Thiamine-Starved Lactococcus lactis for Producing Food-Grade Pyruvate.

Lactococcus lactis is a safe lactic acid bacterium widely used in dairy fermentations. Normally, its main fermentation product is lactic acid; however, L. lactis can be persuaded into producing other compounds, e.g., through genetic engineering. Here, we have explored the possibility of rewiring the metabolism of L. lactis into producing pyruvate without using genetic tools. Depriving the thiamine-auxotrophic and lactate dehydrogenase-deficient L. lactis strain RD1M5 of thiamine efficiently shut down two enzymes at the pyruvate branch, the thiamine pyrophosphate (TPP) dependent pyruvate dehydrogenase (PDHc) and α-acetolactate synthase (ALS). After eliminating the remaining enzyme acting on pyruvate, the highly oxygen-sensitive pyruvate formate lyase (PFL), by simple aeration, the outcome was pyruvate production. Pyruvate could be generated by nongrowing cells and cells growing in a substrate low in thiamine, e.g., Florisil-treated milk. Pyruvate is a precursor for the butter aroma compound diacetyl. Using an α-acetolactate decarboxylase deficient L. lactis strain, pyruvate could be converted to α-acetolactate and diacetyl. Summing up, by starving L. lactis for thiamine, secretion of pyruvate could be attained. The food-grade pyruvate produced has many applications, e.g., as an antioxidant or be used to make butter aroma.

Single-cell resolution of human airway epithelial cells exposed to bronchiolitis obliterans-associated chemicals.

Bronchiolitis obliterans (BO) is a fibrotic lung disease characterized by progressive luminal narrowing and obliteration of the small airways. In the nontransplant population, inhalation exposure to certain chemicals is associated with BO; however, the mechanisms contributing to disease induction remain poorly understood. This study's objective was to use single-cell RNA sequencing for the identification of transcriptomic signatures common to primary human airway epithelial cells after chemical exposure to BO-associated chemicals-diacetyl or nitrogen mustard-to help explain BO induction. Primary airway epithelial cells were cultured at air-liquid interface and exposed to diacetyl, nitrogen mustard, or control vapors. Cultures were dissociated and sequenced for single-cell RNA. Differential gene expression and functional pathway analyses were compared across exposures. In total, 75,663 single cells were captured and sequenced from all exposure conditions. Unbiased clustering identified 11 discrete phenotypes, including 5 basal, 2 ciliated, and 2 secretory cell clusters. With chemical exposure, the proportion of cells assigned to keratin 5+ basal cells decreased, whereas the proportion of cells aligned to secretory cell clusters increased compared with control exposures. Functional pathway analysis identified interferon signaling and antigen processing/presentation as pathways commonly upregulated after diacetyl or nitrogen mustard exposure in a ciliated cell cluster. Conversely, the response of airway basal cells differed significantly with upregulation of the unfolded protein response in diacetyl-exposed basal cells, not seen in nitrogen mustard-exposed cultures. These new insights provide early identification of airway epithelial signatures common to BO-associated chemical exposures.NEW & NOTEWORTHY Bronchiolitis obliterans (BO) is a devastating fibrotic lung disease of the small airways, or bronchioles. This original manuscript uses single-cell RNA sequencing for identifying common signatures of chemically exposed airway epithelial cells in BO induction. Chemical exposure reduced the proportion of keratin 5+ basal cells while increasing the proportion of keratin 4+ suprabasal cells. Functional pathways contributory to these shifts differed significantly across exposures. These new results highlight similarities and differences in BO induction across exposures.

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.

Oxidative Products of Curcumin Rather Than Curcumin Bind to Helicobacter Pylori Virulence Factor VacA and Are Required to Inhibit Its Vacuolation Activity.

