Biosynthesis and role of 3-methylbutanal in cheese by lactic acid bacteria: Major metabolic pathways, enzymes involved, and strategies for control

Revealing the mechanism of flavor improvement of fermented goat milk based on lipid changes

The mechanism of goat milk (GM) flavor improvement based on lipid changes requires understanding. According to sensory evaluation results, the texture, taste, appearance, aroma, and overall acceptability score of Guishan fermented goat milk (GMF) were higher than those of GM. In total, 779 lipid molecules and 121 volatile compounds were formed from the metabolite-lipid level in the GM and GMF, as determined through lipidomics and gas chromatography-mass spectrometry. The key volatile flavor compounds in the GMF were (E,E)-2,4-decadienal, ethyl acetate, acetoin, 2,3-pentanedione, acetic acid, and 2,3-butanedione. Of them, 60 lipids significantly contributed to the flavor profiles of the GMF, based on the correlation analysis. The triacylglycerides (TAGs) 12:0_14:0_16:0 and 13:0_13:0_18:2 contributed to aroma retention, while TAG and phosphatidylethanolamine were identified as key substrates for flavor compound formation during fermentation. Lipids associated with glycerophospholipid and linoleic acid metabolism pathways significantly affected volatile compound formation in the GMF. This study provides an in-depth understanding of the lipids and flavors of the GMF, and this information will be useful for the development of specific GMF products.

Rennets differing in chymosin-to-pepsin ratio shape the metabolomic and sensory profile of Grana Padano PDO cheese during ripening

Utilizing different chymosin and pepsin ratios in cheesemaking may represent a potential strategy to shape the sensory profile of hard cheeses. This study investigated the impact of rennet with varying chymosin and pepsin ratios on the chemical profile and sensory attributes of Grana Padano PDO cheese at different ripening times (10 to 20 months). The research involved the analysis of hard cheese manufactured with distinct calf chymosin percentages (99 %, 95 %, and 83 %), exploiting sensory analyses and untargeted metabolomics to identify marker compounds correlating with specific sensory traits. The results demonstrated that varying the rennet composition significantly affected sensory profile; in particular, the rennet made by 83 % chymosin and 17 % pepsin generated a more complex sensory profile starting from 12 months. AMOPLS and ASCA analysis on untargeted metabolomics signatures revealed that ripening time was the only significant factor when compared with rennet type and the interaction ripening x rennet. Finally, at more advanced ripening times, 3-methylbutanoic acid and homoethone were significantly up-accumulated in cheese samples manufactured with higher pepsin percentages, likely explaining sensory outcomes. This study provides valuable insights into using rennet to tailor the sensory qualities of hard cheeses, underscoring the importance of enzyme selection in cheese manufacturing to drive innovation in the dairy industry.

A Comparison Analysis of Four Different Drying Treatments on the Volatile Organic Compounds of Gardenia Flowers

The gardenia flower not only has extremely high ornamental value but also is an important source of natural food and spices, with a wide range of uses. To support the development of gardenia flower products, this study used headspace gas chromatography-ion mobility spectrometry (HS-GC-IMS) technology to compare and analyze the volatile organic compounds (VOCs) of fresh gardenia flower and those after using four different drying methods (vacuum freeze-drying (VFD), microwave drying (MD), hot-air drying (HAD), and vacuum drying (VD)). The results show that, in terms of shape, the VFD sample is almost identical to fresh gardenia flower, while the HAD, MD, and VD samples show significant changes in appearance with clear wrinkling; a total of 59 volatile organic compounds were detected in the gardenia flower, including 13 terpenes, 18 aldehydes, 4 esters, 8 ketones, 15 alcohols, and 1 sulfide. Principal component analysis (PCA), cluster analysis (CA), and partial least-squares regression analysis (PLS-DA) were performed on the obtained data, and the research found that different drying methods impact the VOCs of the gardenia flower. VFD or MD may be the most effective alternative to traditional sun-drying methods. Considering its drying efficiency and production cost, MD has the widest market prospects.

Onopordum platylepis (Murb.) Murb. as a novel source of thistle rennet: First application to the manufacture of traditional Italian raw ewe's milk cheese

