Citrate metabolism in lactic acid bacteria

Impact of Different Carbon Sources on Volatile Organic Compounds (VOCs) Produced during Fermentation by Levilactobacillus brevis WLP672 Measured Using Proton Transfer Reaction Time-of-Flight Mass Spectrometry (PTR-ToF-MS)

Bacterial fermentation is considered to be a cost-effective means of generating desired flavour compounds from plant-based substrates. However, the wide range of substrates present in plants makes it challenging to understand how individual components impact on flavour volatile organic compound (VOC) production. To simplify this, a defined medium can be used to better understand VOCs production with regard to individual compounds. In the current study, the VOCs produced by the lactic acid bacterium, Levilactobacillus brevis WLP672, growing in a defined medium containing different carbon sources (either glucose (DM), fructose (DMFr) or citrate (DMCi)) under a range of fermentation conditions (time: 0, 7, and 14 days; and temperature: 25 and 35 °C) were assessed using proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS). Among the detected mass peaks (m/z), after 7 days of fermentation, the concentrations of m/z 45.033 (t.i. acetaldehyde), m/z 49.011 (t.i. methanethiol), and m/z 89.060 (t.i. ethyl acetate) were significantly (p < 0.05) higher in DM at 35 °C than all other treatments at either temperature. The knowledge obtained will help to produce desirable LAB fermentation flavour VOCs or VOC mixtures that could be used in developing plant-based analogues with acceptable sensory properties.

Integrating uterine microbiome and metabolome to advance the understanding of the uterine environment in dairy cows with metritis

BACKGROUND

Metritis is a prevalent uterine disease that affects the welfare, fertility, and survival of dairy cows. The uterine microbiome from cows that develop metritis and those that remain healthy do not differ from calving until 2 days postpartum, after which there is a dysbiosis of the uterine microbiome characterized by a shift towards opportunistic pathogens such as Fusobacteriota and Bacteroidota. Whether these opportunistic pathogens proliferate and overtake the uterine commensals could be determined by the type of substrates present in the uterus. The objective of this study was to integrate uterine microbiome and metabolome data to advance the understanding of the uterine environment in dairy cows that develop metritis. Holstein cows (n = 104) had uterine fluid collected at calving and at the day of metritis diagnosis. Cows with metritis (n = 52) were paired with cows without metritis (n = 52) based on days after calving. First, the uterine microbiome and metabolome were evaluated individually, and then integrated using network analyses.

RESULTS

The uterine microbiome did not differ at calving but differed on the day of metritis diagnosis between cows with and without metritis. The uterine metabolome differed both at calving and on the day of metritis diagnosis between cows that did and did not develop metritis. Omics integration was performed between 6 significant bacteria genera and 153 significant metabolites on the day of metritis diagnosis. Integration was not performed at calving because there were no significant differences in the uterine microbiome. A total of 3 bacteria genera (i.e. Fusobacterium, Porphyromonas, and Bacteroides) were strongly correlated with 49 metabolites on the day of metritis diagnosis. Seven of the significant metabolites at calving were among the 49 metabolites strongly correlated with opportunistic pathogenic bacteria on the day of metritis diagnosis. The main metabolites have been associated with attenuation of biofilm formation by commensal bacteria, opportunistic pathogenic bacteria overgrowth, tissue damage and inflammation, immune evasion, and immune dysregulation.

CONCLUSIONS

The data integration presented herein helps advance the understanding of the uterine environment in dairy cows with metritis. The identified metabolites may provide a competitive advantage to the main uterine pathogens Fusobacterium, Porphyromonas and Bacteroides, and may be promising targets for future interventions aiming to reduce opportunistic pathogenic bacteria growth in the uterus.

Decarboxylase activity of the non-starter lactic acid bacterium Loigolactobacillus rennini gives crack defects in Gouda cheese through the production of γ-aminobutyric acid

Ten Gouda cheese wheels with an age of 31 weeks from six different batch productions were affected by a crack defect and displayed an unpleasant off-flavor. To unravel the causes of these defects, the concentrations of free amino acids, other organic acids, volatile organic compounds, and biogenic amines were quantified in zones around the cracks and in zones without cracks, and compared with those of similar Gouda cheeses without crack defect. The Gouda cheeses with cracks had a significantly different metabolome. The production of the non-proteinogenic amino acid γ-aminobutyric acid (GABA) could be unraveled as the key mechanism leading to crack formation, although the production of the biogenic amines cadaverine and putrescine contributed as well. High-throughput amplicon sequencing of the full-length 16S rRNA gene based on whole-community DNA revealed the presence of Loigolactobacillus rennini and Tetragenococcus halophilus as most abundant non-starter lactic acid bacteria in the zones with cracks. Shotgun metagenomic sequencing allowed to obtain a metagenome-assembled genome of both Loil. rennini and T. halophilus. However, only Loil. rennini contained genes necessary for the production of GABA, cadaverine, and putrescine. Metagenetics further revealed the brine and the rennet used during cheese manufacturing as the most plausible inoculation sources of both Loil. rennini and T. halophilus.IMPORTANCECrack defects in Gouda cheeses are still poorly understood, although they can lead to major economic losses in cheese companies. In this study, the bacterial cause of a crack defect in Gouda cheeses was identified, and the pathways involved in the crack formation were unraveled. Moreover, possible contamination sources were identified. The brine bath might be a major source of bacteria with the potential to deteriorate cheese quality, which suggests that cheese producers should regularly investigate the quality and microbial composition of their brines. This study illustrated how a multiphasic approach can understand and mitigate problems in a cheese company.

