Influence of capsular and ropy exopolysaccharide-producing Streptococcus thermophilus on Mozzarella cheese and cheese whey

How Capsular Exopolysaccharides Affect Cell Surface Properties of Lactic Acid Bacteria

Some lactic acid bacteria are able to produce exopolysaccharides that, based on localization, can be distinguished in free and capsular or cell-bound exopolysaccharides (CPS). Up to now, the former were the focus of current research, mainly because of the technofunctional benefits they exhibit on fermented dairy products. On the other hand, CPS affect the surface properties of bacteria cells and thus also the textural properties of fermented foods, but data are very scarce. As the cell surface properties are strongly strain dependent, we present a new approach to investigate the impact of CPS on cell surface hydrophobicity and moisture load. CPS positive and negative Streptococcus thermophilus and Weissella cibaria were subjected to ultrasonication suitable to detach CPS without cell damage. The success of the method was verified by scanning electron and light microscopy as well as by cultivation experiments. Before applying ultrasonication cells with CPS exhibiting an increased hydrophilic character, enhanced moisture load, and faster water adsorption compared to the cells after CPS removal, emphasizing the importance of CPS on the textural properties of fermented products. The ultrasonic treatment did not alter the cell surface properties of the CPS negative strains.

Exopolysaccharides From Streptococcus thermophilus ST538 Modulate the Antiviral Innate Immune Response in Porcine Intestinal Epitheliocytes

It was reported that exopolysaccharides (EPSs) from lactobacilli are able to differentially modulate mucosal antiviral immunity. Although research has described the ability of EPSs derived from Streptococcus thermophilus to modulate the mucosal immune system, their impact on antiviral immunity was less explored. In this work, we investigated the capacity of the EPS-producing S. thermophilus ST538 to modulate the innate antiviral immune response triggered by the activation of the Toll-like receptor 3 (TLR3) in porcine intestinal epitheliocytes (PIE cells). Moreover, in order to study the immunomodulatory potential of S. thermophilus ST538 EPS, we successfully developed two mutant strains through the knockout of the epsB or epsC genes. High-performance liquid chromatography and scanning electron microscopy studies demonstrated that the wild type (WT) strain produced as high as 595 μg/ml of EPS in the skim milk medium, while none of the mutant strains (S. thermophilus ΔepsB and ΔepsC) were able to produce EPS. Studies in PIE cells demonstrated that the EPS of S. thermophilus ST538 is able to significantly improve the expression of interferon β (IFN-β), interleukin 6 (IL-6), and C-X-C motif chemokine 10 (CXCL10) in response to TLR3 stimulation. The role of EPS in the modulation of antiviral immune response in PIE cells was confirmed by comparative studies of cell free culture supernatants and fermented skim milks obtained from S. thermophilus ΔepsB and ΔepsC. These results suggest that S. thermophilus ST538 could be used as an immunobiotic strain for the development of new immunologically functional foods, which might contribute to improve resistance against viral infections.

A 100-Year Review: Cheese production and quality

In the beginning, cheese making in the United States was all art, but embracing science and technology was necessary to make progress in producing a higher quality cheese. Traditional cheese making could not keep up with the demand for cheese, and the development of the factory system was necessary. Cheese quality suffered because of poor-quality milk, but 3 major innovations changed that: refrigeration, commercial starters, and the use of pasteurized milk for cheese making. Although by all accounts cold storage improved cheese quality, it was the improvement of milk quality, pasteurization of milk, and the use of reliable cultures for fermentation that had the biggest effect. Together with use of purified commercial cultures, pasteurization enabled cheese production to be conducted on a fixed time schedule. Fundamental research on the genetics of starter bacteria greatly increased the reliability of fermentation, which in turn made automation feasible. Demand for functionality, machinability, application in baking, and more emphasis on nutritional aspects (low fat and low sodium) of cheese took us back to the fundamental principles of cheese making and resulted in renewed vigor for scientific investigations into the chemical, microbiological, and enzymatic changes that occur during cheese making and ripening. As milk production increased, cheese factories needed to become more efficient. Membrane concentration and separation of milk offered a solution and greatly enhanced plant capacity. Full implementation of membrane processing and use of its full potential have yet to be achieved. Implementation of new technologies, the science of cheese making, and the development of further advances will require highly trained personnel at both the academic and industrial levels. This will be a great challenge to address and overcome.

