Slow Acidification Research Articles

High gamma-aminobutyric acid (GABA) producing Enterococcus malodoratus isolated from protected denomination of origin (PDO) cheese

Four strains of Enterococcus malodoratus were isolated from São Jorge cheese – and showed a high conversion of L-glutamate to GABA, achieving a yield above 50%. These new strains neither fermented lactose nor produced hydrogen sulphide. They produced slow acidification and low amounts of diacetyl, and lacked most enzymatic activities and genes encoding virulence traits. They were sensitive to ampicillin, vancomycin, fluoroquinolones, chloramphenicol and streptogramins, but resistant to penicillin, oxacillin, erytromycin and tetracycline. One strain (SJC62) showed antimicrobial activity against Listeria monocytogenes, Salmonella enterica, Escherichia coli and Pseudomonas aeruginosa; and antifungal activity against Aspergillus caesiellus and Mucor racemosus. The strains also showed probiotic potential in vitro, as they exhibited good resistance to gastrointestinal conditions, high hydrophobicity and mucin adhesion. The spectrum of favourable technological, antimicrobial and probiotic properties, in combination with their high ability to convert glutamic acid into GABA make these strains relevant for use in novel dairy foods.

Variability in the acid adaptation of ten different O157:H7 and non-O157 Escherichia coli strains in orange juice and the impact on UV radiation resistance

This study aimed to assess the impact of adaptation of ten strains of O157:H7 and non-O157 Escherichia coli to low pH (acid shock or slow acidification) and the effects of this exposure or not on the resistance of E. coli strains to UV radiation in orange juice (pH 3.5). The acid-shocked cells were obtained through culture in tryptic soy broth (TSB) with a final pH of 4.8, which was adjusted by hydrochloric, lactic, or citric acid and subsequently inoculated in orange juice at 4 °C for 30 days. No significant differences (p > 0.05) in survival in orange juice were observed between the serotypes O157:H7 and non-O157:H7 for acid-shocked experiments. After slow acidification, where the cells were cultured in TSB supplemented with glucose 1% (TSB + G), a significant increase (p < 0.05) in survival was observed for all strains evaluated. The D-values (radiation dose (J/cm2) necessary to decrease the microbial population by 90%) were determined as the inverse of the slopes of the regressions (k) obtained by plotting log (N/N0). The results show that among the strains tested, E. coli O157:H7 (303/00) and O26:H11 were the most resistant and sensitive strains, respectively. According to our results, the method of acid adaptation contributes to increasing the UV resistance for most of the strains tested.

Soil organic matter priming: The pH effects.

Priming of soil organic matter (SOM) decomposition by microorganisms is a key phenomenon of global carbon (C) cycling. Soil pH is a main factor defining priming effects (PEs) because it (i) controls microbial community composition and activities, including enzyme activities, (ii) defines SOM stabilization and destabilization mechanisms, and (iii) regulates intensities of many biogeochemical processes. In this critical review, we focus on prerequisites and mechanisms of PE depending on pH and assess the global change consequences for PE. The highest PEs were common in soils with pH between 5.5 and 7.5, whereas low molecular weight organic compounds triggered PE mainly in slightly acidic soils. Positive PEs up to 20 times of SOM decomposition before C input were common at pH around 6.5. Negative PEs were common at soil pH below 4.5 or above 7 reflecting a suboptimal environment for microorganisms and specific SOM stabilization mechanisms at low and high pH. Short-term soil acidification (in rhizosphere, after fertilizer application) affects PE by: mineral-SOM complexation, SOM oxidation by iron reduction, enzymatic depolymerization, and pH-dependent changes in nutrient availability. Biological processes of microbial metabolism shift over the short-term, whereas long-term microbial community adaptations to slow acidification are common. The nitrogen fertilization induced soil acidification and land use intensification strongly decrease pH and thus boost the PE. Concluding, soil pH is one of the strongest but up to now disregarded factors of PE, defining SOM decomposition through short-term metabolic adaptation of microbial groups and long-term shift of microbial communities.

