Lactococcus lactis subsp. cremoris strain JFR1 attenuates Salmonella adhesion to human intestinal cells in vitro

Lactococcus lactis subsp. cremoris JFR1 has been studied in reduced fat cheese due to its ability to produce exopolysaccharides (EPS) in situ, contributing to improved textural and organoleptic properties. In this study, the effect of strain JFR1 on virulence gene expression and attachment of Salmonella to HT-29 human colon carcinoma cells was investigated. Overnight cultures of L. lactis subsp. cremoris JFR1 containing EPS, grown in M17 media with 0.5% glucose supplementation, decreased attachment as well as down regulated virulence gene expression in Salmonella enterica subsp. enterica when tested on HT-29 cells. However, EPS isolated from milk fermented with L. lactis subsp. cremoris JFR1 did not affect Salmonella virulence gene expression or attachment to HT-29 cells. These results suggest that EPS does not contribute to the attachment of Salmonella to human intestinal cells. However, the possibility that the isolation process may have affected the structural features of EPS cannot be ruled out.     

Effect of exopolysaccharides and proteolytic activity of Lactococcus lactis subsp. cremoris strains on the viscosity and structure of fermented milks

Two exopolysaccharide (EPS)-producing Lactococcus lactis subsp. cremoris strains (B35 and B891) were used to study the effect of the kinetics of EPS production and bacterial proteolytic activity on the structure of milk gels and the viscosity of stirred milk gels. Strains were grown at 20 °C in milk containing either yeast extract or casitone and at 30 °C in either milk alone or milk containing casitone. Lactococcal counts, pH decrease and production and molecular characteristics (molar mass and radius of gyration) of both EPSs were followed during milk fermentation. The level of proteolysis in the fermented milks was determined after 24 h of incubation. The results obtained showed that the yield of EPS and the timing of EPS production during milk-gel formation were the most important factors that influenced the structure of the milk gels and the viscosity of the stirred product. The proteolytic activity of the strains did not seem to play any significant role.     

Application of ruthenium red and colloidal gold-labeled lectin for the visualization of bacterial exopolysaccharides in Cheddar cheese matrix using transmission electron microscopy

The objective of the present study was to develop a methodology for direct observation of capsular and ropy strains and their exopolysaccharides (EPS) in a Cheddar cheese matrix. Cheddar cheeses with 50% reduced fat were made from milk containing 1.7% fat using mixed starter culture containing either capsule-forming Lactococcus lactis subsp. cremoris (SMQ-461) or ropy L. lactis subsp. cremoris (JRF-1) strains. Control cheese was made using the EPS-negative L. lactis subsp. cremoris (RBL132) strain. Following cheese pressing, samples were taken from each cheese treatment and examined by transmission electron microscopy (TEM). Samples were divided into two series: the first was prepared following the conventional methods (involving fixation, post fixation, dehydration and embedding in resin) and the second with added ruthenium red at 0.15% (w/v) during the fixation, post fixation and washing procedures. Gold-labeled lectin was also used for the visualization and localization of EPS in cheese matrix. Electron micrographs showed that ruthenium red makes it possible to visualize and enhance the resolution of the EPS in a Cheddar matrix compared with the conventional method. The EPS layer of the capsular strain appeared regular and evenly distributed around the cell, whereas the cell-associated EPS layer produced by the ropy strain was longer, more irregular (having a filamentous structure) and unevenly surrounded the cell. EPS released from the ropy strain appeared to form a network-like structure located principally in whey pockets and appeared to interact with the casein matrix and fat globule membrane. Labeling EPS by lectin conjugated to colloidal gold could only be performed with conventional preparation of cheese samples and appeared to react only with the cell surface rather than with liberated EPS. Besides their ability to bind water and increase cheese yield, capsular and ropy strains used in this study appear to have potential autolytic characteristics, which may have an impact on cheese proteolysis, texture and flavor quality.     

