The effect of broad bean diet on structure, flavor and taste of fresh grass carp: A comprehensive study using E-nose, E-tongue, TPA, HS-SPME-GC-MS and LC-MS

Broad bean (Vicia faba L.) has received particular attention with regards to the improvement of flesh meat quality. However, the effect of broad bean diet on structure, flavor and taste of flesh meat is unclear. In present study, E-nose, E-tongue, TPA, HS-SPME-GC-MS, and LC-MS were used to characterize the structure, flavor and taste of grass carp (Ctenopharyngodon idellus) fed with broad bean. Overall, broad bean significantly improved the texture of grass carp muscle, but reduced the overall taste and flavor. The 50 volatile compounds were detected using HS-SPME-GC-MS. The 252 differential metabolites were identified by LC-MS, of which 107 were up-regulated and 145 were down-regulated. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis demonstrated this reduction in taste and flavor was associated with the metabolism of amino acids, lipids and nucleotides. Our findings provide a theoretical basis for improving meat quality and the functional applications of broad bean.

Inoculation of Latilactobacillus sakei with Pichia kluyveri or Saccharomyces boulardii improves flavor compound profiles of salt-free fermented wheat-gluten: Effects from single strain inoculation

Wheat-gluten, the protein-rich portion of wheat, can be processed to produce a highly savory sauce product after solid and liquid-state fermentation (SSF and LSF) with the inoculation of selected lactic acid bacteria (LAB) and yeast under salt-free condition. However, limited research has been done on the impact of different types of microbes in this process. This work studied the flavour impact on fermented wheat-gluten by the single inoculation of Latilactobacillus sakei or one yeast (Saccharomyces boulardii or Pichia kluyveri). Glucose was depleted during LSF in all treatments. Lactic acid production increased over time in L. sakei-fermented samples but not in yeast-fermented samples. Cysteine, serine and arginine remained low over LSF in L. sakei-fermented samples but increased in yeast-fermented samples. More fruity esters such as isoamyl acetate and isobutyl acetate were detected in samples fermented by P. kluyveri, while S. boulardii boosted the production of alcohols such as 3-methyl butanol and 2-phenylethyl alcohol. Principal component analysis revealed a clear difference in volatile profiles of the samples fermented with different strains. Therefore, the fermented sauce can potentially be processed into different flavor directions, and based on the flavor profile, be used in different food applications.

Roasting intervention for the volatile composition of three varieties of nuts originating from Torreya yunnanensis

For the first time, the volatiles of three varieties of fresh and roasted Torreya yunnanensis nuts were investigated by solid-phase microextraction-gas chromatography-mass spectrometry. The results indicated that roasting had the greatest effect on the volatiles of millet capsules. Fresh nuts had many terpenes, esters, and aldehydes, while roasting led to the formation of pyrazines and furans. In subsequent work, short-term low temperature and small sample area exposed to high temperature had a large effect on the increase in some volatiles and was characterized by a green flavor, such as α-pinene, while ultrahigh-temperature (200 and 230°C) resulted in a decrease in the total volatiles with the generation of unpleasant flavors. Finally, the combination of 170°C for 40 min and slight crushing was found to be the best roasting conditions for samples by means of GC-MS and two-dimensional gas chromatography and time-of-flight mass spectrometry (GC × GC/TOF-MS). PRACTICAL APPLICATIONS: Torreya yunnanensis and its nuts have broad development prospects because of their wide use and rich nutrition. However, inappropriate processing and lack of attention to natural materials such as nuts and wood leads to their poor usage. In addition, volatile compounds make a major contribution to the nut aroma, which is an important indicator of their sensory quality. However, no one has applied roasting technology to Torreya yunnanensis nuts or studied the volatile compounds of the roasted nuts. This study revealed the changes in the composition and content of volatile compounds in Torreya yunnanensis nuts before and after roasting, and the influence of different process points, suggesting that they are key contributors to the development of the related products.

Competition between Starter Cultures and Wild Microbial Population in Sausage Fermentation: A Case Study Regarding a Typical Italian Salami (Ventricina)

The work reports a case study describing how the competition wild microflora vs. starter cultures affects the final product characteristics. This study regards an industrial lot of Ventricina, an Italian long-ripened traditional fermented sausages, produced using starter cultures. After ripening, some relevant organoleptic defects (off-odour, crust formation) were observed. Therefore, analyses were carried out in the inner and outer sausage section to explain this phenomenon. Microbiological analyses indicated a high meat batter contamination and metagenomic analyses evidenced the inability of LAB starter cultures to lead the fermentation process. The results of this not controlled fermentation were the accumulation of high levels of biogenic amines (including histamine) and the formation of a volatile profile different if compared with similar products. Indeed, the volatilome analysis revealed unusually high amounts of molecules such as isovaleric acid, propanoic acid, 1-propanol, which can be responsible for off-odours. This study demonstrated that starter culture use needs to be modulated in relation to production parameters to avoid safety and organoleptic concerns.

