The microbiota of ripening cheese is dominated by lactic acid bacteria, which are either added as starters and adjunct cultures or originate from the production and processing environments (nonstarter or NSLAB). After curd formation and pressing, starters reach high numbers, but their viability then decreases due to lactose depletion, salt addition, and low pH and temperature. Starter autolysis releases cellular contents, including nutrients and enzymes, into the cheese matrix. During ripening, NSLAB may attain cell densities up to 8 log cfu per g after 3 to 9 mo. Depending on the species and strain, their metabolic activity may contribute to defects or inconsistency in cheese quality and to the development of typical cheese flavor. The availability of gene and genome sequences has enabled targeted detection of specific cheese microbes and their gene expression over the ripening period. Integrated systems biology is needed to combine the multiple perspectives of post-genomics technologies to elucidate the metabolic interactions among microorganisms. Future research should delve into the variation in cell physiology within the microbial populations, because spatial distribution within the cheese matrix will lead to microenvironments that could affect localized interactions of starters and NSLAB. Microbial community modeling can contribute to improving the efficiency and reduce the cost of food processes such as cheese ripening.

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The potential of Lactobacillus rhamnosus R for producing exopolysaccharide (EPS) when grown on basal minimum medium supplemented with glucose or lactose was investigated. EPS production by L. rhamnosus R is partially growth associated and about 500 mg of EPS per liter was synthesized with both sugars. ؊1 ). It was clearly shown that the amount of EPS produced declined upon prolonged fermentation. Degradation of EPS in fermentation processes was also assessed by measuring its molecular weights and viscosities. As these reductions might have a negative effect on the yield and viscosifying properties of EPS, it was essential to examine possible causes related to this breakdown. The decrease in viscosities and molecular weights of EPS withdrawn at different cultivation times permitted us to suspect the presence of a depolymerizing enzyme in the fermentation medium. Our study on enzymatic production profiles showed a large spectrum of glycohydrolases (␣-D-glucosidase, ␤-D-glucosidase, ␣-D-galactosidase, ␤-D-galactosidase, ␤-D-glucuronidase, and some traces of ␣-L-rhamnosidase). These enzymes were localized, two of them (␣-D-glucosidase and ␤-D-glucuronidase) were partially purified and characterized. When incubated with EPS, these enzymes were capable of lowering the viscosity of the polymer as well as liberating some reducing sugars. Upon prolonged incubation (27 h), the loss of viscosity was increased up to 33%.Exopolysaccharides (EPS) produced by lactic acid bacteria (LAB) have generated increasing attention among researchers for the last few years. LAB are food-grade organisms, and the EPS that they produce contribute to the specific rheology and texture of fermented milk products. These EPS represent safe additives for novel food formulations and may have applications in nonfood products (8).There exist three important groups of EPSs produced by LAB: (i) ␣-glucans, mainly composed of ␣-1,6-and ␣-1,3-linked glucose residues, namely, dextrans produced by Leuconostoc mesenteroides subsp. mesenteroides and L. mesenteroides subsp. dextranicum and mutans produced by Streptococcus mutans and S. sobrinus; (ii) fructans, mainly composed of ␤-2,6-linked fructose molecules, such as levan produced by S. salivarius; (iii) heteropolysaccharides produced by mesophilic (Lactococcus lactis subsp. lactis and L. lactis subsp. cremoris) and thermophilic (Lactobacillus delbrueckii subsp. bulgaricus, L. helveticus, and S. thermophilus) LAB (5). The EPS produced by Lactobacillus rhamnosus belong to third group (14). The sugar composition of the EPS produced by L. rhamnosus R studied in this work, as determined on the hydrolysate by high-pressure liquid chromatography (HPLC) and by gas chromatography of the alditol acetates, is the following: Rha, 4; Glc, 2; and Gal, 1 (M. R. Van Casteren, personal communication).A considerable variation can be observed in EPS quantifications. The amount of EPS reported varies from 25 to 132 mg/ liter for L. lactis subsp. cremoris (26) Many studies showed a decrease in the total EPS amount when incuba...

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The exopolysaccharide (EPS) biosynthesis gene clusters of four Lactobacillus rhamnosus strains consist of chromosomal DNA regions of 18·5 kb encoding 17 ORFs that are highly similar among the strains. However, under identical conditions, EPS production varies considerably among these strains, from 61 to 1611 mg l−1. Fifteen genes are co-transcribed starting from the first promoter upstream of wzd. Nevertheless, five transcription start sites were identified by 5′-RACE PCR analysis, and these were associated with promoter sequences upstream of wzd, rmlA, welE, wzr and wzb. Six potential glycosyltransferase genes were identified that account for the assembly of the heptasaccharide repeat unit containing an unusually high proportion of rhamnose. Four genes involved in the biosynthesis of the sugar nucleotide precursor dTDP-l-rhamnose were identified in the EPS biosynthesis locus, which is unusual for lactic acid bacteria. These four genes are expressed from their own promoter (P2), as well as co-transcribed with the upstream EPS genes, resulting in coordinated production of the rhamnose precursor with the enzymes involved in EPS biosynthesis. This is believed to be the first report demonstrating that the sequence, original organization and transcription of genes encoding EPS production are highly similar among four strains of Lb. rhamnosus, and do not vary with the amount of EPS produced.

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.