Milk fermentation by functional mixed culture producing nisin Z and exopolysaccharides in a fresh cheese model

A nisin Z-producing strain, Lactococcus lactis subsp. lactis biovar. diacetylactis UL719 and two nisin-sensitive cultures, Lactobacillus rhamnosus RW-9595 M producing exopolysaccharide (EPS), and Lc. lactis subsp. cremoris for acidification, were tested in pure and mixed cultures during milk fermentation. The mixed culture of the three strains showed a higher acidifying capacity at 34°C and 38°C, even though populations of Lc. cremoris were largely reduced compared with pure cultures. Bacteriocin production was 3.1–4.6-fold higher in mixed cultures than for pure cultures of Lc. diacetylactis UL719. These data can be explained by commensalism behavior relying on high proteolytic activity of Lc. cremoris and autolysis and nisin Z-induced lysis. In mixed culture, EPS production was 3-fold lower than for Lb. rhamnosus RW-9595 M pure culture. Our data showed that this strain combination, with nisin-producing and sensitive strains, can be used in mixed cultures for manufacture of fresh cheese with improved functional properties.     

Enhancement of functional characteristics of mixed lactic culture producing nisin z and exopolysaccharides during continuous prefermentation of milk with immobilized cells

Antagonistic phenomena between strains often occur in mixed cultures containing a bacteriocinogenic strain. A nisin Z producer (Lactococcus lactis ssp. lactis biovar. diacetylactis UL719) and 2 nisin-sensitive strains for acidification (Lactococcus lactis ssp. cremoris ATCC19257) and exopolysaccharide (EPS) production (Lactobacillus rhamnosus RW-9595M) were immobilized separately in gel beads and used to continuously preferment milk at different temperatures, with pH controlled at 6.0 by fresh milk addition. The process showed high volumetric productivity, with an increase from 8.0 to 12.5 L of prefermented milk per liter of reactor volume and hour as the temperature was increased from 27 to 35°C. Lactococcus lactis ssp. lactis biovar. diacetylactis UL719 counts in prefermented and fermented (22-h batch fermentation) milks were stable during 3 wk of continuous fermentation (8.1 ± 0.1 and 8.9 ± 0.2 log cfu/mL, respectively). The L. lactis ssp. cremoris population (estimated with real-time quantitative PCR) decreased rapidly during the first week of continuous culture to approximately 4.5 log cfu/mL and remained constant afterward. Lactobacillus rhamnosus counts in prefermented and fermented milks significantly increased with prefermentation time, with no temperature effect. Nisin Z reached high titers in fermented milks (from 177 to 363 IU/mL), with EPS concentration in the range from 43 to 178 mg/L. Immobilization and continuous culture led to important physiological changes, with Lb. rhamnosus becoming much more tolerant to nisin Z, and Lb. rhamnosus and L. lactis ssp. lactis biovar. diacetylactis UL719 exhibiting large increases in milk acidification capacity. Our data showed that continuous milk prefermentation with immobilized cells can stimulate the acidification activity of low-acidifying strains and produce fermented milks with improved and controlled functional properties.     

Fresh-cheesemilk formulation fermented by a combination of freeze-dried citrate-positive cultures and exopolysaccharide-producing lactobacilli with liquid lactococcal starters

