Alginate microcapsule size modulates structural, thermal, and metabolic properties and gastrointestinal survival of encapsulated probiotic Lactiplantibacillus plantarum

Alginate, a natural biopolymer, served as the matrix for an advanced probiotic delivery system for Lactiplantibacillus plantarum . Using extrusion-based microencapsulation with 2% sodium alginate, capsules ranging from 100 to 1500 µm were produced via calcium chloride-induced gelation. Alginate content decreased with capsule size, while calcium binding increased (0.58 vs. 0.39 g/g in 200 vs. 400 µm capsules), enhancing thermal stability. Bacterial viability varied significantly (2.4 × 10⁸ to 0.8 × 10⁷ CFU/g), primarily due to a 30-fold reduction in internal capsule volume. Physico-chemical stresses during encapsulation increased unsaturated fatty acid synthesis in smaller capsules. Encapsulated bacteria maintained viability for 300 days at 10 °C. The encapsulated form also retained high viability (>7.7 log CFU/mL) after simulated gastric and intestinal digestion, and markedly improved survival relative to free cells under both gastric acidity (93% vs. 54%) and bile-acid exposure (90% vs. 48%), confirming the protective effect of the alginate matrix under gastrointestinal conditions. Bulk density increased from 270 to 480 kg/m³ with decreasing size, and capsule counts ranged from 18,000 to 1,356,000 per gram. The resulting freeze-dried alginate microcapsules meet probiotic dosage requirements and can be used as stable functional agents in probiotic-enriched foods.