The application of colloidal materials for the formulation of food products is essential and
unavoidable for the creation of innovative foods. In this regard, functional foods are in the scope of
research accompanied with an increasing economic interest in both developing and industrialised
countries. One emerging functional food area is the efficient delivery of health-promoting
probiotics. Although much progress has already been made in the development and understanding
of novel microencapsulation systems, maintaining viability during gastric passage and being
effective at the target site is still an issue for probiotics. On the other hand, one of the foremost
challenges in the production of physically stable foods during the defined shelf life is the
identification of new food-grade ingredients. In this context, the replacement of classical emulsifiers
with solid particles is one of the advancing food research areas, though the number of food-grade
solid particles investigated is still insufficient. Edible probiotic strains can potentially be valorised
as particles similar to micron-sized fat particles in Pickering systems such as ice cream due to their
low calories and their availability in large-scale production.
Therefore, the main objectives of this PhD thesis are an investigation of the use of colloidal
materials for the formulation of food delivery systems intended for probiotics, and the
functionalisation of beneficial living organisms as colloidal building blocks and structure
modulators in food products.
Papers I, II and IV are dedicated to the investigation of generating acid-resistant probiotic
delivery systems for food applications. The layer by layer technique was employed in development
of new multilayer coatings on the surface of individual probiotic cells (L. acidophilus La-5) (Paper
I) or of alginate microbeads (encapsulating L. rhamnosus GG) (Paper IV). The use of chitosan as a
cationic biomaterial was inevitable in formulations. Anionic sulphated β-glucan (custom-made) and
milk proteins (cationic lactoferrin and anionic polymerized polypeptides) were suggested as new
coating biomaterials. They demonstrated potential to endure low pH during gastric passage, as
shown by physical stability, and potential to release the encapsulated probiotics in the intestine, as
shown by changes in the coating structure or their dismantling at simulated experimental conditions.
Moreover, genipin crosslinked macro hydrogels of chitosan and dextran sulphate were examined,
and their properties (rheology, swelling, and morphology) were found to be in strict relation with
the blend ratio of oppositely charged biopolymers and the content of genipin in the formulation
(Paper II). However, the subcentimetre-sized beads of such chemical hydrogels did not allow the
release of bacteria at relevant conditions. Furthermore, the culturability of cells after encapsulation
was found to be not only related to components of formulation, but also to a lack of efficiency in
releasing the cells from the encapsulation system prior to plating for enumeration. Overall, the
results highlighted the challenges to meeting the main criteria for probiotics delivery, underlining
protection at acidic conditions and release at intestinal conditions.
Papers III and V explore the ability of physically and chemically modified La-5 to stabilise
Pickering systems. The physical surface activation of bacteria was done by coating La-5 using milk
proteins such as β-casein and sodium caseinate through electrostatic interactions (Paper III), and the
chemical surface hydrophobisation of bacteria was carried out using octenyl succinic anhydride
(OSA) (Paper V). The foams prepared with protein-coated bacteria in the presence of proteins in the
suspension and with OSA-modified bacteria in the absence of any other surface active molecules
were stable. The foamability and foam stability were found to have opposite behaviour, which is
related to the content of protein and bacteria in the system (Paper III). The chemically modified
bacteria also could stabilise emulsion against coalescence (Paper V). The microstructure of an
emulsion prepared with OSA-modified bacteria revealed the several arrangements of bacteria at the
oil-water interface, suggesting a complex stabilisation mechanism.
In conclusion, this PhD thesis provides new insight by improving existing formulations and
bringing new ideas into the functional food area by introducing novel suitable ingredients.