In this study, for the very first time, aqueous extracts obtained from flowers of spontaneously grown or cultivated Onopordum platylepis (Murb.) Murb. thistles were used as coagulating agents for the pilot-scale manufacture of Caciofiore, a traditional Italian raw ewe's milk cheese. Cheese prototypes were compared to control cheeses curdled with a commercial thistle rennet obtained from flowers of Cynara cardunculus L. After 45 days of ripening under controlled conditions, both the experimental and control cheese prototypes were analyzed for: cheese yield, physico-chemical (pH, titratable acidity, aw, proximate composition), morpho-textural (color and texture), and microbiological parameters (viable cell counts and species composition assessed by Illumina sequencing), as well as volatile profile by SPME-GC-MS. Slight variations in titratable acidity, color, and texture were observed among samples. Based on the results overall collected, neither the yield nor the proximate composition was apparently affected by the type of thistle coagulant. However, the experimental cheese prototypes curdled with extracts from flowers of both spontaneous or cultivated thistles showed 10 % higher values of water-soluble nitrogen compared to the control prototypes. On the other hand, these latter showed slightly higher loads of presumptive lactococci, thermophilic cocci, coliforms, and eumycetes, but lower counts of Escherichia coli. No statistically significant differences were revealed by the metataxonomic analysis of the bacterial and fungal biota. Though most volatile organic compounds (VOCs) were consistent among the prototypes, significant variability was observed in the abundance of some key aroma compounds, such as butanoic, hexanoic, and octanoic acids, ethanol, propan-2-ol, isobutyl acetate, 2-methyl butanoic acid, and 3-methyl butanal. However, further investigations are required to attribute these differences to either the type of coagulant or the metabolic activity of the microorganisms occurring in the analyzed cheese samples. The results overall collected support the potential exploitation of O. platylepis as a novel source of thistle coagulant to produce ewe's milk cheeses.

Graduate Student Literature Review: Network of flavor compounds formation and influence factors in yogurt

Yogurt is popular as a natural and healthy food, but its flavor greatly affects acceptability by consumers. Flavor compounds of yogurt is generally produced by the metabolism of lactose, protein and fat, and the resulting flavors include carbonyls, acids, esters and alcohols, etc. Each flavor compounds could individually provide the corresponding flavor, or it can be combined with other compounds to form a new flavor. The flavor network was formed among the metabolites of milk components, and acetaldehyde, as the central compounds, played a role in connecting the whole network. The flavor compounds can be affected by many factors, such as the use of different raw milks, ways of homogenization, sterilization, fermentation, post ripening, storage condition and packaging materials, etc., which can affect the overall flavor of yogurt. This paper provides an overview of the volatile flavor compounds in yogurt, the pathways of production of the main flavor compounds during yogurt fermentation, and the factors that influence the flavor of yogurt including type of raw milk, processing, and storage. It also tries to provide theoretical guidance for the product of yogurt in ideal flavor, but further research is needed to provide a more comprehensive description of the flavor system of yogurt.

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

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

Discrimination and characterization of volatile organic compounds and nutritional values of three varieties of chopped pepper seeds

Fermented-chopped pepper is a widely consumed condiment in China due to its attractive flavor. Chopped pepper seed (CPS) is the byproduct generated during the production of chopped pepper and is generally discarded as waste. In this study, the volatile organic compounds (VOCs) and nutritional value of three varieties of CPS were investigated. Results indicated that the nutritional compositions of the three CPS varieties exhibited significant differences. All CPS samples contained 17 amino acids and were rich in fatty acids, with unsaturated fatty acids being predominant and accounting for 79 % of the total fatty acids. A total of 53 VOCs were identified by gas chromatography-ion mobility spectrometry, which could be classified into 9 groups, with aldehydes, esters, and alcohols comprising the three largest groups. The three varieties of CPS had remarkably varied aromas whereas there are five key VOCs (i.e., 2-pentylfuran, methional, ethyl 3-methylbutanoate, dimethyl disulfide, and nonanal) in all CPS samples. Network correlation analysis revealed that VOCs are closely correlated with amino and fatty acids. Thus, this study provides a useful basis for understanding the nutritional values and flavor characteristics of different CPS varieties, which could be used as an ingredient and might have great potential in the food industry.

HS-SPME-GC-MS Combined with Orthogonal Partial Least Squares Identification to Analyze the Effect of LPL on Yak Milk's Flavor under Different Storage Temperatures and Times

Flavor is a crucial parameter for assessing the sensory quality of yak milk. However, there is limited information regarding the factors influencing its taste. In this study, the effects of endogenous lipoprotein lipase (LPL) on the volatile flavor components of yak milk under storage conditions of 4 °C, 18 °C and 65 °C were analyzed via headspace solid-phase microextraction gas chromatography-mass spectrometry (HS-SPME-GC-MS) combined with orthogonal partial least-squares (OPSL) discrimination, and the reasons for the changes in yak milk flavors were investigated. Combined with the difference in the changes in volatile flavor substance before and after the action of LPL, LPL was found to have a significant effect on the flavor of fresh yak milk. Fresh milk was best kept at 4 °C for 24 h and pasteurized for more than 24 h. Principal component analysis (PCA) and orthogonal partial least squares discriminant analysis (OPLS-DA) were employed to characterize the volatile components in yak milk under various treatment conditions. Twelve substances with significant influence on yak milk flavor were identified by measuring their VIP values. Notably, 2-nonanone, heptanal, and ethyl caprylate exhibited OAV values greater than 1, indicating their significant contribution to the flavor of yak milk. Conversely, 4-octanone and 2-heptanone displayed OAV values between 0.1 and 1, showing their important role in modifying the flavor of yak milk. These findings can serve as monitoring indicators for assessing the freshness of yak milk.

