Molecular characterisation and antimicrobial activity of bacteria associated with submerged lactic acid cassava fermentation

Molecular identification of microorganisms associated with submerged cassava fermentation was carried out and isolates of lactic acid bacteria (LAB) were examined for antimicrobial activity, including ability to produce antimicrobial peptides as a first step to define starter cultures for controlled cassava fermentations. A total of 75 isolates, including 41 LAB, 31 aerobic bacteria (AB) and three anaerobic bacteria were isolated from unfermented and fermenting cassava roots, cassava leaves and fermented cassava dough and identified by a combination of phenotypic tests and sequencing of 16S rRNA, rpoA, rpoB and pheS genes. Microbial diversity at interspecies and intraspecies level was screened by, respectively, PCR amplification of the 16S-23S rDNA intergenic transcribed spacer (ITS-PCR) and repetitive sequence based PCR (rep-PCR). Antimicrobial activity of LAB cultures and supernatants against indicator bacteria; Escherichia coli, Salmonella enterica serotype Typhimurium (S. Typhimurium), Bacillus cereus and Staphylococcus aureus was studied using agar diffusion tests. Furthermore, inactivation of indicator bacteria was investigated in both liquid medium and during controlled cassava fermentation. Results revealed a diversity of bacterial genera, species and subspecies associated with submerged cassava fermentation. DNA sequencing enabled identification of LAB isolates as Lactobacillus plantarum, Weissella confusa, Weissella paramesenteroides, Lactobacillus rhamnosus, Lactobacillus hilgardii, Lactobacillus paracasei, Leuconostoc mesenteroides, Enterococcus faecium, Enterococcus casseliflavus, and Pediococcus acidilactici. Lactobacillus spp. were the predominant LAB and were present in all cassava samples studied. Aerobic bacteria were predominantly Bacillus spp., including Bacillus subtilis, Bacillus amyloliquefaciens and B. cereus. Other species identified included Staphylococcus pasteuri and Clostridium beijerinkii. Cells, supernatants and cell free supernatants (CFS) of selected LAB isolates were able to inhibit both Gram positive and Gram negative pathogenic bacteria. LAB isolates inactivated all indicator organisms during controlled cassava fermentations, with a 4–6 log reduction after 48 h fermentation. The antimicrobial effect of the LAB was attributed to acid production.     

Probiotic biological strategies to decontaminate aflatoxin M1 in a traditional Iranian fermented milk drink (Doogh)

Aflatoxin M₁ is an important mycotoxin mostly found in milk and dairy products. The main objective of this work was to study the effects of probiotic strains, a probiotic inoculated population, the physiology of probiotic bacteria and final fermentation pH at four consecutive stages on the reduction of 0.500 ppb of free aflatoxin M₁ (AFM₁) in Doogh (a traditional Iranian fermented milk drink). Samples’ biochemical, microbial and AFM₁ binding characteristics were monitored during fermentation and storage (5 °C for 28 days). An immunoaffinity column was used to extract AFM₁ and a high-performance liquid chromatograph with a fluorescence detector was used to measure it. Results showed that Lactobacillus acidophilus was probiotic strain that most reduced free AFM₁. Inoculation of L. acidophilus at 9 log cfu/mL, despite the higher cost, revealed significantly higher free AFM₁ binding capacity than 7 log cfu/mL. Heat-killed (dead) L. acidophilus bacteria reduced less free AFM₁ at the end of storage than viable. Samples with a final fermentation pH of 4.5 bound more free AFM₁ during fermentation and storage than those with a pH of 4.2. It is concluded that inoculation of 7 log cfu/mL L. acidophilus viable cells in Doogh with a final fermentation pH of 4.5 supplied a safety- and health-promoting and cost-effective drink.     