The microbial and metabolite composition of Gouda cheese made from pasteurized milk is determined by the processing chain

Gouda cheeses of different production batches and ripening times often differ in metabolite composition, which may be due to the starter culture mixture applied or the growth of non-starter lactic acid bacteria (NSLAB) upon maturation. Therefore, a single Gouda cheese production batch was systematically investigated from the thermized milk to the mature cheeses, ripened for up to 100 weeks, to identify the main bacterial species and metabolites and their dynamics during the whole production and ripening. As this seemed to be starter culture strain- and NSLAB-dependent, it requested a detailed, longitudinal, and quantitative investigation. Hereto, microbial colony enumeration, high-throughput full-length 16S rRNA gene sequencing, and a metabolomic approach were combined. Culture-dependently, Lactococcus lactis was the most abundant species from its addition as part of the starter culture up to the first two months of cheese ripening. Afterward, the NSLAB Lacticaseibacillus paracasei became the main species during ripening. The milk was a possible inoculation source for the latter species, despite pasteurization. Culture-independently, the starter LAB Lactococcus cremoris and Lc. lactis were the most abundant species in the cheese core throughout the whole fermentation and ripening phases up to 100 weeks. The cheese rind from 40 until 100 weeks of ripening was characterized by a high relative abundance of the NSLAB Tetragenococcus halophilus and Loigolactobacillus rennini, which both came from the brine. These species were linked with the production of the biogenic amines cadaverine and putrescine. The most abundant volatile organic compound was acetoin, an indicator of citrate and lactose fermentation during the production day, whereas the concentrations of free amino acids were an indicator of the ripening time.

Citrate metabolism in lactic acid bacteria: is there a beneficial effect for Oenococcus oeni in wine?

Lactic acid bacteria (LAB) are Gram positive bacteria frequently used in the food industry for fermentation, mainly transformation of carbohydrates into lactic acid. In addition, these bacteria also have the capacity to metabolize citrate, an organic acid commonly found in food products. Its fermentation leads to the production of 4-carbon compounds such as diacetyl, resulting in a buttery flavor desired in dairy products. Citrate metabolism is known to have several beneficial effects on LAB physiology. Nevertheless, a controversial effect of citrate has been described on the acid tolerance of the wine bacterium Oenococcus oeni. This observation raises questions about the effect of citrate on the capacity of O. oeni to conduct malolactic fermentation in highly acidic wines. This review aims to summarize the current understanding of citrate metabolism in LAB, with a focus on the wine bacterium O. oeni. Metabolism with the related enzymes is detailed, as are the involved genes organized in cit loci. The known systems of cit locus expression regulation are also described. Finally, the beneficial effects of citrate catabolism on LAB physiology are reported and the negative impact observed in O. oeni is discussed.

Microbial interactions shape cheese flavour formation

Cheese fermentation and flavour formation are the result of complex biochemical reactions driven by the activity of multiple microorganisms. Here, we studied the roles of microbial interactions in flavour formation in a year-long Cheddar cheese making process, using a commercial starter culture containing Streptococcus thermophilus and Lactococcus strains. By using an experimental strategy whereby certain strains were left out from the starter culture, we show that S. thermophilus has a crucial role in boosting Lactococcus growth and shaping flavour compound profile. Controlled milk fermentations with systematic exclusion of single Lactococcus strains, combined with genomics, genome-scale metabolic modelling, and metatranscriptomics, indicated that S. thermophilus proteolytic activity relieves nitrogen limitation for Lactococcus and boosts de novo nucleotide biosynthesis. While S. thermophilus had large contribution to the flavour profile, Lactococcus cremoris also played a role by limiting diacetyl and acetoin formation, which otherwise results in an off-flavour when in excess. This off-flavour control could be attributed to the metabolic re-routing of citrate by L. cremoris from diacetyl and acetoin towards α-ketoglutarate. Further, closely related Lactococcus lactis strains exhibited different interaction patterns with S. thermophilus, highlighting the significance of strain specificity in cheese making. Our results highlight the crucial roles of competitive and cooperative microbial interactions in shaping cheese flavour profile.

Microbial survival mechanisms within serpentinizing Mariana forearc sediments

Marine deep subsurface sediment is often a microbial environment under energy-limited conditions. However, microbial life has been found to persist and even thrive in deep subsurface environments. The Mariana forearc represents an ideal location for determining how microbial life can withstand extreme conditions including pH 10-12.5 and depleted nutrients. The International Ocean Discovery Program Expedition 366 to the Mariana Convergent Margin sampled three serpentinizing seamounts located along the Mariana forearc chain with elevated concentrations of methane, hydrogen, and sulfide. Across all three seamount summits, the most abundant transcripts were for cellular maintenance such as cell wall and membrane repair, and the most abundant metabolic pathways were the Entner-Doudoroff pathway and tricarboxylic acid cycle. At flank samples, sulfur cycling involving taurine assimilation dominated the metatranscriptomes. The in situ activity of these pathways was supported by the detection of their metabolic intermediates. All samples had transcripts from all three domains of Bacteria, Archaea, and Eukarya, dominated by Burkholderiales, Deinococcales, and Pseudomonales, as well as the fungal group Opisthokonta. All samples contained transcripts for aerobic methane oxidation (pmoABC) and denitrification (nirKS). The Mariana forearc microbial communities show activity not only consistent with basic survival mechanisms, but also coupled metabolic reactions.

The rotation of primary starter culture mixtures results in batch-to-batch variations during Gouda cheese production

Industrial production of Gouda cheeses mostly relies on a rotated use of different mixed-strain lactic acid bacteria starter cultures to avoid phage infections. However, it is unknown how the application of these different starter culture mixtures affect the organoleptic properties of the final cheeses. Therefore, the present study assessed the impact of three different starter culture mixtures on the batch-to-batch variations among Gouda cheeses from 23 different batch productions in the same dairy company. Both the cores and rinds of all these cheeses were investigated after 36, 45, 75, and 100 weeks of ripening by metagenetics based on high-throughput full-length 16S rRNA gene sequencing accompanied with an amplicon sequence variant (ASV) approach as well as metabolite target analysis of non-volatile and volatile organic compounds. Up to 75 weeks of ripening, the acidifying Lactococcus cremoris and Lactococcus lactis were the most abundant bacterial species in the cheese cores. The relative abundance of Leuconostoc pseudomesenteroides was significantly different for each starter culture mixture. This impacted the concentrations of some key metabolites, such as acetoin produced from citrate, and the relative abundance of non-starter lactic acid bacteria (NSLAB). Cheeses with the least Leuc. pseudomesenteroides contained more NSLAB, such as Lacticaseibacillus paracasei that was taken over by Tetragenococcus halophilus and Loigolactobacillus rennini upon ripening time. Taken together, the results indicated a minor role of leuconostocs in aroma formation but a major impact on the growth of NSLAB. The relative abundance of T. halophilus (high) and Loil. rennini (low) increased with ripening time from rind to core. Two main ASV clusters of T. halophilus could be distinguished, which were differently correlated with some metabolites, both beneficial (regarding aroma formation) and undesirable ones (biogenic amines). A well-chosen T. halophilus strain could be a candidate adjunct culture for Gouda cheese production.