Metabolic cross-feeding enhances branched-chain aldehydes production in a synthetic community of fermented sausages

Microbial interactions play an important role in regulating the metabolic function of fermented food communities, especially the production of key flavor compounds. However, little is known about specific molecular mechanisms that regulate the production of key flavor compounds through microbial interactions. Here, we designed a synthetic consortium containing Debaryomyces hansenii D1, Staphylococcus xylosus S1, and Pediococcus pentosaceus PP1 to explore the mechanism of the microbial interactions underlying the branched-chain aldehydes production. In this consortium, firstly, D. hansenii secreted amino acids that promoted the growth of P. pentosaceus and S. xylosus. Specifically, D. hansenii D1 secreted alanine, aspartate, glutamate, glutamine, glycine, phenylalanine, serine, and threonine, which were the primary nutrients for bacterial growth. P. pentosaceus PP1 utilized all these eight amino acids through cross-feeding, whereas S. xylosus S1 did not utilize aspartate and serine. Furthermore, D. hansenii D1 promoted the production of branched-chain aldehydes from S. xylosus and P. pentosaceus through cross-feeding of α-keto acids (intermediate metabolites). Thus, the accumulation of 2-methyl-butanal was promoted in all co-culture. Overall, this work revealed the mechanism by which D. hansenii and bacteria cross-feed to produce branched-chain aldehydes in fermented sausages.

Regulation of the pleiotropic transcriptional regulator CodY on the conversion of branched-chain amino acids into branched-chain aldehydes in Lactococcus lactis

IMPORTANCE

Branched-chain aldehydes are the primary compounds that contribute to the nutty flavor in cheddar cheese. Lactococcus lactis, which is often applied as primary starter culture, is a significant contributor to the nutty flavor of cheddar cheese due to its ability of conversion of BCAAs into branched-chain aldehydes. In the present study, we found that the regulatory role of CodY is crucial for the conversion. CodY acts as a pleiotropic transcriptional regulator via binding to various regulatory regions of key genes. The results presented valuable knowledge into the role of CodY on the regulation and biosynthetic pathway of branched-chain amino acids and the related aldehydes. Furthermore, it provided new insight for increasing the nutty flavor produced during the manufacture and ripening of cheese.

Integrated-Omics Study on the Transcriptomic and Metabolic Changes of Bacillus licheniformis, a Main Microorganism of Fermented Soybeans, According to Alkaline pH and Osmotic Stress

Bacillus licheniformis has been widely utilized in the food industry as well as various agricultural industries. In particular, it is a main microorganism of fermented soybeans. In this study, the changes of the metabolome and transcriptome of B. licheniformis KACC15844, which had been isolated from fermented soybeans, were investigated depending on alkaline pH (BP) and a high salt concentration (BS) using an integrated-omics technology, focusing on leucine metabolism. Overall, carbohydrate (glycolysis, sugar transport, and overflow) and amino acid (proline, glycine betaine, and serine) metabolisms were strongly associated with BS, while fatty acid metabolism, malate utilization, and branched-chain amino acid-derived volatiles were closely related to BP, in both gene and metabolic expressions. In particular, in leucine metabolism, the formation of 3-methylbutanoic acid, which has strong cheesy odor notes, was markedly increased in BP compared to the other samples. This study provided information on how specific culture conditions can affect gene expressions and metabolite formations in B. licheniformis using an integrated-omics approach.

Characterization of a fermented dairy, sour cream: Lipolysis and the release profile of flavor compounds

Lipolysis and flavor development during fermentation of sour cream were studied by evaluating the physicochemical changes, sensory differences and volatile components. The fermentation caused significant changes in pH, viable count and sensory evaluation. The peroxide value (POV) decreased after reaching the maximum value of 1.07 meq/kg at 15 h, while thiobarbituric acid reactive substances (TBARS) increased continuously with the accumulation of secondary oxidation products. The Free fatty acids (FFAs) in sour cream were mainly myristic, palmitic and stearic. GC-IMS was used to identify the flavor properties. A total of 31 volatile compounds were identified, among which the contents of characteristic aromatic substances such as ethyl acetate, 1-octen-3-one and hexanoic acid were increased. The results suggest that lipid changes and flavor formation in sour cream are influenced by fermentation time. Furthermore, flavor compounds may be related to lipolysis such as 1-octen-3-one and 2- heptanol were also observed.