The effect of proteolytic activity of starter cultures on technologically important properties of yogurt

In this study, the effects of proteolytic activity of yogurt starter bacteria on physicochemical and technological properties of yogurt were investigated. Moreover, impact of proteolytic activity and production of exopolysaccharide (EPS) on the performance of each strain were screened. In order to compare the textural properties of yogurt samples, four parameters were evaluated: syneresis, water-holding capacity, cohesiveness, and hardness. Results showed that strains with high proteolytic activity had lower acidifying activity during fermentation and storage. Samples containing EPS-producing starter cultures had low proteolytic activity except samples K, L, and M. These differences related to nature and characteristics of each strain. Counts of starter cultures in samples produced using strains with high proteolytic activity were higher than other samples. Textural analysis data showed significant differences (p < .05) among strains in the four tested parameters. Strains with high proteolytic activity showed lower texture properties than other samples. Evaluation of sensory characteristics also showed samples prepared using strains with low or medium proteolytic activity and produced with EPS-producing strains have higher overall acceptability than other samples. Accordingly, microbial, physicochemical, and sensory properties of produced yogurts confirm that proteolytic activity is one of the most effective factors in quality of product and performance of each strain.

Effect of adjuncts on microbiological and chemical properties of Scamorza cheese

Scamorza is a semi-hard, pasta filata cheese resembling low-moisture Mozzarella cheese, with a short ripening time (<30d). Scamorza has a bland flavor and, to provide diversification from similar cheeses, it was manufactured using 2 types of milk in the current study: 100% Italian Friesian milk (F) or 90% F and 10% Jersey cow milk (mixed, M), and 2 types of starter: Streptococcus thermophilus or S. thermophilus with peptidolytic Lactococcus lactis, Lactobacillus helveticus, and Lactobacillus paracasei strains as adjuncts). The cheeses were ripened for 30d. The adjunct did not significantly affect acid production or growth of the primary starter; 2 of the species used in the adjunct (Lb. paracasei and Lb. helveticus) rapidly colonized the cheese and persisted until the end of ripening, whereas the counts of nonstarter lactic acid bacteria in the control cheese were low until the end of ripening. The use of adjuncts affected pH, microbial composition (as assessed by both culture-dependent and culture-independent methods), total free amino acid content, and volatile profile (measured using an electronic nose), whereas milk type had only a minor effect. Although differences in primary proteolysis were found, they were probably indirect and related to the effects on pH and moisture. We conclude that, even with a short ripening time (30d), use of a peptidolytic adjunct may significantly affect important features of Scamorza and may be used to create a product that is measurably different from competing products.

Genomic insights into high exopolysaccharide-producing dairy starter bacterium Streptococcus thermophilus ASCC 1275

Streptococcus thermophilus ASCC 1275 (ST 1275), a typical dairy starter bacterium, yields the highest known amount (~1,000 mg/L) of exopolysaccharide (EPS) in milk among the species of S. thermophilus. The addition of this starter in milk fermentation exhibited texture modifying properties for fermented dairy foods such as yogurt and cheese in the presence of EPS as its important metabolite. In this genomic study, a novel eps gene cluster for EPS assembly of repeating unit has been reported. It contains two-pair epsC-epsD genes which are assigned to determine the chain length of EPS. This also suggests this organism produces two types of EPSs – capsular and ropy EPS, as observed in our previous studies. Additionally, ST 1275 appears to exhibit effective proteolysis system and sophisticated stress response systems to stressful conditions, and has the highest number of four separate CRISPR/Cas loci. These features may be conducive to milk adaptation of this starter and against undesirable bacteriophage infections which leads to failure of milk fermentation. Insights into the genome of ST 1275 suggest that this strain may be a model high EPS-producing dairy starter.

Structure and properties of a (1→3)-β-D-glucan from ultrasound-degraded exopolysaccharides of a medicinal fungus

A high molecular weight (MW) exopolysaccaride (EPS) fraction EPS1 was isolated from the fermentation broth of a medicinal fungus Cordyceps sinensis Cs-HK1 and partially degraded by high-intensity ultrasound (US) into a lower MW fraction EPS1U. EPS1U exhibited a single, symmetric peak on size exclusion chromatography with an average MW of 730kDa by light scattering analysis. It had a much lower intrinsic viscosity (1.7 versus 15.6dL/g) but a much higher solubility in water (77.5 versus 5.1g/L) than EPS1. Based on methylation analysis and NMR spectrometry, the structure of EPS1U was deduced as a (1→3)-β-d-glucan with glucose side chains attached to O-6 position at the branching points. EPS1U showed a high moisture absorption capability comparable to chitosan and urea, suggesting its potential as a moisturizing agent for food and cosmeceutical application. This is the first report on a high MW (1→3)-β-d-glucan isolated from EPS produced by Cordyceps sinensis.