Vitality, fermentation, aroma profile, and digestive tolerance of the newly selected Lactiplantibacillus plantarum and Lacticaseibacillus paracasei in fermented apple juice

Background

To enrich the probiotic lactic acid bacteria (LAB) strains and expand the commercialization of new fermented juice products, we have identified two LAB strains with excellent potential in fermenting apple juice from pickles.

Methods

The two strains were morphologically, physiologically, and genetically characterized. The strains' fermentation performance and alterations in volatile aroma components of apple juice and ability to survive in a simulated gastrointestinal environment were evaluated.

Results

Two strains were identified as Lacticaseibacillus paracasei (WFC 414) and Lactiplantibacillus plantarum (WFC 502). The growth of WFC 414 and WFC 502 in apple juice for 48 h reached 8.81 and 9.33 log CFU/mL, respectively. Furthermore, 92% and 95% survival rates were achieved in 2 h simulated gastric juice, and 80.7 and 83.6% survival rates in 4 h simulated intestinal juice. During the fermentation, WFC 414 and WFC 502 reduced the soluble sugars and total polyphenols in apple juice, and consumed malic acid to produce large amounts of lactic acid (3.48 and 5.94 mg/mL). In addition, the esters and aldehydes were reduced, and the production of alcohols, acids and ketones was elevated in the apple juice fermented by both strains.

Conclusion

These results show that WFC 414 and WFC 502 have great potential applications in the fermented fruit juice industry.

Fermented butter aroma for plant-based applications

Plant-based dairy alternatives are gaining increasing interest, e.g. alternatives to yoghurt, cheese, and butter. In all these products butter flavor (diacetyl + acetoin) plays an important role. We previously have reported efficient butter flavor formation from low value dairy side streams using a dairy isolate of Lactococcus lactis deficient in lactate dehydrogenase. Here, we have tested the ability of this strain, RD1M5, to form butter flavor in plant milks based on oat and soy. We found that oat milk, with its high sugar content, supported more efficient production of butter aroma, when compared to soy milk. When supplemented with glucose, efficient butter aroma production was achieved in soy milk as well. We also carried out an extended adaptive laboratory evolution of the dairy strain in oat milk. After two months of adaptation, we obtained a strain with enhanced capacity for producing butter aroma. Despite of its high sugar content, RD1M5 and its adapted version only metabolized approximately 10% of the fermentable sugars available in the oat milk, which we found was due to amino acid starvation and partly starvation for vitamins. The study demonstrates that dairy cultures have great potential for use in plant-based fermentations.

Effect of Six Lactic Acid Bacteria Strains on Physicochemical Characteristics, Antioxidant Activities and Sensory Properties of Fermented Orange Juices

Six lactic acid bacteria strains were used to study the effects on physicochemical characteristics, antioxidant activities and sensory properties of fermented orange juices. All strains exhibited good growth in orange juice. Of these fermentations, some bioactive compositions (e.g., vitamin C, shikimic acid) and aroma-active compounds (e.g., linalool, 3-carene, ethyl 3-hydroxyhexanoate, etc.) significantly increased in Lactiplantibacillus plantarum and Lactobacillus acidophilus samples. DPPH free radical scavenging rates in L. plantarum and Lacticaseibacillus paracasei samples increased to 80.25% and 77.83%, respectively. Forty-three volatile profiles were identified, including 28 aroma-active compounds. 7 key factors significantly influencing sensory flavors of the juices were revealed, including D-limonene, linalool, ethyl butyrate, ethanol, β-caryophyllene, organic acids and SSC/TA ratio. The orange juice fermented by L. paracasei, with more optimization aroma-active compounds such as D-limonene, β-caryophyllene, terpinolene and β-myrcene, exhibited more desirable aroma flavors such as orange-like, green, woody and lilac incense, and gained the highest sensory score. Generally, L. paracasei fermentation presented better aroma flavors and overall acceptability, meanwhile enhancing antioxidant activities.

Improvement of antioxidant properties of jujube puree by biotransformation of polyphenols via Streptococcus thermophilus fermentation

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Streptococcus thermophilus enriched polyphenols in fermented jujube puree.

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Fermentation improved jujube puree DPPH scavenging capability by 26%.

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12 phenolics were identified as differential metabolites.

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Fermentation could be a promising approach to improve jujube phenolic quality.

To investigate the effect of lactic acid bacteria fermentation on jujube bioactivity, Streptococcus thermophilus was used to ferment jujube puree. The number of viable bacteria cells, physicochemical properties, phenolics profile and antioxidant capacity were analyzed, and their correlation were investigated. Streptococcus thermophilus exhibited a high growth capacity in jujube puree, and significantly (p < 0.05) increased the total phenolics content, 2,2-Diphenyl-1-picrylhydrazyl (DPPH) scavenging activity and reducing power after 48 h fermentation, while 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) scavenging activity was decreased. 12 differentially metabolized polyphenols were identified in fermented jujube puree. Upregulated phenolics exhibited a positive correlation with DPPH radical-scavenging ability and reducing power. This work demonstrated that Streptococcus thermophilus fermentation can be an effective method with great practical application potential to improve the antioxidant activity in jujube puree by modifying the phenolic compositional quantity and quality.