Whole soybean as probiotic lactic acid bacteria carrier food in solid-state fermentation

For the purpose of preparing lactic acid bacteria (LAB) carrier food, the solid-state fermentation of whole soybean was performed using Bifidobacterium animalis 937, Lactobacillus casei Zhang and Lactobacillus plantarum P-8 mixed with Bacillus subtilis natto, respectively. The physicochemical properties, the amino nitrogen content and peptide molecular weight distribution of the fermented whole soybean products were examined during this process. After 48 h of fermentation, the viable counts of the three samples were 1.41 × 10⁸ CFU/g (B. animalis 937), 1.74 × 10¹⁰ CFU/g (L. casei Zhang) and 2.19 × 10¹⁰ CFU/g (L. plantarum P-8), with the pH declined rapidly from 6.32 to 5.78, 5.60 and 5.44 at the early stage of the fermentation and increased to 6.71, 6.47 and 6.60 at the later stage of the fermentation. The fermentation caused a sharply increase in the content of the free amino nitrogen from 99.7 μmol/g to 301.9 μmol/g, 390.1 μmol/g and 529.1 μmol/g in the solid fermented soybean products, due to the multiplication of microorganism and the effect of enzyme system. Furthermore, the levels of soybean peptide with molecular weight less than 1000 Da increased 30.7%, 71.2% and 81.3% relative to that of the control group at 48 h. The result of the present work implied that whole soybean fermented by LAB can provide the different probiotics for the host, and there is potential to develope nutritious fermented soybean products.     

Degradation of the minor Fusarium mycotoxin beauvericin by intracellular enzymes of Saccharomyces cerevisiae

Beauvericin (BEA) is a cyclic depsipeptide with antibiotic and insecticidal effects. It was discovered for the first time from the fungus Beauveria bassiana, but more significantly, is produced by several Fusarium strains, and considered a contaminant of several cereals like corn, wheat and barley.This study investigated the degradation of BEA by intracellular raw enzymes of four strains of Saccharomyces cerevisiae, named LO9, YE5, A34, and A17. The BEA at 25 mg/kg in a model solution and in corn flour was co-incubated with the raw enzymes from the four yeast strains, respectively. The reduction of BEA was evaluated using liquid chromatography coupled to a diode array detector (LC–DAD); the products formed during the co-incubation were determined by liquid chromatography coupled to mass spectrometry-linear ion trap (LC–MS-LIT). In model solution BEA reduction ranged from 83 to 100%. In corn flour treated with the intracellular raw enzymes, the BEA degradation was from 66 to 91%. A product resulted from the BEA degradation was identified.     

Prevention of bread mould spoilage by using lactic acid bacteria with antifungal properties

The ability of lactic acid bacteria (LAB) to inhibit Aspergillus, Fusarium, and Penicillium, the main contaminants in bread, was evaluated. Only four strains (Lactobacillus plantarum CRL 778, Lactobacillus reuteri CRL 1100, and Lactobacillus brevis CRL 772 and CRL 796) from 95 strains tested displayed antifungal activity. The major antifungal compounds were acetic and phenyllactic acids. The fermentation quotient (FQ = 2.0) and the leaven volume (80 cm³) of doughs with LB and yeasts were higher than doughs without LB. The inclusion of antifungal LAB strains in the starter culture allowed a reduction in the concentration of calcium propionate by 50% while still attaining a shelf life similar to that of traditional bread containing 0.4% CP.     

Biodiversity,dynamics and antimicrobial activity of lactic acid bacteria involved in the fermentation of maize flour for doklu production in Côte d'Ivoire