Biochemical changes and microbial community dynamics during spontaneous fermentation of Zhacai, a traditional pickled mustard tuber from China

Zhacai is a traditional fermented vegetable that has been consumed in China for centuries. It is currently manufactured by spontaneous fermentation and therefore mostly relies on the activities of autochthonous microorganisms. Here, we characterized microbial community dynamics and associated biochemical changes in 12% salted Zhacai during a 90-day spontaneous fermentation process using high-throughput sequencing and chromatography-based approaches to identify associations between microorganisms and fermentation characteristics. Amplicon sequencing targeting bacterial 16S rRNA genes revealed that bacterial communities were dominated by halophilic bacteria (HAB, i.e., Halomonas and Idiomarina) and lactic acid bacteria (LAB, i.e., Lactobacillus-related genera and Weissella) after 30 days of fermentation. In addition, the relative abundances of the fungal genera Debaryomyces, Sterigmatomyces, and Sporidiobolus increased as fermentation progressed. Concomitantly, pH decreased while titratable acidity increased during fermentation, along with associated variation in biochemical profiles. Overall, the levels of organic acids (i.e., lactic and acetic acid), free amino acids (i.e., alanine, lysine, and glutamic acid), and volatiles (i.e., alcohols, esters, aldehydes, and ketones) increased in mature Zhacai. In addition, the abundances of Lactobacillus-related species, Halomonas spp., Idiomarina loihiensis, as well as that of the yeast Debaryomyces hansenii, were strongly correlated with increased concentrations of organic acids, amino acids, biogenic amines, and volatiles. This study provides new detailed insights into the succession of microbial communities and their potential roles in Zhacai fermentation.

Effect of storage temperature on volatile marker compounds in cured loins fermented with Staphylococcus carnosus by brine injection

In this study, the influence of low (5 °C), intermediate (15 °C) and high (25 °C) storage temperatures on the profile of volatile compounds of North European cured loins fermented with Staphylococcus carnosus strains was investigated. In this context, proteolytic activity, bacterial growth, key volatile compounds and sensory attributes were studied. In conclusion, storage temperature significantly affected the volatile marker compounds. A multiple regression indicated significant effects of seven volatile compounds (acetophenone, benzaldehyde, butanone, 3-methylbutanal, 1-octen-3-ol, nonanal and pentanone) on the overall odor (R2 = 95.9%) and overall flavor (R2 = 81.1%). The sum of the marker volatiles aldehydes, ketones and alcohol increased with rising temperatures and the highest amounts of the odor active 3-methylbutanal up to 155 and 166 ng/g meat were detected in high temperature-stored loins. Moreover, the addition of S. carnosus strain LTH 3838 showed maximum effect at 5 °C-storage temperature in comparison to the control.

Dynamics of microbial communities, texture and flavor in Suan zuo yu during fermentation

Suan zuo yu is a traditional Chinese fermented product. We explored the microbial diversity, the dynamic changes of texture and flavor compounds at different fermentation times (up to 24 d). Results showed that Weissella and Lactobacillus may play a vital role in fermentation especially for the flavor. At the end of fermentation, the taste activity value of Asp, Glu, and His were 21.61, 17.29 and 7.73, respectively. The bound water increased gradually indicated by low-field nuclear magnetic resonance, and the hardness was also increased. During the whole fermentation process, the myosin heavy chain protein and actin decreased obviously. Gas chromatography-mass spectrometry showed that a total of 80 volatile compounds were detected, and 6 alcohols, 6 aldehydes and 6 esters increased significantly, which mainly contributed to the flavor of Suan zuo yu. This study provides a theoretical basis for the industrial production of fermented fish.

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.

Volatile flavour components and the mechanisms underlying their production in golden pompano (Trachinotus blochii) fillets subjected to different drying methods: A comparative study using an electronic nose, an electronic tongue and SDE-GC-MS

The impacts of the vacuum freeze (VFD), hot air (HAD), microwave (MD) and vacuum microwave (VMD) drying on the flavour of golden pompano fillets were evaluated using an electronic nose (E-nose), an electronic tongue (E-tongue) and simultaneous distillation extraction (SDE) - gas chromatography - mass spectrometry (GC-MS). The results showed that the E-nose and E-tongue systems could effectively differentiate volatile compounds of four samples. A total of 86 volatile flavour components were identified in the dried fillets; the main flavour components contained hydrocarbons (39), aldehydes (15), esters (10) and alcohols (9). HAD, MD and VMD processing promoted a gradual reduction in ketones and the generation of esters, while the fillets that were processed by VFD contained more hydrocarbon (29.68%) and alcohol (2.64%) compounds. The volatile compounds of dried golden pompano fillets were developed through four potential pathways, including the Maillard reaction, lipid oxidation and degradation, protein hydrolysis, and Strecker degradation.