Characterization of metabolic pathways for biosynthesis of the flavor compound 3-methylbutanal by Lactococcus lactis

3-Methylbutanal is a key volatile compound that imparts a nutty flavor to Cheddar cheese. Lactococcus lactis has been successfully applied as a starter to increase the level of 3-methylbutanal produced during the ripening of cheese. However, the mechanism of action and genetic diversity of this bacterium for 3-methylbutanal biosynthesis remains unclear. In this study, we investigated the association between the L. lactis genotype and phenotype in the biosynthesis of 3-methylbutanal via both direct and indirect pathways. Fourteen strains of L. lactis were screened for the capacity to produce 3-methylbutanal, and strain 408 (>140 μM) produced the highest among all tested strains, which exhibited both α-keto acid decarboxylase and α-ketoacid dehydrogenase activities. Furthermore, the results of a sodium meta-arsenite inhibition experiment showed that the 3-methylbutanal-producing capacities of each strain declined to various degrees. The kdcA gene, which encodes the direct pathway component α-ketoacid decarboxylase, was detected in 4 of the 14 strains, of which only strain 408 contained the full-length gene. We then characterized the genes associated with the indirect pathway by detecting the expression levels of the pdh gene cluster, ack, and pta, which were expressed at relatively higher levels in a high-yield strain than in a low-yield strain. As a result, these L. lactis strains were divided into 3 categories according to gene diversity, gene expression, and 3-methylbutanal production. The results of this study refine our knowledge of the genetic determinants of 3-methylbutanal biosynthesis in L. lactis and explain the effect of both synthesis pathways on 3-methylbutanal production.

Formation of 3-Methylbutanal and 3-Methylbutan-1-ol Recognized as Malty during Fermentation in Swiss Raclette-Type Cheese, Reconstituted Milk, and de Man, Rogosa, and Sharpe Broth

This work aimed to determine the formation over time of 3-methylbutanal and 3-methylbutan-1-ol recognized as malty during the manufacture of Raclette-type cheese and the fermention of reconstituted skim milk, and filter-sterilized MRS broth. Using dynamic headspace-vacuum transfer in trap extraction followed by gas chromatography coupled with mass spectrometry-olfactometry (DHS-VTT-GC-MS-O) as a screening method for the malty compounds, five compounds (2-methylpropanal, 2- and 3-methylbutanal, and 2- and 3-methylbutan-1-ol) were identified as potential compounds causing the malty aroma in starter culture development and Raclette-type cheeses. Focus on compounds having a predominant sensorial effect (3-methylbutanal and 3-methylbutan-1-ol), spikings of leucine, ¹³C-labeled leucine, α-ketoisocaproic acid, and α-ketoglutaric acid provided a better understanding of their formation pathway. This study highlighted the discrepancies in the formation of 3-methylbutanal and 3-methylbutan-1-ol between the growth media; namely, despite the presence of free leucine available in MRS and the addition of an excess, no increase of the target compounds was observed. The concentration of these compounds in MRS increased only when α-ketoglutaric acid or α-ketoisocaproic acid was added, and a preference for the pathway to α-hydroxyisocaproic acid instead of 3-methylbutanal was shown. In addition, a formation of 3-methylbutanal when the bacteria were not yet active was observed when spiking α-ketoisocaproic acid, which potentially indicates that this part of the metabolism could take place extracellularly. These results could potentially unveil other, not-yet-identified reactants, directly influencing the production of compounds responsible for the malty aroma in Raclette cheese.

Evaluation of the Perceptual Interactions among Aldehydes in a Cheddar Cheese Matrix According to Odor Threshold and Aroma Intensity

To evaluate the contributions of 3-methylbutanal, 2-methylbutanal, 2-methylpropanal, and benzaldehyde in cheddar cheese models, the threshold values, optimal concentration ranges, and perceptual actions of these compounds were determined at various concentrations. The thresholds for 3-methylbutanal, 2-methylbutanal, 2-methylpropanal, and benzaldehyde in the cheese matrix were 150.31, 175.39, 150.66, and 500.21 μg/kg, respectively, which were significantly higher than the corresponding values in water. The optimal concentration ranges of these aldehydes were determined as 150–300, 175–325, 150–350, and 500–1500 μg/kg, respectively. Based on the results of the threshold method and Feller’s model, five binary mixtures were found to have synergistic effects, and only the pair of 2-methylpropanal and benzaldehyde was determined to have a masking effect. In addition, the synergistic olfactory effects between the four ternary mixtures and the quaternary mixture of these aldehydes were also assesSsed using Feller’s model. In a σ-τ plot analysis, synergism was usually observed when these odor pairs were at their threshold levels. In summary, the results suggested that perceptual interactions among these aldehydes exist in a cheese model variably with different concentrations and threshold ratios. This study will be helpful to a further understanding of the nutty aroma and improving the aroma quality of cheddar cheese.