Milk formulation (4% fat and 5% protein) prepared to simulate fresh cheese production was inoculated with: (1) 10⁷ cfu mL⁻¹ of fresh liquid starters of Lactococcus lactis ssp. lactis T1 and Lc. lactis ssp. cremoris T2, (2) a freeze-dried exopolysaccharide-producing (EPS) strain of Lactobacillus rhamnosus RW-9595M, and (3) freeze-dried Leuconostoc cremoris LM057 or Lc. lactis ssp. lactis var. diacetylactis MD089 strains. The effect of inoculation rate of the freeze-dried starters (between 10⁶ and 10⁷ cfu mL⁻¹) and incubation temperature (between 23.5 and 36.5 °C) on evolution of pH and the various populations during fermentation was examined. Texture (apparent viscosity, syneresis potential) and chemical composition (diacetyl, acetaldehyde) of the fermented milks were also determined. Milk was incubated until a pH of 4.6 was obtained, which required between 6 and 10 h depending on temperature.In the range of inoculation levels used, there was no significant effect of the presence of lactobacilli, Ln. cremoris or Lc. lactis ssp. lactis var. diacetilactis on the growth of the lactococci. There was a direct correlation between the inoculation rates of the freeze-dried cultures and their final populations in the fermented milks. The growth of the cultures were also affected by temperature, Ln. cremoris growing less as incubation temperature increased, while the opposite was noted with Lb. rhamnosus. The apparent viscosity of the fermented milk was significantly affected by incubation temperature, but there was no correlation between apparent viscosity and the final population in lactobacilli. Of the three variables studied, the highest correlation with diacetyl content was obtained with the inoculation level of the Leuconostoc strain.     

Intragenomic 16S rRNA gene heterogeneity in Lactococcus lactis subsp. cremoris

Partial 16S rRNA gene sequencing of Lactococcus lactis subsp. cremoris strains from our collection identified strains containing sequences diagnostic of both subspecies L. lactis subsp. lactis and L. lactis subsp. cremoris together in the same strain. The presence of a plasmid-encoded 16S rRNA pseudogene partly explained this result. Twenty-four out of 46 L. lactis subsp. cremoris strains tested by PCR contained this pseudogene. However, further analysis showed that five of these 24 strains also contained chromosomal 16S rRNA genes with sequences typical of L. lactis subsp. lactis. Genetic and phenotypic tests indicated these strains were otherwise normal L. lactis subsp. cremoris strains. Past recombination events between the 16S rRNA pseudogene and chromosomal 16S rRNA genes may explain this phenomenon. Genomic heterogeneity for both 16S rRNA and other gene sequences was observed for L. lactis subsp. cremoris SK11 from different laboratories, indicating caution is needed when integrating data from diverse sources for nominally the same strain.     

Fat-free yogurt made using a galactose-positive exopolysaccharide-producing recombinant strain of Streptococcus thermophilus

To prevent textural defects in low-fat and fat-free yogurts, fat substitutes are routinely added to milk. In situ production of exopolysaccharides (EPS) by starter cultures is an acknowledged alternative to the addition of biothickeners. With the aim of increasing in situ EPS production, a recombinant galactose-positive EPS⁺Streptococcus thermophilus strain, RD-534-S1, was generated and compared with the parent galactose-negative EPS⁺ strain RD-534. The RD-534-S1 strain produced up to 84 mg/L of EPS during a single-strain milk fermentation process, which represented 1.3 times more than the EPS produced by strain RD-534. Under conditions that mimic industrial yogurt production, the starter culture consisting of RD-534-S1 and (EPS⁻) Lactobacillus bulgaricus L210R strain (RD-534-S1/L210R) led to an EPS production increase of 1.65-fold as compared with RD-534-S1 alone. However, the amount of EPS produced did not differ from that found in yogurts produced using an isogenic starter culture that included the parent S. thermophilus strain RD-534 and Lb. bulgaricus L210R (RD-534/L210R). Moreover, the gel characteristics of set-style yogurt and the rheological properties of stirred-style yogurt produced using RD-534-S1/L210R were similar to the values obtained for yogurts made with RD-534/L210R. In conclusion, it is possible to increase the production of EPS by ropy S. thermophilus strains through genetic engineering of galactose metabolism. However, when used in combination with Lb. bulgaricus for yogurt manufacture, the EPS overproduction of recombinant strain is not significant.     

