Fast Acidification Research Articles

Microbiological Safety and Functional Properties of a Fermented Nut-Based Product

Fermented nut-based products, obtained after soaking and fermentation, are gaining increasing interest as animal food substitutes because of ethical, environmental and health reasons. In these products, Lactic Acid Bacteria (LAB) perform the fermentation, leading to matrix acidification and contributing to controlling spoilage and pathogenic microbiota. In this work, LAB strains isolated from an artisanal product and combined with a commercial strain were added as starter cultures during nut soaking to produce a cheese-like fermented plant-based product. Three different LAB consortia were used in challenge tests at laboratory scale against Listeria monocytogenes, Escherichia coli or Salmonella Enteritidis, inoculated in nuts at 5 log CFU/g, and monitored for pathogen survival and matrix acidification. The combination of Lactiplantibacillus plantarum 82 and Leuc. carnosum 4010 resulted in faster acidification (pH value < 4.4 after 18 h instead of 48 h) and the reduction of target pathogens; L. monocytogenes was already absent after seven days from production, and the counts of E. coli or S. Enteritidis were lower with respect to other samples. Thus, this microbial consortium was used for a pilot-scale production in which, beyond safety, the fermented plant-based product was also characterized for aroma profile and phenolic compounds, parameters that are known to be affected by LAB fermentation. The results showed an enhancement of the aroma profile, with an accumulation of molecules able to confer cheese-like notes (i.e., acetoin and diacetyl) and higher phenolic content, as well as the presence of compounds (i.e., phenyllactic acid and hydroxyphenyllactic acid) that could exert antimicrobial activity. This study allowed us to set up a guided fermentation for a cheese-like vegan product, guaranteeing safety and improving aromatic and functional features.

Deciphering the metabolism of Lactobacillus delbrueckii subsp. delbrueckii during soy juice fermentation using phenotypic and transcriptional analysis.

To reduce the environmental and health concerns related to food, an alternative diet is recommended, containing 50% of plant-based proteins. Soy juice, which is protein rich, is a relevant alternative to animal milk, for the production of yogurt-like products. However, soy "beany" and "green" off-flavors limit the consumption of such products. The lactic acid bacteria (LAB) used for fermentation can help to improve the organoleptic properties of soy products. But metabolic data concerning LAB adapted to soy juice are lacking. The aim of this study was, thus, to decipher the metabolism of Lactobacillus delbrueckii subsp. delbrueckii during fermentation of a soy juice, based on a multidisciplinary approach. This result will contribute to give tracks for a relevant selection of starter. Indeed, the improvement of the organoleptic properties of these types of products could help to promote plant-based proteins in our diet.

Acid Fracturing Technology and Effect Evaluation of Carbonate Horizontal Well in Fuman Oilfield

Since the deep and ultra-deep carbonate rocks in the Fuman oilfield have made continuous breakthroughs, the acidification and fracturing of the reservoir on the south bank of Tarim River have been faced with problems such as faster acidification and shorter effective acidification fracture length with the increased burial depth and formation temperature, which seriously restrict the effect of acidification and fracturing. At present, the Fuman oilfield has achieved remarkable results after acid fracturing construction. In this paper, the acid fracturing construction effects of multiple reservoir reconstruction Wells in Tahe Yueman, Fuyuan, Guole, and Manshen blocks are evaluated and classified by analyzing acid fracturing construction curves, using fracture static pressure fitting and fracture conductivity curve analysis. The characteristics and distribution law of carbonate reservoirs corresponding to different construction curves are studied, and various situations of artificial fractures communicating with natural fracture-cavity systems are analyzed. The research results can accurately guide the design optimization and development of acid pressure.

