentation process. Orthogonal partial least squares screening and correlation analysis derived several significant correlations, including 48 pairs of positive correlations and 19 pairs of negative correlations. Among them, Acetobacter spp., Clostridium spp. and Sporolactobacillus spp. have 15, 14, 20 significant correlation pairs, respectively, and are considered the most important bacterial genera post-fermentation. Volatile substances become abundant with increasing fermentation time. LAB are excessive after more than 120 days but cause a drastic reduction in volatile ester levels. Thus, the post-fermentation time should be restricted to 120 days, which retains the highest concentrations of volatile esters in RSS. Overall, these findings provide a theoretical basis to determine an optimal post-fermentation time duration, and identify essential bacteria for manufacturing high-quality starter material to shorten the RSS post-fermentation processing time.

Enzymes and microorganisms jointly promote the fermentation of rapeseed cake

Rapeseed cake is a by-product of rapeseed oil separation. The nutritional components of rapeseed cake mainly include a variety of carbohydrates, proteins, and minerals. In order to improve the conversion rate of rapeseed cake, we studied the physicochemical properties, the structure of microbial communities, and the composition of metabolites in rapeseed cake after enzymatic fermentation. The results showed that the addition of enzymatic preparation increased microbial diversity. The relative abundance of Bacillus, Lysinibacillus, Empedobacter, Debaryomyces, Hyphopichia, and Komagataella in enzymatic fermentation was significantly higher than that in natural fermentation. Unlike natural fermentation, microbial diversity during enzymatic fermentation is specific, which improves the efficiency of fermentation. Otherwise, enzymatic fermentation promotes the conversion of macromolecular substances in rapeseed cake, which increases small metabolites, such as fatty acids, organic acids, amino acids and their derivatives. The metabolite enrichment pathway is mostly concentrated in sugar metabolism and fatty acid metabolism. In conclusion, after adding enzymatic preparation, enzymes and microorganisms jointly promote the transformation of macromolecules during the fermentation of rapeseed cake, which laid a good foundation for further utilization of rapeseed cake.