Modeling of starter cultures growth for improved Thai sausage fermentation and cost estimating for sausage preparation and transportation

The purpose of this study was to improve Thai fermented sausage flavor by adding starter cultures (i.e., Pediococcus pentosaceus, Pediococcus acidilactici, Weissella cibaria, Lactobacillus plantarum, Lactobacillus pentosus, and Lactobacillus sakei) as compared with naturally fermented sausage. The predictive mathematical models for growth of P. acidilactici and natural lactic acid bacteria (LAB) in Thai fermented sausage were developed to obtain specific prepared sausage quality. Furthermore, comparisons of sausage preparation and transportation cost between nonrefrigerated and refrigerated trucks were studied. The concentration of 3-methyl-butanoic acid synthesized from LAB inoculated sausage was higher than in the control sample which contributed to the flavor forming. Moreover, the proposed unstructured kinetic models of Thai fermented sausage substrates and products describing the consumption of total protein and glucose, and the production of nonprotein nitrogen responsible for flavor enhancer, lactic acid and formic acid concentration were successfully fitted with two selected experimental data sets of the in situ fermentation of Thai fermented sausage. Finally, the transportation of inoculated sausages in a nonrefrigerated truck by combining fermentation process and transportation was more cost efficient for delivering sausages in a long distance.

Effect of temperature and pH on the community dynamics of coagulase-negative staphylococci during spontaneous meat fermentation in a model system

Coagulase-negative staphylococci (CNS) contribute to the product quality of fermented meats. In spontaneously fermented meats, CNS communities are variable and difficult to predict, as their compositions depend on a superposed combination of different processing factors. To partially disentangle this superposition, a meat model system was used to study the influence of temperature and pH on the CNS community dynamics. Therefore, cured pork mince was prepared that was divided into three batches of different initial acidity levels, namely pH 5.7, pH 5.5, and pH 5.3. These three batches were incubated at three different temperatures, namely 23 °C, 30 °C, and 37 °C. Hence, the experimental set-up resulted in nine combinations of different temperature and initial pH values. Samples were analysed after 3 and 14 days to monitor pH, colony counts, and species diversity of the CNS communities, based on mannitol-salt-phenol-red agar (MSA) medium. At conditions of mild acidity (pH 5.7) and low temperature (23 °C), as often encountered during artisan-type meat fermentations, a co-prevalence of Staphylococcus xylosus, Staphylococcus equorum, and Staphylococcus saprophyticus occurred. At the same initial pH but higher incubation temperatures (30 °C and 37 °C), Staphylococcus lugdunensis became the prevailing CNS species, besides S. saprophyticus (30 °C) and the coagulase-positive species Staphylococcus aureus (37 °C). When the initial pH was set at 5.5, S. saprophyticus was the prevailing CNS species at both 23 °C and 30 °C, but it was replaced by Staphylococcus epidermidis and Staphylococcus simulans at 37 °C after 3 and 14 days, respectively. At the most acidic conditions (pH 5.3), CNS counts declined and many of the MSA isolates were of non-staphylococcal nature. Among others, Staphylococcus carnosus (23 °C), Staphylococcus warneri (30 °C), and S. epidermidis (37 °C) were found. Overall, the results of the present study indicated that the processing factors temperature and pH had a clear impact on the shaping of staphylococcal communities during meat fermentation.

Application of Baechu-Kimchi Powder and GABA-Producing Lactic Acid Bacteria for the Production of Functional Fermented Sausages

This study was carried out to determine the physicochemical, microbiological, and quality characteristics of a new type of fermented sausage manufactured by incorporating Baechu-kimchi powder and gamma-aminobutyric acid (GABA)-producing lactic acid bacteria (LAB). The LAB count was at the maximum level by day nine of ripening in inoculated sausages, accompanied by a rapid decrease in the pH. The addition of kimchi powder decreased the lightness (L*) and increased the redness (a*) and, yellowness (b*) values, while also significantly increasing the hardness and chewiness of the sausage (p<0.05). Moreover, although the thiobarbituric acid reactive substances values increased in all samples during the study period, this increase was lower in the kimchi-treated samples, indicating a reduction in lipid oxidation. Overall, our results show that the addition of Baechu-kimchi powder to sausages reduced the off-flavor properties and improved the taste profile of the fermented sausage in sensory evaluations. The GABA content of all fermented sausages increased from 17.42-25.14 mg/kg on the third day of fermentation to 60.95-61.47 mg/kg on the thirtieth day. These results demonstrate that Baechu-kimchi powder and GABA-producing LAB could be functional materials in fermented sausage to improve quality characteristics.