Evaluation of the synergistic olfactory effects of diacetyl, acetaldehyde, and acetoin in a yogurt matrix using odor threshold, aroma intensity, and electronic nose analyses

Despite intensive analyses of yogurt flavor, the synergistic effects of the key aroma compounds on sensory responses and their optimum concentration ranges remain less well-documented. This study investigated the odor thresholds, optimum concentration ranges, and perceptual actions of diacetyl, acetaldehyde, and acetoin in a yogurt matrix. Our results show that the odor thresholds of diacetyl, acetaldehyde, and acetoin in the yogurt matrix were 5.43, 15.4, and 29.0 mg/L, respectively, which were significantly higher than the corresponding values in water. The optimum diacetyl, acetaldehyde, and acetoin concentration ranges were found to be 6.65 to 9.12, 25.9 to 35.5, and 37.3 to 49.9 mg/L, respectively. In Feller's additive model, the addition of each compound led to a significant reduction in their odor threshold in the yogurt matrix, thus demonstrating the synergistic effects of the compounds. In the σ-τ plot, various concentrations of compounds were associated with various degrees of additive behavior with respect to the aroma intensity of the yogurt matrix, thus demonstrating the synergism among these compounds in increasing the overall aroma intensity. The optimal simultaneous concentration ratio of diacetyl:acetaldehyde:acetoin was determined to be 4.00:16.0:32.0 mg/L. The specific synergistic effects were also confirmed by an electronic nose analysis and aroma profile comparison. In summary, these 3 aroma compounds exhibited synergistic effects in a yogurt matrix, thus providing a theoretical basis for the enhancement of flavors in dairy products.

Influence of must yeast-assimilable nitrogen content on fruity aroma variation during malolactic fermentation in red wine

This study assessed the impact of must yeast-assimilable nitrogen (YAN) content and lactic acid bacteria (LAB) strains used for malolactic fermentation (MLF) on the formation of substituted esters, as well as the corresponding precursors (substituted acids), to investigate the modulation of fruity expression in red wines. In microvinification experiments, a Merlot must was fermented with an initial YAN content of 111 mg/L, or supplemented up to 165 and 220 mg/L. Two Oenococcus oeni LAB strains were used for MLF. Analytical methods were used to quantify substituted esters, as well as the corresponding acids, including, any enantiomeric forms. YAN supplementation of the must significantly increased concentrations of substituted esters of short- and branched-chain alkyl fatty acids produced during alcoholic fermentation (AF) (up to 67% in samples with the highest nitrogen content) and substituted esters of hydroxycarboxylic acids generated during MLF (up to 58% in samples with the highest nitrogen content). YAN supplementation in the must did not affect substituted acid formation during AF. After MLF, short- and branched-chain alkyl fatty acid levels increased in wines made from musts with the highest nitrogen content (up to 56% in samples with the highest nitrogen content), whereas concentrations of hydroxycarboxylic acids increased (up to 55%) independently of the initial YAN content, highlighting the important role of MLF. (2S)-2-hydroxy-4-methylpentanoic acid was only found in wines after malolactic fermentation, suggesting different pathways for each enantiomer and opening up new prospects for the study of bacterial metabolisms. Moreover, sensory profiles revealed a significant increase in black-berry- and jammy-fruit aromas during MLF and a strong positive correlation between these aromas and the production of substituted esters following must nitrogen supplementation and MLF. Aromatic reconstitutions revealed that variations in the concentrations of substituted esters after MLF impacted the fruity aroma of red wines.

Short communication: Sensory characteristics and volatile organic compound profile of high-moisture mozzarella made by traditional and direct acidification technology

In the present study, the sensory characteristics and the volatile organic compound (VOC) profiles of high-moisture mozzarella made by different acidification techniques were compared. The cheeses were manufactured at the same dairy by fermentation by autochthonous natural whey starter (traditional backslopping method) and direct acidification with citric acid (the most used industrial technology). Three cheesemaking trials were performed from February to June using raw milk from a single farm. The mozzarella samples were subjected to assessment of the chemical, microbiological, and sensory characteristics and to VOC analysis by headspace solid-phase microextraction coupled to gas chromatography-mass spectrometry. The relevant microbiological differences found between the 2 types of products were reflected in different sensory and VOC profiles. The cheeses were clearly discriminated by the panel, and traditional mozzarella had higher intensity attributes. The most discriminating descriptors were elasticity, overall odor and taste intensity, sour milk and fruity/vegetable odors, sour taste, and aftertaste. Even though some variability was observed among the trials, the VOC profile of traditional product was always much more complex than that obtained by direct acidification. In both products the carbonyl compounds were the most abundant chemical class, accounting for about 50% of the total. In detail, 51 compounds were identified in the entire set of samples, and their contribution to cheese aroma was roughly estimated by calculating the odor activity values on the basis of the odor thresholds available in the literature. The results allowed hypothesizing that only 12 of them could play a primary role. The most important among the odor-active compounds was 3-methyl-butanal that can both derive from metabolism of lactic acid bacteria and yeasts. The results of the study may be very useful in view of European Union PDO labeling of the traditional product, in terms of protecting it from imitations.