Doklu is a maize-based spontaneously fermented dough produced and consumed in parts of West Africa, particularly in Côte d'Ivoire. The characterization of the microbial ecosystem of doklu was carried out using a polyphasic approach. First, culture-dependent methods were used for bacterial enumeration and the phenotypic and molecular identification of 250 lactic acid bacteria (LAB) isolates. Then, culture-independent methods, including PCR-TTGE (V3 region of the 16S rRNA gene), provided a fingerprinting of bacterial DNA directly extracted from doklu. Bio preservative abilities were also tested and strains producing antimicrobial compounds were genotyped using PFGE. During maize dough fermentation, LAB became dominant and their load increased from 4.2 ± 0.2 log CFU/g to 9.0 ± 0.7 log CFU/g only after 48 h. Culture-dependent methods highlighted the presence of five LAB groups with the species Lactobacillus plantarum (28%), Lactobacillus fermentum (41.6%), Pediococcus acidilactici (6.8%), Pediococcus pentosaceus (18%) et Weissella cibaria (5.6%), succeeding during the fermentation. Lb. fermentum being practically the only species present at the end of fermentation, is with Lb. plantarum, the predominant species of fermenting dough. Culture-independent analysis underlined the undoubted role of Lb. fermentum, actively involved in the dough fermentation. These Lb. fermentum species, with a diversity of strains also showed important antimicrobial activity, due to production of bacteriocins. Being able to produce antimicrobial compounds, Lb. fermentum species may act as both bio protective culture as well as fermenting agent in cereal products and could be exploited to create functional starter cultures.     

Characterization of the dominant bacterial communities of traditional fermentation starters for Hong Qu glutinous rice wine by means of MALDI-TOF mass spectrometry fingerprinting, 16S rRNA gene sequencing and species-specific PCRs

The main goal of this work was to investigate the bacterial flora present in the traditional fermentation starters using matrix-assisted laser desorption/ionisation time of flight (MALDI-TOF) mass spectrometry fingerprinting combined with 16S rRNA gene sequencing and species-specific PCRs. A total of 1314 bacterial strains were obtained from ten kinds of representative wine fermentation starters (5 Hong Qu and 5 Yao Qu) and classified by MALDI-TOF mass spectrometry. Sixteen different bacterial species belonging to six different genera (Bacillus, Staphylococcus, Leuconostoc, Pediococcus, Lactobacillus and Lactococcus) were identified with high accuracy by comparing them with generated spectra of MALDI Biotyper database. Comparison of 16S rRNA gene phylogeny and MALDI-TOF mass spectrometry analysis showed that MALDI-TOF mass spectrometry fingerprinting was a good complementary approach to 16S rRNA sequencing and even a more powerful tool in the accurate differentiation and classification of Bacillus species, especially for differentiating between Bacillus subtilis, Bacillus amyloliquefaciens and Bacillus methylotrophicus. MALDI-TOF mass spectrometry was also found to provide valuable information at species levels for the genus of Lactobacillus. The identification revealed that most strains isolated from traditional fermentation starters were B. amyloliquefaciens (31.81%), followed by Bacillus megaterium (16.89%), Lc. lactis subsp. lactis (8.60%) and B. subtilis (8.14%). In the culture-dependent quantitative analysis, 69.01% of the detected isolates were Bacillus species, and 30.44% of the detected isolates belonged to the class of lactic acid bacteria (LAB), including Leu. mesenteroides, Pediococcus pentosaceus, Lactobacillus plantarum, Lactobacillus paracasei, Lactobacillus brevis, Pediococcus acidilactici, Lactobacillus coryniformis and Lc. lactis subsp. lactis. This study provides detailed evaluation of bacterial communities in the traditional fermentation starters for Hong Qu glutinous rice wine using a fast and cost-effective technique—MALDI-TOF mass spectrometry fingerprinting. Information provided by this study is fundamental and useful to develop effective strategies for the selection of beneficial bacterial strains and improve of quality of Hong Qu glutinous rice wine.     