Anti-inflammatory effects of pulsed magnetic field on the immune system

When an inflammatory response occurs in the body, primary immune cells, such as macrophages, phagocytize and present antigens, and secondary immune cells, such as T cells, produce antibodies against the presented antigens to prevent infection in the body. Abnormalities in the immune system releases pro-inflammatory cytokines such as TNF-α and IL-6, leading to rheumatoid arthritis and sepsis. This study aimed to confirm the change in inflammatory activity across innate immunity and acquired immunity while investigating the effects of pulsed magnetic field (PMF) on the immune system to regulate excessive inflammation. Raw 264.7 and EL4 cells were co-cultured in a DMEM medium using permeable cell culture inserts and inflammation was induced using LPS. Our PMF stimulator has the maximum intensity of 4700 G at a transition time of 136 μs with pulse intervals of 1 Hz. In order to confirm anti-inflammatory effects of PMF, the level of IL-6, pH and mitochondrial respiration rate (MRR) were measured using ELISA, pH meter and MTT assay, respectively. It was observed in vitro that inflammation-induced macrophages induce activation of T cells, as evidenced by the faster acidification in pH, increased IL-6 expression, and enhanced MRR, compared to T cells. But after PMF stimulation, pH levels improved, the concentration of IL-6 reduced, and MRR decreased in both macrophages and T-cells, which indicates inflammation relief. Our results show that macrophages, the primary immune cells, are first involved in the inflammatory response and recovery and influence T cells, the secondary immune cells. Therefore PMF stimulus is thought to affect the immune system of the human body by balancing the activation and suppression of immune cells and improve anti-inflammation. For clinical use, non-invasive PMF can be developed as a medical devices modulating immune system with optimizing the PMF conditions such as pulse shape, duration, or repetition rate.

Diminishing neuronal acidification by channelrhodopsins with low proton conduction.

Many channelrhodopsins are permeable to protons. We found that in neurons, activation of a high-current channelrhodopsin, CheRiff, led to significant acidification, with faster acidification in the dendrites than in the soma. Experiments with patterned optogenetic stimulation in monolayers of HEK cells established that the acidification was due to proton transport through the opsin, rather than through other voltage-dependent channels. We identified and characterized two opsins which showed large photocurrents, but small proton permeability, PsCatCh2.0 and ChR2-3M. PsCatCh2.0 showed excellent response kinetics and was also spectrally compatible with simultaneous voltage imaging with QuasAr6a. Stimulation-evoked acidification is a possible source of disruptions to cell health in scientific and prospective therapeutic applications of optogenetics. Channelrhodopsins with low proton permeability are a promising strategy for avoiding these problems.

Diminishing neuronal acidification by channelrhodopsins with low proton conduction

Many channelrhodopsins are permeable to protons. We found that in neurons, activation of a high-current channelrhodopsin, CheRiff, led to significant acidification, with faster acidification in the dendrites than in the soma. Experiments with patterned optogenetic stimulation in monolayers of HEK cells established that the acidification was due to proton transport through the opsin, rather than through other voltage-dependent channels. We identified and characterized two opsins which showed large photocurrents, but small proton permeability, PsCatCh2.0 and ChR2-3M. PsCatCh2.0 showed excellent response kinetics and was also spectrally compatible with simultaneous voltage imaging with QuasAr6a. Stimulation-evoked acidification is a possible source of disruptions to cell health in scientific and prospective therapeutic applications of optogenetics. Channelrhodopsins with low proton permeability are a promising strategy for avoiding these problems.

Continuous co-generation of biohydrogen and biomethane through two-stage anaerobic digestion of hydrothermally pretreated food waste

Increasing accumulation of food waste (FW) exerts pressure on resource conservation and environmental protection, while conventional anaerobic digestion (AD) of raw FW for bioenergy recovery and waste minimization often encounters limitations such as fast acidification and process failure. Herein, mild hydrothermal pretreatment (HTP: 140 °C, 20 min) of raw FW was employed to advance subsequent AD performances in a long-term continuous mode. With a decreased overall hydraulic retention time (HRT) from 30 to 18 days, the two-stage system digesting HTP-treated FW maintained very stable and robust biohydrogen (38.1 mL/gVS) and biomethane (439.6 mL/gVS) co-production, whereas both the two-stage system digesting raw FW and the one-stage system digesting HTP-treated FW exhibited deteriorating trends with significantly lower energy yields by 61.1% and 28.1%, respectively. Microbial community analyses revealed the less involvement of carbohydrate-fermenting and proteolytic microbes in the second-stage biomethane reactors as a result of the enhanced hydrolysis and acidogenesis of FW through HTP and first-stage biohydrogen fermentation. A proximate energy balance assessment indicated that the energy gain of increased biogas generation offset the extra heat requirement for the HTP of FW, while a significant treatment capacity enhancement of 66.7% was achieved given the HRT reduction from 30 to 18 days through this integrated system.