Metabolic Conversions by Lactic Acid Bacteria during Plant Protein Fermentations

To secure a sustainable food supply for the rapidly growing global population, great efforts towards a plant-based diet are underway. However, the use of plant proteins comes with several challenges, such as improvement or removal of undesired flavours, and generation of desired texture properties. Fermentation holds large potential to alter these properties, but compared to dairy fermentations, our knowledge on strain properties in different plant-based substrates is still limited. Here, we explored different lactic acid bacteria for their ability to grow, produce flavour compounds, or remove off-flavour compounds from different plant proteins. For this, 151 LAB strains from dairy and non-dairy origins were cultured in plant protein plus coconut oil emulsions supplemented with glucose. Pea, chickpea, mung, fava, and soybean proteins were used in the study and bacterial strains for screening included the genera Streptococcus, Lactococcus, Lactobacillus, and Leuconostoc. Efficient, high throughput, screening on plant proteins was developed and strains were assessed for their ability to (i) acidify and decrease the pH; (ii) express key enzymes involved in the formation of amino acid derived flavours, which included PepN (aminopeptidase N), PepXP (X-prolyl dipeptidyl peptidase), EstA (esterase), BcAT (branched chain aminotransferase), CBL (cystathione beta lyase), and ArAT (aromatic aminotransferase); and (iii) improve the overall aroma profile by generating dairy/cheesy notes and decreasing off flavours. Suitable screening conditions were determined, and highlighted the importance that a sufficient heat treatment must be applied to samples containing plant proteins, prior to fermentation, as an outgrowth of spore forming Bacillus cereus was observed if the material was only pasteurised. Enzyme activities for strains measured in rich broth vs. a buffered protein solution showed little-to-no correlation, which illustrated the importance of screening conditions to obtain predictive enzyme measurements. Aroma formation analysis allowed to identify strains that were able to increase key aromas such as diacetyl, acetoin, 2- and 3-methyl butanol, and 2,3-pentanedione, as well as decrease the off-flavours hexanal, pentanal, and nonanal. Our findings illustrate the importance of strain specific differences in the assessed functionalities and how a methodical approach to screening LAB can be applied to select suitable microorganisms that show promise in fermentation of plant proteins when applied in non-dairy cheese applications.

Identification of Potential Citrate Metabolism Pathways in Carnobacterium maltaromaticum

In the present study, we describe the identification of potential citrate metabolism pathways for the lactic acid bacterium (LAB) Carnobacterium maltaromaticum. A phenotypic assay indicated that four of six C. maltaromaticum strains showed weak (Cm 6-1 and ATCC 35586) or even delayed (Cm 3-1 and Cm 5-1) citrate utilization activity. The remaining two strains, Cm 4-1 and Cm 1-2 gave negative results. Additional analysis showed no or very limited utilization of citrate in media containing 1% glucose and 22 or 30 mM citrate and inoculated with Cm 6-1 or ATCC 35586. Two potential pathways of citrate metabolism were identified by bioinformatics analyses in C. maltaromaticum including either oxaloacetate (pathway 1) or tricarboxylic compounds such as isocitrate and α-ketoglutarate (pathway 2) as intermediates. Genes encoding pathway 1 were present in two out of six strains while pathway 2 included genes present in all six strains. The two potential citrate metabolism pathways in C. maltaromaticum may potentially affect the sensory profiles of milk and soft cheeses subjected to growth with this species.

Wiring Up Along Electrodes for Biofilm Formation

Millimeter-length cables of bacteria were discovered growing along a graphite-rod electrode serving as an anode of a microbial electrolysis cell (MEC). The MEC had been inoculated with a culture of Fe-reducing microorganisms enriched from a polluted river sediment (Reconquista river, Argentina) and was operated at laboratory controlled conditions for 18 days at an anode poised potential of 240 mV (vs. Ag/AgCl), followed by 23 days at 480 mV (vs. Ag/AgCl). Anode samples were collected for scanning electron microscopy, phylogenetic and electrochemical analyses. The cables were composed of a succession of bacteria covered by a membranous sheath and were distinct from the known "cable-bacteria" (family Desulfobulbaceae). Apparently, the formation of the cables began with the interaction of the cells via nanotubes mostly located at the cell poles. The cables seemed to be further widened by the fusion between them. 16S rRNA gene sequence analysis confirmed the presence of a microbial community composed of six genera, including Shewanella, a well-characterized electrogenic bacteria. The formation of the cables might be a way of colonizing a polarized surface, as determined by the observation of electrodes extracted at different times of MEC operation. Since the cables of bacteria were distinct from any previously described, the results suggest that bacteria capable of forming cables are more diverse in nature than already thought. This diversity might render different electrical properties that could be exploited for various applications.

Dynamics of the Fermentation Process and Chemical Profiling of Pomegranate (Punica granatum L.) Wines Obtained by Different Cultivar×Yeast Combinations

Pomegranate (Punica granatum L.) is one of the historical tree crops in the Mediterranean region and is nowadays commercialized for its beneficial properties in the form of fruits, juice, jams and, in some East countries, as fermented juice (pomegranate wine). However, pomegranate wines are not established as a common beverage in Western countries. In this work, we produced pomegranate wines using two cultivars and two yeasts (Saccharomyces cerevisiae strain Clos and S. cerevisiae ex-bayanus strain EC1118) with contrasting characteristics. A comprehensive chemical profile of the wines was obtained. Notable differences were observed in the function of the cultivars and the yeasts. Different cultivar×yeast combinations provided wines with clearly different chemical profiles and specific features in the patterns of organic acids, phenolics, and volatile compounds. This highlights the opportunity to obtain tailored pomegranate wines with desired chemical profiles and, consequently, sensory properties, through management optimization of pomegranate winemaking. In this view, pomegranate wines have the potential to become an established beverage in Western countries.

Investigation of the possibility of fermentation of red grape juice and rice flour by Lactobacillus plantarum and Lactobacillus casei

The aim of the current study was to evaluate the possibility of the bacterial growth and substrate metabolism during the fermentation of red grape juice and the mixture of red grape juice and rice flour solution using Lactobacillus plantarum and Lactobacillus casei. In recent years, cereal‐based beverages have been used as functional compounds such as antioxidants, dietary fiber, minerals, probiotics, and vitamins in diets. In this research, fermentation of red grape juice (media 1) and 1:1 mixture of red grape juice and rice flour solution (media 2) by two strains of gram positive and homofermentative lactic acid bacteria: L. plantarum and L. casei (individually and mixed) was examined. Fermentation was carried out at 37°C for 48 hr. Microbial population, pH, acidity, sugar, and organic acid metabolism were measured during the fermentation period. Data showed that in media 2 fermented with mixed culture of both L. plantarum and L. casei, acidity and microbial population increased sharply at the initial stages of fermentation, and the most percentage of lactic acid production occurred. Red grape juice fermented with mixture of L. plantarum and L. casei showed the most sugar consumption (p < .05). Results indicated that the use of the mixture of red grape juice and rice flour solution can be a proper substrate for producing lactic acid.

