Sichuan paocai fermented by mixed-starter culture of lactic acid bacteria

Analysis of Pickled Cucumber Products, Based on Microbial Diversity and Flavor Substance Detection

Changes to the microbial community during pickled cucumber fermentation were studied using the 16S rDNA technique. The changes of volatile organic compounds (VOCs) during pickled cucumber fermentation were studied by gas chromatograph-ion mobility spectrometry. At the phylum level, Cyanophyta and Proteobacteria were the dominant flora in the natural fermentation group, and Firmicutes were the dominant flora in the added-bacteria fermentation group. At the generic level, the addition of Lactobacillus led to changes in the community of the bacteria in the added-bacterial fermentation group and decreased the species abundance of other bacteria. In total, 75 volatile organic compounds were identified from naturally fermented pickled cucumber, and 60 volatile organic compounds were identified from fermented pickled cucumber with bacterial addition. The main metabolites were esters, aldehydes, acids, alcohols, ketones, alkanes, nitriles, and alkenes. These metabolites will bring their unique aroma components to the pickled cucumber. Metabolomic analysis of the O2PLS model showed that Weissella and Lactobacillus were closely and positively correlated with nine alcohols, six esters, five aldehydes, four acids, three ketones, and one pyrazine. Pseudomonas and norank_f_Mitochondria show a close positive correlation with four kinds of alcohols, two kinds of esters, one kind of aldehyde, and one kind of nitrile.

Screening and characterization of lactic acid bacteria and fermentation of gamma-aminobutyric acid-enriched bamboo shoots

In order to produce fermented bamboo shoots with functional properties, two strains of lactic acid bacteria were selected for inoculation and fermentation. One strain, Lactiplantibacillus plantarum R1, exhibited prominent potential probiotic properties (including gastrointestinal condition tolerance, adhesion ability, antimicrobial ability, and antibiotic resistance), while the other, Levilactobacillus brevis R2, demonstrated the capability of high γ-aminobutyric acid (GABA) production (913.99 ± 14.2 mg/L). The synergistic inoculation of both strains during bamboo shoot fermentation led to a remarkable increase in GABA content (382.31 ± 12.17 mg/kg), surpassing that of naturally fermented bamboo shoots by more than 4.5 times and outperforming mono-inoculated fermentation. Simultaneously, the nitrite content was maintained at a safe level (5.96 ± 1.81 mg/kg). Besides, inoculated fermented bamboo shoots exhibited an increased crude fiber content (16.58 ± 0.04 g/100 g) and reduced fat content (0.39 ± 0.02 g/100 g). Sensory evaluation results indicated a high overall acceptability for the synergistically inoculated fermented bamboo shoots. This study may provide a strategy for the safe and rapid fermentation of bamboo shoots and lay the groundwork for the development of functional vegetable products enriched with GABA.

Isolation of Aroma-Producing Wickerhamomyces anomalus Yeast and Analysis of Its Typical Flavoring Metabolites

In this study, 21 strains of aroma-producing yeast were isolated from Sichuan paocai juice of farmers in western, eastern and southern Sichuan. One strain, Y3, with the best aroma-producing characteristics, was screened using an olfactory method and a total ester titration method, and was identified as Wickerhamomyces anomalus. The total ester content of Y3 fermentation broth was as high as 1.22 g/L, and there was no white colonies or film on the surface. Meanwhile, the Y3 strain could tolerate 14% salt concentration conditions and grow well in a pH range of 3-4. Through sensory analysis, the fermented mustard with a ratio of Lactiplantibacillus plantarum to Y3 of 1:1 showed the highest overall acceptability. Ethyl acetate with its fruit and wine flavor was also detected in the fermented Sichuan paocai juice with a mixed bacteria ratio of 1:1, analyzed with SPME-GC-MS technology, as well as phenylethyl alcohol, isobutyl alcohol, isothiocyanate eaters, myrcene and dimethyl disulfide. These contributed greatly to the unique flavor of Sichuan paocai. In general, Wickerhamomyces anomalus Y3 enhanced the aroma of the fermented Sichuan paocai.

Evaluation and comparison of physicochemical properties, volatile substances, and microbial communities of leaf mustard (Brassica juncea var. multiceps) under natural and inoculated fermentation

