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The objective of this research was to determine the effect of exopolysaccharide (EPS) production by lactic acid bacteria on the microstructure and rheology of Karish cheese, a soft acid coagulated cheese made using skim milk. An EPS-producing strain of Streptococcus thermophilus, and its EPS non-producing genetic variant were used to make comparable batches of the cheese. EPS in cheese was visualized by cryo-SEM as a large, dense, filamentous mass. Cheese made with the EPS non-producing culture was characterized by a dense protein network with smaller pores compared to that prepared with the EPS-producing culture. High elastic and viscous moduli measured by dynamic rheology were observed for EPS negative cheese and was attributed to its dense protein network. Creep test experiments demonstrated that cheese prepared with the EPS non-producing strain was more rigid and recovered its deformation, while cheese made using the EPS producing culture was more deformable. These results indicate that EPS-producing cultures can improve the physical properties of Karish cheese by reducing undesirable rigidity.

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Twenty-one Lactobacillus isolates from “Sha’a” (a maize – based fermented beverage) and “Kossam” (traditionally fermented cow milk) were selected in accordance with their antagonistic activities and tested for their bacteriocinogenic potential as well as safety properties. These isolates were preliminarily identified as Lactobacillus plantarum (62%), Lactobacillus rhamnosus (24%), Lactobacillus fermentum (10%) and Lactobacillus coprophilus (4%) based on phenotypic characteristics and rep-PCR genomic fingerprinting. Twelve (57.1%) out of the 21 strains tested were found to be bacteriocin producers, as revealed by the sensitivity of their antimicrobial substances to proteolytic enzymes (Trypsin, Proteinase K) and inhibition of other Lactobacillus spp. These bacteriocinogenic strains showed no positive haemolytic and gelatinase activities and proved to be sensitive to penicillin G, ampicillin, tetracycline, erythromycin, amoxicillin, chloramphenicol, co-trimoxazole and doxycyclin, but resistant to ciprofloxacin and gentamicin. The bacteriocins showed a broad inhibitory activity against Gram-positive and Gram-negative pathogenic bacteria, several of which are classified as especially dangerous by the World Health Organization, as well as Multidrug-resistant strains. These include Staphylococcus aureus, Salmonella enterica subsp. enterica serovare Typhi, Bacillus cereus, Streptococcus mutans, Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae and Shigella flexneri. These Lactobacillus strains are promising candidates for use as protective cultures in food fermentation.

-Wild LAB (188 strains) isolated from artisanal Egyptian Ras cheese including Lactococcus (15), Lactobacillus (95), Enterococcus (77) and Pediococcus (1) were characterised for biochemical and production characteristics related to their technological performance. All Lactococcus (subsp. lactis and cremoris) were able to grow at 40 °C and in the presence of 4% NaCl. Six L. lactis subsp. lactis, one strain of subsp. cremoris and 95% of Lactobacillus strains were salt-tolerant, as they were able to grow in 6.5% NaCl. Twenty percent of lactococci, 21% of lactobacilli and 43% of enterococci showed a medium acidification rate, one strain of Lb. delbrueckii subsp. lactis and E. faecium exhibited a fast acidifying ability. Most lactobacilli revealed higher aminopeptidase and autolytic activity when compared with other strains. Twenty-seven percent of tested strains appeared to have antimicrobial activity and the highest inhibitory activity was observed among enterococci (13% of 188 strains). Exopolysaccharides (capsule) were detected in 4.3% of the lactobacilli. Most strains were able to produce a high yield of biomass after growth, showed a good biomass separation and were not affected by lyophilisation treatment. Strains which showed outstanding characteristics were screened for their flavour-forming abilities in milk cultures. Ninety-one strains, 7 Lactococcus, 52 Lactobacillus and 32 Enterococcus, were selected for an application in Ras cheese according to their production and technological criteria: salt tolerance, ability to grow at 40-45°C, acid production, proteolytic activity, flavour and bacteriocin production. These strains are currently being tested in pilot-scale cheese trials to improve Egyptian Ras cheese.

The intracellular peptide hydrolase activities of Leuconostoc species were determined using various aminopeptidase, dipeptidase, carboxypeptidase substrates, and casein. The activities were separated by disc gel electrophoresis. All strains had aminopeptidase activity as determined with amino acid ß-napthylamides and dipeptidase. All strains also showed bands or true dipeptidase activities on a large number of dipeptidase.