Amino acid conversions by coagulase-negative staphylococci in a rich medium: Assessment of inter- and intraspecies heterogeneity

The ability of coagulase-negative staphylococci (CNS) to convert amino acids into volatile compounds and biogenic amines was investigated after 24h and 48 h of incubation in a rich medium (brain heart infusion). Volatile compounds were measured with static-headspace gas chromatography and mass spectrometry (SH-GC-MS); biogenic amine measurements were carried out with a newly developed method based on ultra-performance liquid chromatography coupled to tandem mass spectrometry (UPLC-MS/MS). In total, 56 CNS strains from five different species were used, namely Staphylococcus carnosus, Staphylococcus epidermidis, Staphylococcus equorum, Staphylococcus saprophyticus, and Staphylococcus xylosus. With respect to the production of volatile compounds, the leucine-derived 3-methyl butanol was produced over time by most CNS strains, up to 52 μM for S. xylosus W1-1 after 48 h of incubation. The average production by strains of S. xylosus was significantly higher than for strains of S. carnosus, whereas strains of S. epidermidis turned out to be poor producers. Yet, differences between species were blurred to a large degree because of the high strain variability. A few strains also produced 3-methyl butanal on top of the amount that was already present in the medium background, although most CNS led to a decrease of this compound. Concerning biogenic amines, the average total concentrations per species remained below 100 μM after 48 h of incubation. The most abundant variant was 2-phenylethylamine (PEA), especially within S. carnosus (average of 65 μM after 48 h of incubation). Yet, some individual strains were able to produce higher concentrations, as found for the PEA production of 295 μM by S. epidermidis ATCC 12228 after 48 h of incubation. The insights obtained during this study indicate heterogeneity and are of importance in view of both starter culture development and the evaluation of a spontaneously established CNS microbiota in artisan-type meat fermentations.

A hydrolase from Lactobacillus sakei moonlights as a transaminase

Enzymatic transamination of amino acids yields α-keto acids and is the initial step for the production of volatile compounds that contribute to the sensory perception of fermented foods such as salami. Lactobacillus sakei is one of the lactic acid bacterial strains commonly used in starter cultures. Although the genome sequence of L. sakei 23K lacks genes encoding typical branched-chain amino acid transaminases, transamination activity and the formation of amino acid-derived volatile metabolites could be demonstrated. A protein purified from L. sakei is held responsible for the transamination activity. By heterologous expression of the corresponding gene in Escherichia coli, we were able to characterize the transamination side activity of an enzyme annotated as a putative acylphosphatase (AcP). A transamination side activity of hen egg white lysozyme (HEWL) was also discovered. Both enzymes showed substrate specificity toward branched-chain and aromatic amino acids. AcP also accepted l-methionine. Activity was optimal at neutral pH for both enzymes, whereas AcP showed a significantly higher temperature optimum (55°C) than that of HEWL (37°C). Kinetic parameters revealed high affinity toward l-leucine for AcP (K(m) = 1.85 mM) and toward l-isoleucine for HEWL (K(m) = 3.79 mM). AcP seems to play a major role in the metabolism of amino acids in L. sakei.

Effect of Carnobacterium piscicola on aroma formation in sausage mince

The metabolites from leucine degradation play a key role in sausage aroma. Leucine catabolism is particularly important in Carnobacterium piscicola. The purpose of the study was to determine the effect of C. piscicola inoculation on the aroma of a sausage model and on leucine catabolism. Sensory analysis and implantation of Carnobacterium were performed. Aromatic compound production from (3)H leucine added to the sausage model was measured by high-performance liquid chromatography. After 18 days of ripening, the level of Carnobacterium species was 10-fold higher in sausages inoculated with C. piscicola than in the control sausages. Addition of C. piscicola enhanced the development of sausage aroma. The leucine was 10% more degraded in the models inoculated with C. piscicola. 3-Methyl butanoic acid was the main metabolite detected and the highest production of α-ketoisocaproic acid and hydroxy α-ketoisocaproic acid were detected in the sausage model inoculated with C. piscicola.