In vitro and in situ screening of lactic acid bacteria and propionibacteria antifungal activities against bakery product spoilage molds

In the agri-food context, bioprotective cultures represent an interesting alternative to chemical preservatives. The aim of this study was to evaluate the in vitro and in situ antifungal activity of lactic acid bacteria (LAB) and propionibacteria against bakery product spoilage fungi. Firstly, the biodiversity of fungal contaminant in pound cakes and milk bread rolls, as well as the resistance to chemical preservatives of representative isolates (n = 21), were studied. Aspergillus and Eurotium species were the most dominant spoilage fungi and the most resistant towards the tested chemical preservatives. They were followed by Penicillium, Cladosporium and Wallemia spp. Secondly, an in vitro screening showed that the most active isolates against selected fungal targets belonged to the Leuconostoc spp., Lactobacillus reuteri and Lactobacillus buchneri groups among the 270 tested LAB, as well as to the Propionibacterium freudenreichi and Propionibacterium acidipropionici species for the 50 tested propionibacteria. Finally, in situ tests showed that cultures of isolates belonging to the Leuconostoc citreum, Lactobacillus sakei, Lactobacillus plantarum, Lactobacillus spicheri, L. reuteri and Lactobacillus brevis species could delay one or several target fungal growths after bakery product surface spraying. Moreover, different strain cultures led to delayed fungal growths after incorporation in milk bread rolls preparation. The combination of in vitro and in situ approaches allowed for the identification of bacteria exhibiting antifungal activity, providing future prospects for use as bioprotective cultures in bakery products.     

In vitro antifungal activity of lactic acid bacteria against mycotoxigenic fungi and their application in loaf bread shelf life improvement

Food spoilage caused by mycotoxigenic fungi represents an important food safety problem. Lactic acid bacteria (LAB) are used as starter cultures in a larger number of food products. In this study, 16 strains of LAB were cultivated in MRS broth under anaerobiosis. Then, cell free supernatants were obtained by centrifugation and their antifungal activity against Aspergillus parasiticus and Penicillium expansum was tested using the disc-diffusion method. Furthermore, the LABs that showed in vitro antifungal activity were used in bread fermentation with yeast in order to study fungal growth inhibition and aflatoxin (AF) reduction in processed bread previously inoculated with A. parasiticus. The compounds present in the fermented medium of six LAB strains induced inhibition of P. expansum growth, whereas five probiotic strains produced antifungal compounds against A. parasiticus. The analysis by liquid chromatography coupled to mass spectrometry in tandem showed a reduction of the AF content in bread samples fermented with yeast and LABs. The reduction of AFs ranged from 84.1 to 99.9%. Moreover, bread sample studies showed a shelf life increase of about 3–4 days.     

Starter culture fermentation of Chinese sauerkraut: Growth,acidification and metabolic analyses

Chinese sauerkraut is a kind of traditional and typical fermented food. Four lactic acid bacteria (LAB) strains, Leuconostoc mesenteroides NCU1426, Lactococcus lactis NCU1315, Lactobacillus plantarum NCU1121 and Lactobacillus casei NCU1222 isolated from Chinese sauerkraut, were used in single starter cultures. Microbiological changes and pH values were monitored during fermentation. Metabolic substrates and products during the fermentation were examined using high performance liquid chromatography (HPLC) technology. Results have shown that Leu. mesenteroides and Lc. lactis grew faster, produced lactic acid earlier and were poorly acid-resistant, whereas Lb. plantarum and Lb. casei produced much more lactic acid throughout fermentation and showed better acid-tolerance. Two Lactococcus had outstanding performance in sucrose utilization while the other two Lactobacillus were likely to use glucose and fructose during fermentation. Unexpectedly, Leu. mesenteroides and Lc. lactis showed weak citric acid metabolism in fermentation. All the four LAB strains were able to utilize malic acid in early fermentation. In conclusion, these LAB strains have shown notable differences in growth and fermentative properties during starter culture fermentation of Chinese sauerkraut, probably resulting from LAB fermentative function and a mixture of complex substrates.     