The distribution of rennet activity between the cheese aging process and whey is not influenced by the association of enzymes with caseins

The division of rennet in cheesemaking is split between the curd and whey, influencing the taste and texture of aged cheeses. Our study aimed to examine how raising the protein concentration in reconstituted skim milk (up to 8.8 %) affects the distribution of calf rennet activity (RA) in rennet curds produced through two methods: renting only and renneting with glucono-δ-lactone (GDL) to achieve slow acidification. The distribution of rennet activity (RA) into curds increased as the concentration of skim milk rose, ranging from 8.6 % to 29.1 % without acidification, and from 6.5 % to 19.4 % when combined with slow acidification. This increase seemed to be related to the retention of moisture and protein. Surprisingly, the concentration of residual RA in the whey (measured in international milk clotting units, IMCU/mL) remained unaffected and remained consistent with the initial IMCU/mL of milk. This suggests that the division of RA between curd and whey is not influenced by the association of enzymes with caseins (CNs). Instead, it is possible that the strength of interactions between CNs themselves plays a significant role. These findings could be valuable for research focused on enhancing the cheese aging process.

Tailoring an egg white protein double network emulsion gel as a novel fat substitute for improving freeze-thaw stability of minced meat gel

This work aimed to investigate the effect of double network emulsion gel (DNEG) as a substitute for pork back fat (PBF) on the freeze-thaw stability of minced meat gel (MMG). We used slow acidification with glucono-δ-lactone (GDL) to induce egg white protein (EWP) gelation, while CaCO3 was affected by acidification to release Ca2+ to cross-link sodium alginate (SA), resulting in the gradual formation of DNEG networks. The MMG prepared using DNEG substitute for PBF had fine hardness and color after 5 freeze-thaw cycles (FTCs). This was due to the excellent plasticity of DNEG, which made it more capable of filling the voids of minced meat products, thus reducing the space for ice crystal growth. As a result, MMG prepared with DNEG could pass through the maximum ice crystal formation zone faster and avoid damage due to the formation of larger ice crystals. Notably, DNEG provided a barrier for the migration of immobilized water to bulk water in the sample. After several freeze-thaw treatments, the water holding capacity of samples prepared with DNEG decreased significantly slower, which slowed down the oxidation and decomposition of meat proteins. In summary, this study revealed the role of fat substitute DNEG as a favorable filling material for improving the freeze-thaw stability of minced meat products, and provided a technical reference for the development of healthy and storable meat products.

Structural dependence of concentrated skim milk curd on micellar restructuring

This study was conducted to establish an understanding of how milk concentration modulates the rennet curd structure. Rennet-induced gelation and renneting under slow acidification achieved using glucono-δ-lactone (GDL) and structural properties of reconstituted skim milk gels at two concentration levels (9 and 25 % total solids) were studied by measuring variations in (a) viscoelastic behaviour, (b) micellar size, charge density, diffusivity, and (c) hydrophobicity using dynamic rheometry, dynamic light scattering and fluorimetry, respectively. Concentrated milk showed a greater estimated hydrodynamic radius of casein micelles, lower zeta (ζ)-potential, ratio of serum to total Calcium (Ca) and charge density and increased surface hydrophobicity, all supporting the view that micellar restructuring particularly sub-particle transfer takes place and contributes to rapid gelation. Moreover, hydrophobic interactions occurred very quickly (within 5 min in combined gels, 10 min for renneting only), demonstrating their pivotal role during the flocculation stage. All gels exhibited a solid viscoelastic character as the elastic modulus (G′) was greater than loss modulus (G″) while both G′ and tan δ (G''/G′) were frequency-dependent. Frequency sweeps classified the concentrated gels into three stiffness categories caused by the level of rennet or GDL as rigid, hard and soft, whereas an increased flow-like behaviour (high tan δ), restricted diffusion and excessive water retention revealed limited structural rearrangements (contraction & macrosyneresis) during curd ageing. Acidification increased the diffusion rate in control curd, thus, enhanced contractive rearrangements, macrosyneresis and curd strength. Findings suggest that micellar restructuring induced by milk concentration is the principal modulator of the curd structure.