Cooperation between Lactococcus lactis NRRL B-50571 and NRRL B-50572 for Aroma Formation in Fermented Milk

The aim of the present study was to characterize the aroma and volatile profiles of milk fermented by wild Lactococcus lactis NRRL B-50571 (FM-571) and NRRL B-50572 (FM-572) and co-fermented with both strains (co-FM). Milks fermented by these strains have been reported to have an antihypertensive effect, yet their sensory characteristics, which are of great importance for consumer acceptance of functional foods, have not been studied. In the study, volatiles were determined using solid-phase microextraction gas chromatography mass spectrometry (SPME-GC-MS) and aroma was determined by quantitative descriptive sensory analysis (QDA). Volatile compounds identified in FM-571, FM-572, and co-FM were mainly acids, alcohols, aldehydes, and ketones. FM-571 showed higher total relative volatile abundance than FM-572 or co-FM. Furthermore, the concentrations of specific amino acids (aa) were lower in FM-571 and co-FM than in FM-572. Thus, these results suggested that FM-571 or co-FM are more efficient in transforming specific aa into the corresponding volatiles than FM-572. Indeed, several alcohols and aldehydes, associated with the catabolism of these aa, were found in FM-571 and co-FM, but not in FM-572. Additionally, QDA showed that FM-571 and co-FM presented higher yeasty and cheesy aroma descriptors than FM-572. Also, total aroma intensity scores for FM-571 were higher than those for co-FM or FM-572. Thus, results suggested that the combination of these two specific wild L. lactis strains may complement amino acid catabolic routes that resulted in the enhancement or attenuation of aroma production of single strains, presenting new possibilities for the preparation of custom-made starter cultures.

Physicochemical, Sensorial and Microbiological Characterization of PoroCheese, an Artisanal Mexican Cheese Made from Raw Milk

Poro cheese is a regional product originally from the area of Los Rios, Tabasco in Mexico. In the context of preserving the heritage of Poro cheese and protecting the specific characteristics that define its typicity through an origin designation, the present study was conducted to establish a general profile of Poro cheese by characterizing their physicochemical, textural, rheological, sensorial and microbiological characteristics. Differences in moisture, proteins, fats, NaCl, titrable acidity, pH, color texture and rheology amongst cheese factories were observed and ranges were established. Fifteen descriptors were generated to provide a descriptive analysis, eight of which were significantly different amongst the factories with no differences in the global acceptability of cheese. The favorite cheese had the highest scores for aroma attributes. Conventional and molecular methods were used to identify the main microorganisms, for which Lactobacillus plantarum, L. fermentum, L. farciminis and L. rhamnosus were the main microorganisms found in Porocheese. The obtained data constituted the parameters for characterizing Poro cheese, which will strongly help to support its origin appellation request process.

An electricalchemical method to detect the branch-chain aminotransferases activity in lactic acid bacteria

In this study, an electrochemical system was established to detect the branched-chain amino acid aminotransferase (BCAT) activity in lactic acid bacteria (LAB). A nanocomposite of chitosan (CS) with multi-walled carbon nanotubes (MWCNTs) was synthesized, and the composite solution were uniformly spread over the glassy carbon electrode (GCE) surface by drop-casting to fabricate an electrochemical biosensor. The composite was characterized by scanning electron microscopy (SEM) and cyclic voltammetry (TEM). Results indicated that the MWCNTs-CS/GCE electrode exhibited higher stability and sensitivity, compared with the GCE electrode. The linear response for nicotinamide adenine dinucleotide (NADH) was 1.0-9.0 μM and the response limit was 0.12 µM. The system effectively and sensitively detected the BCAT activity by NADH concentration in the LAB culture, comparing with the optical method. The culture condition of LAB was optimized by using this system, evidencing that established method was available to detect the BCAT activity of LAB.