Genomic analysis of Leuconostoc citreum TR116 with metabolic reconstruction and the effects of fructose on gene expression for mannitol production

The species Leuconostoc citreum is often isolated from grain and vegetable fermentations such as sourdough, sauerkraut and kimchi. Lc. citreum has seen an increase in its use as a starter culture for various fermentations and food applications. The strain Lc. citreum TR116 has been applied previously in this laboratory aimed at sugar depletion through metabolism resulting in the reduction of fructose to mannitol, a polyol considered as a sweet carbohydrate. Besides reducing sugar, TR116 showed flavour modulating characteristics and contributes to the extension of microbial shelf life. In order to obtain a better understanding of this strain and to fully use its set of abilities, the genome of Lc. citreum TR116 was sequenced using the Illumina MiSeq, assembly with SPAdes and annotated by the Prokaryotic Genome Annotation Pipeline. Metabolic reconstruction was employed to elucidate carbohydrate, organic acid and amino acid metabolism in the strain. Of particular interest was the gene expression analysis ascertained the influence of fructose on the genes mdh and manX involved in the uptake of fructose and its conversion to mannitol. This investigation, the first in Lc. citreum, illustrates the metabolic processes involved in fermentation used by this strain and demonstrates that in the presence of fructose, expression of the genes mdh and manX is increased. The resulting transparency of the skill set of TR116 contributes highly to future functionalisation of food systems and food ingredients.

Genome-based selection and application of food-grade microbes for chickpea milk fermentation towards increased L-lysine content, elimination of indigestible sugars, and improved flavour

Background

Plant-based milk alternatives are more popular than ever, and chickpea-based milks are among the most commercially relevant products. Unfortunately, limited nutritional value because of low levels of the essential amino acid l-lysine, low digestibility and unpleasant taste are challenges that must be addressed to improve product quality and meet consumer expectations.

Results

Using in-silico screening and food safety classifications, 31 strains were selected as potential l-lysine producers from approximately 2,500 potential candidates. Beneficially, 30% of the isolates significantly accumulated amino acids (up to 1.4 mM) during chickpea milk fermentation, increasing the natural level by up to 43%. The best-performing strains, B. amyloliquefaciens NCC 156 and L. paracasei subsp. paracasei NCC 2511, were tested further. De novo lysine biosynthesis was demonstrated in both strains by ¹³C metabolic pathway analysis. Spiking small amounts of citrate into the fermentation significantly activated l-lysine biosynthesis in NCC 156 and stimulated growth. Both microbes revealed additional benefits in eliminating indigestible sugars such as stachyose and raffinose and converting off-flavour aldehydes into the corresponding alcohols and acids with fruity and sweet notes.

Conclusions

B. amyloliquefaciens NCC 156 and L. paracasei subsp. paracasei NCC 2511 emerged as multi-benefit microbes for chickpea milk fermentation with strong potential for industrial processing of the plant material. Given the high number of l-lysine-producing isolates identified in silico, this concept appears promising to support strain selection for food fermentation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01595-2.

Improvements in makkhan (traditional Indian cultured butter) production: a review

Since Vedic times, traditional Indian cultured butter or makkhan has been one of the most demanding and extensively used food items in the village households of Indian subcontinent. Its processing and overall quality suffers due to the use of conventional practices, which has probably discouraged the production of product in organized sectors. No scientific intervention has taken place to improve the product quality and process since the inception of makkhan making. As an initiative towards the improvement, the present study is focused to prepare a detail scientific background on chemistry, quality attributes, utilization, preparation methods, and storage of product for identifying challenges and scopes of overall improvement in production status. To validate the opportunities identified for the improvements in production various approaches especially mechanized approaches are suggested in this review.

Role of lactic acid bacteria in flavor development in traditional Chinese fermented foods: A review

Traditional Chinese fermented foods are favored by consumers due to their unique flavor, texture and nutritional values. A large number of microorganisms participate in the process of fermentation, especially lactic acid bacteria (LAB), which are present in almost all fermented foods and contribute to flavor development. The formation process of flavor is complex and involves the biochemical conversion of various food components. It is very important to fully understand the conversion process to direct the flavor formation in foods. A comprehensive link between the LAB community and the flavor formation in traditional Chinese fermented foods is reviewed. The main mechanisms involved in the flavor formation dominated by LAB are carbohydrate metabolism, proteolysis and amino acid catabolism, and lipolysis and fatty acid metabolism. This review highlights some useful novel approaches for flavor enhancement, including the application of functional starter cultures and metabolic engineering, which may provide significant advances toward improving the flavor of fermented foods for a promising market.

Lemon juice and apple juice used as source of citrate and malate, respectively, enhance the formation of buttery aroma compounds and/or organic acids during Type 2 and Type 3 sourdough productions performed with Companilactobacillus crustorum LMG 23699

Extra ingredients are often used in traditional sourdough production recipes by artisan bakeries. These ingredients may be the source of microorganisms or stimulate the growth and/or the metabolic activities of the microorganisms added to or naturally present in the flour-water mixture. The present study examined the influence of the addition of lemon juice or apple juice as source of citrate or malate, respectively, on the growth and activity of the citrate- and malate-positive Companilactobacillus crustorum LMG 23699 strain (formerly known as Lactobacillus crustorum LMG 23699), used to initiate firm (dough yield of 200) wheat sourdough productions, and on the flavour of the baked goods produced. Three fermentation strategies were applied, namely one-step long fermentation sourdough production processes with the addition of juice at the start (Type 2) and backslopped fermentations with the addition of juice either only at the start of the sourdough productions or at the start of the sourdough productions and at the beginning of each subsequent refreshment step during the whole backslopping process (both Type 3). It turned out that the starter culture strain used prevailed during all sourdough productions performed. Yeasts were particularly present in Type 3 sourdough productions, although lemon juice retarded their growth. Due to high yeast activity, high concentrations of ethanol and glycerol were produced toward the end of the sourdough productions. Addition of lemon juice stimulated the production of lactic acid, acetic acid, and the buttery flavour compounds acetoin and diacetyl, because of citrate conversion, during the Type 2 and Type 3 sourdough productions. In Type 3 sourdough productions, these compounds were found in higher concentrations only when lemon juice was added at each backslopping step. Alternatively, the addition of apple juice led to high concentrations of lactic acid because of malolactic fermentation in both Type 2 and Type 3 sourdough productions. Moreover, the addition of apple juice increased the initial concentrations of the carbohydrates (fructose, glucose, and sucrose) and sugar alcohols (mannitol and sorbitol), which were exhausted upon backslopping or accumulated in the sourdough matrix, respectively. Baked goods produced using sourdoughs obtained from the Type 2 and Type 3 sourdough productions with the addition of juice at each backslopping step were significantly different in flavour from doughs supplemented with the respective juices and lactic acid and/or Type 3 sourdough productions with the addition of juice only at the start.