Long-term additions of ammonium nitrate to montane forest ecosystems may cause limited soil acidification, even in the presence of soil carbonate

Abstract. Nitrogen (N) deposition has decreased in the last decades in Europe but in many cases remains higher than the critical loads, i.e., higher than what could be considered safe for biodiversity and ecosystem functioning. The main concerns about N deposition are eutrophication and acidification. In a long-term experiment (1994 to present) in a montane (1200 m a.s.l.) coniferous forest in Alptal, central Switzerland, we simulated increased N deposition by adding NH4NO3 to rainwater. This treatment consisted of an additional N input of 22 kg ha−1 yr−1 to the 12 kg ha−1 yr−1 ambient bulk deposition rate or 17 kg N ha−1 yr−1 throughfall rate. The treatment was applied simultaneously to a small catchment area and to plots in a replicated block experiment (n=5). The site has a carbonate-rich parent material and is thus not particularly at risk of acidification. Nevertheless, we examined soil acidification as affected both by ambient and experimentally increased N deposition. In the 2.5 decades since the beginning of the study, nitrate (NO3-) and especially sulfate (SO42-) concentrations decreased in precipitation, while pH increased by slightly more than 1 unit. In the same time period a reduction in pH of the soil was measured. The exchangeable acidity in the soil increased, especially in the N-addition treatment. This was mainly observed on small mounds because the drier mounds are less well buffered than wet depressions. This trend, however, was limited in time, as exchangeable acidity later declined again to reach values not much higher than 26 years before. This was also the case in the N-addition treatment and can be considered a progressive recovery mainly due to the reduced acid inputs and, at this site with a carbonate-rich subsoil, to the biological cycling of base cations. The pH of the runoff from the experimental catchments decreased by 0.3 units, both in the control and under N addition. Decreasing Ca2+ and increasing Al3+ and Fe2+ concentrations in runoff also show that the recovery observed in the exchangeable soil acidity is not yet able to stop the slow acidification of water leaving the catchments. However, with the runoff water pH remaining above 7, this trend is not alarming for water quality or for the health of water bodies. Future monitoring will be necessary to see if and when a recovery takes place in the soil and runoff pH.

Fermented Milk Produced with Goat Milk Enriched with PUFA Omega-3 by Supplementation of Diet with Extruded Linseed

This research aimed to evaluate goat milk rich in Omega-3 PUFA from animals fed extruded flaxseed as a raw material to produce fermented milk using beneficial lactobacilli. Experimental fermented milks were produced using Lacticaseibacillus paracasei Shirota and two potential probiotic lactobacilli, namely Lacticaseibacillus rhamnosus A2 and Lacticaseibacillus paracasei FS109. The fermented milks were produced using milk from goats fed a basal diet without any supplementation (CON) and milk from goats supplemented with 200 g/d of extruded linseed (LIN). All lactobacilli tested grew well both in CON and LIN milk, reaching high numbers during fermentation. The colony count ranged between 8 and 10 Log CFU/mL, despite slow acidification activity, which occurred especially in milk fermented by L. FS109. By contrast, an undesired post-acidification occurred, more pronounced in CON than in LIN milk, which still highlighted the strong acid-tolerance of L. Shirota and L. rhamnosus A2 in particular. This research showed that goat milk enriched in PUFA had no negative effect on the viability of the tested Lactobacilli. Both values of L. Shirota and L. rhamnosus “live cells” throughout the cold storage of the products were higher than those recommended to guarantee the quality of fermented milk products, making them beneficial to consumers’ health.