Generation of flavour compounds in fermented sausages-the influence of curing ingredients, Staphylococcus starter culture and ripening time

The volatile profiles of fermented sausages made with either Staphylococcus xylosus or Staphylococcus carnosus starter cultures were studied with regard to the influence of salt concentration, ripening time and three different combinations of curing ingredients-nitrate, nitrite or nitrite/ascorbate. Emphasis was laid on volatile compounds originating from degradation of branched-chain amino acids. Volatile compounds were collected using dynamic headspace sampling and were identified by gas chromatography/mass spectrometry (GC/MS). Development in water activity, water loss and pH was monitored throughout maturation. Curing salts had a pronounced effect on the level of volatile compounds. In particular, curing with nitrate instead of nitrite resulted in a striking difference. Generally, nitrate increased the level of volatile compounds compared to nitrite, whereas ascorbate had only a small influence. The concentration level of NaCl had a considerable effect on the amount of volatile compounds but the effect was highly related to the ripening stage. Most compounds, but not all, increased in concentration as ripening proceeded. Major differences in the development of volatile compounds were observed depending on whether S. xylosus or S. carnosus were used as starter culture. In particular the effects of nitrate was much more predominant in the sausages made with S. carnosus than S.xylosus.

Species diversity and metabolic impact of the microbiota are low in spontaneously acidified Belgian sausages with an added starter culture of Staphylococcus carnosus

Quality of fermented sausages is affected by acidifying lactic acid bacteria (LAB) and colour- and flavour-promoting coagulase-negative staphylococci (CNS), whether or not used as starter culture. Artisan fermented sausages are often perceived as superior to industrial variants, partially because of the specific microbiota due to spontaneous acidification, which may be considered as an artisan characteristic. Therefore, two kinds of spontaneously acidified Belgian sausages were prepared (Belgian-type salami and Boulogne sausage), but with addition of a Staphylococcus carnosus culture. The Belgian-type salami was made from pork and beef, whereas the Boulogne sausage contained pork and horse meat. In all cases, Lactobacillus sakei was the dominant LAB species present on the raw materials and during fermentation, whereas enterococci remained present in the background. Enterobacteriaceae vanished after fermentation. The CNS species diversity on the raw materials was large and differed between the pork, beef, and horse meat. Nevertheless, this species diversity was annihilated during fermentation by the added S. carnosus culture. The volatiles fraction was mainly composed of aldehydes that originated from lipid oxidation and spices-derived compounds. Aromatic compounds that are typically associated to CNS activity, such as end-products from the metabolism of branched-chain amino acids, were not present in the Belgian-type salami and only marginally present in the Boulogne sausage. In conclusion, spontaneous acidification of Belgian-type fermented sausages leads to dominance of L. sakei and is no guarantee for bacterial contribution to the aroma profile when S. carnosus is added as a starter culture.

Spoilage-related activity of Carnobacterium maltaromaticum strains in air-stored and vacuum-packed meat

One hundred three isolates of Carnobacterium spp. from raw meat were analyzed by random amplification of polymorphic DNA (RAPD) and PCR and were identified by 16S rRNA gene sequencing. Forty-five strains of Carnobacterium maltaromaticum were characterized for their growth capabilities at different temperatures, NaCl concentrations, and pH values and for in vitro lipolytic and proteolytic activities. Moreover, their spoilage potential in meat was investigated by analyzing the release of volatile organic compounds (VOCs) in meat stored in air or vacuum packs. Almost all the strains were able to grow at 4, 10, and 20°C, at pH values of 6 to 9, and in the presence of 2.5% NaCl. The release of VOCs by each strain in beef stored at 4°C in air and vacuum packs was evaluated by headspace solid-phase microextraction (HS-SPME)-gas chromatography-mass spectrometry (GC-MS) analysis. All the meat samples inoculated and stored in air showed higher numbers of VOCs than the vacuum-packed meat samples. Acetoin, 1-octen-3-ol, and butanoic acid were the compounds most frequently found under both storage conditions. The contaminated meat samples were evaluated by a sensory panel; the results indicated that for all sensory odors, no effect of strain was significant (P > 0.05). The storage conditions significantly affected (P < 0.05) the perception of dairy, spoiled-meat, and mozzarella cheese odors, which were more intense in meat stored in air than in vacuum packs but were never very intense. In conclusion, different strains of C. maltaromaticum can grow efficiently in meat stored at low temperatures both in air and in vacuum packs, producing volatile molecules with low sensory impacts, with a negligible contribution to meat spoilage overall.

Metabolic pathway of α-ketoglutarate in Lactobacillus sanfranciscensis and Lactobacillus reuteri during sourdough fermentation

AIM

To identify metabolites of α-ketoglutarate (α-KG) in Lactobacillus sanfranciscensis and Lactobacillus reuteri in modified MRS and sourdough.