Utilization of ram horn hydrolysate as a supplement for recovery of heat- and freeze-injured bacteria

Tryptic soy agar (TSA) has been widely and successfully used as a resuscitation medium for injured bacteria. The purpose of this study was to examine the use of ram horn hydrolysate (RHH) as a supplement for recovery of injured bacteria, to compare it to TSA and to evaluate its usefulness as a possible alternative supplement in microbial media. The TSA was enriched with RHH at various concentrations. 2% RHH was found to be optimal for growth of healthy bacteria. The enriched medium (TSA + 20 ml RHH in 1–l) was termed enriched tryptic soy agar (ETSA). TSA and ETSA were tested for the recovery of heat- and freeze-stressed bacteria. Four species of bacteria (Escherichia coli, Listeria monocytogenes, Salmonella typhimurium and Pseudomonas aeruginosa) were heat-treated at 55 °C in physiological saline by a capillary tube method. For the freeze-injury studies, 5 ml of the tryptic soy broth cultures were placed in a freezer at −25 °C for 7 days. The numbers of uninjured bacteria on TSA were evaluated as control. The number of viable bacteria on TSA and ETSA was determined by the pour plate method. Recovery rates of bacteria on ETSA were higher than that of TSA. The RHH appears to be a valuable supplement for microbial media.     

Characterization and selection of Lactobacillus brevis starter for nitrite degradation of Chinese pickle

To improve the safety of Chinese pickles, lactic acid bacteria were isolated from traditional fermented foods to test for nitrite degradation during the pickling process. Lactobacillus brevis AR123 was screened out as a starter culture for pickle fermentation. L. brevis AR123 has a high tolerance to nitrite and salt. During pickle fermentation, the nitrite peak appeared from 12 to 24 h in the pickle juice and was delayed to 24–48 h in the pickles. In 6% salt, L. brevis AR123 and a commercial starter could rapidly lower the pH of the pickles, and the nitrite contents were 0.83 mg kg⁻¹ and 3.05 mg kg⁻¹, respectively, in pickles at 72 h. Meanwhile, no obvious nitrite peak was observed for the pickle inoculated with a mixed starter; it had a low nitrite level (0.88 mg kg⁻¹ at 72 h). However, the concentration of nitrite was as high as 17.92 mg kg⁻¹ in the spontaneous fermentation group. During pickle fermentation, Lactic acid bacteria (LAB) population increased significantly when salt concentration was 4%–8%, and sensory analysis shown that the L. brevis AR123 inoculated pickle exhibited better sensory evaluation results than spontaneous fermentation pickle. In conclusion, inoculation with L. brevis AR123 alone or combined with a commercial starter (mixed starter) is effective in removing nitrites and enhancing the sensory properties of pickle. LAB could accelerate degradation of nitrite significantly and shorten the fermentation time for pickle process.     

Reduction of ochratoxin A during the fermentation of Italian red wine Moscato

The interaction between ochratoxin A (OTA) and 16 yeast strains of Saccharomyces cerevisiae during alcoholic fermentation of the Italian wine Moscato was studied. Levels of OTA in the fermentation liquid, on the cell walls and on the internal part of the cells were determined using a high-performance liquid chromatography (HPLC) system with a fluorescence detector. Results showed that yeast cells adsorbed the mycotoxin on the external and internal part of the cells and also that OTA presence did not affect the alcoholic fermentation in any tested yeast. All the yeasts analyzed in this study showed a significant reduction in the OTA content during the fermentation process.     

Metabolites produced during the growth of probiotics in cocoa supplementation and the limited role of cocoa in host-enteric bacterial pathogen interactions

Cocoa contains various compounds that can significantly affect the growth of a broad range of bacteria, and have multiple human health-promoting properties. In this study, the effects of cocoa powder on the growth of Lactobacillus, common milk resident bacteria, and three major foodborne enteric bacterial pathogens; enterohemorrhagic Escherichia coli O157:H7 (EHEC), Salmonella enterica serovar Typhimurium, and Listeria monocytogenes, were investigated in vitro. Significant (p < 0.05) growth stimulation on beneficial bacteria including Lactobacillus and other resident bacteria in milk was observed in the presence of 3% cocoa powder. In contrast, growth of three foodborne enteric pathogens was significantly (p < 0.05) inhibited within 9 h, but no stimulation was found with longer incubation. In addition, cocoa powder significantly (p < 0.05) inhibited adhesion to and invasion of INT407 cells by these bacterial pathogens in a dose dependent manner. These results suggest that addition of cocoa into dairy products could improve the beneficial effect of probiotics by stimulating their growth, without raising the risk of cross-contamination with enteric pathogens.     