Effect of Respiratory Growth on the Metabolite Production and Stress Robustness of Lactobacillus casei N87 Cultivated in Cheese Whey Permeate Medium

Cheese whey permeate (WP) is a low-cost feedstock used for the production of biomass and metabolites from several lactic acid bacteria (LAB) strains. In this study, Lactobacillus casei N87 was cultivated in an optimized WP medium (WPM) to evaluate the effect of anaerobic and respiratory conditions on the growth performances (kinetics, biomass yield), consumption of sugars (lactose, galactose, glucose) and citrate, metabolite production [organic acids, volatile organic compounds (VOCs)] and stress survival (oxidative, heat, freezing, freeze-drying). The transcription of genes involved in the main pathways for pyruvate conversion was quantified through Real Time-PCR to elucidate the metabolic shifts due to respiratory state. Cultivation in WPM induced a diauxic growth in both anaerobic and respiratory conditions, and L. casei N87 effectively consumed the lactose and galactose present in WPM. Genomic information suggested that membrane PTS system and tagatose-6-P pathway mediated the metabolism of lactose and galactose in L. casei N87. Respiration did not affect specific growth rate and biomass production, but significantly altered the pyruvate conversion pathways, reducing lactate accumulation and promoting the formation of acetate, acetoin and diacetyl to ensure the redox balance. Ethanol was not produced under either cultivation. Pyruvate oxidase (pox), acetate kinase (ack), α-acetolactate decarboxylase (ald), acetolactate synthase (als) and oxaloacetate decarboxylase (oad) genes were up-regulated under respiration, while L-lactate dehydrogenase (ldh), pyruvate formate lyase (pfl), pyruvate carboxylase (pyc), and phosphate acetyltransferase (pta) were down regulated by oxygen. Transcription analysis was consistent with metabolite production, confirming that POX-ACK and ALS-ALD were the alternative pathways activated under aerobic cultivation. Respiratory growth affected the production of volatile compounds useful for the development of aroma profile in several fermented foods, and promoted the survival of L. casei N87 to oxidative stresses and long-term storage. This study confirmed that the respiration-based technology coupled with cultivation on low-cost medium may be effectively exploited to produce competitive and functional starter and/or adjunct cultures. Our results, additionally, provided further information on the activation and regulation of metabolic pathways in homofermentative LAB grown under respiratory promoting conditions.

One complete and three draft genome sequences of four Brochothrix thermosphacta strains, CD 337, TAP 175, BSAS1 3 and EBP 3070

Brochothrix thermosphacta is one of the dominant bacterial species associated with spoilage of chilled meat and seafood products through the production of various metabolites responsible for off-odors. However, metabolic pathways leading to meat and seafood spoilage are not all well known. The production of spoiling molecules seems to depend both on strains and on food matrix. Several B. thermosphacta genome sequences have been reported, all issued from meat isolates. Here, we report four genome sequences, one complete and three as drafts. The four B. thermosphacta strains CD 337, TAP 175, BSAS1 3, and EBP 3070 were isolated from different ecological niches (seafood or meat products either spoiled or not and bovine slaughterhouse). These strains known as phenotypically and genetically different were selected to represent intraspecies diversity. CD 337 genome is 2,594,337 bp long, complete and circular, containing 2593 protein coding sequences and 28 RNA genes. TAP 175, BSAS1 3, and EBP 3070 genomes are arranged in 57, 83, and 71 contigs, containing 2515, 2668, and 2611 protein-coding sequences, respectively. These genomes were compared with two other B. thermosphacta complete genome sequences. The main genome content differences between strains are phages, plasmids, restriction/modification systems, and cell surface functions, suggesting a similar metabolic potential but a different niche adaptation capacity.

Influence of Monascus purpureus BD-M-4 on the physicochemical properties, proteolysis and volatile compounds of surface mould-ripened cheese

The effect of adjunct culture Monascus purpureus BD-M-4 on the physicochemical, proteolytic, and lipolytic properties of surface mold-ripened cheese were evaluated. During the maturation of Monascus-fermented cheese, the total microbiota count and the content of soluble nitrogen increased steadily, whereas the total protein content showed no significant difference. Moreover, a 17-fold increase in total free amino acids was observed in Monascus-fermented cheese. The use of adjunct culture M. purpureus BD-M-4 in the production of surface-ripened cheeses did not show a significant effect on the total fat content in the ripening period, nor did it change the lipolysis of cheese during ripening. Compared to 52 volatile compounds of the control cheese, a total of 62 compounds were detected in M. purpureus-fermented cheese, including 16 acids, 16 ketones, 11 alcohols, 5 aldehydes, 11 esters, and 3 unclassified compounds.