Evaluation of the yield,molar mass of exopolysaccharides,and rheological properties of gels formed during fermentation of milk by Streptococcus thermophilus strains St-143 and ST-10255y

The yield and chemical structures of exopolysaccharides (EPS) produced by many strains of Streptococcus thermophilus have been characterized. However, the kinetics (or production profile) for EPS during milk fermentation is not clear. In this study, we investigated whether any differences existed in the yield and molar mass of EPS when milk was fermented at the same acidification rate by 2 strains of S. thermophilus (St-143 and ST-10255y). The type of EPS produced by these 2 strains is different. Milk samples were analyzed for EPS concentration every 30 min during a fermentation period of 270 min (final pH 4.5) by using a modified quantification method, which was faster and validated for its recovery of added EPS. Rheological properties of milks during fermentation were also analyzed using small-strain dynamic oscillatory rheology. For the determination of molar mass, EPS extracts were isolated by ultrafiltration of whey obtained during fermentation of milk to pH values 5.2, 4.9, 4.7, and 4.5, and molar mass was analyzed using size-exclusion chromatography–multi-angle laser light scattering. During fermentation, both strains appeared to start producing significant amounts of EPS after about ～150 min, which corresponded to pH ～5.3, which was close to the point of gelation. During the remainder of the fermentation process (150–270 min), the EPS concentration from strains St-143 and ST-10255y significantly increased from 30 to 72 mg/L and from 26 to 56 mg/L, respectively. The quantity of EPS recovered by our modified method was estimated to represent ～60% of the total EPS added to milk. The molar mass of EPS produced by both strains appeared to slightly decrease during fermentation. At pH 5.2, EPS from St-143 and ST-10255y had molar masses of 2.9 × 10⁶ and 1.4 × 10⁶ g/mol, respectively, which decreased to 1.6 × 10⁶ and 0.8 × 10⁶ g/mol, respectively, when the pH of milk was 4.5. Distinct differences were apparent in the rheological properties of gels fermented by the 2 strains. At the end of fermentation, St-143 fermented milk had weaker gels with storage modulus (G′) value at pH 4.6 of 26 Pa, whereas gels made with ST-10255y were stiffer with a G′ value at pH 4.6 of 82 Pa. For St-143 gels, maximum loss tangent (LT_max) values were higher (0.50) and occurred earlier (at a higher pH value) than the LT_max values (0.46) for gels from ST-10255y strain. Because the fermentation conditions were identical for both strains, the observed changes in rheological properties could be due to the differences in chemical structures and molar mass of the EPS produced by these 2 S. thermophilus strains.     

Efficacy of four conjugal lactococcal phage resistance plasmids against phage in commercial Lactococcus lactis subsp. cremoris cheese starter strains

The efficacy of four lactococcal phage resistance plasmids (pNP40, pMU1311, pDI60 and pKP100) against phage was assessed after their conjugal transfer to four commercial Lactococcus lactis subsp. cremoris cheese starter strains and to the plasmid-free strain L. lactis subsp. cremoris MG1363. In MG1363, only pNP40 conferred resistance to prolate phages c2 and 643. Highest levels of resistance to small isometric phages in MG1363 occurred when pNP40 was stacked together with pMU1311 or pDI60. In the four starter strains, the plasmids conferred varying levels of resistance to small isometric phages. Growth and acidification rates in milk of most transconjugants derived from the starter strains decreased, but this was not always due to loss of plasmid-encoded cell wall proteinase (lactocepin) activity. Only one transconjugant grew during repeated subculture in milk with addition of factory wheys containing phages. This and the presence of bacteriocins encoded on pMU1311 and pDI60 limited application of the plasmids to protect L. lactis subsp. cremoris starters against phages in industry. However, some of the plasmids could be useful in extending the industry life of starters where fast acid production is not required or where bacteriocin production is acceptable.     

Microbial characterisation and stability of a farmhouse natural fermented milk from Spain

This work reports the microbial characterisation of a farmhouse natural fermented milk (NFM) with good sensorial properties produced in Spain. Culturing and denaturing gradient gel electrophoresis (DGGE) analyses showed thatLactococcus lactissubsp.lactis and L. lactissubsp.cremoris(approximate levels of 10⁹ cfu/mL) were dominant in this NFM, whileLactobacillus plantarumappeared at a lower level (10⁶–10⁷ cfu/mL). Repetitive extragenic palindromic (REP)‐PCR typing of the isolates identified single strains each ofLc. lactissubsp.lactis, Lc. lactissubsp.cremorisandLb. plantarum. These three strains formed a stable microbial association which has been maintained for at least some decades.     