METHODS AND RESULTS

Lactobacillus sanfranciscensis and L. reuteri were grown with additional α-KG in mMRS and in wheat sourdough. In mMRS, α-KG was used as an electron acceptor and converted to 2-hydroxyglutarate (2-OHG) by both organisms. Production of 2-OHG was identified by high performance liquid chromatography (HPLC) and confirmed by gas chromatography (GC). Crude cell extracts of L. sanfranciscensis and L. reuteri grown with or without α-KG exhibited OHG dehydrogenase activity of 6.3 ± 0.3, 2.3 ± 0.9, 1.2 ± 0.2, and 1.1 ± 0.1 mmol l(-1) NADH (min x mg protein)(-1), respectively. The presence of phenylalanine and citrate in addition to α-KG partially redirected the use of α-KG from electron acceptor to amino group acceptor. In wheat sourdoughs, α-KG was predominantly used as electron acceptor and converted to 2-OHG.

CONCLUSIONS

Lactobacillus sanfranciscensis and L. reuteri utilize α-KG as electron acceptor. Alternative use of α-KG as amino group acceptor occurs in the presence of abundant amino donors and citrate.

SIGNIFICANCE AND IMPACT OF THE STUDY

The use of α-KG as electron acceptor in heterofermentative lactobacilli impacts the formation of flavour volatiles through the transamination pathway.

Branched chain aldehydes: production and breakdown pathways and relevance for flavour in foods

Branched aldehydes, such as 2-methyl propanal and 2- and 3-methyl butanal, are important flavour compounds in many food products, both fermented and non-fermented (heat-treated) products. The production and degradation of these aldehydes from amino acids is described and reviewed extensively in literature. This paper reviews aspects influencing the formation of these aldehydes at the level of metabolic conversions, microbial and food composition. Special emphasis was on 3-methyl butanal and its presence in various food products. Knowledge gained about the generation pathways of these flavour compounds is essential for being able to control the formation of desired levels of these aldehydes.

Carnobacterium: positive and negative effects in the environment and in foods

The genus Carnobacterium contains nine species, but only C. divergens and C. maltaromaticum are frequently isolated from natural environments and foods. They are tolerant to freezing/thawing and high pressure and able to grow at low temperatures, anaerobically and with increased CO₂ concentrations. They metabolize arginine and various carbohydrates, including chitin, and this may improve their survival in the environment. Carnobacterium divergens and C. maltaromaticum have been extensively studied as protective cultures in order to inhibit growth of Listeria monocytogenes in fish and meat products. Several carnobacterial bacteriocins are known, and parameters that affect their production have been described. Currently, however, no isolates are commercially applied as protective cultures. Carnobacteria can spoil chilled foods, but spoilage activity shows intraspecies and interspecies variation. The responsible spoilage metabolites are not well characterized, but branched alcohols and aldehydes play a partial role. Their production of tyramine in foods is critical for susceptible individuals, but carnobacteria are not otherwise human pathogens. Carnobacterium maltaromaticum can be a fish pathogen, although carnobacteria are also suggested as probiotic cultures for use in aquaculture. Representative genome sequences are not yet available, but would be valuable to answer questions associated with fundamental and applied aspects of this important genus.

Diversity of microorganisms in the environment and dry fermented sausages of small traditional French processing units

Naturally fermented sausages produced in nine traditional French processing units and their environmental surfaces were characterised by microbial and physico-chemical analyses. Salmonella and Staphylococcus aureus were not detected in the environment whereas Listeria monocytogenes was detected in four samples. Staphylococcus/Kocuria, lactic acid bacteria (LAB), Enterobacteriaceae, Pseudomonas, yeasts/moulds and enterococci contaminated the surfaces of two processing units, indicating insufficient cleaning and disinfection procedures. The final sausages did not present any health risk in seven of the processing units. In two of the processing units, the final sausages were contaminated with S. aureus and L. monocytogenes, respectively, at levels exceeding the maximum tolerable limit. Staphylococcus/Kocuria and LAB grew well in the products. Biogenic amines were found in the majority of the final products. Their occurrence was associated with high numbers of lactic acid bacteria and enterococci. The study outlined the processing and microbial diversities of French naturally fermented sausages.

Carbohydrate, peptide and lipid metabolism of lactic acid bacteria in sourdough

The metabolic pathways of lactic acid bacteria that influence bread quality are coupled to the central carbon flux by the availability of cofactors influencing the cellular and environmental redox potential. Homo- and heterofermentative metabolism differ fundamentally with respect to the requirement for regeneration of reduced cofactors, NADH or NADPH. The utilization of co-substrates such as oxygen or fructose as electron acceptors by obligate heterofermentative lactobacilli is coupled to an increased production of acetate in dough. Recently, several oxidoreductases involved in cofactor regeneration were characterized and glutathione and short-chain aldehydes derived from lipid oxidation were identified as substrates for cofactor regeneration by Lactobacillus sanfranciscensis. Based on the different metabolic requirements for cofactor regeneration, homo- and heterofermentative lactobacilli exert divergent effects on redox-reactions in sourdough that influence bread quality beyond the formation of acetate. Proteolysis, followed by peptide or amino acid metabolism by LAB is one of the key routes of flavour formation in bread flavour, and enables the strain-specific formation of antifungal metabolites. Peptide metabolism as well as the metabolism of cysteine, arginine, and phenylalanine in Lactobacillus plantarum, L. sanfranciscensis, and Lactobacillus pontis is increasingly understood and these insights provide new opportunities for the directed application of sourdough LAB for improved bread quality.