Curcumin is a hydrophobic polyphenol derived from turmeric with potent anti-oxidant, anti-microbial, anti-inflammatory and anti-carcinogenic effects. Curcumin is degraded into various derivatives under in vitro and in vivo conditions, and it appears that its degradation may be responsible for the pharmacological effects of curcumin. The primary risk factor for the cause of gastric cancer is Helicobacter pylori (H. pylori). A virulence factor vacuolating cytotoxic A (VacA) is secreted by H. pylori as a 88 kDa monomer (p88), which can be fragmented into a 33 kDa N-terminal domain (p33) and a 55 kDa C-terminal domain (p55). Recently it has been reported that curcumin oxidation is required to inhibit the activity of another major H.pylori toxin CagA. We performed molecular docking of curcumin and its oxidative derivatives with p33 and p55 domains of VacA. Further, we have examined the effect of the oxidation of curcumin on the vacuolation activity of VacA protein. We observed the binding of curcumin to the p55 domain of VacA at five different sites with moderate binding affinities. Curcumin did not bind to p33 domain of VacA. Remarkably, cyclobutyl cyclopentadione and dihydroxy cyclopentadione, which are oxidized products of curcumin, showed a higher binding affinity with VacA protein at all sites except one as compared to parent curcumin itself. However, cyclobutyl cyclopentadione showed a significant binding affinity for the active site 5 of the p55 protein. Active site five (312-422) of p55 domain of VacA plays a crucial role in VacA-mediated vacuole formation. Invitro experiments showed that curcumin inhibited the vacuolation activity of H. pylori in human gastric cell line AGS cells whereas acetyl and diacetyl curcumin, which cannot be oxidized, failed to inhibit the vacuolation in AGS cells after H. pylori infection. Here our data showed that oxidation is essential for the activity of curcumin in inhibiting the vacuolation activity of H. pylori. Synthesis of these oxidized curcumin derivatives could potentially provide new therapeutic drug molecules for inhibiting H. pylori-mediated pathogenesis.

Developmental history modulates adult olfactory behavioral preferences via regulation of chemoreceptor expression in Caenorhabditiselegans.

Developmental experiences play critical roles in shaping adult physiology and behavior. We and others previously showed that adult Caenorhabditiselegans which transiently experienced dauer arrest during development (postdauer) exhibit distinct gene expression profiles as compared to control adults which bypassed the dauer stage. In particular, the expression patterns of subsets of chemoreceptor genes are markedly altered in postdauer adults. Whether altered chemoreceptor levels drive behavioral plasticity in postdauer adults is unknown. Here, we show that postdauer adults exhibit enhanced attraction to a panel of food-related attractive volatile odorants including the bacterially produced chemical diacetyl. Diacetyl-evoked responses in the AWA olfactory neuron pair are increased in both dauer larvae and postdauer adults, and we find that these increased responses are correlated with upregulation of the diacetyl receptor ODR-10 in AWA likely via both transcriptional and posttranscriptional mechanisms. We show that transcriptional upregulation of odr-10 expression in dauer larvae is in part mediated by the DAF-16 FOXO transcription factor. Via transcriptional profiling of sorted populations of AWA neurons from control and postdauer animals, we further show that the expression of a subset of additional chemoreceptor genes in AWA is regulated similarly to odr-10 in postdauer animals. Our results suggest that developmental experiences may be encoded at the level of olfactory receptor regulation, and provide a simple mechanism by which C. elegans is able to precisely modulate its behavioral preferences as a function of its current and past experiences.

Behavioral Forgetting of Olfactory Learning Is Mediated by Interneuron-Regulated Network Plasticity in Caenorhabditis elegans.

Forgetting is important for animals to manage acquired memories to enable adaptation to changing environments; however, the neural network in mechanisms of forgetting is not fully understood. To understand the mechanisms underlying forgetting, we examined olfactory adaptation, a form of associative learning, in Caenorhabditis elegans The forgetting of diacetyl olfactory adaptation in C. elegans is regulated by secreted signals from AWC sensory neurons via the TIR-1/JNK-1 pathway. These signals cause a decline of the sensory memory trace in AWA neurons, where diacetyl is mainly sensed. To further understand the neural network that regulates this forgetting, we investigated the function of interneurons downstream of AWA and AWC neurons. We found that a pair of interneurons, AIA, is indispensable for the proper regulation of behavioral forgetting of diacetyl olfactory adaptation. Loss or inactivation of AIA caused the impairment of the chemotaxis recovery after adaptation without causing severe chemotaxis defects in the naive animal. AWA Ca2+ imaging analyses suggested that loss or inactivation of AIA interneurons did not affect the decline of the sensory memory trace after the recovery. Furthermore, AIA responses to diacetyl were observed in naive animals and after the recovery, but not just after the conditioning, suggesting that AIA responses after the recovery are required for the chemotaxis to diacetyl. We propose that the functional neuronal circuit for attractive chemotaxis to diacetyl is changed temporally at the recovery phase so that AIA interneurons are required for chemotaxis, although AIAs are dispensable for attractive chemotaxis to diacetyl in naive animals.