Cholesterol Lowering Activity of Ropy Fermented Milk

The effect of ropy fermented milk on serum cholesterol in rats was studied. Basic diets containing slime-forming Lactococcus lactis subsp. cremoris SBT 0495, its non slime-forming variant SBT 1275, and acidified reconstituted skim milk, with 0.5% cholesterol added, were fed to F-344 rats for 7 days. Serum cholesterol level of rats fed the ropy fermented milk were the lowest among the three treatments. The serum high-density lipoprotein (HDL) cholesterol/total cholesterol ratio of rats fed ropy fermented milk was the highest. Slime materials produced by L. lactis subsp. cremoris SBT 0495 had a beneficial effect on rat cholesterol metabolism.     

Effect of protein supplementation on the rheological characteristics of milk permeates fermented with exopolysaccharide-producing Lactococcus lactis subsp. cremoris

This research studied the effect of addition of whey proteins on the rheological properties of ultrafiltration permeate fermented with the exopolysaccharide (EPS)-producing strain Lactococcus lactis subsp. cremoris JFR1. Milk permeates containing 8% solids and various levels of added whey proteins (0, 2, 4, 6 and 8%) were fermented for 12 h at 30 °C. The rheological properties of the fermented samples were then evaluated and compared to controls fermented with a non-EPS producing strain. Scanning electron microscopy was also employed to confirm the existence of interactions between whey protein aggregates and EPS. The presence of EPS considerably increased the viscosity and viscoelastic properties of the media, especially in samples containing >2% whey protein added. The results obtained demonstrate the importance of EPS–protein interactions in structure formation and may help explain the viscosifying mechanism of EPS in fermented dairy products. Production of highly viscous material could potentially be employed in the future as a novel fiber-rich functional ingredient in dairy products.     

Effect of fermentation temperature on the properties of exopolysaccharides and the acid gelation behavior for milk fermented by Streptococcus thermophilus strains DGCC7785 and St-143

We investigated the effect of fermenting milk with 2 strains (DGCC7785 and St-143) of Streptococcus thermophilus, which are known to produce different types of exopolysaccharide (EPS) structures. The yields and physical properties of these ropy EPS were monitored during the fermentation of milk at different temperatures. We wanted to understand how these types of EPS properties affected yogurt gelation. Reconstituted skim milk was fermented at 33, 39, or 45°C until pH values reached 5.2, 4.9, 4.7, and 4.5. Molar mass of ropy EPS samples was determined using size exclusion chromatography coupled with multiangle laser light scattering. Rheological properties of fermented milk gels were analyzed using small-strain dynamic oscillatory measurements. In both strains, concentrations of ropy EPS increased during fermentation and at all temperatures. Fermentation times, by both strains, were shortest at 45°C and longest at 33°C. For both strains, molar mass of ropy EPS ranged from 2 to 4 × 10⁶ g/mol during fermentation. A major proteinaceous contaminant that was co-isolated with the ropy EPS fraction by our isolation method was identified as a milk-derived phosphoglycoprotein PP3. Increase in fermentation temperature from 33 to 45°C significantly decreased the storage modulus values (from 170 to 41 Pa) for milk gelled by strain DGCC7785, whereas the gels made with St-143 had very low storage modulus values (11–17 Pa) regardless of fermentation temperatures. For both strains, the values of maximum loss tangent in the milk gels increased with fermentation temperature; the maximum loss tangent occurred at higher pH values when milk was fermented by strain DGCC7785. The specific type of EPS produced appeared to be responsible for the differences in yogurt texture rather than the concentration or molar mass of the EPS.     

