The Influence of Hazelnut Milk Fortification on Quality Attributes of Probiotic Yogurt

This study evaluated probiotic yogurt fortified with varying proportions of hazelnut milk (10%, 20%, 30%, 40%, and 50%) for its physicochemical, textural, microbiological, and bioactive properties during refrigerated storage. The incorporation of hazelnut milk and the storage duration significantly influenced the overall quality attributes of yogurt. Increasing the proportion of hazelnut milk from 0% to 50% significantly influenced pH levels (rising from 4.22 to 5.16), likely due to the reduced buffering capacity of plant-based milk. Yogurt samples produced with 50% hazelnut milk exhibited the highest dry matter (15.22%), protein content (4.11%), and total phenolic content, along with the strongest DPPH radical scavenging activity. Total phenolic content ranged from 30.12 to 155.29 mg/L, while antioxidant activity increased from 7.27% to 46.59% with higher hazelnut milk concentrations, indicating enhanced bioactive properties. Microbiological analysis revealed improved probiotic viability at higher hazelnut milk levels (30%–50%), with total lactic acid bacteria counts increasing from 7.38 to 8.58 log CFU/g during storage. Microstructural analysis showed that increased hazelnut milk content resulted in a more homogeneous and smoother protein network. The values for hardness (from 0.43 to 1.01), consistency (from 4.22 to 10.63), internal stickiness (from −0.10 to −0.46), and viscosity (from −0.03 to −0.51) of the yogurt samples changed significantly depending on the proportion of hazelnut milk. Textural parameters—including hardness (0.43–1.01), consistency (4.22–10.63), internal stickiness (−0.10 to −0.46), and viscosity index (−0.03 to −0.51)—also varied significantly with hazelnut milk addition, indicating firmer, thicker, and less adhesive yogurts with modified flow properties. Among all formulations, the 50% hazelnut milk-enriched sample demonstrated the most favorable overall quality. These findings suggest that hazelnut milk fortification can effectively improve the functional, structural, and microbiological quality of probiotic yogurt, although its high pH and lower acidity levels may influence fermentation dynamics and shelf stability.