Purification and Structure Study on Exopolysaccharides Produced by Lactobacillus paracasei KL1-Liu from Tibetan Kefir

To investigate the pure conditions of exopolysaccharides produced by Lactobacillus paracasei KL1-Liu from Tibetan Kefir, and to analyse the structure, we used Multi-level single-factor test to purify EPS by Sepharose CL-6B. And the purity of EPS was detected by UV scan and high performance liquid chromatogram (HPLC). EPS molecular weight and monosaccharide composition were determined by HPLC. Results: Adoption phosphate buffer gradient elution 0.02-0.10 mol/L, the velocity 0.25 mL/min, the sample concentration 1.0 mg/mL, the sample capacity 1.0 mL. Under this purification conditions, components EPSa and EPSb were obtained. The purities of EPSa and EPSb were 82.82% and 91.74% respectively, which were 1.4 and 1.5 times of the pre-purification. Purity Test results showed that EPSa and EPSb polysaccharide were single components, basically no nucleic acid and protein in them. Structural analysis revealed that the molecular weight of EPSa and EPSb were 4.60×104 Da and 2.12×104 Da detected by HPLC. EPSa monosaccharide components were glucose and rhamnose, and the molar ratio was 1:0.68. EPSb were composed of glucose, xylose and rhamnose, and the molar ratio was 1:0.77:0.69.

Production of volatile compounds in reconstituted milk reduced-fat cheese and the physicochemical properties as affected by exopolysaccharide-producing strain

The application of the exopolysaccharide-producing strains for improving the texture and technical properties of reduced-fat cheese looks very promising. Streptococcus thermophilus TM11 was evaluated for production of reduced-fat cheese using reconstituted milk powder (CRMP). The physicochemical analysis of fresh and stored cheeses showed that this strain slightly increased moisture content resulting in cheese with higher yield and lower protein content compared to the direct acidified cheese. The volatiles of cheese were determined by SPME and GC equipped with a mass spectrometer. The results indicated that the major compounds included aldehydes, ketones and acids, whereas, alcohols and branched-chain aldehydes that contribute to exciting and harsh flavors were not found in CRMP. By the textural profile analysis, we found the cheese made with S. thermophilus TM11 had lower cohesiveness, resilience and higher adhesiveness than the direct acidified cheese, and had similar hardness. Further, S. thermophilus TM11 greatly changed the protein matrix with more opened cavities according to observation by scanning electron microscopy. Consequently, use of S. thermophilus TM11 could endow CRMP with the novel and suitable flavor properties and improved texture quality.

Protein-protein interactions among the components of the biosynthetic machinery responsible for exopolysaccharide production in Streptococcus thermophilus MR-1C

AIM

This study identified protein-protein interactions among the biosynthetic machinery responsible for exopolysaccharide (EPS) production in Streptococcus thermophilus MR-1C.

METHODS AND RESULTS

Protein-protein interactions were investigated using the yeast two-hybrid system. A strong protein-protein interaction was detected between the transmembrane activation protein Wzd and the protein tyrosine kinase Wze. Weaker protein-protein interactions were detected between two duplicate Wze proteins and between Wze and the phosphotyrosine phosphatase Wzh. Protein-protein interactions involving a Wzd/Wze fusion protein and Wzd and Wze may indicate that these proteins form multi-protein complexes. All combinations of the Wzh, Wzd, Wze, Wzg (regulation), CpsE (glycosyl-1-phosphate transferase), CpsS (polymerization), CpsL (unknown), CpsW (regulation) and CpsU (membrane translocation) were analysed for protein-protein interactions but no additional interactions were discovered using the yeast two-hybrid system.

CONCLUSIONS

Interactions among the phosphotyrosine phosphatase, tyrosine kinase, and transmembrane activation protein are important in the regulation of capsule biosynthesis in Strep. thermophilus MR-1C.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides some valuable insight into the organization and interactions between the many proteins involved in EPS production. A better understanding of this process may facilitate the genetic manipulation of capsule production to impart desirable properties to dairy starter cultures.