Due to the uncontrolled fermentation process and unstable quality of naturally fermented leaf mustard, inoculated fermentation is receiving more attention. Here, the physicochemical properties, volatile compounds, and microbial community in leaf mustard under natural fermentation (NF) and inoculated fermentation (IF) were analyzed and compared. The contents of total acid, crude fiber, and nitrite of leaf mustard were measured. Headspace-solid phase microextraction-gas chromatography-mass spectrometry and orthogonal projection on latent structure-discriminant analysis were used to analyze the differences of volatile compounds in NF and IF leaf mustard. Moreover, Illumina MiSeq high-throughput sequencing technology was employed to reveal the composition of microbiota. The results showed that the nitrite content in leaf mustard after IF (3.69 mg/kg) was significantly lower than that after NF (4.43 mg/kg). A total of 31 and 25 kinds of volatile components were identified in IF and NF, respectively. Among the detected compounds, 11 compounds caused the differences between IF and NF leaf mustard. The results of inter-group difference analysis showed that there were significant differences in fungal flora between IF and NF samples. Saccharomycetes, Kazachstania, and Ascomycota were the landmark microorganisms in IF leaf mustard and the landmark microorganisms in NF were Mortierellomycota, Sordariomycetes, and Eurotiomycetes. The abundance of probiotics (such as Lactobacillus) in IF leaf mustard (51.22%) was higher than that in NF (35.20%) and the abundance of harmful molds (such as Mortierella and Aspergillus) was opposite. Therefore, IF leaf mustard showed the potential to reduce the content of nitrite and harmful molds and increase the beneficial volatile compounds and probiotics. PRACTICAL APPLICATION: Leaf mustard of inoculated fermentation (IF) showed better fermented characteristics than natural fermentation in terms of lower nitrite content, greater beneficial volatile substances, and better potential for increasing probiotics and reducing harmful molds. These results provided a theoretical basis for IF leaf mustard and contributed to the industrial production of fermented leaf mustard.

Cold Plasma Controls Nitrite Hazards by Modulating Microbial Communities in Pickled Radish

The hazard of nitrite caused by microorganisms is the main food safety problem in the pickle production. To seek a method to control the nitrite hazards of pickles by regulating microbial community without additional substances, we focused on cold plasma because Gram-negative and Gram-positive bacteria have different degrees of sensitivity to the sterilization of cold plasma. Using radish pickles as the experimental object, based on colony counting, dynamic monitoring of pH and nitrite, qPCR and high-throughput sequencing, it was found that when the raw material was treated with dielectric barrier discharge (DBD) cold plasma at 40 kV for 60 s, Gram-negative bacteria with the potential to produce nitrite were preferentially sterilized. Meanwhile, Gram-positive bacteria dominated by the lactic acid bacteria were retained to accelerate the acid production rate, initiate the self-degradation of nitrite in advance and significantly reduce the peak value and accumulation of nitrite during the fermentation process of pickled radish. This study preliminarily verified that DBD cold plasma can inhibit the nitrite generation and accelerate the self-degradation of nitrite by regulating the structure and abundance of microbial community in radish pickles, which provides an important reference for the control of nitrite hazards in the fermentation process of pickles without additives.

Effect of Levilactobacillus brevis as a starter on the flavor quality of radish paocai

The aim of this study was to investigate the effectiveness of Levilactobacillus brevis on the fermentation kinetics and flavor quality of radish paocai. Compared with spontaneous fermentation (SF), the radish paocai of inoculated fermentation (IF) using Levilactobacillus brevis PL6-1 as a starter could rapidly utilize sugar to produce acid, thus accelerating the fermentation process. The texture including hardness, chewiness, and springiness of the IF were all higher than that of the SF, and the IF paocai showed higher L value in color. L. brevis PL6-1 as a starter could increase the final levels of metabolites of mannitol (5.43 mg/g), lactic acid (543.44 mg/100 g) and acetic acid (87.79 mg/100 g). Fifteen volatile organic compounds (VOCs) were identified as key aroma-active compounds in radish paocai and 8 differential VOCs were considered as the potential markers. L. brevis PL6-1 could improve the levels of 1,8-cineole, 1-hexanol, hexanoic acid, 2-methoxy-4-vinylphenol, and eugenol, giving the radish paocai floral, sweet, and sour aroma, and reduce the unpleasant odor of garlic, onion, and pungent, contributed by erucin, diallyl disulfide, and allyl trisulfide. Sensory evaluation results showed that the appearance, taste, texture, and overall acceptability of IF paocai were all better than the SF group. Therefore, L. brevis PL6-1 could be a potential starter to improve the flavor and sensory quality for radish paocai fermentation.

