Effects of Different Fermentation Strains on the Flavor Characteristics of Fermented Soybean Curd

Soybean fermentation: Microbial ecology and starter culture technology

Fermented soybean products, including Soya sauce, Tempeh, Miso, and Natto have been consumed for decades, mainly in Asian countries. Beans are processed using either solid-state fermentation, submerged fermentation, or a sequential of both methods. Traditional ways are still used to conduct the fermentation processes, which, depending on the fermented products, might take a few days or even years to complete. Diverse microorganisms were detected during fermentation in various processes with Bacillus species or filamentous fungi being the two main dominant functional groups. Microbial activities were essential to increase the bean's digestibility, nutritional value, and sensory quality, as well as lower its antinutritive factors. The scientific understanding of fermentation microbial communities, their enzymes, and their metabolic activities, however, still requires further development. The use of a starter culture is crucial, to control the fermentation process and ensure product consistency. A broad understanding of the spontaneous fermentation ecology, biochemistry, and the current starter culture technology is essential to facilitate further improvement and meet the needs of the current extending and sustainable economy. This review covers what is currently known about these aspects and reveals the limited available information, along with the possible directions for future starter culture design in soybean fermentation.

Fermented foods: Harnessing their potential to modulate the microbiota-gut-brain axis for mental health

Over the past two decades, whole food supplementation strategies have been leveraged to target mental health. In addition, there has been increasing attention on the ability of gut microbes, so called psychobiotics, to positively impact behaviour though the microbiota-gut-brain axis. Fermented foods offer themselves as a combined whole food microbiota modulating intervention. Indeed, they contain potentially beneficial microbes, microbial metabolites and other bioactives, which are being harnessed to target the microbiota-gut-brain axis for positive benefits. This review highlights the diverse nature of fermented foods in terms of the raw materials used and type of fermentation employed, and summarises their potential to shape composition of the gut microbiota, the gut to brain communication pathways including the immune system and, ultimately, modulate the microbiota-gut-brain axis. Throughout, we identify knowledge gaps and challenges faced in designing human studies for investigating the mental health-promoting potential of individual fermented foods or components thereof. Importantly, we also suggest solutions that can advance understanding of the therapeutic merit of fermented foods to modulate the microbiota-gut-brain axis.

Revealing the formation mechanisms of key flavors in fermented broad bean paste

Pixian Douban (PXDB) is a popular Chinese condiment for its distinctive flavor. Broad bean fermentation (Meju) is the most important process in the formation of flavor substances. Key flavors were analyzed qualitatively and quantitatively, and metagenomic technology was applied to study the microbial diversity during broad bean fermentation. In addition, the main metabolic pathways of key flavors were explored. Results indicated that Staphylococcus_gallinarum was the main microorganism in the microbial community, accounting for 39.13%, followed by Lactobacillus_agilis, accounting for 13.76%. Aspergillus_flavus was the fungus with the highest species abundance, accounting for 3.02%. The KEGG Pathway enrichment analysis showed that carbohydrate metabolism and amino acid metabolism were the main metabolic pathways. Glycoside hydrolase and glycosyltransferase genes were the most abundant, accounting for more than 70% of the total number of active enzyme genes. A total of 113 enzymes related to key flavors and 39 microorganisms corresponding to enzymes were annotated. And Staphylococcus_gallinarum, Lactobacillus_agilis, Weissella_confusa, Pediococcus_acidilactici, Staphylococcus_kloosii, Aspergillus_oryzae, and Aspergillus_flavus played a key role in the metabolic pathway. This study reveals the formation mechanism of key flavors in fermented broad bean, it is important for guiding the industrial production of PXDB and improving product quality.

