Spray Drying of Probiotic Foods: A Critical Review of Microbial Viability Optimisation across Strain, Formulation, Process and Storage

Abstract

Spray drying is attractive for manufacturing probiotic food ingredients because it is continuous, scalable and generally less costly than freeze drying, yet the combined thermal, dehydration, oxidative and mechanical stresses can sharply reduce cultivable cell numbers and compromise subsequent functionality. This critical narrative review evaluates strategies for preserving microbial viability across the complete spray-drying lifecycle rather than treating carrier selection or dryer temperature as isolated variables. Literature published from 1 January 1995 to 30 May 2026 was identified through biomedical and multidisciplinary scholarly indexes, DOI metadata searching and citation chaining. Evidence was appraised according to strain definition, process transparency, viability methodology, storage design, food relevance and scale. The synthesis indicates that no universally protective formulation or operating window exists. Survival is governed by interactions among strain physiology, growth phase, stress adaptation, feed composition, droplet thermal history, residual water, oxygen exposure and the physical state of the dried matrix. Low-molecular-weight sugars can stabilise membranes and proteins, whereas dairy or plant proteins and polysaccharides can form protective interfacial and glassy structures; composite matrices often outperform single carriers, but may reduce cell density, increase viscosity or impair sensory and release characteristics. Outlet temperature is a useful but incomplete proxy for cellular exposure because residence-time distribution, solids content, atomisation and evaporative cooling modify the actual trajectory. Storage stability depends more consistently on water activity, water mobility, glass-transition margin, oxygen control and package integrity than on moisture content alone. The evidence is weakened by strain heterogeneity, inconsistent calculation of survival, reliance on colony counts without sublethal-injury assessment, short ambient-storage studies and limited pilot-scale validation. A defensible optimisation strategy therefore requires multi-objective design that jointly targets viable-cell recovery, dose density, powder yield, shelf-life, gastrointestinal performance, food compatibility and manufacturability. Progress depends on harmonised reporting, mechanistic single-droplet and cellular measurements, scale-aware modelling and validation in real foods through the end of shelf-life.