Lactic acid bacteria with a strong antioxidant function isolated from "Jiangshui," pickles, and feces

Excessive free radicals and iron death lead to oxidative damage, which is one of the main causes of aging and diseases. In this field of antioxidation, developing new, safe, and efficient antioxidants is the main research focus. Lactic acid bacteria (LAB) are natural antioxidants with good antioxidant activity and can regulate gastrointestinal microecological balance and immunity. In this study, 15 LAB strains from fermented foods ("Jiangshui" and pickles) or feces were evaluated in terms of their antioxidant attributes. Strains with strong antioxidant capacity were preliminarily screened by the following tests: 2,2-diphenyl-1-picrylhydrazyl (DPPH), hydroxyl radical, superoxide anion radical scavenging capacity; ferrous ion chelating assay; hydrogen peroxide tolerance capacity. Then, the adhesion of the screened strains to the intestinal tract was examined using hydrophobic and auto-aggregation tests. The safety of the strains was analyzed based on their minimum inhibitory concentration and hemolysis, and 16S rRNA was used for molecular biological identification. Antimicrobial activity tests showed them probiotic function. The cell-free supernatant of selected strains were used to explore the protective effect against oxidative damage cells. The scavenging rate of DPPH, hydroxyl radicals, and ferrous ion-chelating of 15 strains ranged from 28.81-82.75%, 6.54-68.52%, and 9.46-17.92%, respectively, the scavenging superoxide anion scavenging activity all exceeded 10%. According to all the antioxidant-related tests, strains possessing high antioxidant activities J2-4, J2-5, J2-9, YP-1, and W-4 were screened, these five strains demonstrated tolerance to 2 mM hydrogen peroxide. J2-4, J2-5, and J2-9 were Lactobacillus fermentans and γ-hemolytic (non-hemolytic). YP-1 and W-4 were Lactobacillus paracasei and α-hemolytic (grass-green hemolytic). Although L. paracasei has been proven as a safe probiotic without hemolytic characteristics, the hemolytic characteristics of YP-1 and W-4 should be further studied. Due to the weak hydrophobicity and antimicrobial activity of J2-4, finally, we selected J2-5, J2-9 for cell experiment, J2-5 and J2-9 showed an excellent ability that resistant to oxidative damage by increasing SOD, CAT, T-AOC activity of 293T cells. Therefore, J2-5, and J2-9 strains from fermented foods "Jiangshui" could be used as potential antioxidants for functional food, health care, and skincare.

Effect of dense phase carbon dioxide treatment on the flavor, texture, and quality changes in new-paocai

This study investigated the effect of dense phase carbon dioxide (DPCD) treatment on the organoleptic properties of new-paocai. Optimal DPCD treatment (25 MPa/40 °C/40 min) was determined by conducting single-factor and orthogonal experiments with the sensory, bactericidal, and electronic eye evaluations. DPCD treatment (25 MPa/40 °C/40 min) did not significantly affect the nitrite, pH, total acid, and organic acid of the new-paocai brine, and the texture of the radish slices did not display substantial changes. Gas chromatography-mass spectrometry (GC-MS) was employed to characterize the new-paocai brine flavor, revealing 63 and 60 respective flavor compounds with and without DPCD treatment. In addition, DPCD treatment significantly reduced the total organic volatile compound content in the paocai from 48.182 μg/mL to 35.952 μg/mL, DPCD has a great influence on volatile flavor substances. The electronic nose (E-nose) effectively distinguished the flavor differences in the new-paocai brine with and without DPCD treatment. This study combined new food processing technology with traditional food production, could provide a new idea for pickle production technology.

Enhanced mass transfer of pulsed vacuum pressure pickling and changes in quality of sour bamboo shoots

Sour bamboo shoot is a traditional Chinese fermented vegetable food. The traditional pickling method of sour bamboo shoots has the disadvantages of being time-consuming, inhomogeneous, and difficult to control. Pulsed vacuum pressure pickling (PVPP) technology uses pulsed vacuum pressure to enhance the pickling efficiency significantly. To demonstrate the effects of salt content and PVPP technical parameters on the fermentation of bamboo shoots, the sample salinity, pH value, color, crunchiness and chewiness, nitrite content, and lactic acid bacteria content during the pickling process were investigated. The salt content inside the bamboo shoots gradually increased to the equilibrium point during the pickling process. The pickling efficiency of bamboo shoots under PVPP technology increased by 34.1% compared to the traditional control groups. Meanwhile, the uniform salt distribution under PVPP technology also obtained better performance in comparison with the traditional groups. The pH value declined slowly from 5.96 to 3.70 with the extension of pickling time and sour flavor accumulated progressively. No significant differences were found in the color values (L^*, a^*, and b^*) and the crunchiness of the bamboo shoot under different salt solution concentrations, vacuum pressure, and pulsation frequency ratio conditions. Colony-forming unit of lactic acid bacteria (CFU of LAB) decreased, to begin with, and then increased until the 6th day, followed by a declining trend in volatility. The nitrate content of bamboo shoots samples under PVPP treatments did not exceed the safety standard (<20 mg/kg) during the whole fermentation process, which proves the safety of PVPP technology. In conclusion, PVPP technology can safely replace the traditional method with better quality performance. The optimal PVPP processing conditions (vacuum pressure 60 kPa, 10 min vacuum pressure time vs. 4 min atmospheric pressure time, salt solution concentration 6%) have been recommended for pickling bamboo shoots with high product quality.