Characterisation of the Aroma Profile and Dynamic Changes in the Flavour of Stinky Tofu during Storage

Stinky tofu is a traditional Chinese food with wide consumption in China. Nevertheless, the dynamic changes in the flavour of stinky tofu during storage have yet to be investigated. In this study, the flavour changes of stinky tofu over six different storage periods were comprehensively analysed through sensory, electronic nose and gas chromatography-mass spectrometry (GC-MS) analyses. The results of the sensory and electronic nose analyses confirmed the changes in the flavour of stinky tofu across different storage periods. In the GC-MS analysis, 60 volatile compounds were detected during storage, and the odour activity values indicated that 29 of these 60 compounds significantly contributed to the aroma profile. During storage, the alcohol concentration of the stinky tofu gradually decreased while the acid and ester concentrations increased. According to a partial least squares analysis, 2-phenylethyl acetate, 2-phenylethyl propanoate, p-cresol, and phenylethyl alcohol, which were detected after 10 days of storage, promoting the release of an overripe apple-like odour from the stinky tofu. Findings regarding the flavour changes and characteristics of stinky tofu during different storage periods can provide a potential reference for recognising the quality of these products.

Microbiology, flavor formation, and bioactivity of fermented soybean curd (furu): A review

Soybeans are an important plant-based food but its beany flavor and anti-nutritional factors limit its consumption. Fermentation is an effective way to improve its flavor and nutrition. Furu is a popular fermented soybean curd and mainly manufactured in Asia, which has been consumed for thousands of years as an appetizer because of its attractive flavors. This review first classifies furu products on the basis of various factors; then, the microorganisms involved in its fermentation and their various functions are discussed. The mechanisms for the formation of aroma and taste compounds during fermentation are also discussed; and the microbial metabolites and their bioactivities are analyzed. Finally, future prospects and challenges are introduced and further research is proposed. This information is needed to protect the regional characteristics of furu and to regulate its consistent quality. The current information suggests that more in vivo experiments and further clinical trials are needed to confirm its safety and the microbial community needs to be optimized and standardized for each type of furu to improve the production process.

Sensory and Volatile Compounds Characteristics of the Sauce in Bean Paste Fish Treated with Ultra-High-Pressure and Representative Thermal Sterilization

This study investigated the differences between three sterilized samples to reveal the unique aroma characteristics of the sauce in bean paste fish by multiple analysis methodologies. Samples were subjected to pasteurized (PS), high-temperature sterilization (HTS), and ultra-high-pressure treatment (UHP) tests. The UHP had a higher sensory evaluation and could better maintain the original flavor of the sample. A total of 92, 83, 85, and 76 volatile compounds were detected via comprehensive two-dimensional gas chromatography-mass spectrometry (GC×GC-MS) techniques in the control (CT), PS, HTS, and UHP groups, respectively. According to the analysis of gas chromatograph-olfactometry and odor activity value, 7 compounds were considered to have an aromatic influence on the sauces, in which four compounds (1,8-Cineole, Linalool, Hexanal, and Dimethyl trisulfide) exhibited a positive contribution to the aroma of the sauces. PLS-DA results showed that the UHP group positively correlated with volatiles (Isoamylol and 1-Octen-3-ol), color, and gloss. In general, the UHP treatment could retain the original state and flavor of the sauce, showing a high similarity to the control group. The HTS significantly altered the flavor and status of the samples.

Application progress of intelligent flavor sensing system in the production process of fermented foods based on the flavor properties

Fermented foods are sensitive to the production conditions because of microbial and enzymatic activities, which requires intelligent flavor sensing system (IFSS) to monitor and optimize the production process based on the flavor properties. As the simulation system of human olfaction and gustation, IFSS has been widely used in the field of food with the characteristics of nondestructive, pollution-free, and real-time detection. This paper reviews the application of IFSS in the control of fermentation, ripening, and shelf life, and the potential in the identification of quality differences and flavor-producing microbes in fermented foods. The survey found that electronic nose (tongue) is suitable to monitor fermentation process and identify food authenticity in real time based on the changes of flavor profile. Gas chromatography-ion mobility spectrometry and nuclear magnetic resonance technology can be used to analyze the flavor metabolism of fermented foods at various production stages and explore the correlation between flavor substances and microorganisms.