Impact of lactic acid fermentation on sensory and chemical quality of dairy analogues prepared from lupine (Lupinus angustifolius L.) seeds

Lupine (Lupinus sp.) is a valuable source of plant proteins. There is little knowledge on the impact of food processing on composition and sensory properties of lupine products. In this research, we investigated the impact of fermentation with five starters of lactic acid bacteria on the sensory quality and flavor-active compounds in dairy analogues prepared from sweet lupine (Lupinus angustifolius L.). The sensory qualities of unfermented and fermented products were studied with generic descriptive analysis and affective tests. Acids and sugars were analyzed with GC-FID and volatiles with HS-SPME-GC-MS and GC-O. Fermentation increased sourness and 'vinegar' odor and reduced the 'beany' odor and flavor as well as the unpleasantness of flavor. Formation of volatiles during the fermentation was dependent on the starters. However, all fermentations increased the contents of lactic, acetic, and hexanoic acids, while reducing the contents of hexanal, described as 'grassy' in the unfermented lupine sample.

Differential Analysis of Stress Tolerance and Transcriptome of Probiotic Lacticaseibacillus casei Zhang Produced from Solid-State (SSF-SW) and Liquid-State (LSF-MRS) Fermentations

The property differences between bacteria produced from solid-state and liquid-state fermentations have always been the focus of attention. This study analyzed the stress tolerance and transcriptomic differences of the probiotic Lacticaseibacillus casei Zhang produced from solid-state and liquid-state fermentations under no direct stress. The total biomass of L. casei Zhang generated from liquid-state fermentation with MRS medium (LSF-MRS) was 2.24 times as much as that from solid-state fermentation with soybean meal-wheat bran (SSF-SW) medium. Interestingly, NaCl, H2O2, and ethanol stress tolerances and the survival rate after L. casei Zhang agent preparation from SSF-SW fermentation were significantly higher than those from LSF-MRS fermentation. The global transcriptomic analysis revealed that in L. casei Zhang produced from SSF-SW fermentation, carbohydrate transport, gluconeogenesis, inositol phosphate metabolism were promoted, that pentose phosphate pathway was up-regulated to produce more NADPH, that citrate transport and fermentation was extremely significantly promoted to produce pyruvate and ATP, and that pyruvate metabolism was widely up-regulated to form lactate, acetate, ethanol, and succinate from pyruvate and acetyl-CoA, whereas glycolysis was suppressed, and fatty acid biosynthesis was suppressed. Moreover, in response to adverse stresses, some genes encoding aquaporins (GlpF), superoxide dismutase (SOD), nitroreductase, iron homeostasis-related proteins, trehalose operon repressor TreR, alcohol dehydrogenase (ADH), and TetR/AcrR family transcriptional regulators were up-regulated in L. casei Zhang produced from SSF-SW fermentation. Our findings provide novel insight into the differences in growth performance, carbon and lipid metabolisms, and stress tolerance between L. casei Zhang from solid-state and liquid-state fermentations.

The Role of Yeasts and Lactic Acid Bacteria on the Metabolism of Organic Acids during Winemaking

The main role of acidity and pH is to confer microbial stability to wines. No less relevant, they also preserve the color and sensory properties of wines. Tartaric and malic acids are generally the most prominent acids in wines, while others such as succinic, citric, lactic, and pyruvic can exist in minor concentrations. Multiple reactions occur during winemaking and processing, resulting in changes in the concentration of these acids in wines. Two major groups of microorganisms are involved in such modifications: the wine yeasts, particularly strains of Saccharomyces cerevisiae, which carry out alcoholic fermentation; and lactic acid bacteria, which commonly conduct malolactic fermentation. This review examines various such modifications that occur in the pre-existing acids of grape berries and in others that result from this microbial activity as a means to elucidate the link between microbial diversity and wine composition.

Characterization and ACE Inhibitory Activity of Fermented Milk with Probiotic Lactobacillus plantarum K25 as Analyzed by GC-MS-Based Metabolomics Approach

Addition of probiotics to yogurt with desired health benefits is gaining increasing attention. To further understand the effect of probiotic Lactobacillus plantarum on the quality and function of fermented milk, probiotic fermented milk (PFM) made with probiotic L. plantarum K25 and yogurt starter (L. delbrueckii ssp. bulgaricus and Streptococcus thermophilus) was compared with the control fermented milk (FM) made with only the yogurt starter. The probiotic strain was shown to survive well with a viable count of 7.1 ± 0.1 log CFU/g in the PFM sample after 21 days of storage at 4°C. The strain was shown to promote formation of volatiles such as acetoin and 2,3-butanediol with milk fragrance, and it did not cause post-acidification during refrigerated storage. Metabolomics analysis by GC-MS datasets coupled with multivariate statistical analysis showed that addition of L. plantarum K25 increased formation of over 20 metabolites detected in fermented milk, among which γ-aminobutyric acid was the most prominent. Together with several other metabolites with relatively high levels in fermented milk such as glyceric acid, malic acid, succinic acid, glycine, alanine, ribose, and 1,3-dihydroxyacetone, they might play important roles in the probiotic function of L. plantarum K25. Further assay of the bioactivity of the PFM sample showed significant (p < 0.05) increase of ACE inhibitory activity from 22.3% at day 1 to 49.3% at day 21 of the refrigerated storage. Therefore, probiotic L. plantarum K25 could be explored for potential application in functional dairy products.