Staphylococcal community of a small unit manufacturing traditional dry fermented sausages

The level and the diversity of the staphylococcal community occurring in the environment and meat products of a small unit manufacturing traditional dry fermented sausages were investigated at two seasons: winter and spring. Gram-positive cocci were enumerated and a collection of 412 Staphylococcus isolates was made. Multiplex PCR, pulse-field gel electrophoresis (PFGE) and sequencing of the sodA gene were used to identify and characterize the isolates. High counts of Staphylococcus were found in final traditional sausages, reaching about 6 log CFU/g in winter and about 8 log CFU/g in spring. In the environment, the counts varied from 2 log to 7 log/100 cm(2), the higher colonisation being observed on the surface of the drying and cold rooms, cutting tables and the butcher's block. The combination of the three methods allowed the identification of seven species of Staphylococcus in spring and five in winter. S. equorum and S. succinus dominated both in environment and in meat products, 49% and 33% of the isolates, respectively. The other identified species were in decreasing order S. saprophyticus (6%), S. xylosus (5%), S. carnosus (5%), S. simulans (1%) and S. warneri (1%). The two species S. xylosus and S. carnosus were sporadically isolated during the spring. PFGE allowed the assignment of S. equorum to eight pulsotypes showing a wide diversity among this species. But the entire environment and the meat products were dominated by one pulsotype. For S. succinus, three pulsotypes were found with one dominant mainly isolated during the spring sampling. This study highlighted the diversity of staphylococci isolated in the environment and the meat products of a small processing unit manufacturing traditional dry fermented sausages. The S. equorum and S. succinus species rarely described in meat products and never in the environment had great capacity to colonise the entire small processing unit and the meat products.

Functional meat starter cultures for improved sausage fermentation

Starter cultures that initiate rapid acidification of the raw meat batter and that lead to a desirable sensory quality of the end-product are used for the production of fermented sausages. Recently, the use of new, functional starter cultures with an industrially or nutritionally important functionality is being explored. Functional starter cultures offer an additional functionality compared to classical starter cultures and represent a way of improving and optimising the sausage fermentation process and achieving tastier, safer, and healthier products. Examples include microorganisms that generate aroma compounds, health-promoting molecules, bacteriocins or other antimicrobials, contribute to cured meat colour, possess probiotic qualities, or lack negative properties such as the production of biogenic amines and toxic compounds. The vast quantity of artisan fermented sausages from different origins represents a treasure chest of biodiversity that can be exploited to create such functional starter cultures.

Branched-chain fatty acid biosynthesis in a branched-chain amino acid aminotransferase mutant of Staphylococcus carnosus

Fatty acid biosynthesis by a mutant strain of Staphylococcus carnosus deficient in branched-chain amino acid aminotransferase (IlvE) activity was analysed. This mutant was unable to produce the appropriate branched-chain alpha-ketoacid precursors for branched-chain fatty acid biosynthesis from the amino acids valine, isoleucine, and leucine, and required the short branched chain acids 2-methylbutanoic acid or 2-methylpropanoic acid for growth in a defined medium. The isoleucine related metabolites, alpha-keto-beta-methylvaleric acid and 2-methylbutanal also served as growth factors. Growth in rich medium and growth in defined medium supplemented with 2-methylpropanoic acid lead to extensive alteration of the fatty acid composition in the cell membrane. In rich medium, a change from 51.7% to 17.1% anteiso-C15:0, and from 3.6% to 33.9% iso-C14:0 fatty acids as compared to the wild-type strain was observed. Despite the deficiency in IlvE activity, the mutant strain was still able to produce the short chain carboxylic acids, 3-methylbutanoic acid and 2-methylpropanoic acid when cultivated in rich medium. Supplementation experiments employing deuterated glucose induced the valine biosynthetic pathway for 2-methylpropanoic acid production, revealing that the IlvE protein plays an important, but not essential role in the biosynthesis of branched-chain fatty acids and secondary metabolites in S. carnosus.

