الفهرس 
Lactic acid bacteriaOnly 14 pages are availabe for public view
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Abstract
Lactic acid bacteria are generally recognized as safe microorganisms and play an important role in food and feed fermentation. Generally, they are used as starter culture for production of fermented food products, biocontrol agent for food preservation or as probiotics for improving the human health. However, viability and functionality of these bacteria could be affected during food manufacturing, storage and transition through gastrointestinal tract. Microencapsulation technology appeared to be a useful approach for enhancing the viability of lactic acid bacteria under detrimental environmental conditions that could be encountered during food processing and during transition through gastrointestinal tract.
In the present study, the survivability of Lactobacillus plantarum and Streptococcus thermophilus microencapsulated with alginate (Alg) combined with skim milk (Sm), dextrin (Dex), denatured whey protein (DWP) or coated with chitosan (Ch) was evaluated after heat treatments and in presence of some food additives such as salt and organic acids, during storage and under simulated gastrointestinal condition. Moreover, the encapsulated cells were evaluated for production of different bioactive compounds such as exopolysaccharides and antimicrobial substances.
Our findings demonstrate that the encapsulation efficiency of the four formulated capsules (Alg-Sm, Alg-Dex, Alg-Ch, and Alg-DWP) loaded with L. plantarum was ranged between 98.11% and 94.94% and that loaded with S. thermophilus was ranged between 94.39% and 92.13%. The particle size of the capsules loaded with L. plantarum in the dry state ranged between 501.54 and 800.739 μm while in the wet state it ranged between 1492 and 1635 μm. Also, the capsules loaded with S. thermophilus in the dry state ranged between 859.72 μm and 1006.23 μm while in the wet state it ranged between 1137 and 1745 μm.
The SEM images of the microcapsules loaded with L. plantarum and S. thermophilus indicate that all the produced capsules were irregular in shape with rough surface. They are also appeared as drop-like shape with small tail.
Our results showed that there is a differential behavior between L. plantarum and S. thermophilus among the different encapsulating agents toward the different stress conditions. This means that each encapsulating agent can secure protection to the encapsulated LAB in a specific stress evaluation.
Only Alg-Sm capsule excelled the other encapsulating agents in protecting L. plantarum at heat treatment reached to 65 oC for 30 min, however, all the encapsulating agents showed good protection to S. thermophilus under the same condition.
Alg-DWP was significantly the promising capsule for maintaining the survivability of L. plantarum and S. thermophilus in case of freezing storage up to 3 months. During the refrigerated storage at ≈8 ᵒC for 4 weeks, L. plantarum was capable of growing in all the encapsulating agents, however, the cells encapsulated with Alg-Ch showed the best growth. On the other hand, Alg-Sm and Alg-Dex were the most efficient capsules for maintaining the survivability of S. thermophilus during the refrigerated storage.
Alg-Sm, Alg-Ch, Alg-DWP maintain the survivability of L. plantarum in presence of NaCl up to 5% after incubation for 24 h at 37 oC, whereas Alg-DWP, Alg-Dex, Alg-Sm apply the same mission for S. thermophilus. However, only Alg-Ch and Alg-Dex that provides better survivability for L. plantarum and S. thermophilus, respectively during the refrigerated storage at ≈8 oC for one month in presence of 5% NaCl.
Tolerance of the microencapsulated cells toward organic acids was varied depending on the type of the organic acid. For instance, Alg-Dex and Alg-DWP were significantly the most appropriate encapsulating agents for maintaining the survivability of L. plantarum and S. thermophilus, respectively in presence of citric acid up to 2%. However, Alg-Sm and Alg-Ch were the most promising for protecting L. plantarum and S. thermophilus, respectively from the adverse effect of lactic acid concentration up to 2%. On the other hand, Alg-Ch excels the other encapsulating agents for enhancing the survivability of L. plantarum and S. thermophilus in presence of 2% ascorbic acid.
Alg-Dex and Alg-DWP significantly provide better protection for L. plantarum and S. thermophilus, respectively when they present in a medium containing the byproduct of other lactic acid bacteria like L. bulgaricus, L. acidophilus, L. rhamnosus or Leu. mesenteroides.
Under simulated gastric condition, Alg-Ch secured protection for the L. plantarum and S. thermophilus. However, Alg-Sm and Alg-DWP were significantly more efficient for securing L. plantarum and S. thermophilus, respectively under simulated intestinal condition.
The results revealed that microencapsulation does not affect the metabolic activity of the entrapped cells as there is no significant difference between the free and the encapsulated cells in production of exopolysaccharides or antimicrobial substances. However, S. thermophilus encapsulated with Alg-DWP produced a greater amount of exopolysaccharides than that produced by the free cells and the cells entrapped within the other encapsulating agents. Our results also show that only the cell free supernatant of L. plantarum grown in MRS broth medium for 72 h at 37 oC was effective against Escherichia coli O157: H7, Bacillus cereus and Staphylococcus aureus.
In conclusion, Alg-Sm and Alg-DWP proved to be the most promising encapsulating agents that maintain the survivability of L. plantarum and S. thermophilus, respectively under almost all the stress conditions adopted in the current study. Overall, the type of final application, the potential stress and the selected strain are the factors that will determine the appropriate type of the encapsulating agent.