Probiotic Sheep Milk: Physicochemical Properties of Fermented Milk and Viability of Bacteria Under Simulated Gastrointestinal Conditions.

BACKGROUND/OBJECTIVES: Within the spectrum of lactic acid bacteria, Lacticaseibacillus casei and Lactobacillus johnsonii are of particular technological and nutritional significance. Protein fortification of fermented dairy systems offers dual benefits: it improves product quality while enhancing probiotic resilience. Supplementary proteins supply bioavailable nitrogen and peptides that stimulate bacterial metabolism and contribute to a viscoelastic gel matrix that buffers cells against gastric acidity and bile salts. The aim of this study was to clarify the functional potential of such formulations by assessing probiotic survival under in vitro digestion simulating oral, gastric, and intestinal phases. METHODS: Sheep milk was fermented with L. casei 431 or L. johnsonii LJ in the presence of whey protein isolate (WPI), soy protein isolate (SPI), or pea protein isolate (PPI) at concentrations of 1.5% and 3.0%. Physicochemical parameters (pH, titratable acidity, color, syneresis), organoleptic properties, and microbiological counts were evaluated. The viability of L. casei and L. johnsonii was determined at each digestion stage, and probiotic survival rates were calculated. RESULTS: Samples with L. johnsonii consistently exhibited lower pH values compared to L. casei. Across both bacterial strains, the addition of 1.5% protein isolate more effectively limited syneresis than 3.0%, regardless of protein type. Samples fortified with WPI at 1.5% (JW1.5) and 3.0% (JW3.0) were rated highest by the panel, demonstrating smooth, homogeneous textures without grittiness. The greatest bacterial survival (>70%) was observed in WPI-fortified samples (JW1.5, JW3.0) and in SPI-fortified JS3. CONCLUSIONS: Protein isolates of diverse origins are suitable for the enrichment of fermented sheep milk, with 1.5% supplementation proving optimal. Such formulations maintained desirable fermentation dynamics and, in most cases, significantly improved the survival of L. casei and L. johnsonii under simulated gastrointestinal conditions, underscoring their potential in the development of functional probiotic dairy products.