HIGH PRESSURE FOR MILK SAFETY AND SUBSQUENT YOGURT FERMENTATION

High Pressure Processing (HPP) is a nonthermal alternative to thermal pasteurization (TP) for milk. This work aimed to study the impact of HPP versus TP on milk for subsequent yogurt production. Initially, a pathogen challenge test was conducted on raw milk inoculated with Salmonella enterica and Staphylococcus aureus to compare two HPP treatments (550 MPa/10 min vs. 600 MPa/8 min) immediately and after 7 days of storage. S. enterica was inactivated below the detection limit (DL, <2.7 Log CFU/g) by both treatments, while S. aureus showed higher barotolerance, showing a greater reduction at 600 MPa/8 min (3.18 Log CFU/g) than at 550 MPa/10 min (1.59 Log CFU/g). Consequently, 600 MPa/8 min was selected for yogurt production. Regarding endogenous microbiota in non-inoculated milks, raw milk had high initial microbial counts, including psychrotrophic bacteria (5.67 Log CFU/g), yeasts and molds (4.13 Log CFU/g), Enterobacteriaceae (4.43 Log CFU/g), pathogens like S. enterica and S. aureus (~4.1–4.4 Log CFU/g) and LAB (~5.30–5.66 Log CFU/g). Both TP and HPP reduced this endogenous microbiota below DL. However, HPP outperformed TP by inactivating psychrotrophic bacteria to below DL, whereas TP showed 3.70 Log CFU/g (quantification limit, QL). TP (92 °C, 2 min), and HPP milks were then inoculated with a commercial starter culture and fermented at 37, 40, and 43 °C until pH 4.6. Higher temperatures accelerated fermentation, with HPP milk consistently fermenting faster than TP milk. At the end of fermentation, viable counts of the starter cultures Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus reached >9.00 Log CFU/g, across all conditions. Modified Gompertz modeling yielded good fits (R² >0.99), with HPP milk showing higher maximum acidification rates (Vmax) than TP milk at 37, 40, and 43 °C (0.5986, 0.6609, and 0.6560 vs. 0.5393, 0.6171, and 0.5563 pH/h), despite longer apparent lag-times (1.575–1.247 vs. 1.239–0.365 h), overall resulting in lower fermentation time (pH 4.6) for HPP milk. In conclusion, HPP at 600 MPa/8 min ensures microbial safety and significantly accelerates subsequent milk fermentation kinetics across all evaluated temperatures compared to TP, showing potential as a safe and highly efficient alternative for yogurt production.