Effect of starter culture on growth of Staphylococcus aureus in sucuk

The effects of starter cultures, starter A (Pediococcus acidilactici + Lactobacillus curvatus + Staphylococcus xylosus), starter B (Lactobacillus sakei + Staphylococcus carnosus) and ripening period on growth of S. aureus were investigated in sucuk (Turkish dry-fermented sausage) production. Sucuk batters were inoculated with S. aureus (10⁶/g). S. aureus, Micrococcus/Staphylococcus, lactic acid bacteria and Enterobacteriaceae counts were determined during ripening, and pH and a_w were analyzed. Both starter cultures and ripening period had significant effects on the S. aureus, lactic acid bacteria, Micrococcus/Staphylococcus, Enterobacteriaceae counts and pH value (P < 0.01). The numbers of S. aureus increased by 1 log on the 3rd day in the control group, whereas a reduction was observed during ripening period in sucuk with starter cultures.     

Presence of Fusarium emerging mycotoxins in tiger-nuts commercialized in Spain

Tiger-nuts are tubers from the plant Cyperus esculentus which are commercialized in some regions of West and Central Africa, USA and Spain. These tubers have a high microbial charge such as bacteria, moulds and yeasts which could remain until the finished product. In this study we examined 47 samples of tiger-nut tubers purchased from Spanish supermarkets for contamination with the emerging Fusarium mycotoxins: Enniatins ENs (EN A, EN A₁, EN B and EN B₁), beauvericin (BEA) and fusaproliferin (FUS). The extraction of the samples was carried out with methanol using an Ultraturrax homogenizer. Mycotoxins were analyzed with a liquid chromatography (LC) coupled to a diode array detector (DAD).The percentage of contaminate samples with one or more emerging mycotoxins was 21.3%. EN A₁ result the most common EN found with the highest prevalence of 17% with levels ranging from 32.3 to 4400 μg/g. EN B₁ and BEA were present simultaneously in 5 samples, whereas the maximum contamination observed of the EN B₁ was of 346 μg/g. The lowest incidence were observed for the ENs A and B evidenced only in one sample of the total studied. Only one emerging mycotoxin, fusaproliferin (FUS), has not been detected in any sample studied.The present work is the first one ever drafted on the presence of the emerging Fusarium mycotoxins in Spanish tiger-nuts.     

First report on the presence of emerging Fusarium mycotoxins enniatins (A,A1, B,B1), beauvericin and fusaproliferin in rice on the Moroccan retail markets

Seventy samples of rice purchased from local markets in six cities from Morocco (Rabat, Casablanca, Kénitra, Mohammadia, Tanger and Errachidia) were analyzed for the presence of six emerging mycotoxins: four enniatins ENs (ENA, ENA₁, ENB and ENB₁), beauvericin (BEA) and fusaproliferin (FUS). Samples were extracted with a mixture of acetonitrile/water (85/15, v/v) by using an ultra-turrax homogenizer. Mycotoxins were then identified and quantified with liquid chromatography (LC) coupled to diode array detector (DAD). Positive samples were confirmed with an LC-MS/MS. Analytical results showed that BEA was present in 75.7% of total analyzed samples. BEA levels varied between 3.8 and 26.3 mg/kg. The frequencies of contamination of samples with total ENs and FUS were 50% and 4.3%, respectively. Among the ENs, ENB was the mycotoxin much more found (30% of total samples), while ENB₁, ENA and ENA₁ were found in 24.6%, 22.8% and 5.7% of total samples, respectively. The high ENs value was registered in a rice sample from kénitra (448.7 mg/kg of ENA₁). This is the first study that describes the presence of emerging Fusarium mycotoxins in rice available in Morocco.     