Large Increase in Dissolved Inorganic Carbon in the East Sea (Japan Sea) From 1999 to 2019

The East Sea (also known as the Japan Sea; hereafter, EJS) has its own deep overturning circulation, that operates over a centurial timescale compared with a millennial timescale in the ocean. This allows the EJS to be used as a natural laboratory for investigating potential future changes in the oceanic system. Dissolved inorganic carbon (DIC), total alkalinity (TA), and pH were measured in 2019 in a wide area of the EJS to investigate the characteristics and changes of the carbonate system since the last extensive survey in 1999. In the layer below ∼1,000 m, DIC and apparent oxygen utilization (AOU) was uniform implying rapid horizontal mixing within a few years. Since 1999, DIC concentration increased by ∼11 μmol kg–1 in the layer deeper than 500 m. This increase accompanied a commensurate increase in AOU with the canonical ratio of 1.3, indicating that the accumulation of DIC was supplied from organic matter decomposition. This observation is consistent with a previous study suggesting that the slowed deep water ventilation was the cause of the increase in AOU and fast acidification. In the EJS, increase in DIC from the surface water to deep waters is much higher than that in TA, which is caused by high primary productivity and export production together with low rates of CaCO3 export. Thus, the DIC/TA ratio of deep waters, an indicator of vulnerability to acidification, is high. A recently reported change in deep water ventilation, namely, re-initiation of deep water formation reaching deeper depths to the Deep Water and the Bottom Water, implies that unexpected changes in the carbonate system may be detected in the future, which needs to be further monitored.

Selected physicochemical, technological, and hygienic characteristics of artisanal and sausages produced with functional starter culture

Fast acidification is one of the main factors of microbial stability of dry fermented sausages. Development of functional starter cultures for improving safety of sausages without altering their quality is under way. This study compared aspects of physicochemical, technological, hygienic and instrumental quality of sausages produced with or without functional starter culture. Finished sausages with starter had lower water activity and lower levels of enterobacteria and lactic acid bacteria, compared to artisanal ones. During most of the ripening, pH was lower in sausages with starter, but in the final products, the same pH was observed in both groups of sausages. In sausages with starter lower redness was determined than in artisanal sausages, while other parameters, colour and chewiness, did not differ significantly. Use of starter culture improved physicochemical, technological and hygienic characteristics of the final products.

Probing Dynamic Features of Phagosome Maturation in Macrophage using Au@MnOx @SiO2 Nanoparticles as pH-Sensitive Plasmonic Nanoprobes.

Phagosome maturation in macrophage is essential to the clearance of pathogenic materials in host defence but the dynamic features remain difficult to be measured in real time. Herein, we reported the multilayered Au@MnOx @SiO2 nanoparticle as a robust pH-sensitive plasmonic nanosensor for monitoring the dynamic acidification features over the phagosome maturation process in macrophage under darkfield microscopy. For this multilayered nanosensor, the gold nanoparticle core plays a role of signal reporter, the MnOx shell and the outmost SiO2 act as the sensing layer and the protecting layer, respectively. After subject to the acidic buffer solution, the MnOx layer in the multilayered nanoprobe could be decomposed rapidly, resulting in a remarkable spectral shift and color change under darkfield microscopy. We demonstrated this nanosensor for the investigation of single phagosome acidification dynamics by monitoring the color changes of nanoprobes after phagocytosis over time. The nanoprobes after phagocytosized in macrophage displayed a slight color change within the first hour and then cost several minutes to change from red to green in the next stage, indicating the phagosome undergoes a slow first and then fast acidification feature as well as a slow-to-fast acidification translation over the phagosome maturation process. Moreover, we validated that the slow-to-fast acidification translation was dependent on the activation of V-ATPase from the ATP depletion assay. We believed that this nanosensor is promising for studying the dynamic acidification features as well as disorders in phagosome maturation in phagocytic cells, which might provide valuable information for understanding the disease pathogenesis related to phagosome dysfunctions.

Analysis of the response of the cell membrane of Saccharomyces cerevisiae during the detoxification of common lignocellulosic inhibitors

Gaining an in-depth understanding of the response of Saccharomyces cerevisiae to the different inhibitors generated during the pretreatment of lignocellulosic material is driving the development of new strains with higher inhibitor tolerances. The objective of this study is to assess, using flow cytometry, how three common inhibitors (vanillin, furfural, and acetic acid) affect the membrane potential, the membrane permeability and the concentration of reactive oxygen species (ROS) during the different fermentations. The membrane potential decreased during the detoxification phase and reflected on the different mechanisms of the toxicity of the inhibitors. While vanillin and furfural caused a metabolic inhibition and a gradual depolarization, acetic acid toxicity was related to fast acidification of the cytosol, causing an immediate depolarization. In the absence of acetic acid, ethanol increased membrane permeability, indicating a possible acquired tolerance to ethanol due to an adaptive response to acetic acid. The intracellular ROS concentration also increased in the presence of the inhibitors, indicating oxidative stress. Measuring these features with flow cytometry allows a real-time assessment of the stress of a cell culture, which can be used in the development of new yeast strains and to design new propagation strategies to pre-adapt the cell cultures to the inhibitors.