Determination of microbial volatile organic compounds from Staphylococcus pasteuri against Tuber borchii using solid-phase microextraction and gas chromatography/ion trap mass spectrometry

The mycelium of Tuber borchii Vittad., a commercial truffle species, is used as a model system for in vitro ectomycorrhizal synthesis, infected seedling production and biotechnological applications. Our fungal cultures were accidentally contaminated with a Staphylococcus pasteuri strain, showing a strong antifungal activity against T. borchii mycelium. In order to identify the antifungal volatile agents produced by S. pasteuri, solid-phase microextraction (SPME) with gas chromatography and mass spectrometry (GC/MS) was used. Using this method 65 microbial volatile organic compounds (MVOCs), synthesized by this bacterium in either single or in fungal-bacterial dual culture, were identified. SPME combined with GC/MS may be a useful method for the determination of MVOCs involved in the antifungal activity. These results showed that bacteria with unusual biological activities could be a major problem during large-scale production of inoculum for truffle-infected seedling.

The pH-unrelated influence of salt, temperature and manganese on aroma formation by Staphylococcus xylosus and Staphylococcus carnosus in a fermented meat model system

The influence of manganese (0.01-0.1-1.0 microg/g), temperature (15-24 degrees C) and salt (3-4% w/w) on volatile formation in model minces inoculated with Pediococcus pentosaceus and either Staphylococcus xylosus or Staphylococcus carnosus was studied in a full factorial experiment. In order to study the direct, pH-unrelated effect of the parameters, data were analysed by use of multiple linear regression and partial least-squares regression both before and after transformation of the volatile responses into pH-orthogonal (pH-unrelated) responses. By using the pH-orthogonalised data, the overall interpretability of the experiment was increased, and new cause-and-effect relations were suggested. Approximately 50% of the total variance in volatile levels was due to differences caused by S. xylosus and S. carnosus, and another 30% was related to differences in pH development. The remaining 20% covered pH-orthogonal effects of manganese, temperature and salt plus the experimental noise. From this, it was concluded that most of the variation in volatile profiles caused by manganese, temperature and salt was in fact directly or indirectly caused by changes in lactic acid bacterial activity and pH.

On-line monitoring of important organoleptic methyl-branched aldehydes during batch fermentation of starter culture Staphylococcus xylosus reveal new insight into their production in a model fermentation

A small fermentor (55 mL) was directly interfaced to a membrane inlet mass spectrometer for continuous on-line monitoring of oxygen and volatile metabolites during batch fermentations of the starter culture Staphylococcus xylosus. Using this technique, we were able to correlate production of the very important flavor compounds 2-methylbutanal, 3-methylbutanal, and 2-methylpropanal with various growth conditions. We found that the aldehydes were present in the culture broth only as transient metabolites. They were produced in the exponential growth phase, reached a maximum concentration when the culture became anaerobic, and then they rapidly disappeared from the culture medium. This general pattern was observed for three different strains of S. xylosus and S. carnosus. Small amounts of inoculum or increased exposure to oxygen were found to favor production of the aldehydes as a result of a longer aerobic growth period. Growing S. xylosus under conditions resembling those in a fermented sausage revealed that NaCl (5%) increased aldehyde production considerably, whereas KNO(3) (0.03%) or NaNO(2) (0.03%) had little effect. A lowering of pH from 7.2 to 6.0 reduced cell density, but had a minor affect on aldehyde production.

Catabolism of leucine to branched-chain fatty acids in Staphylococcus xylosus

AIMS

Staphylococcus xylosus is an important starter culture in the production of flavours from the branched-chain amino acids leucine, valine and isoleucine in fermented meat products. The sensorially most important flavour compounds are the branched-chain aldehydes and acids derived from the corresponding amino acids and this paper intends to perspectivate these flavour compounds in the context of leucine metabolism.

METHODS AND RESULTS

GC and GC/MS analysis combined with stable isotope labelling was used to study leucine catabolism. This amino acid together with valine and isoleucine was used as precursors for the production of branched-chain fatty acids for cell membrane biosynthesis during growth. A 83.3% of the cellular fatty acids were branched. The dominating fatty acid was anteiso-C(15:0) that constituted 55% of the fatty acids. A pyridoxal 5'-phosphate and alpha-ketoacid dependent reaction catalysed the deamination of leucine, valine and isoleucine into their corresponding alpha-ketoacids. As alpha-amino group acceptor alpha-keto-beta-methylvaleric acid and alpha-ketoisovaleric acid was much more efficient than alpha-ketoglutarate. The sensorially and metabolic key intermediate on the pathway to the branched-chain fatty acids, 3-methylbutanoic acid was produced from leucine at the onset of the stationary growth phase and then, when the growth medium became scarce in leucine, from the oxidation of glucose via pyruvate.

CONCLUSIONS

This paper demonstrates that the sensorially important branched-chain aldehydes and acids are important intermediates on the metabolic route leading to branched-chain fatty acids for cell membrane biosynthesis.

SIGNIFICANCE AND IMPACT OF THE STUDY

The metabolic information obtained is extremely important in connection with a future biotechnological design of starter cultures for production of fermented meat.