Technological characterization of lactic acid bacteria isolated from Armada cheese (a Spanish goats’ milk cheese)

Thirty-one strains of lactic acid bacteria (LAB) isolated from Armada cheese, Sobado variety, (eight strains of Lactococcus lactis subsp. lactis, four strains of Lactococcus lactis subsp. cremoris, two strains of L. lactis subsp. lactis biovar. diacetylactis, two strains of Leuconostoc mesenteroides subsp. mesenteroides, two strains of Leuconostoc mesenteroides subsp. dextranicum, five strains of Lactobacillus plantarum, six strains of Lactobacillus casei subsp. casei and two strains of Lactobacillus brevis) were screened for their acidifying capacity and enzymatic activity, that included the rapid API-ZYM system, the proteolytic activity, the amino-, di-, and carboxypeptidase activity and the caseinolytic activity. The strains of L. lactis subsp. lactis exhibited the highest acidifying and proteolytic activity. Lipase and esterase activity was practically non-existent for lactococci and lactobacilli; a certain esterase activity was observed among leuconostoc. The highest aminopeptidase activity was demonstrated by the cell-free extract (CFE) of some strains of L. plantarum, L. casei subsp. casei and L. mesenteroides subsp. dextranicum. The CFEs of L. lactis subsp. cremoris and L. lactis subsp. lactis possessed carboxypeptidase and dipeptidase activities, at levels depending on the strain. Appreciable caseinolytic activity was detected for the CFE of L. plantarum and those some lactococci.     

A centrifugation-based clearing method allows high-throughput acidification and growth-rate measurements in milk

The turbidity of milk prohibits the use of optical density measurements for strain characterizations. This often limits research to laboratory media. Here, we cleared milk through centrifugation to remove insoluble milk solids. This resulted in a clear liquid phase, termed milk serum, in which optical density measurements can be used to track microbial growth until a pH of 5.2 is reached. At pH 5.2 coagulation of the soluble protein occurs, making the medium opaque again. We found that behavior in milk serum was predictive of that in milk for 39 Lactococcus lactis (R² = 0.81) and to a lesser extent for 42 Lactiplantibacillus plantarum (formerly Lactobacillus plantarum; R² = 0.49) strains. Hence, milk serum can be used as an optically clear alternative to milk for comparison of microbial growth and metabolic characteristics. Characterization of the growth rate, specific acidification rate for optical density at a wavelength of 600 nm, and the amount of acid produced per unit of biomass for all these strains in milk serum, showed that almost all strains could grow in milk, with higher specific acidification and growth rates of Lc. lactis strains compared with Lb. plantarum strains. Nondairy Lc. lactis isolates had a lower growth and specific acidification rate than dairy isolates. The amount of acid produced per unit biomass was relatively high and similar for Lc. lactis dairy and nondairy isolates, as opposed to Lb. plantarum isolates. Lactococcus lactis ssp. lactis showed slightly lower growth and acidification rates when compared with ssp. cremoris. For Lc. lactis strains a doubling of the specific acidification rate occurred with a doubling of the maximum growth rate. This relation was not found for Lb. plantarum strains, where the acidification rate remained relatively constant across 39 strains with growth rates ranging from 0.2 h^?1 to 0.3 h^?1. We conclude that milk serum is a valuable alternative to milk for high-throughput strain characterization during milk fermentation.     

Amino acid catabolism by Lactococcus lactis during milk fermentation

Two wild-type Lactococcus lactis strains isolated from naturally Tunisian fermented milk (Leben), and one laboratory strain, were used to investigate the ability of L. lactis to transform amino acids into aroma compounds during milk fermentation. The α-ketoacid acceptor used for leucine transamination, the first step of catabolism, was identified by gas chromatography/mass spectrometry analysis of the ¹⁵N-labelled amino acids that formed from ¹⁵N-labelled leucine in fermented milk. Furthermore, the amino acids produced or catabolized by the laboratory strain via transamination were identified by comparing the free amino acids in milk fermented with the wild-type strain and the double mutant for aromatic and branched-chain aminotransferases, which cannot transaminate amino acids. The three L. lactis strains strongly catabolized leucine and valine during milk fermentation. The principal amino acid formed via leucine and valine transamination was glutamate indicating that α-ketoglutarate was the principal α-ketoacid acceptor and was generated during milk fermentation.     