Comparative Genomics of Lactobacillus acidipiscis ACA-DC 1533 Isolated From Traditional Greek Kopanisti Cheese Against Species Within the Lactobacillus salivarius Clade

Lactobacillus acidipiscis belongs to the Lactobacillus salivarius clade and it is found in a variety of fermented foods. Strain ACA-DC 1533 was isolated from traditional Greek Kopanisti cheese and among the available L. acidipiscis genomes it is the only one with a fully sequenced chromosome. L. acidipiscis strains exhibited a high degree of conservation at the genome level. Investigation of the distribution of prophages and Clustered Regularly Interspaced Short Palindromic Repeats (CRISPRs) among the three strains suggests the potential existence of lineages within the species. Based on the presence/absence patterns of these genomic traits, strain ACA-DC 1533 seems to be more related to strain JCM 10692^T than strain KCTC 13900. Interestingly, strains ACA-DC 1533 and JCM 10692^T which lack CRISPRs, carry two similar prophages. In contrast, strain KCTC 13900 seems to have acquired immunity to these prophages according to the sequences of spacers in its CRISPRs. Nonetheless, strain KCTC 13900 has a prophage that is absent from strains ACA-DC 1533 and JCM 10692^T. Furthermore, comparative genomic analysis was performed among L. acidipiscis ACA-DC 1533, L. salivarius UCC118 and Lactobacillus ruminis ATCC 27782. The chromosomes of the three species lack long-range synteny. Important differences were also determined in the number of glycobiome related proteins, proteolytic enzymes, transporters, insertion sequences and regulatory proteins. Moreover, no obvious genomic traits supporting a probiotic potential of L. acidipiscis ACA-DC 1533 were detected when compared to the probiotic L. salivarius UCC118. However, the existence of more than one glycine-betaine transporter within the genome of ACA-DC 1533 may explain the ability of L. acidipiscis to grow in fermented foods containing high salt concentrations. Finally, in silico analysis of the L. acidipiscis ACA-DC 1533 genome revealed pathways that could underpin the production of major volatile compounds during the catabolism of amino acids that may contribute to the typical piquant flavors of Kopanisti cheese.

Characteristics of Milk Fermented by Streptococcus thermophilus MGA45-4 and the Profiles of Associated Volatile Compounds during Fermentation and Storage

The lactic acid bacterium Streptococcus thermophilus is a major starter culture for the production of dairy products. In this study, the physiochemical characteristics of milk fermented by the MGA45-4 isolate of S. thermophilus were analyzed. Our data indicate that milk fermented using S. thermophilus MGA45-4 maintained a high viable cell count (8.86 log10 colony-forming units/mL), and a relatively high pH (4.4), viscosity (834.33 mPa·s), and water holding capacity (40.85%) during 14 days of storage. By analyzing the volatile compound profile using solid-phase microextraction and gas chromatography/mass spectrometry, we identified 73 volatile compounds in the fermented milk product, including five carboxylic acids, 21 aldehydes, 13 ketones, 16 alcohols, five esters, and 13 aromatic carbohydrates. According to the odor activity values, 11 of these volatile compounds were found to play a key role in producing the characteristic flavor of fermented milk, particularly octanal, nonanal, hexanal, 2,3-butanedione, and 1-octen-3-ol, which had the highest odor activity values among all compounds analyzed. These findings thus provide more insights in the chemical/molecular characteristics of milk fermented using S. thermophilus, which may provide a basis for improving dairy product flavor/odor during the process of fermentation and storage.

The production characteristics of volatile organic compounds and their relation to growth status of Staphylococcus aureus in milk environment

In our previous research, 3-methyl-butanal and 3-methyl-butanoic acid were identified as representative and specific volatile organic compounds released by Staphylococcus aureus in broth. In this study, we explored the production of the 2 volatiles and their correlation to Staph. aureus growth in milk under different conditions. We found significant correlations between the production of 3-methyl-butanoic acid and cell counts of 5 Staph. aureus strains in sterile milk, and there were no obvious differences for its production among 5 tested strains. The intensities of the 2 volatiles were similar and positively correlated with bacterial counts in cultures at 25°C and 37°C despite delayed production of volatiles at 25°C; however, neither compound could be detected at 4°C. The production of 3-methyl-butanoic acid was strongly correlated with growth of Staph. aureus mixed with Streptococcus agalactiae, Escherichia coli O157:H7, and Shigella flexneri, whereas correlations for 3-methyl-butanal were not statistically significant. Compared with the monoculture of Staph. aureus, in mixed cultures, production of 3-methyl-butanal was decreased and that of 3-methyl-butanoic acid was comparatively higher. In pasteurized and raw milks, production of 3-methyl-butanoic acid was correlated with growth of Staph. aureus, and 3-methyl-butanoic acid could be detected when Staph. aureus populations reached 10⁶ to 10⁷ cfu/mL in pasteurized milk and 10⁵ to 10⁶ cfu/mL in raw milk; the correlations for 3-methyl-butanal were not statistically significant. Our results suggest that 3-methyl-butanoic acid is a more suitable marker for high counts of Staph. aureus in milk, whereas 3-methyl-butanal is a transient metabolite and easily depressed by environmental factors.

The synthesis of two long-chain N-hydroxy amino coumarin compounds and their applications in the analysis of aldehydes

Two long-chain N-substituted coumaryl hydroxylamines were synthesized, which can serve as excellent probes for the analysis of various aldehydes.