Gel properties of acid-induced gels obtained at room temperature and based on common bean proteins and xanthan gum

This work investigates the effect of pH (4.0, 4.5) and the protein-polysaccharide ratio (1:1–5:1) on the intermolecular interactions and gel properties of food hydrogels based on bean protein isolate (BPI) and xanthan gum (XG). Mixed hydrogels induced by acidification with glucone-δ-lactone (GDL) were obtained at room temperature, without the use of crosslinking agents, and using 1% (w/v) of total biopolymer content. The studies on intermolecular forces by a protein solubility assay and FTIR analyses showed that BPI promotes electrostatic associations with XG that depend on pH and ratio. Interestingly, hydrophobic interactions were predominant at ratios 1:1 and 2:1 even when no denaturing heat treatment was employed. In addition, the absence of heating throughout the gelation process caused no changes in the secondary structure of bean proteins as demonstrated by Raman spectroscopy. Concerning gel properties, hydrogels at pH 4.5 showed higher water holding capacity (up to 98.09%) than gels at pH 4.0 (up to 95.65%). Despite this observation, more compact and stronger gel structures were assembled at pH 4.0 as shown by SEM, dynamic oscillatory measurements, and texture profile analysis. Regarding the effect of ratio, the lowest ratios showed the best performance on the evaluated gel properties highlighting that almost all the gels with the 1:1–3:1 ratio exhibited the typical rheological behavior of strong gels. These results indicate that acidification with GDL of BPI:XG systems at ratios 1:1–3:1 at pH 4.0/4.5 and room temperature produce promising mixed hydrogels for functional foods. • Hydrogels were obtained through slow acidification of BPI:XG mixtures. • Mixed hydrogels were obtained at room temperature and without crosslinking agents. • Electrostatic and hydrophobic interactions drove the formation of mixed hydrogels. • Gels at low ratios (1:1–3:1) and both pH 4 and 4.5 showed strong-gel-character. • The food gels obtained exhibited excellent rheological and textural properties.

The supervision and multi-sectoral guarantee mechanism of the global marine sulphur limit—assessment from Chinese shipping industry

To significantly reduce sulfur oxides emissions from fossil fuel-powered ships, reduce air pollution in ports and slow ocean acidification, the International Maritime Organization (IMO) has imposed the new 0.50%m/m limit (reduced from 3.50%m/m in the past) on sulphur in ships’ fuel oil. This has given rise to a host of issues regarding fuel replenishment operations, safe operation management, maritime regulation, and coordinated governance of air and climate. In response to ocean acidification and climate change, regulations on the use of low-sulfur oil or alternative fuels by ships greatly reduce sulfur oxide emissions, but have no significant impact on reducing greenhouse gas emissions. In fact, the refining process for low-sulfur fuels and the use of the gas cleaning system on ships both increase energy consumption and carbon dioxide emissions. To ensure the decarbonization process of shipping industry, there is an urgent need for a conceptual change in global ocean governance so as to promote the coordinated governance of air pollution and climate change. China’s conception of “a maritime community with a shared future” provides a new model for global ocean governance. The Chinese government has formulated regulations at different levels to promote the coordinated management of atmospheric pollutants and greenhouse gas emissions. Regarding supervision of sulfur oxide emissions from ships, this study proposes to build a multi-department collaborative supervision mechanism from marine fuel life cycle to enhance sulfur oxide monitoring and risk control capabilities. Specific measures of the proposed supervision mechanism include: the joint supervision of compliant fuel supply, the compliant fuel information disclosure platform, a joint law enforcement mechanism for atmospheric pollution, the ability of intelligent ship exhaust monitoring, and the construction of port power infrastructure.

Bioremediation of trichloroethylene-polluted groundwater using emulsified castor oil for slow carbon release and acidification control.