Antimicrobial activity of d-3-phenyllactic acid produced by fed-batch process against Salmonella enterica

Five lactic acid bacteria (LAB) were studied for the ability to produce 3-phenyllactic acid (PLA), a novel antimicrobial compound derived from the metabolism of phenylalanine (Phe). The highest amount of PLA (1.38 ± 0.048 mM) was produced by Lactobacillus plantarum CECT-221. A 3∗∗(2-0) full factorial design studding different initial Phe and glucose concentrations revealed the inhibitory effect of high starting glucose concentrations in spite of being the most significant variable. Fed-batch methodologies were assessed since no product inhibition was observed with PLA concentrations lower than 6 mM. Fed-batch fermentations of L. plantarum in Erlenmeyer Flasks without pH-control were unsuccessful; however fed-batch fermentations in a 2 L Biorreactor with pH-control improved considerably the stoichiometric parameters. Starting with 0.6 g L⁻¹ Phe, PLA was continuously accumulated reaching a maximum amount of 4.25 ± 0.18 mM after 158 h of fermentation (Q_P = 0.00258 g L⁻¹ h⁻¹). A GC-FID analysis of the metabolites obtained in the Phe metabolism by LAB showed the presence of small amounts of benzaldehyde and phenylacetaldehyde. Exhausted broths containing mixtures of PLA and other organic acids resulted to be an effective antimicrobial against the pathogen Salmonella reducing his growth in 63.2% in 24 h of incubation, thus, opening new perspective to preserve foods and feedstuffs with natural antimicrobials, avoiding food-borne diseases caused by Salmonella.     

Plantaricin MG active against Gram-negative bacteria produced by Lactobacillus plantarum KLDS1.0391 isolated from “Jiaoke”, a traditional fermented cream from China

A bacteriocin named plantaricin MG produced by Lactobacillus plantarum KLDS1.0391 which was isolated from “Jiaoke”, a traditional, naturally fermented cream from Inner Mongolia in China is reported in this article. Plantaricin MG was purified by ammonium sulfate precipitation followed by sequential gel filtration chromatography and reverse-phase chromatography. Mass spectrometry analysis showed the mass of plantaricin MG to be approximately 2180 Da. The bacteriocin showed a broad inhibitory activity against Gram-positive and Gram-negative bacteria including Listeria monocytogenes, Staphylococcus aureus, Salmonella typhimurium and Escherichia coli. The bacteriocin was extremely heat-stable (30 min at 121 °C) and remained active after incubation at pH 2.0–10.0. It was found to be sensitive to proteolytic enzymes (pepsin, trypsin, papain, α-chymotrypsin, proteinase K, Neutrase and Alcalase). The mode of action of plantaricin MG was identified as bactericidal.     

Fermented tofu: Enhancement of keeping quality and sensorial properties

In this study a new way to produce tofu by means of soymilk fermentation by specific lactic acid bacteria, Lactobacillus casei and Lactobacillus acidophilus alone or in combination, and subsequent precipitation has been developed, in order to prevent undesired microbial and chemical spoilage, as well as improve the stability and the quality of the product. In particular, the combination L. casei and L. acidophilus generated tofu having shelf life exceeding 20 days and able to prevent the growth of the spoilage strains inoculated. This fermented tofu was characterized by the production of antimicrobial molecules, such as acetic acid, limonene, 2-nonen-1-ol, 1-nonanol, 2(5H)-furanone, benzyl alcohol, phenylethyl alcohol and heptanoic acid. Depending on the Lactobacillus species used, the fermentation process generated different metabolites profiles and sensorial properties. Another promising properties conferred by the lactic acid bacteria fermentation was the inhibition of unsaturated fatty acids (UFAs) peroxidation or reduction of the aldehydes originated to their corresponding alcohols.