Machine learning predicts and provides insights into milk acidification rates of Lactococcus lactis.

Lactococcus lactis strains are important components in industrial starter cultures for cheese manufacturing. They have many strain-dependent properties, which affect the final product. Here, we explored the use of machine learning to create systematic, high-throughput screening methods for these properties. Fast acidification of milk is such a strain-dependent property. To predict the maximum hourly acidification rate (Vmax), we trained Random Forest (RF) models on four different genomic representations: Presence/absence of gene families, counts of Pfam domains, the 8 nucleotide long subsequences of their DNA (8-mers), and the 9 nucleotide long subsequences of their DNA (9-mers). Vmax was measured at different temperatures, volumes, and in the presence or absence of yeast extract. These conditions were added as features in each RF model. The four models were trained on 257 strains, and the correlation between the measured Vmax and the predicted Vmax was evaluated with Pearson Correlation Coefficients (PC) on a separate dataset of 85 strains. The models all had high PC scores: 0.83 (gene presence/absence model), 0.84 (Pfam domain model), 0.76 (8-mer model), and 0.85 (9-mer model). The models all based their predictions on relevant genetic features and showed consensus on systems for lactose metabolism, degradation of casein, and pH stress response. Each model also predicted a set of features not found by the other models.

Effects of using different O2 scavengers on the qualitative attributes of bifidus yogurt during refrigerated storage

Among the factors that adversely influence the viability of probiotics, the oxygen content of the product and the permeation of oxygen molecules through the packaging system have a noticeable role in the viability loss during the manufacture and storage of fermented milk products. The objective of this study was to examine the qualitative attributes of probiotic yogurt containing different O2 scavengers, including the commercial O2 absorber and cysteine-ascorbic acid. Bifidobacterium lactis BIA-7 and B. longum BIA-8 were used as probiotic strains for the production of bio-yogurts. The biochemical parameters, including the changes in pH, titratable acidity, redox potential and incubation time, were determined throughout the fermentation period at 30-min intervals. Also, the changes in viable count, pH, redox potential, titratable acidity, and dissolved oxygen were evaluated at 7-day intervals during the 28 days of refrigerated storage. In addition, the evaluation of rheological and sensory properties measured in the freshly made samples was carried out. The results showed that the utilization of different oxygen scavengers has an effective impact on the decrement of oxygen content and improvement of probiotic viability. As such, the population of B. lactis in the treatments containing various oxygen scavengers was maintained above 7 log CFU/mL throughout the refrigerated storage. Notwithstanding the effective function of cysteine-ascorbic acid in the enhancement of viability, the containing treatments had not only weaker gel structure probably due to short incubation time (360 min) and fast acidification [22.20–22.35 (˚D/min) × 10-2], but also lower sensory acceptance. Overall, the yogurt treatment containing commercial O2 scavenger and B. lactis indicated a great potential for the industrial applications. To the best of our knowledge, there is no study on the efficiency of commercial O2 absorber as a potential factor to maintain the viability of probiotics in yogurt.

The role of histidine dipeptides on postmortem acidification of broiler muscles with different energy metabolism

It is generally held that the content of several free amino acids and dipeptides is closely related to the energy-supplying metabolism of skeletal muscles. Metabolic characteristics of muscles are involved in the variability of meat quality due to their ability to influence the patterns of energy metabolism not only in living animal but also during postmortem time. Within this context, this study aimed at establishing whether the concentration of histidine dipeptides can affect muscle postmortem metabolism, examining the glycolytic pathway of 3 chicken muscles (pectoralis major, extensor iliotibialis lateralis, and gastrocnemius internus as glycolytic, intermediate, and oxidative-type, respectively) selected based on their histidine dipeptides content and ultimate pH. Thus, a total of 8 carcasses were obtained from the same flock of broiler chickens (Ross 308 strain, females, 49 d of age, 2.8 kg body weight at slaughter) and selected immediately after evisceration from the line of a commercial processing plant. Meat samples of about 1 cm3 were excised from bone-in muscles at 15, 60, 120, and 1,440 min postmortem, instantly frozen in liquid nitrogen and used for the determination of pH, glycolytic metabolites, buffering capacity as well as histidine dipeptides content through 1H-NMR. Overall results suggest that glycolysis in leg muscles ceased already after 2 h postmortem, whereas in breast muscle continued until 24 h, when it exhibited significantly lower pH values (P < 0.05). However, considering its remarkable glycolytic potential, pectoralis major muscle should have exhibited a greater and faster acidification, suggesting that its higher (P < 0.05) histidine dipeptides' content might have prevented a potentially stronger acidification process. Accordingly, breast muscle also showed greater (P < 0.05) buffering ability in the pH range 6.0–7.0. Therefore, anserine and carnosine, being highly positively correlated with muscle's buffering capacity (P < 0.001), might play a role in regulating postmortem pH decline, thus exerting an effect on muscle metabolism during prerigor phase and the quality of the forthcoming meat. Overall results also suggest that total histidine dipeptides content along with muscular ultimate pH represent good indicators for the energy-supplying metabolism of chicken muscles.