In this study, the emulsified castor oil (ECO) substrate was developed for a long-term supplement of biodegradable carbon with pH buffering capacity to anaerobically bioremediate trichloroethylene (TCE)-polluted groundwater. The ECO was produced by mixing castor oil, surfactants (sapindales and soya lecithin [SL]), vitamin complex, and a citrate/sodium phosphate dibasic buffer system together for slow carbon release. Results of the emulsification experiments and microcosm tests indicate that ECO emulsion had uniform small droplets (diameter = 539 nm) with stable oil-in-water characteristics. ECO had a long-lasting, dispersive, negative zeta potential (-13 mv), and biodegradable properties (viscosity = 357 cp). Approximately 97% of TCE could be removed with ECO supplement after a 95-day operational period without the accumulation of TCE dechlorination byproducts (dichloroethylene and vinyl chloride). The buffer system could neutralize acidified groundwater, and citrate could be served as a primary substrate. ECO addition caused an abrupt TCE adsorption at the initial stage and the subsequent removal of adsorbed TCE. Results from the next generation sequences and real-time polymerase chain reaction (PCR) indicate that the increased microbial communities and TCE-degrading bacterial consortia were observed after ECO addition. ECO could be used as a pH-control and carbon substrate to enhance anaerobic TCE biodegradation effectively. PRACTITIONER POINTS: Emulsified castor oil (ECO) contains castor oil, surfactants, and buffer for a slow carbon release and pH control. ECO can be a long-term carbon source for trichloroethylene (TCE) dechlorination without causing acidification. TCE removal after ECO addition is due to adsorption and reductive dechlorination mechanisms.

Aerobic stability of alfalfa silage and methods of its improvement

Alfalfa is a non-silage crop, the silage of which became possible after the effect of drying was detected. A special feature of alfalfa is the intensive proteolysis that occurs during silage, and the prolonged development of undesirable microflora due to the slow acidification of feed. The objective of the research was to determine the effectiveness of the use of Biotrof, Biotrof 111, Biotrof 2+ and Biotal Axfast NS Gold in the silage of dried (37.07% SV) alfalfa mass. It was found that the use of these drugs contributed to the rapid acidification of feed to a pH of 3.97-4.08, against a pH of 4.96 in silage without additives, which led to the suppression of the vital activity of undesirable bacteria, a decrease in the breakdown of nutrients to gaseous products by 1.7-2.3 times and the accumulation of ammonia by 1.5-4.0 times. Due to the high resistance of alfalfa silage to aerobic spoilage, the use of bacterial preparations did not lead to an improvement in the aerobic stability of the silage, but by restraining the development of some yeasts, including pathogenic ones, and fungi of the Aspergillus sp. species, it helped to improve the sanitary status of the feed. Yeasts of the genera C. gattii and D. hansenii serve as marker organisms, an increase in the number of which indicates the occurrence of aerobic spoilage in the feed.

A comparison of the acid gelation properties of nonfat cow, sheep, and goat milk with standardized protein contents

Skimmed cow, sheep, and goat milks were adjusted to the same total protein and dry matter level, heat treated and glucono delta lactone (GDL) was added for slow acidification. Acid coagulation was monitored by measuring the turbidity and viscosity every 10 min and pH was measured every min throughout the incubation. Physicochemical composition (total solids, fat, protein, casein, bound and soluble Ca, P, Fe, Mg, K, Na), pH, acidity, SDS-PAGE analysis of the milk samples and viscosity, serum separation, and SEM images of the acidified milk gels were determined. During incubation, the highest decrease in pH was observed with cow milk, while sheep milk pH was higher and was above pH 4.6 at the end of incubation. Cow milk gels had the highest viscosity, lowest serum separation, and more homogenous microstructure. Sheep milk gels had a lower viscosity than cow milk and goat milk formed the weakest acid gels of all. Novelty impact statement Acid gelation of the milk is essential for the production of many dairy products that involve fermentation such as yogurt. Previous studies showed major differences between cow, sheep, and goat milk yogurts; however, there is not much study that compares acid gelation and gel properties of standardized cow, sheep, and goat milks. We found significant differences between acid gelation and gel properties of cow, sheep, and goat milks even if their total protein, total solids, and fat contents were adjusted to the same level; thus our findings highlight the importance of the differences in type and ratio of the protein fractions.

