2.1. Sample Preparation

Commercial whole milk powder (Nestlé, Portugal) was reconstituted at 10% (w/v) and pasteurized (90 °C, 20 min) [8], followed by inoculation with commercial lyophilized culture kefir (A-Vogel, Switzerland) in a proportion of 0.7 g for 1 L of milk, as indicated on the label. The samples were packed under aseptic conditions in polyamide-polyethylene bags (Plásticos Macar Lda., Santo Tirso, Portugal), previously sterilized with UV radiation, which were manually heat sealed to minimize the amount of air inside the bags. The inoculum was composed of both bacteria (Lactococcus lactis subsp. lactis, Lactococcus lactis subsp. cremoris, Lactococcus lactis subsp. lactis biovar diacetylactis, Leuconostoc mesenteroides subsp. cremoris, Lactobacillus acidophilus, Streptococcus thermophilus, Lactobacillus kefyr) and yeasts (Kluyveromyces marxianus var. marxianus and Saccharomyces unisporus).