Lactobacillus plantarum and Streptococcus thermophilus as starter cultures for a donkey milk fermented beverage

Donkey milk is recently gaining attention due to its nutraceutical properties. Its low casein content does not allow caseification, so the production of a fermented milk would represent an alternative way to increase donkey milk shelf life. The aim of this study was to investigate the possibility of employing selected Streptococcus thermophilus and Lactobacillus plantarum isolates for the production of a novel donkey milk fermented beverage. Lysozyme resistance and the ability to acidify donkey milk were chosen as main selection parameters. Different fermented beverages (C1–C9) were produced, each with a specific combination of isolates, and stored at refrigerated conditions for 35 days. The pH values and viability of the isolates were weekly assessed. In addition, sensory analysis was performed. Both S. thermophilus and L. plantarum showed a high degree of resistance to lysozyme with a Minimum Bactericidal Concentration > 6.4 mg/mL for 100% of S. thermophilus and 96% of L. plantarum. S. thermophilus and L. plantarum showed the ability to acidify donkey milk in 24 h at 37 °C, with an average ΔpH value of 2.91 ± 0.16 and 1.78 ± 0.66, respectively. Four L. plantarum and two S. thermophilus were chosen for the production of fermented milks. Those containing the association S. thermophilus/L. plantarum (C1–C4) reached a pH lower than 4.5 after 18 h of fermentation and showed microbial loads higher than 7.00 log cfu/mL until the end of the storage period. Moreover, comparing the microbial loads of samples containing both species and those containing S. thermophilus alone (C5), we highlighted the ability of L. plantarum to stimulate S. thermophilus replication. This boosted replication of S. thermophilus allowed to reach an appropriate pH in a time frame fitting the production schedule. This was not observed for samples containing a single species (C5–C9). Thus, L. plantarum strains seem to be good candidates in the production of a novel type of fermented milk, not only for their probiotic potential, but also for the enhancing effect on S. thermophilus growth.