Characterization and pre-industrial validation of Streptococcus thermophilus strains to be used as starter cultures for Crescenza, an Italian soft cheese

The aim of this work was to search for new candidate strains to be included in a culture for Crescenza, a rindless soft cheese, today produced mainly at industrial level using selected starter cultures composed of S. thermophilus. Performance testing was applied to 29 pre-selected strains and a scoring approach was developed to identify the most suitable candidates to be employed in Crescenza cheesemaking. Eight S. thermophilus strains fulfilling most of the desired properties (e.g., high phage resistance, fast acidification rate, no growth below 20 °C, NaCl sensibility, no post acidification at 4 °C) were selected. These strains were grouped in pairs to design different starter culture formulations, which were preliminary tested for the production of Crescenza cheeses at laboratory scale. Two couples of binary cultures (designed Phage rotation 1 and Phage rotation 2) were finally designed and used as starters in pilot scale cheesemaking. The combinations, especially those designed in Phage rotation 1, appeared to be suitable for Crescenza production and showed mutual similarity in terms of strain characteristics, technological performance, and cheese quality. The selection method and ranking approach presented in this work may be adapted to other species of LAB showing traits of industrial interest.

Starter culture for curly kale juice fermentation selected using principal component analysis

Multivariate statistical analysis was used to select potential starter cultures to produce fermented curly kale juice. The technological and functional properties of juices fermented using 10 autochthonous lactic acid bacteria isolated from spontaneously fermented curly kale were examined based on their capability of fast acidification while maintaining high numbers of viable cells, antioxidant activity, and antimicrobial effects against Gram positive and Gram negative bacteria. The outcome of the analysis of these parameters was used as input data for a principal component analysis (PCA) to select the most promising strains. The PCA showed that three strains Lactobacillus paraplantarum G2114 (GenBank ID: KY883562), Lb. plantarum 332 (GenBank ID: KY883559), and Pediococcus pentosaceus 2211 (GenBank ID: KY883563) showed a similarity to Lb. plantarum NRRL B-4496 used as a reference strain in terms of functional and technological properties and its ability to ferment vegetables. These strains were selected as starter culture for further fermentations. Samples of curly kale juice fermented with a multistrain formulation showed fast acidification with a high number of viable cells, high level of antioxidant activity and antimicrobial effects against some pathogenic bacteria. Furthermore, the multistrain formulation resulted in improved results in comparison with the control and autochthonous bacteria strains tested individually.

Slowing of the Time Course of Acidification Decreases the Acid-Sensing Ion Channel 1a Current Amplitude and Modulates Action Potential Firing in Neurons.

Acid-sensing ion channels (ASICs) are H+-activated neuronal Na+ channels. They are involved in fear behavior, learning, neurodegeneration after ischemic stroke and in pain sensation. ASIC activation has so far been studied only with fast pH changes, although the pH changes associated with many roles of ASICs are slow. It is currently not known whether slow pH changes can open ASICs at all. Here, we investigated to which extent slow pH changes can activate ASIC1a channels and induce action potential signaling. To this end, ASIC1a current amplitudes and charge transport in transfected Chinese hamster ovary cells, and ASIC-mediated action potential signaling in cultured cortical neurons were measured in response to defined pH ramps of 1–40 s duration from pH 7.4 to pH 6.6 or 6.0. A kinetic model of the ASIC1a current was developed and integrated into the Hodgkin-Huxley action potential model. Interestingly, whereas the ASIC1a current amplitude decreased with slower pH ramps, action potential firing was higher upon intermediate than fast acidification in cortical neurons. Indeed, fast pH changes (<4 s) induced short action potential bursts, while pH changes of intermediate speed (4–10 s) induced longer bursts. Slower pH changes (>10 s) did in many experiments not generate action potentials. Computer simulations corroborated these observations. We provide here the first description of ASIC function in response to defined slow pH changes. Our study shows that ASIC1a currents, and neuronal activity induced by ASIC1a currents, strongly depend on the speed of pH changes. Importantly, with pH changes that take >10 s to complete, ASIC1a activation is inefficient. Therefore, it is likely that currently unknown modulatory mechanisms allow ASIC activity in situations such as ischemia and inflammation.