Lysis of starters in UF cheeses: Behaviour of mesophilic lactococci and thermophilic lactobacilli

The objective of this work was to study the autolytic behaviour of strains of mesophilic (Lactococcus lactis subsp. lactis and L. lactis subsp. cremoris) and thermophilic lactic acid bacteria (Lactobacillus helveticus, Lb. delbrueckii subsp. lactis and Streptococcus thermophilus) in UF cheese. Cheeses were made from a UF-retentate (milk concentrated by a factor of 6) of microfiltered milk (0.8 μm pore size membrane) using the following starter systems: (1) single inocula of autolytic strains of L. lactis (US3, AM2 or AM1), non-autolytic strains of L. lactis (AM2-C or CNRZ-144), (2) a co-inocula of strains of Lb. helveticus (ITG-LH1, CNRZ-32 or CNRZ-303), Lb. delbrueckii subsp. lactis (ITG-LL14 or ITG-LL51) with the same strain of S. thermophilus CNRZ-1358. Cell viability was monitored over a 28 day ripening period by enumeration on selective media. Degree of lysis was determined by the measurement of the intracellular marker lactate dehydrogenase (LDH) activity, and also by immunodetection of intracellular proteins with species specific antibodies. In UF cheeses, lysis of autolytic strains of L. lactis was significantly delayed, showing release of intracellular components after 21 days of ripening. No lysis was observed for non-autolytic L. lactis strains or for S. thermophilus. Lysis of thermophilic lactobacilli (Lb. helveticus, Lb. delbrueckii), was observed from the start of ripening, but the onset and the level of lysis observed was strain and species dependent.     

The effect of using an exopolysaccharide‐producing culture on the physicochemical properties of low‐fat and reduced‐fat Kasar cheeses

A mixed starter culture containing exopolysaccharide (EPS)‐producing strains of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus was combined with Lactobacillus helveticus LH301 and used in the manufacture of low‐fat and reduced‐fat Kasar cheeses. For comparison, low‐fat (C10) and reduced‐fat (C20) cheeses were made using EPS‐producing (EPS⁺) starter strain and EPS‐non‐producing (EPS^?) starter strain. The physicochemical properties of the cheeses were assessed in terms of chemical composition, texture, microstructure and microbial content over 90 days. Cheeses made with EPS‐producing culture (EPS10 and EPS20) had lower protein contents than control cheeses with 10% and 20% fat in dry basis (C10 and C20). Scanning electron microscopy images showed that using EPS‐producing culture resulted in a less compact protein matrix and sponge‐like structure in the cheese samples. In general, cheeses made using EPS‐producing culture had lower total viable counts. This could be related to the reduced survivability of EPS‐producing cells in the cheese matrix during ripening due to autolysis ability.     

Stability of autolytic lactococci during starter production and storage in commercial whey-based media

Lactococcus lactis ssp. cremoris strains with different autolytic activities were grown in milk and in a commercial whey-based medium. Fermentation under pH control substantially improved total and viable counts of strain NM33-7; highest populations were obtained on the commercial whey-based medium and the cultures had the highest content of viable cells (80%). Cultures grown in milk without pH control gave only 46% viable cells. Once fermentation was completed, cooling prevented a significant decrease in viable counts. Losses in viability of NM33-7 upon extended incubation at 30 °C or storage at 4 °C were higher when the cultures were prepared on whey-based medium than when prepared on milk without pH control. In pH-controlled fermentations, the autolytic activity of NM33-7 was higher when grown on whey-based medium than when grown on milk. A negative correlation was obtained between the autolytic activity of the Lc. lactis ssp. cremoris strains and their subsequent stability during storage.