Effect of Coconut Powder as a Drying Carrier Medium on Long-Term Preservation of Lactobacillus plantarum and Streptococcus thermophilus for Development of Starter Cultures With Potential Application in Traditionally Fermented Foods

Fermentation is of great importance to the food industry as it extends the shelf life of food while imparting desirable nutritional and sensory properties. However, the unpredictable fermentation period, inconsistency of the final product quality, and contamination with spoilage microorganisms of spontaneously fermented food remain a challenge. The aim of this study was to explore the storage stability of Lactobacillus plantarum and Streptococcus thermophilus prepared by freeze-drying using coconut powder (CCN) (25%) as a carrier medium in comparison to skim milk powder (SMP). Freeze-dried lactic acid bacteria (LAB) were evaluated for cell viability, vitality, moisture content, water activity (a_w), acidification kinetics, cell morphology, and glass transition temperature (Tg). In general, LAB stored at 4°C showed better survival after 112 days than LAB stored at room temperature (RT). With cell viability, the reduction in S. thermophilus and L. plantarum counts was not significantly (p < 0.05) affected by the type of carrier media (CCN or SMP) after 112 days at 4°C. SMP supported better cell vitality (approx. 2.0) at 4°C than CCN after 112 days for L. plantarum. The moisture content and a_w were within acceptable levels throughout storage on all samples. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) depicted a normal cell structure at 4°C for S. thermophilus and L. plantarum in SMP and CCN, while negative changes were observed at RT. The Tg on all samples was above 75°C, and this suggested long-term stability of the LAB strains. Overall, our findings show that CCN shows similar protection of L. plantarum and S. thermophilus strains to that of SMP during storage at 4°C. Further work is needed to confirm the fermentation performance of CCN-preserved strains.