Physicochemical characteristics and flavor profiles of fermented fish sauce (budu) during fermentation in commercial manufacturing plant

This study was done by dynamically monitoring the changes in bacterial composition, physicochemical characteristics, and volatile substances during fermentation of fermented fish sauce (budu). The degree of hydrolysis, TCA-soluble peptides, and nitrogen contents increased as the fermentation time progressed. A continuous decrease in peroxide value and thiobarbituric acid reactive substances was noted over 60 days of fermentation. A total of 44 volatile compounds were detected, and increases in volatile compounds, such as 2-methylbutanal, 3-methylbutanal, benzaldehyde, and 2-ethyl furan, with low odor thresholds values, might contribute to budu's flavor of the final product. Additionally, the relationship between evolving microbiota and the formation of flavor compounds was analyzed, and halophilic lactic acid bacteria was identified to be the most important bacterial contributing to flavor and aroma development. This finding will provide important information for improving the quality of budu in terms of flavor characteristics.

E-sensing and nanoscale-sensing devices associated with data processing algorithms applied to food quality control: a systematic review

Devices of human-based senses such as e-noses, e-tongues and e-eyes can be used to analyze different compounds in several food matrices. These sensors allow the detection of one or more compounds present in complex food samples, and the responses obtained can be used for several goals when different chemometric tools are applied. In this systematic review, we used Preferred Reporting Items for Systematic Reviews and Meta-Analysis guidelines, to address issues such as e-sensing with chemometric methods for food quality control (FQC). A total of 109 eligible articles were selected from PubMed, Scopus and Web of Science. Thus, we predicted that the association between e-sensing and chemometric tools is essential for FQC. Most studies have applied preliminary approaches like exploratory analysis, while the classification/regression methods have been less investigated. It is worth mentioning that non-linear methods based on artificial intelligence/machine learning, in most cases, had classification/regression performances superior to non-liner, although their applications were seen less often. Another approach that has generated promising results is the data fusion between e-sensing devices or in conjunction with other analytical techniques. Furthermore, some future trends in the application of miniaturized devices and nanoscale sensors are also discussed.

Metatranscriptomic approach reveals the functional and enzyme dynamics of core microbes during noni fruit fermentation

Noni (Morinda citrifolia L.) has been recognized as an important herb for treating various physiological disorders worldwide. Fermented noni fruit juice, established as a novel food in European Union, is the most important noni product. However, the structure, functions and enzyme profiles of microbiome during fermentation remain unclear. The metatranscriptomic was used to comprehensively explore the active microbial community and key metabolic function. Acetobacter sp., Acetobacter aceti and Gluconobacter sp. were the major microorganisms and appeared in succession during fermentation. According to principal components analysis (PCA) of metabolism-related unigenes by KEGG database, the fermentation process was divided into three stages and almost completed at the end of the second stage. Furthermore, carbohydrate-active enzymes (CAZymes) and the expression of key enzymes in major metabolic pathways were analyzed systematically. Analysis by HS-SPME-GC-MS and odor active value (OAV) revealed that butanoic acid and hexanoic acid were the main volatile compounds for the unpleasant odor of fermented noni fruit juice. The microbiome in the fermentation process lacked key enzymes that degrade butanoic acid and hexanoic acid, which imparted rancid and sweat odor. This study provides theoretical basis for product improvement and new product development, thus promoting the development of noni food industry.

Correlation between microbial communities and key flavors during post-fermentation of Pixian broad bean paste

Pixian broad bean paste (PBP) is a traditional Chinese condiment, famous for its distinctive flavor. Microbial communities play a vital role in producing the unique flavor of PBP, and a significant accumulation of these volatile flavors occurs during the post-fermentation stage of its production. However, little is known about the relationship between the microbes and flavor compounds in PBP. In this study, high-throughput sequencing (HTS) analysis revealed that Leuconostoc (8.30%), Lactobacillus (7.05%), Weissella (5.80%) and Staphylococcus (4.03%) were the dominant bacterial genera, while the most prevalent yeast genera were Zygosaccharomyces (41.45%) and Pichia (5.83%). Gradual accumulations of free amino acids (glutamic acid and asparagine), organic acids (malic acid and tartaric acid), and unique volatiles (aldehydes, phenols and pyrazines) were evident throughout the post-fermentation process. Analysis of the Pearson's correlation coefficients between 66 key microbes and the key flavors was investigated. Nine core microbes were identified based on the linear discriminant analysis (LDA) scores ≥ 4 (or an average abundance >0.1%) and a high correlation with at least two flavor categories (P < 0.05, |ρ| > 0.8), namely Kosakonia, Kazachstania, Debaryomyces, Lactobacillus, Myroides, Stenotrophomonas, Ochrobactrum, Wohlfahrtiimonas, and Lactococcus genera. These results provide a clearer insight into microbial succession during PBP post-fermentation, thereby contributing to further quality improvement of PBP.