Influence of Transglutaminase on Glucono-δ-lactone-Induced Soy Protein Gels

Enzymatic cross-linking of proteins can be used to create a variety of foods with appealing textures and facilitate further processing such as slicing or packaging. Transglutaminase (TG), a protein-glutamine-γ-glutamyltransferase, has been used in the production of processed meat and fish products. However, it has a reaction optimum at pH 7 and is therefore typically unsuitable for application in up-and-coming acidic food products such as fermented vegan meat product alternatives. To determine whether a simultaneous addition of slowly acidifying glucono-δ-lactone (GDL) and TG can facilitate protein cross-linking prior to the pH becoming too low, TG and GDL were added separately or in combination to soy protein suspensions composed of 10–15% soy protein concentrates or isolates (SPI). Texture analysis showed that SPI gels at pH 5–6, induced by GDL and TG combined, were harder than only GDL-induced gels and as hard as TG-induced gels at the optimum pH of 7. Decreased tan δ values and confocal laser scanning microscopy images indicated that protein cross-linking had taken place in combined gels. The results suggest an initial network formation induced by TG, which later in the acidification is being supplemented by an agglomeration of proteins due to weakened electrostatic repulsion. Taken together, the combination of slow acidification by GDL and TG addition leads to acidic gels with enhanced textural properties and shelf life.

Rheology of protein-stabilised emulsion gels envisioned as composite networks 1– Comparison of pure droplet gels and protein gels

HypothesisProtein-stabilised emulsion gels can be studied in the theoretical framework of colloidal gels, because both protein assemblies and droplets may be considered as soft colloids. These particles differ in their nature, size and softness, and these differences may have an influence on the rheological properties of the gels they form. ExperimentsPure gels made of milk proteins (sodium caseinate), or of sub-micron protein-stabilised droplets, were prepared by slow acidification of suspensions at various concentrations. Their microstructure was characterised, their viscoelasticity, both in the linear and non-linear regime, and their frequency dependence were measured, and the behaviour of the two types of gels was compared. FindingsProtein gels and droplet gels were found to have broadly similar microstructure and rheological properties when compared at fixed volume fraction, a parameter derived from the study of the viscosity of the suspensions formed by proteins and by droplets. The viscoelasticity displayed a power law behaviour in concentration, as did the storage modulus in frequency. Additionally, strain hardening was found to occur at low concentration. These behaviours differed slightly between protein gels and droplet gels, showing that some specific properties of the primary colloidal particles play a role in the development of the rheological properties of the gels.

Biodiversity and dairy traits of lactic acid bacteria from foliage of aromatic plants before and after dehydration process monitored by a smart sensors system.

The main hypothesis of this work was to evaluate the presence of lactic acid bacteria (LAB) intrinsically resistant to plant essential oils in sage (Salvia officinalis L.) and laurel (Laurus nobilis), for future applications in functional cheese production by addition of aromatic herbs. The effect of the drying process on the viability of LAB was evaluated with three biomass densities (3, 4 and 5kg/m2). The drying densities did not affect weight loss, but influenced the levels of LAB of sage and laurel. A total of 10 different strains of Enterococcus faecium, Enterococcus mundtii, Enterococcus raffinosus and Leuconostoc mesenteroides were identified from laurel, while sage did not host any LAB species. In particular, L. mesenteroides was the only species sensitive to the heat treatment. Only five strains, all enterococci, were resistant to at least one antibiotic, even though no strain showed gelatinase or haemolytic activity. The investigation on the technological traits useful in cheese making demonstrated that all LAB can be considered non starter LAB, because they were characterized by a slow acidification capacity (the pH was still above 6.00 after 3 d) and a very limited autolysis (the maximum decrease of the optical density at 599nm was barely 0.2).