Molecular mechanism of two functional protein structure changes under 2,3-butanedione-induced oxidative stress and apoptosis effects in the hepatocytes.

Food security has become closely watched with the occurrence of a series of food safety incidents in recent years. The widespread adoption of 2,3-butanedione (BUT), as a food additive, is an unpreventable significant risk factor to food security. Based on this, mouse hepatocyte AML-12 cells and two functional proteins (bovine serum albumin and lysozyme) were utilized as targeted receptors to study the adverse effects of BUT at the cellular and molecular levels. Results suggested that BUT could disrupt the redox balance of AML-12 cells, reducing glutathione (GSH) activity fell to 87.18 %, which cannot offset the production of reactive oxygen species (ROS). Meanwhile, the increasement of lipid peroxidation and malondialdehyde (MDA) levels were observed. The mitochondrial membrane function was also abnormal due to the excessive accumulation of ROS and eventually leads to cell apoptosis and death. At the molecular level, the exposure of BUT could alter the skeleton and secondary structure of bovine serum albumin (BSA) and lysozyme (LYZ), and it could statically quench the intrinsic fluorescence of proteins. The combined experiments confirmed proved the potentially toxic effects of BUT accumulation on the detoxification organ, providing theoretical support for the liver diseases caused by BUT exposure, and a reference for the risk assessment of occupational exposure of BUT.

Harnessing diversity of Lactococcus lactis from raw goat milk: Design of an indigenous starter for the production of Rocamadour, a French PDO cheese.

Twenty-four strains of Lactococcus lactis isolated from raw goat milk collected in the Rocamadour PDO area were analysed by MLST typing and phenotypic characterisation. The strains were combined to design an indigenous starter for the production of Rocamadour PDO cheese. The strains were divided into three classes based on their technological properties: acidifying and proteolytic strains in class I (12/24 strains), slightly acidifying and non-proteolytic strains in class II (2/24 strains), and non-acidifying and non-proteolytic strains in class III (10/24 strains). Interestingly, all but three strains (21/24) produced diacetyl/acetoin despite not having citrate metabolism genes, as would classically be expected for the production of these aroma compounds. Three strains (EIP07A, EIP13D, and EIP20B) were selected for the indigenous starter based on the following inclusion/exclusion criteria: (i) no negative interactions between included strains, (ii) ability to metabolize lactose and at least one strain with the prtP gene and/or capable of producing diacetyl/acetoin, and (iii) selected strains derived from different farms to maximise genetic and phenotypic diversity. Despite consisting exclusively of L. lactis strains, the designed indigenous starter allowed reproducible cheese production with performances similar to those obtained with an industrial starter and with the sensory qualities expected of Rocamadour PDO cheese.

Volatile and soluble metabolite profiles in surface-ripened cheeses with Aspergillus oryzae and Aspergillus sojae.

To determine the impact of traditional koji molds on chemical characteristics of soft-type natural cheese, novel surface mold-ripened cheeses with Aspergillus oryzae and Aspergillus sojae were studied by non-targeted metabolite profiling. Comprehensive water-soluble and volatile metabolite profiles of koji cheese were evaluated among five Aspergillus strains and other mold-ripened cheeses. Time-course changes in the metabolite profiles and degrading enzyme activities were also compared with those of an industrial Penicillium candidum starter culture. Koji cheeses differed from Camembert, Brie, and blue cheeses in higher lactic acid, amino acid, and acetoin levels and lower methyl ketone and volatile fatty acid levels. Time-course analysis revealed the associations of rapid accumulations of glutamic, aspartic, and 3-methylbutanoic acids and 3-methylbutanal with higher proteolytic activity, and methyl ketone and fatty acid derivative suppressions with lower lipolytic activity. Ethyl butanoate, diacetyl, and malic acid also characterized koji cheeses as strain-dependent metabolites. This study highlighted the key compositional difference derived from cheese ripening with Aspergillus strains. The findings could help quality improvements of koji cheese product.

Post-translational modifications to hemidesmosomes in human airway epithelial cells following diacetyl exposure.