Nutritional requirements for Lactobacillus vini growth in sugarcane derivative substrate of ethanol fermentation

Lactobacillus vini is a bacterial contaminant found in industrial environments of winemaking and fuel-ethanol fermentation. However, there has been no standard analysis of its physiology that can pinpoint its adaptive traits to these kinds of environments. In view of this lack of information, the aim of this study is to determine the nutritional factors that lead to the growth of L. vini in the industrial plants of fuel-ethanol. First of all, the limited growth of this bacterium was studied in the industrial substrate, which was improved by nutritional supplementation with amino acids, and its homofermentative status was confirmed. Metabolite analysis showed that citrate is a growth factor of paramount importance for this bacterium in industrial processes through pyruvate metabolization, and increases ATP production and biomass formation. Furthermore,e acetate uptake, either from the medium or generated from citrate metabolism, was assimilated for biomass production. Hence, a metabolic model was designed to describe the role of citrate and acetate in the growth of L. vini that could be tested on other lactobacilli.

Improvement of the Sensory Characteristics of Goat Milk Yogurt

The sensory characteristics of goat yogurt could be greatly improved by integrating to a culture typical of yogurt starters, Streptococcus thermophilus, Lactobacillus delbrueckii spp. bulgaricus, with cultures of Leuconostoc lactis. The addition of L. lactis requires the use of the definition of  "fermented milk" to the product, although it is prepared according to the same technology. The experimental plan has designed the use of inoculation with a bacterial concentration of at least 109 , for each microbial species considered, with the aim of increasing the probiotic effect of yogurt and fermented milk. Descriptive analysis of the samples was conducted using the profile sheet obtained applying the consensus profiling method (UNI EN ISO 13299:2010) by a trained panel. The acceptability of our products was evaluated by 62 consumers through 7-points hedonic scale. The results of this study indicated that the addition of L. lactis to the traditional yogurt starter produced a noticeable improvement to the sensory characteristics of fermented goat milk. PRACTICAL APPLICATION: To improve the sensory characteristics of goat's yogurt to make it more acceptable to the consumer and increase market demand we tested the integration of Leuconostoc lactis to typical starter cultures. The experimentation carried out tested different combinations of fermenting microorganisms with the aim of developing a better flavor as compared to that of traditional yogurt, which is often unpleasant to consumers. The addition of L. lactis to the starter culture of the goat's milk results in less aggressive aromatic notes and greater appreciation as confirmed by consumers who tested the product.

Effect of different Lactobacillus species on volatile and nonvolatile flavor compounds in juices fermentation

Lactobacillus is the dominant genus during fruit and vegetable juices (FVFs) fermentation, which are the key factors for taste and flavor. This study was performed to investigate the effects of different Lactobacillus spp. on profile of volatile flavor compounds and nonvolatile taste compounds in FVFs fermentation. A total of 14 compounds were identified as discriminant flavor and taste markers for fermented FVFs via gas chromatography-mass spectrometry (GC-MS)-based multimarker profiling. The PCA score plot and PLS-DA showed that different FVFs were divided into three distinct types, suggesting that the different species significantly affect the volatile and nonvolatile compounds profiles of FVFs. Lactobacillus casei and Lactobacillus rhamnosus (Type A FVFs) might make a greater contribution to the umami taste. Lactobacillus plantarum and Lactobacillus acidophilus (Type B FVFs) make a greater contribution to the sour taste. Lactobacillus fermentum may be an potential critical contributor to produce volatile compounds. We reveal that different Lactobacillus strains play different roles in modifying these compounds related to flavor and taste features.

Screening of aroma-producing lactic acid bacteria and their application in improving the aromatic profile of yogurt

The development of yogurt flavor is a complicated and dynamic biochemical process. In addition to traditional starter cultures, adjunct cultures could also make significant contributions to the flavor profiles of yogurt. In the current study, two Lactobacillus plantarum strains (1-33 and 1-34) were isolated based on their abilities to produce acetaldehyde and diacetyl. In co-fermentation with traditional starters, these isolated strains were able to maintain viability without affecting the yogurt's acidification profiles. Furthermore, they positively influenced the aroma quality of the yogurt samples. They promoted the formation of volatile metabolites, especially acetaldehyde, diacetyl, and acetoin, which are recognized as characteristic compounds. The results of this work provide novel knowledge about the contributions of isolated strains on the flavor profiles of yogurt, which will help to improve the organoleptic properties of the final products. PRACTICAL APPLICATIONS: Using lactic acid bacteria (LAB) as adjunct cultures co-fermented with traditional yogurt starter cultures can increase the quantities of flavor compounds in yogurt. This study enriches our understanding of the effects of adjunct cultures on yogurt flavor. Researchers and manufacturers that specialize in yogurt making can use the results of this study to improve the aromatic profile and organoleptic quality of yogurt.

Metabolomic and transcriptomic changes underlying cold and anaerobic stresses after storage of table grapes

The currently accepted paradigm is that fruits and vegetables should be consumed fresh and that their quality deteriorates during storage; however, there are indications that some metabolic properties can, in fact, be improved. We examined the effects of low temperature and high-CO₂ conditions on table grapes, Vitis vinifera L. cv. 'Superior Seedless'. Berries were sampled at harvest (T0) and after low-temperature storage for 6 weeks under either normal atmosphere conditions (TC) or under an O₂ level of 5 kPa and elevated CO₂ levels of 5, 10 or 15 kPa (T5, T10, T15). Accumulation of 10 stilbenes, including E-ε-viniferin, E-miyabenol C and piceatannol, significantly increased under TC treatment as compared to T0 or T15. Sensory analysis demonstrated that elevated CO₂ elicited dose-dependent off-flavor accumulation. These changes were accompanied by an accumulation of 12 volatile metabolites, e.g., ethyl acetate and diacetyl, that imparted disagreeable flavors to fresh fruit. Transcriptome analysis revealed enrichment of genes involved in pyruvate metabolism and the phenylpropanoid pathway. One of the transcription factors induced at low temperature but not under high CO₂ was VvMYB14, which regulates stilbene biosynthesis. Our findings reveal the potential to alter the levels of targeted metabolites in stored produce through understanding the effects of postharvest treatments.