Electrostatic hydrogels formed by gelatin and carrageenan induced by acidification: Rheological and structural characterization

Abstract The rheological, chemical and structural characteristics of electrostatic hydrogels formed by gelatin and carrageenan (Gel:Car) were studied in this work. Hydrogels were formed at 1:1, 1:2, 1:4, 1:8, 2:1, 4:1 and 8:1 protein:polysaccharide ratios. The influence of the ratio between the biopolymers on the rheological characteristics of hydrogels was evaluated. The hydrogels were characterized by large and small deformation, water holding capacity and microstructural analysis. All hydrogels showed viscoelastic characteristics, and the Burger model was appropriate to explain the viscoelastic behavior of hydrogels formed. The hydrogel that presented the best viscoelastic and mechanical characteristics was the 1:2 Gel:Car showing that good amount of carrageenan associated with slow acidification results the formation of more junction zones due to complementary interactions between the gelatin triple helix and the carrageenan double helix. When carrageenan (1:4 and 1:8) or gelatin (2:1, 4:1 and 8:1) was in excess there was a suppression of the gelatin structure modification significantly reducing the density of the junction zones negatively impacting the flexibility and hardness of these hydrogels. All samples showed a good water holding capacity, retaining between 80 % and 90 % of water in the interior, making these hydrogels an attractive material for food applications because they can improve food texture, increasing water retention capacity, which will produce greater durability of food. However, its structure and its mechanism of formation can also be explored in protecting active molecules in its structure or in the manufacture of new hydrogels through the slow acidification, such as in yogurt manufacturing.

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.

Effects of dissimilatory iron reduction on acetate production from the anaerobic fermentation of waste activated sludge under alkaline conditions

Anaerobic digestion of waste activated sludge (WAS) to produce acetate has recently attracted growing interest, while the slow hydrolytic acidification of sludge and the consumption of acetate by methanogens both decrease the yield of acetate. In this study, Fe3O4 was added to a WAS anaerobic digester under alkaline conditions (pH = 10). The concentration of short-chain fatty acids (SCFA) during WAS anaerobic fermentation was found to be affected positively by Fe3O4. The maximal SCFA production of the Fe3O4-added digester was 3619.4 mg/L, while the maximal SCFA production in the control was 2899.7 mg/L. The increase of SCFA with Fe3O4 was mainly resulted from the increase in acetate accumulation (accounting for 90%), because Fe3O4 stimulated dissimilatory iron reduction (DIR) that participated in the decomposition of complex organics and the transformation of pronionate and butyrate into acetate. Further investigation showed that each step of hydrolytic-acidification process was promoted except the homoacetogenesis. The activity of enzymes and abundance of microbes relevant to hydrolysis and acidification were in agreement with the above results.

The Influence of Meat Batter Composition and Sausage Diameter on Microbiota and Sensory Traits of Artisanal Wild Boar Meat Sausages.

SUMMARYIn this study, the influence of meat batter composition and sausage diameter on the development of microbiota and sensory traits of traditional, spontaneously fermented wild boar meat sausages are evaluated. This research also demonstrates how principal component analysis (PCA) can be used to relate product sensory properties to particular microbial genotype and to select potential starter or adjunct culture. Generally, similar microbiological results were obtained in all types of products. The undesirable microbiota was either not detected at any sausage production stage or its number decreased below the detection limit in ripened sausages. The low growth rate of lactic acid bacteria (LAB) was consistent with the obtained pH and slow acidification rate. Although no differences in the composition of LAB species were noticed between sausage types (50S=50% wild boar meat in small casing, 50L=50% wild boar meat in large casing, 100S=100% wild boar meat in small casing), a clear separation based on LAB genotypes could be observed. Upon quantitative descriptive analysis, significant differences in sensory attributes between sausage types were established. According to the PCA, the overall acceptability traits of sausages are closely linked to one Leuconostoc mesenteroides genotype (LM_4). Of all tested technological properties, LM_4 strains showed remarkable acidification ability, lowering the pH from pH=5.41 to 3.74, and pronounced proteolytic activity on skimmed milk as well as antagonistic activity against Staphylococcus aureus (DSM 20231) and Brochothrix thermosphacta (LMG 17208). Lipolytic and haemolytic activities were not detected, and all analyzed strains were susceptible to tested antibiotics and possessed no biogenic amine genes.