Diacetyl (DA; 2,3-butanedione) is a highly reactive alpha (α)-diketone. Inhalation exposure to DA can cause significant airway epithelial cell injury, however, the mechanisms of toxicity remain poorly understood. The purpose of these experiments was to assess for changes in abundance and distribution of hemidesmosome-associated proteins following DA exposure that contribute to DA-induced epithelial toxicity. Human bronchial epithelial cells were grown in submerged cultures and exposed to three occupationally-relevant concentrations of DA (5.7, 8.6, or 11.4 mM) for 1 h. Following DA exposure, epithelial cells were cultured for 4 days to monitor for cell viability by MTT and WST-1 assays as well as for changes in cellular distribution and relative abundance of multiple hemidesmosome-associated proteins, including keratin 5 (KRT5), plectin (PLEC), integrin alpha 6 (ITGα6) and integrin beta 4 (ITGß4). Significant toxicity developed in airway epithelial cells exposed to DA at concentrations ≥ 8.6 mM. DA exposure resulted in post-translational modifications to hemidesmosome-associated proteins with KRT5 crosslinking and ITGß4 cleavage. Following DA exposure at 5.7 mM, these post-translational modifications to KRT5 resolved with time. Conversely, at DA concentrations ≥ 8.6 mM, modifications to KRT5 persisted in culture with decreased total abundance and perinuclear aggregation of hemidesmosome-associated proteins. Significant post-translational modifications to hemidesmosome-associated proteins develop in airway epithelial cells exposed to DA. At DA concentrations ≥ 8.6 mM, these hemidesmosome modifications persist in culture. Future work targeting hemidesmosome-associated protein modifications may prevent the development of lung disease following DA exposure.

Chemoinformatic Exploration of "Bioinspired Metabolomes" Illuminates Diacetyl Assembly Pathways Toward Nesteretal A-Like Cage Molecules.

An appealing and challenging cage structure along with an unusual biosynthetic pathway prompted us to explore an expeditious bioinspired one-pot total synthesis of nesteretal A. An unconventional strategy was chosen, and a cascade reaction starting from diacetyl was studied. Under organocatalytic conditions mimicking an aldolase, nesteretal A and a related cage analogue were anticipated by in silico metabolization, detected, targeted, and characterized.

Repetitive diacetyl vapor exposure promotes ubiquitin proteasome stress and precedes bronchiolitis obliterans pathology.

Bronchiolitis obliterans (BO) is a devastating lung disease seen commonly after lung transplant, following severe respiratory tract infection or chemical inhalation exposure. Diacetyl (DA; 2,3-butanedione) is a highly reactive alpha-diketone known to cause BO when inhaled, however, the mechanisms of how inhalation exposure leads to BO development remains poorly understood. In the current work, we combined two clinically relevant models for studying the pathogenesis of DA-induced BO: (1) an in vivo rat model of repetitive DA vapor exposures with recovery and (2) an in vitro model of primary human airway epithelial cells exposed to pure DA vapors. Rats exposed to 5 consecutive days 200 parts-per-million DA 6 h per day had worsening survival, persistent hypoxemia, poor weight gain, and histologic evidence of BO 14 days after DA exposure cessation. At the end of exposure, increased expression of the ubiquitin stress protein ubiquitin-C accumulated within DA-exposed rat lung homogenates and localized primarily to the airway epithelium, the primary site of BO development. Lung proteasome activity increased concurrently with ubiquitin-C expression after DA exposure, supportive of significant proteasome stress. In primary human airway cultures, global proteomics identified 519 significantly modified proteins in DA-exposed samples relative to controls with common pathways of the ubiquitin proteasome system, endosomal reticulum transport, and response to unfolded protein pathways being upregulated and cell-cell adhesion and oxidation-reduction pathways being downregulated. Collectively, these two models suggest that diacetyl inhalation exposure causes abundant protein damage and subsequent ubiquitin proteasome stress prior to the development of chemical-induced BO pathology.

Toxic stress-specific cytoprotective responses regulate learned behavioral decisions in C. elegans.