Aroma formation in retentostat co-cultures of Lactococcus lactis and Leuconostoc mesenteroides

Lactococcus lactis subsp. lactis biovar diacetylactis and Leuconostoc mesenteroides are considered to be the main aroma producers in Dutch-type cheeses. Both species of lactic acid bacteria were grown in retentostat mono- and co-cultures to investigate their interaction at near-zero growth rates and to determine if co-cultivation enhances the aroma complexity compared to single species performance. During retentostat mono-cultures, the growth rates of both species decreased to less than 0.001 h^-1 and a large fraction of the cells became viable but not culturable. Compared to Lc. mesenteroides, L. lactis reached a 3.4-fold higher biomass concentration caused by i) a higher ATP yield on substrate, ii) a higher biomass yield on ATP and iii) a lower maintenance requirement (m_ATP). Dynamic models estimated that the m_ATP of both species decreased approximately 7-fold at near-zero growth rates compared to high growth rates. Extension of these models by assuming equal substrate distribution resulted in excellent prediction of the biomass accumulation in retentostat co-cultures with L. lactis dominating (100:1) as observed in ripened cheese. Despite its low abundance (∼1%), Lc. mesenteroides contributed to aroma production in co-cultures as indicated by the presence of all 5 specific Lc. mesenteroides compounds. This study provides insights in the production of cheese aroma compounds outside the cheese matrix by co-cultures of L. lactis and Lc. mesenteroides, which could be used as food supplements in dairy or non-dairy products.

Lactic acid bacteria as starter cultures: An update in their metabolism and genetics

Lactic acid bacteria (LAB) are members of an heterogenous group of bacteria which plays a significant role in a variety of fermentation processes. The general description of the bacteria included in the group is gram-positive, non-sporing, non-respiring cocci or rods. An overview of the genetics of lactococci, Streptococcus thermophilus, lactobacilli, pediococci, leuconostocs, enterococci and oenococciis presented with special reference to their metabolic traits. The three main pathways in which LAB are involved in the manufacture of fermented foods and the development of their flavour, are (a) glycolysis (fermentation of sugars), (b) lipolysis (degradation of fat) and (c) proteolysis (degradation of proteins). Although the major metabolic action is the production of lactic acid from the fermentation of carbohydrates, that is, the acidification of the food, LAB are involved in the production of many beneficial compounds such as organic acids, polyols, exopolysaccharides and antimicrobial compounds, and thus have a great number of applications in the food industry (i.e. starter cultures). With the advances in the genetics, molecular biology, physiology, and biochemistry and the reveal and publication of the complete genome sequence of a great number of LAB, new insights and applications for these bacteria have appeared and a variety of commercial starter, functional, bio-protective and probiotic cultures with desirable properties have marketed.

Potential of three probiotic lactobacilli in transforming star fruit juice into functional beverages

The star fruit is popularly cultivated and consumed in Southeast Asia due to its high antioxidant capacity and various nutrients. In this study, three commercial probiotic strains (Lactobacillus helveticus L10, Lactobacillus paracasei L26, and Lactobacillus rhamnosus HN001) were evaluated in star fruit juice fermentation and all strains grew well with the final cell counts of around 108 CFU/ml. The star fruit juice fermented by L. rhamnosus produced the highest amount of lactic acid, resulting in a significant lower pH (4.41) than that of L. helveticus (4.76) and L. paracasei (4.71). Most of aldehydes and esters endogenous in star fruit juice decreased to low or undetectable levels, while ketones, alcohols, and fatty acids were produced at varying levels that could impart different aroma notes to the beverages. Therefore, the selection of appropriate probiotics can be an alternative way to develop new functional beverages from star fruit juice with specific aroma notes.

Batch Electrodialysis of Lactic Acid Obtained from Lab Fermentation

The aim of this work was to develop the method of lactic acid (LA) separation from fermented whey. CMI-7000 Cation Exchange Membrane and AMI-7001 Anion Exchange Membrane were employed in electrodialysis process. Experiments showed that the selected membranes separated organic acids effectively (including LA) from other organic ingredients present in medium. Selecting an appropriate volume of a receiving chamber could lead to LA concentration. Moreover, membrane fouling during separation was investigated. This phenomenon is negligible which is the main advantage of this process. As it was shown during batch processes, with the voltage increase, the rate of electrodialysis increases as well. It prompts to a reduction of residence time in electrodialyzer during a continuous separation.

The addition of citrate stimulates the production of acetoin and diacetyl by a citrate-positive Lactobacillus crustorum strain during wheat sourdough fermentation

In traditional sourdough fermentation recipes of artisan bakeries, often extra ingredients are added to the flour-water mixture. This may accentuate the aroma and taste of the baked products produced from such sourdoughs. This is made possible, for instance, by stimulating certain microbial activities during fermentation. This study examined the effects of the addition of citrate (a food-grade organic acid present in milk and lemons) on wheat sourdough fermentation in the presence of a citrate-positive, homofermentative, lactic acid bacterial starter culture strain, namely Lactobacillus crustorum LMG 23699. Both liquid and firm wheat sourdoughs were produced. The starter culture strain was able to steer all wheat sourdough fermentations performed, as it always prevailed due to its competitiveness, as shown through culture-dependent microbiological plating and culture-independent bacterial community profiling. Moreover, it possessed all enzyme-encoding genes (as unraveled through genome mining) necessary to convert citrate into desirable compounds such as lactic acid, acetic acid, succinate, acetoin, diacetyl, and 2,3-butanediol. Indeed, citrate addition to the wheat flour-water mixture had an impact on the sourdough fermentation dynamics and thus on the aroma profile of the liquid and firm sourdoughs produced and breads made thereof. A higher final pH, higher total titratable acidity values, and low yeast counts were found in wheat sourdoughs produced with citrate. In particular, the starter culture strain added converted the supplemented citrate into more l-lactic acid as well as acetoin and diacetyl (buttery aroma compounds), which was independent of the dough yield. The buttery aroma compounds were also accentuated in the concomitant breads produced. Further, organic acid production was stimulated in the sourdoughs, whereas increased pyrazine concentrations occurred in the breads. Consequently, citrate supplementation to wheat sourdoughs could be of interest to produce baked goods with enhanced buttery aroma compounds and notes.