Molecular understanding of whey protein-based matrices for the preservation of saccharomyces boulardii cells in spray dried microcapsules
Creators: name: Duong, Thi
Publisher: name: RMIT University
Year: 2024

description: Work in this PhD Thesis focused on molecular understanding of whey protein based matrices for the preservation of Saccharomyces boulardii cells in spray dried microcapsules. The yeast S. boulardii is considered to be a biotherapeutic agent as it renders protection against antibiotic-induced diarrhoea, ulcerative colitis, and Crohn’s disease. It is typically administered freeze-dried, i.e. in an unprotected state, in capsules, and there is little knowledge regarding its microencapsulation, as a means to improving survival and hence viability during gastrointestinal passage. To this end, fundamental understanding of the structural behaviour and molecular interactions of gelling biopolymers is required in order to develop a capsule for efficient preservation of the encapsulated microorganism.<br><br>Whey protein isolate is a readily available and inexpensive byproduct of the cheese manufacturing industry and it was used as the main polymeric ingredient for making capsules. Screening a broad range of protein concentrations and added calcium chloride (to induce network formation) provided the basic relationship between physicochemical behaviour of the coating material and high capsulant quality, as judged from scanning electron microscopy images. A relatively low regime of inlet temperatures on the spray dryer (70 to 90°C) was used to enhance probiotic survivability after it was confirmed that the moisture content of microcapsules was according to experience (4 to 5%, w/w). The additional purpose of using this temperature regime was to evaluate spray drying conditions in relation to the technofunctionality of whey protein that denatures in these thermal conditions, as judged from micro differential scanning calorimetry and Fourier transform infrared spectroscopy. Rapid crust formation in microcapsules, due to denaturation of whey protein with increasing temperature, leads to the highest percentage of active microorganisms following spray drying with the inlet temperature of 90°C. <br><br>Further protection of the yeast was achieved by examining the integrity of the coating matrix in relation to pH of the feed solution. Whey protein at pH close to its isoelectric point exhibits early structure formation prior to extensive aggregation with increasing temperature, as judged from work on small deformation dynamic oscillation in shear. From a mechanistic perspective, it is the kinetics of incipient gelation that improves the survival and hence the viability of the yeast, as opposed to the final mechanical strength in a mature polymeric network. Rapid crust formation has the potential to shield early the core material and reduce the extent of thermal shock transmitting through the interior of the high-solid matrix. It creates an uneven surface on spray dried capsules characterised by indentations and microfractures, as indicated in microscopy images, allowing adequate vapour release that otherwise would have killed the yeast via pressure build-up. The last chapter of the Thesis introduces dietary fibre (sodium alginate and chitosan) in the whey protein matrix, and examines the effect of the composite gel on the viability of Saccharomyces boulardii cells for potential applications in spray dried food at near neutral pH.