BACKGROUND: Recognition of stress and mobilization of adequate "fight-or-flight" responses is key for survival and health. Previous studies have shown that exposure of Caenorhabditis elegans to pathogens or toxins simultaneously stimulates cellular stress and detoxification responses and aversive behavior. However, whether a coordinated regulation exists between cytoprotective stress responses and behavioral defenses remains unclear. RESULTS: Here, we show that exposure of C. elegans to high concentrations of naturally attractive food-derived odors, benzaldehyde and diacetyl, induces toxicity and food avoidance behavior. Benzaldehyde preconditioning activates systemic cytoprotective stress responses involving DAF-16/FOXO, SKN-1/Nrf2, and Hsp90 in non-neuronal cells, which confer both physiological (increased survival) and behavioral tolerance (reduced food avoidance) to benzaldehyde exposure. Benzaldehyde preconditioning also elicits behavioral cross-tolerance to the structurally similar methyl-salicylate, but not to the structurally unrelated diacetyl. In contrast, diacetyl preconditioning augments diacetyl avoidance, weakens physiological diacetyl tolerance, and does not induce apparent molecular defenses. The inter-tissue connection between cellular and behavioral defenses is mediated by JNK-like stress-activated protein kinases and the neuropeptide Y receptor NPR-1. Reinforcement of the stressful experiences using spaced training forms stable stress-specific memories. Memory retrieval by the olfactory cues leads to avoidance of food contaminated by diacetyl and context-dependent behavioral decision to avoid benzaldehyde only if there is an alternative, food-indicative odor. CONCLUSIONS: Our study reveals a regulatory link between conserved cytoprotective stress responses and behavioral avoidance, which underlies "fight-or-flight" responses and facilitates self-protection in real and anticipated stresses. These findings imply that variations in the efficiency of physiological protection during past episodes of stress might shape current behavioral decisions.

A potential flavor culture: Lactobacillus harbinensis M1 improves the organoleptic quality of fermented soymilk by high production of 2,3-butanedione and acetoin.

Lactic acid bacteria (LAB) are commonly used in soymilk fermentation to improve health-related functionality, but their contribution to sensory qualities is less valued. We characterized Lactobacillus harbinensis M1, Lactobacillus mucosae M2, Lactobacillus fermentum M4, Lactobacillus casei M8 and Lactobacillus rhamnosus C1 from naturally-fermented tofu whey, along with Streptococcus thermophilus ST3 from kefir XPL-1 fermented soymilk, to investigate their potential as starter cultures of fermented soymilk. They were characterized for antibiotic susceptibility, probiotic potential and their performance as starter cultures. All the LABs showed sensitivity to the tested antibiotics. L. casei M8 had strongest tolerance to synthetic gastrointestinal juice (<1.0 log CFU/mL loss), as well as antagonistic effects towards five food-borne pathogens. GC/MS analysis showed that L. harbinensis M1 produced significantly higher abundance (P < 0.05) of 2,3-butanedione (2.45 ppm) and acetoin (44.30 ppm), thus improving the overall sensory acceptability of fermented soymilk. The coding genes for the synthesis of 2,3-butanedione/acetoin (alsS, alsD, butA) were predicted from the whole-genome. A co-culture of L. harbinensis M1 and L. casei M8 produced a fermented soymilk product with both markedly improved flavor and good probiotic potential. It appears that L. harbinensis M1 has much potential for improving the organoleptic properties of fermented soymilk.

Identification and analysis of new α- and ß-hydroxy ketones related to the formation of 3-methyl-2,4-nonanedione in musts and red wines.

The formation of 3-methyl-2,4-nonanedione (MND) during red wine aging can contribute to the premature evolution of aroma, characterized by the loss of fresh fruit and development of dried fruit flavors. The identification of two new hydroxy ketones, 2-hydroxy-3-methylnonan-4-one (syn- and anti-ketol diastereoisomers) and 3-hydroxy-3-methyl-2,4-nonanedione (HMND), prompted the investigation of the precursors and pathways through which MND is produced and evolves. An HS-SPME-GC-MS method was optimized for their quantitation in numerous must and wine samples, providing insight into the evolution of MND, HMND, and ketols through alcoholic fermentation and wine aging. Alcoholic fermentation resulted in a significant decrease in MND and HMND and the simultaneous appearance of ketol diastereoisomers. The analysis of 167 dry red wines revealed significant increases in MND and anti-ketol contents through aging and a significant positive correlation between MND and anti-ketols. Additional experiments demonstrated that ketols are precursors to MND during red wine oxidation.

Technological properties of Lactococcus lactis subsp. lactis bv. diacetylactis obtained from dairy and non-dairy niches.