Detection and quantification of capsular exopolysaccharides from Streptococcus thermophilus using lectin probes

The aim of this work was to use fluorescently labeled lectins to develop a convenient and reliable method to determine the relative abundance of capsular polysaccharides (CPS) at the surface of Streptococcus thermophilus MR-1C cells. Fluorescein isothiocyanate-labeled peanut agglutinin isolated from Arachis hypogaea was found to interact specifically with the CPS of Strep. thermophilus MR-1C. This labeled lectin was then used as an effective probe to detect and quantify CPS. A fluorescence-based lectin-binding assay was successfully applied to follow the accumulation of CPS during the growth of Strep. thermophilus MR-1C in milk and in M17 broth supplemented with lactose. Our results showed that in both media, CPS production by Strep. thermophilus MR-1C began during the exponential phase of growth and continued for several hours after the culture reached the stationary growth phase.

Galactose metabolism and capsule formation in a recombinant strain of Streptococcus thermophilus with a galactose-fermenting phenotype

The capsule-producing, galactose-negative Streptococcus thermophilus MR-1C strain was first transformed with a low-copy plasmid containing a functional galK gene from Streptococcus salivarius to generate a recombinant galactose-fermenting Strep. thermophilus strain named MR-AAC. Then, we compared the functional properties of Strep. thermophilus MR-AAC with those of the parent MR-1C strain when used as starter for fermented products and cheese. In lactose-supplemented laboratory medium, MR-AAC metabolized galactose, but only when the amount of lactose was less than 0.1% (wt/vol). After 7 h of fermentation, the medium was almost depleted of galactose. The parent strain, MR-1C, showed the same pattern, except that the concentration of galactose decreased by only 25% during the same period. It was found that, during milk fermentation and Mozzarella cheese production, the galactose-fermenting phenotype was not expressed by MR-AAC and this strain expelled galactose into the medium at a level similar to the parent MR-1C strain. In milk and in lactose-supplemented medium, capsular exopolysaccharide production occurred mainly during the late exponential phase and the stationary growth phase with similar kinetics between MR-1C and MR-AAC.

Molecular identification and typing of natural whey starter cultures and microbiological and compositional properties of related traditional Mozzarella cheeses

Cell numbers of presumptive lactic acid bacteria varied markedly between 7 natural whey starter cultures (NWSC) used for producing traditional cows' milk Mozzarella cheeses in the Apulia region of Southern Italy. Taxonomic identification revealed a large diversity at species level, including mesophilic and thermophilic lactobacilli, lactococci, streptococci and enterococci. Randomly Amplified Polymorphic DNA (RAPD-PCR), analysis showed the biodiversity among the strains and, for lactobacilli, some relationships with provenience of the natural starter. Cell numbers of presumptive lactic acid bacteria in the corresponding Mozzarella cheeses were similar or higher than those found in the corresponding NWSC. RAPD-PCR analyses showed that most of the strains in cheese originated from the starter. The gross composition varied markedly between the 7 Mozzarella cheeses and ranged from 53-64% moisture, 17-23% protein, 13-20% fat and 0.50-1.61% salt. The values of pH for several samples were above 6.0. As shown by urea-PAGE of the pH 4.6-insoluble nitrogen fractions, cheese samples were characterized by differences in alpha(S1)- and beta-casein hydrolysis. Cheeses also differed with respect to secondary proteolysis as shown by Principal Component Analysis (PCA) of data from RP-HPLC of the pH 4.6-soluble, pH 4.6-70% ethanol-soluble and 70% ethanol-insoluble nitrogen fractions. These differences were attributed to the different microbial composition of the NWSC. Strain selection and optimization of a protocol for producing a natural whey starter culture to be used by dairy factories of the Apulia region appears to be a pre-requisite to standardize the major traits distinguishing this cheese variety.

Improvement of Texture and Structure of Reduced-Fat Cheddar Cheese by Exopolysaccharide-Producing Lactococci

