Pre-acidification of ultrafiltered (UF) milk has been proposed as a strategy to overcome its intrinsically high buffering capacity (BC) and enhance lactic acid bacteria (LAB) fermentation. In this study, fresh cow milk (FCM), UF milk without acidification (UFM), and UF milk pre-acidified to pH 6.0 with lactic (UFL), acetic (UFA), or citric (UFC) acids were inoculated (1 % v/v) with Lactobacillus acidophilus, Lactobacillus delbrueckii subsp. Bulgaricus, Lactobacillus casei, or Streptococcus thermophilus and incubated at strain-specific optimal temperatures for 8 h. Kinetic parameters, TA%, pH, pH change rate (ΔpH), and strain viable count (SVC), were measured at 0, 2, 4, and 8 h. Multivariate analyses (principal component analysis PCA and K-means clustering) were performed to elucidate the combined effects of pre-acidification and incubation time on LAB activity. Results showed that pre-acidified treatments (UFL, UFA, UFC) began with significantly higher TA% and lower pH than UFM and FCM, and exhibited markedly greater increases in TA%, larger ΔpH, and higher SVC over 8 h. UFM maintained near-constant pH and minimal ΔpH, confirming its strong BC. PCA revealed that the PC1 explained over 94 % of variance in each kinetic parameter, with pre-acidified samples (especially at 4–8 h) and FCM diverging sharply from UFM along the acidification axis. K-means clustering consistently grouped samples by incubation time and treatment: early timepoints and UFM clustered together, while late timepoints of pre-acidified samples formed distinct high-acidification clusters. These findings demonstrate that pre-acidification of UF milk effectively reduces its BC, accelerates pH decline, and promotes LAB growth, offering a practical approach to optimize acidification kinetics of UF milk in the production of fermented dairy products, particularly hard and semi-hard cheeses.
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