Lactococcus lactis subsp. lactis bv. diacetylactis strains are often used as starter cultures by the dairy industry due to their production of acetoin and diacetyl, important substances that add buttery flavor notes in dairy products. Twenty-three L. lactis subsp. lactis isolates were obtained from dairy products (milk and cheese) and dairy farms (silage), identified at a biovar level, fingerprinted by rep-PCR and characterized for some technological features. Fifteen isolates presented molecular and phenotypical (diacetyl and citrate) characteristics coherent with L. lactis subsp. lactis bv. diacetylactis and rep-PCR allowed the identification of 12 distinct profiles (minimum similarity of 90%). Based on technological features, only two isolates were not able to coagulate skim milk and 10 were able to produce proteases. All isolates were able to acidify skim milk: two isolates, in special, presented high acidifying ability due to their ability in reducing more than two pH units after 24 h. All isolates were also able to grow at different NaCl concentrations (0 to 10%, w/v), and isolates obtained from peanut and grass silages presented the highest NaCl tolerance (10%, w/v). These results indicate that the L. lactis subsp. lactis bv. diacetylactis isolates presented interesting technological features for potential application in fermented foods production. Despite presenting promising technological features, the isolates must be assessed according to their safety before being considered as starter cultures.

Reliability of an interneuron response depends on an integrated sensory state.

The central nervous system transforms sensory information into representations that are salient to the animal. Here we define the logic of this transformation in a Caenorhabditis elegans integrating interneuron. AIA interneurons receive input from multiple chemosensory neurons that detect attractive odors. We show that reliable AIA responses require the coincidence of two sensory inputs: activation of AWA olfactory neurons that are activated by attractive odors, and inhibition of one or more chemosensory neurons that are inhibited by attractive odors. AWA activates AIA through an electrical synapse, while the disinhibitory pathway acts through glutamatergic chemical synapses. AIA interneurons have bistable electrophysiological properties consistent with their calcium dynamics, suggesting that AIA activation is a stereotyped response to an integrated stimulus. Our results indicate that AIA interneurons combine sensory information using AND-gate logic, requiring coordinated activity from multiple chemosensory neurons. We propose that AIA encodes positive valence based on an integrated sensory state.

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.

Melanoidins from Coffee, Cocoa, and Bread Are Able to Scavenge α-Dicarbonyl Compounds under Simulated Physiological Conditions.

Free amino residues react with α-dicarbonyl compounds (DCs) contributing to the formation of advanced glycation end products (AGEs). Phenolic compounds can scavenge DCs, thus controlling the dietary carbonyl load. This study showed that high-molecular weight cocoa melanoidins (HMW-COM), HMW bread melanoidins (HMW-BM), and especially HMW coffee melanoidins (HMW-CM) are effective DC scavengers. HMW-CM (1 mg/mL) scavenged more than 40% DCs within 2 h under simulated physiological conditions, suggesting some physiological relevance. Partial acid hydrolysis of HMW-CM decreased the dicarbonyl trapping capacity, demonstrating that the ability to react with glyoxal, methylglyoxal (MGO), and diacetyl was mainly because of polyphenols bound to macromolecules. Caffeic acid (CA) and 3-caffeoylquinic acid showed a DC-scavenging kinetic profile similar to that of HMW-CM, while mass spectrometry data confirmed that hydroxyalkylation and aromatic substitution reactions led to the formation of a stable adduct between CA and MGO. These findings corroborated the idea that antioxidant-rich indigestible materials could limit carbonyl stress and AGE formation across the gastrointestinal tract.

Influence of sugars and pH on the citrate metabolism of different lactic acid bacteria strains in a synthetic wine matrix.

AIMS: This study investigated the influence of sugars (glucose and fructose) and pH on the gene expression of citE (citrate lyase ß-subunit) and the subsequent formation of metabolites associated with citrate metabolism. METHODS AND RESULTS: Different levels of glucose (2·5, 50 and 115 g l-1 ), fructose (2·5, 50 and 115 g l-1 ) and pH (3·0, 3·5, 4·0 and 5·0) were evaluated for their effect on citE expression in four different lactic acid bacteria strains. Two Oenococcus oeni strains and two Lactobacillus plantarum strains were used, of which one strain of each species screened positive for the citE gene. Among the factors tested, fructose had the biggest influence on the relative expression of citE in O. oeni. In addition, the citrate-positive strains produced high concentrations of diacetyl and acetoin. CONCLUSIONS: This study gives an overview of how sugar, pH and different lactic acid bacteria strains influence citE gene expression and the formation of metabolites associated with citrate metabolism closely linked to malolactic fermentation (MLF). SIGNIFICANCE AND IMPACT OF THE STUDY: These results can be used to make informed decisions regarding MLF when aiming to create a wine with a buttery aroma or not.