Low-Temperature Vacuum Spray Drying of Live Biotherapeutic Products : Formulation, Process Development and Data-Driven Analysis Using Probiotic Bacteria

Sammanfattning

Low-temperature vacuum spray drying (VSD) is a potential drying approach for probiotic and live biotherapeutic product (LBP) formulations. This thesis investigated VSD for Limosilactobacillus reuteri DSM 17938 as a model probiotic strain, with the aim of identifying a feasible lab-scale operating window and using data-driven analysis to support process understanding. Sucrose–maltodextrin formulations were processed using a Pilotech YC-2000 vacuum spray dryer under varied chamber pressures, inlet temperatures, and formulation compositions. The dried powders were characterised by outlet temperature, water activity, moisture content, powder recovery, particle morphology, particle size distribution, and bacterial viability. Bacterial viability was assessed by CFU counting before drying, after drying, and after 18 days of room-temperature storage. Empirical and machine-learning-assisted models were also applied to explore relationships between process variables, powder properties, and bacterial survival. Across the bacteria-containing batches, outlet temperature ranged from 38.6 to 65.8℃, water activity from 0.145 to 0.272, and after-drying viability from approximately 3% to 31%. Higher outlet-temperature conditions generally reduced water activity but were associated with greater viability loss, whereas milder drying improved after-drying viability but did not always produce the lowest water activity. Batch 0316A, produced at 60 kPa, 80℃ inlet temperature, and a 90:10 sucrose:maltodextrin ratio, showed the most favourable overall performance, with approximately 26% after-drying viability, 81% storage retention, and 21% final survival. Data-driven analysis supported this operating-window interpretation. Outlet temperature was described by an empirical linear model, water activity was analysed using Ridge regression, and after-drying viability was explored using Random Forest modelling. The modelling results indicated that bacterial survival was strongly linked to the outlet-temperature-related drying state, but remained exploratory due to the limited dataset size. Overall, this study demonstrates that low-temperature VSD can produce L. reuteri-containing powders with measurable viability after drying and short-term storage, and identifies a promising process–formulation region for further optimisation of probiotic and LBP drying processes.