Co-fermentation Involving Saccharomyces cerevisiae and Lactobacillus Species Tolerant to Brewing-Related Stress Factors for Controlled and Rapid Production of Sour Beer.

Increasing popularity of sour beer urges the development of novel solutions for controlled fermentations both for fast acidification and consistency in product flavor and quality. One possible approach is the use of Saccharomyces cerevisiae in co-fermentation with Lactobacillus species, which produce lactic acid as a major end-product of carbohydrate catabolism. The ability of lactobacilli to ferment beer is determined by their capacity to sustain brewing-related stresses, including hop iso-α acids, low pH and ethanol. Here, we evaluated the tolerance of Lactobacillus brevis BSO464 and Lactobacillus buchneri CD034 to beer conditions and different fermentation strategies as well as their use in the brewing process in mixed fermentation with a brewer’s yeast, S. cerevisiae US-05. Results were compared with those obtained with a commercial Lactobacillus plantarum (WildBrewTM Sour Pitch), a strain commonly used for kettle souring. In pure cultures, the three strains showed varying susceptibility to stresses, with L. brevis being the most resistant and L. plantarum displaying the lowest stress tolerance. When in co-fermentation with S. cerevisiae, both L. plantarum and L. brevis were able to generate sour beer in as little as 21 days, and their presence positively influenced the composition of flavor-active compounds. Both sour beers were sensorially different from each other and from a reference beer fermented by S. cerevisiae alone. While the beer produced with L. plantarum had an increased intensity in fruity odor and dried fruit odor, the L. brevis beer had a higher total flavor intensity, acidic taste and astringency. Remarkably, the beer generated with L. brevis was perceived as comparable to a commercial sour beer in multiple sensory attributes. Taken together, this study demonstrates the feasibility of using L. brevis BSO464 and L. plantarum in co-fermentation with S. cerevisiae for controlled sour beer production with shortened production time.

Characterization of the microbial communities and their correlations with chemical profiles in assorted vegetable Sichuan pickles

The Sichuan pickles of chili peppers, cowpeas and radishes were separately fermented for 12 days in order to identify the distinct microfloras and their correlations with the metabolites in different products. The chili pickle presented a slow fermentation process, high bacterial diversity within six phyla, and sixteen marker genera. Moreover, free amino acids accumulated in chili pickle and its main volatiles were alcohols and esters. Fast acidification and limited bacterial diversity within three phyla were found in Sichuan cowpea and radish pickles. The cowpea pickle was characterized by Lactobacillus and Pediococcus, as well as alcohols and alkenes. The radish pickle featured Lactococcus and Fructobacillus, as well as sulfides and aldehydes. Correlation analysis indicated that the metabolites, especially volatiles, were closely associated with not only the dominant bacteria but also those in low abundance.

Isolation and characterization of Streptococcus thermophilus possessing prtS gene from raw milk in Japan.

Streptococcus thermophilus is widely used for producing fermented dairy products such as yogurt and cheese. Some S. thermophilus strains possessing the cell-wall protease PrtS show high proteolytic activity and fast acidification properties, which are very useful in industrial starters. However, few S. thermophilus strains possessing the prtS gene have been isolated from the environment. To clarify whether or not S. thermophilus strains possessing the prtS gene are present in Japan, we isolated S. thermophilus from raw milk collected in Japan from 2011 to 2017 and investigated the strains for the presence of prtS by PCR. A total of 172 S. thermophilus strains were isolated, and 59 strains were confirmed to possess prtS. We measured fermentation times of 59 prtS-positive strains in skim milk broth and found that 53 strains showed fast acidification properties, finishing fermentation within 10 hr. However, the remaining 6 prtS-positive strains showed slow acidification properties, and they had several amino acid mutations in PrtS compared with fast acidifying S. thermophilus LMD-9 and 4F44. These results demonstrate that S. thermophilus strains possessing prtS are prevalent in Japan and that some prtS-positive strains could lose their fast acidifying properties through mutations in PrtS.