Comparative aroma and taste profiles of oil furu (soybean curd) fermented with different mucor strains

The effects of different mucor strains (Mucor racemosus, Actinomucor, and Mucor wutungkiao) on aroma and taste profiles based on proteolysis, lipolysis, and their catabolism in oil furu were studied. Gas chromatography-mass spectrometry and relative odor activity were used to monitor the changes of key volatile compounds and the differences in the characteristic aroma contents of oil furu. Using principal component analysis, the different fermentation strains had an effect on aroma profiles. The volatile compounds from metabolism of protein and fatty acid contributed to the aroma of oil furu with different contribution from the different strains, presumably due to their different enzymes. The electronic tongue and free amino acid profiles also showed strain differences of taste. Based on these results, optimization of the amount of each of the different mucor strains during cofermentation might achieve better flavor.

Effect of static-state fermentation on volatile composition in rapeseed meal

BACKGROUND

Fermented rapeseed meal has been used as an alternative protein source for animal feed, but the volatile compounds and how their contents change during fermentation have not been reported. To clarify the effect of static-state fermentation on its aroma, the volatile compounds of rapeseed meal during different stages of fermentation were analyzed using an electronic nose system and headspace solid-phase microextraction-gas chromatography-mass spectrometry.

RESULTS

The results suggested that the volatile compounds in the raw rapeseed meal, mostly hydrocarbons and some aldehydes, were lost. The levels of the volatile compounds resulting from microbial metabolism, especially pyrazines, greatly increased during fermentation. Nonanal was the dominant volatile measured in the headspace of raw rapeseed meal. However, the volatile compounds found at high concentrations in rapeseed meal after 5 days of fermentation were tetramethylpyrazine, followed by butanoic acid, benzenepropanenitrile, 2-methylbutanoic acid, trimethylamine, 2,3,5-trimethyl-6-ethylpyrazine, and 2,3,5-trimethylpyrazine.

CONCLUSION

The fermentation process could significantly change the composition and content of volatile compounds in rapeseed meal. The results may provide reference data for studies on the choice of fermentation period and formation mechanism of flavor substances in fermented rapeseed meal. © 2020 Society of Chemical Industry.

Evaluation of bacterial and fungal communities during the fermentation of Baixi sufu, a traditional spicy fermented bean curd

BACKGROUND

Baixi sufu (BS) is a traditional Chinese spicy fermented bean curd manufactured with a natural starter. In this study, the bacterial and fungal communities during BS fermentation were determined by culture and by the culture-independent method of high-throughput sequencing (HTS). Correlation analyses were performed to select the microorganisms potentially contributing to this fermentation.

RESULTS

During the fermentation of BS, 162 bacterial and 97 fungal strains were isolated and identified, and a total of 268 314 bacterial and 287 844 fungal high-quality sequences were analyzed. In general, lactic acid bacteria (LAB), especially Enterococcus and Lactococcus, were dominant in the early stage of fermentation, and spore-forming bacteria, especially Bacillus spp., became the predominant bacteria by the end of fermentation. Geotrichum, Mortierella, and unclassified Ascomycota, were the major fungal populations, which could not be detected in the final product. Correlation analyses indicated that Enterococcus, Bacillus, Geotrichum, and unclassified Ascomycota correlated significantly and positively with amino nitrogen. However, due to the sporulation characteristics of Bacillus, they may have little effect on BS ripening. The presence of Bifidobacterium spp. in sufu is reported for the first time, but the excessive counts of the Bacillus cereus group (>10⁵  CFU g^-1 ) indicate a potential hazard to consumers.

CONCLUSION

The profiles obtained from this study will contribute to the development of autochthonous starter cultures to control BS fermentation, and may lead to the development of novel strategies to shorten the fermentation time of sufu products. © 2019 Society of Chemical Industry.