Proteolytic Activity of Commercial Thermophilic Starter Cultures and Changes in Protein Fractions and Free Amino Acids in Organic and Conventional Fermented Milk

ABSTRACT

Starter culture (SC) composition and milk matrix characteristics shape fermentation and proteolysis‐related changes in fermented milk (FM). This study evaluated the effects of milk origin (organic (ORG) and conventional (CNV)), commercial thermophilic SCs composition, and 7‐day refrigerated storage on bacterial viability, proteolytic activity, protein fraction (PF) hydrolysis, and free amino acid (FAA) accumulation. ORG and CNV pasteurized milk collected over 1 year (twice outdoor and indoor period) was fermented with commercial starters containing Lactobacillus delbrueckii subsp. bulgaricus , Lactobacillus helveticus , and Streptococcus thermophilus . Fermentation reduced pH to 4.60–4.63 and increased proteolytic activity from 68.50–72.00 AU in pasteurized milk to 140.83–169.67 AU in FM. After storage, proteolytic activity further increased to 187.33–234.00 AU, with the highest value in CNV/TCC‐20. Nonprotein nitrogen compounds increased from 0.29 g 100 g ⁻¹ to 0.33–0.42 g 100 g ⁻¹ after storage. Total identified PFs content decreased by 8.0%–24.2% after fermentation, with the greatest reduction in CNV/YFL902, and TCC‐20 samples showed further decreases of approximately 22%–24% during storage. Total FAAs content increased by 6.7%–30.4% after fermentation, but after storage in all samples, except YFL902, decreased by 18.1%–46.8%. Overall, SC composition and storage time dominated proteolysis‐related changes, while milk origin mainly modulated selected storage responses.