The objective of this study was to evaluate the effect of capsular and ropy exopolysaccharide (EPS)-producing strains of Lactococcus lactis ssp. cremoris on textural and microstructural attributes during ripening of 50%-reduced-fat Cheddar cheese. Cheeses were manufactured with added capsule- or ropy-forming strains individually or in combination. For comparison, reduced-fat cheese with or without lecithin added at 0.2% (wt/vol) to cheese milk and full-fat cheeses were made using EPS-nonproducing starter, and all cheeses were ripened at 7 degrees C for 6 mo. Exopolysaccharide-producing strains increased cheese moisture retention by 3.6 to 4.8% and cheese yield by 0.28 to 1.19 kg/100 kg compared with control cheese, whereas lecithin-containing cheese retained 1.4% higher moisture and had 0.37 kg/100 kg higher yield over the control cheese. Texture profile analyses for 0-d-old cheeses revealed that cheeses with EPS-producing strains had less firm, springy, and cohesive texture but were more brittle than control cheeses. However, these effects became less pronounced after 6 mo of ripening. Using transmission electron microscopy, fresh and aged cheeses with added EPS-producing strains showed a less compact protein matrix through which larger whey pockets were dispersed compared with control cheese. The numerical analysis of transmission electron microscopy images showed that the area in the cheese matrix occupied by protein was smaller in cheeses with added EPS-producing strains than in control cheese. On the other hand, lecithin had little impact on both cheese texture and microstructure; after 6 mo, cheese containing lecithin showed a texture profile very close to that of control reduced-fat cheese. The protein-occupied area in the cheese matrix did not appear to be significantly affected by lecithin addition. Exopolysaccharide-producing strains could contribute to the modification of cheese texture and microstructure and thus modify the functional properties of reduced-fat Cheddar cheese.

Low-Fat Mozzarella as Influenced by Microbial Exopolysaccharides, Preacidification, and Whey Protein Concentrate

Low-fat Mozzarella cheeses containing 6% fat were made by preacidification of milk, preacidification combined with exopolysaccharide- (EPS-) producing starter, used independently or as a coculture with non-EPS starter, and preacidification combined with whey protein concentrate (WPC) and EPS. The impact of these treatments on moisture retention, changes in texture profile analysis, cheese melt, stretch, and on pizza bake performance were investigated over 45 d of storage at 4 degrees C. Preacidified cheeses without EPS (control) had the lowest moisture content (53.75%). These cheeses were hardest and exhibited greatest springiness and chewiness. The meltability and stretchability of these cheeses increased most during the first 28 d of storage. The moisture content in cheeses increased to 55.08, 54.79, and 55.82% with EPS starter (containing 41.18 mg/g of EPS), coculturing (containing 28.61 mg/g of EPS), and WPC (containing 44.23 mg/g of EPS), respectively. Exopolysaccharide reduced hardness, springiness, and chewiness of low-fat cheeses made with preacidified milk in general and such cheeses exhibited an increase in cohesiveness and meltability. Although stretch distance was similar in all cheeses, those containing EPS were softer than the control. Cocultured cheeses exhibited the greatest meltability. Cheeses containing WPC were softest in general; however, hardness remained unchanged over 45 d. Cheeses made with WPC had the least increase in meltability over time. Incorporation of WPC did not reduce surface scorching or increase shred fusion of cheese shreds during pizza baking; however, there was an improvement in these properties between d 7 and 45. Coating of the cheese shreds with oil was necessary for adequate browning, melt, and flow characteristics in all cheese types.

Fermentation conditions affecting the bacterial growth and exopolysaccharide production by Streptococcus thermophilus ST 111 in milk-based medium

AIMS

To study the effect of different fermentation conditions and to model the effect of temperature and pH on different biokinetic parameters of bacterial growth and exopolysaccharides (EPS) production of Streptococcus thermophilus ST 111 in milk-based medium.

METHODS AND RESULTS

The influence of temperature and pH was studied through fermentation and modelling. Fermentations under non-pH controlled conditions with S. thermophilus ST 111 indicated that the EPS production was low in milk medium, even if additional nitrogen sources were supplemented. Under pH-controlled conditions, addition of whey protein hydrolysate to the milk medium resulted in a fivefold increase of the EPS production. This medium did not contain polysaccharides interfering with EPS isolation. Primary and secondary modelling of different fermentations revealed an optimum temperature and pH of 40 degrees C and constant pH 6.2, respectively, for growth in milk medium supplemented with whey protein hydrolysate. Maximum EPS production was observed in the range of 32-42 degrees C and constant pH 5.5-6.6. Whereas growth and maximum EPS production were clearly influenced by temperature and pH, the specific EPS production was only affected by stress conditions (T = 49 degrees C).

CONCLUSIONS

Addition of whey protein hydrolysate to milk medium resulted in an increased growth and EPS production of S. thermophilus ST 111 under pH-controlled conditions. A modelling approach allowed studying the influence of temperature and pH on the kinetics of both growth and EPS production.

