*2.1. Preparation of the Inocula for Kinetics Study*

Stock cultures of the tested microorganisms (*Bifidobacterium bifidum* and *Lactobacillus casei*) were maintained in cryovials at −40 ◦C with glycerol (20% *v*/*v*) used as a cryoprotective agent. For the revival of each microorganism, one cryovial was transferred into 10 mL MRS broth (Merck, 1.10661, Darmstadt, Germany) and incubated at 37 ◦C for 24 h; for the revival of *Bifidobacterium bifidum*, 3% *v*/*v* of L-cysteine HCl sol. was added to MRS broth to achieve anaerobic environment. For growth and use in the kinetic experiments, 100 μL of the above inocula were transferred individually into 10 mL MRS broth (with 3% *v*/*v* of L-cysteine HCl sol. for *Bifidobacterium bifidum*) and incubated at 37 ◦C for 18−20 h. The final suspensions were transferred into 90 mL MRS broth with modified pH value to 4.80 (HCl sol. 1 M) or 6.50 (Na2HPO4/NaH2PO4 buffer sol. 0.1 M), representing the model system of low and high pH value respectively, and served as the inocula for viability loss experiments (microbial cells in stationary

phase and initial plate counted at approximately 109 CFU/mL). The selection of MRS growth medium instead of a milk-based medium was based on the fact that the viability of the tested microorganisms will not be affected by the possible presence of any protective agent (e.g., lipids) of the bacterial cells.