SIGNIFICANCE AND IMPACT OF THE STUDY

The use of an appropriate milk-based medium and a combined model of temperature and pH can be of practical importance for the production of yoghurt or other fermented milks as well as for process optimization of the large-scale production of starter strains to be used for their EPS production.

Exopolysaccharide biosynthesis by Lactobacillus helveticus ATCC 15807

Exopolysaccharide (EPS) production and the activities of the enzymes involved in sugar nucleotide biosynthesis in Lactobacillus helveticus ATCC 15807 under controlled pH conditions were investigated. Batch fermentations using lactose as energy source showed higher EPS synthesis by L. helveticus ATCC 15807 at pH 4.5 with respect to pH 6.2, the enzyme alpha-phosphoglucomutase (alpha-PGM) being correlated with both total and specific EPS production. When glucose was used as carbon source instead of lactose, the lower EPS synthesis obtained was linked to a decrease in alpha-PGM and galactose 1-phosphate-uridyltransferase (GalT) activities, the reduction of the latter being more pronounced. Higher EPS production by L. helveticus ATCC 15807 at the acidic constant pH of 4.5 requires that both alpha-PGM and GalT activities are high. These enzymes are needed to synthesize UDP-glucose and UDP-galactose for supplying the corresponding monomers for EPS biosynthesis. Although differences are observed in EPS production by this strain regarding the energy source (lactose or glucose), the monomeric composition of the polymers produced is independent of the carbohydrate used. The obtained results contribute to a better understanding of the physiological factors that affect EPS biosynthesis by lactobacilli, which could help in the correct handling of the fermentation parameters within the fermented dairy industry.

Biodiversity of exopolysaccharides produced by Streptococcus thermophilus strains is reflected in their production and their molecular and functional characteristics

Twenty-six lactic acid bacterium strains isolated from European dairy products were identified as Streptococcus thermophilus and characterized by bacterial growth and exopolysaccharide (EPS)-producing capacity in milk and enriched milk medium. In addition, the acidification rates of the different strains were compared with their milk clotting behaviors. The majority of the strains grew better when yeast extract and peptone were added to the milk medium, although the presence of interfering glucomannans was shown, making this medium unsuitable for EPS screening. EPS production was found to be strain dependent, with the majority of the strains producing between 20 and 100 mg of polymer dry mass per liter of fermented milk medium. Furthermore, no straightforward relationship between the apparent viscosity and EPS production could be detected in fermented milk medium. An analysis of the molecular masses of the isolated EPS by gel permeation chromatography revealed a large variety, ranging from 10 to >2,000 kDa. A distinction could be made between high-molecular-mass EPS (>1,000 kDa) and low-molecular-mass EPS (<1,000 kDa). Based on the molecular size of the EPS, three groups of EPS-producing strains were distinguished. Monomer analysis of the EPS by high-performance anion-exchange chromatography with amperometric detection was demonstrated to be a fast and simple method. All of the EPS from the S. thermophilus strains tested were classified into six groups according to their monomer compositions. Apart from galactose and glucose, other monomers, such as (N-acetyl)galactosamine, (N-acetyl)glucosamine, and rhamnose, were also found as repeating unit constituents. Three strains were found to produce EPS containing (N-acetyl)glucosamine, which to our knowledge was never found before in an EPS from S. thermophilus. Furthermore, within each group, differences in monomer ratios were observed, indicating possible novel EPS structures. Finally, large differences between the consistencies of EPS solutions from five different strains were assigned to differences in their molecular masses and structures.

Biochemistry, genetics, and applications of exopolysaccharide production in Streptococcus thermophilus: a review

Many strains of Streptococcus thermophilus synthesize extracellular polysaccharides. These molecules may be produced as capsules that are tightly associated with the cell, or they may be liberated into the medium as a loose slime (i.e., "ropy" polysaccharide). Although the presence of exopolysaccharide does not confer any obvious advantage to growth or survival of S. thermophilus in milk, in situ production by this species or other dairy lactic acid bacteria typically imparts a desirable "ropy" or viscous texture to fermented milk products. Recent work has also shown that exopolysaccharide-producing S. thermophilus can enhance the functional properties of Mozzarella cheese, but they are not phage-proof. As our understanding of the genetics, physiology, and functionality of bacterial exopolysaccharides continues to improve, novel applications for polysaccharides and polysaccharide-producing cultures are likely to emerge inside and outside the dairy industry. This article provides an overview of biochemistry, genetics, and applications of exopolysaccharide production in S. thermophilus.