Comprehensive microbial stability assessment of freeze-dried millet-based yogurt mix as a space food for extended shelf-life

Abstract

Microbial safety and nutritional stability are critical factors for food systems designed for long-duration space missions. This study explores a proof-of-concept freeze-dried yogurt formulation incorporating finger millet (Eleusine coracana) and strawberry (Fragaria × ananassa), aimed at developing a product with extended shelf-life potential. The yogurt was fermented with Streptococcus thermophilus and Lactobacillus bulgaricus, followed by the freeze-drying process. The product demonstrated fungal stability and cytotoxic safety over 60 days of storage. Formulations were optimized using Response Surface Methodology (RSM), and ten samples were analyzed for microbial safety, cytotoxicity, and sensory acceptance. Storage stability was assessed under controlled ambient conditions (25 ± 2 °C, 60% RH) over 60 days. Microbiological tests included total bacterial count (TBC), yeast and mould count (TYMC), and pathogen detection (E. coli, Salmonella). Cytotoxicity was evaluated using MTT assay on Vero cell lines. All samples remained within acceptable microbial safety limits, with no pathogenic growth detected. For all samples, cell viability remained above 94%, and IC₅₀ values exceeded 100 μg mL⁻¹. The optimized formulation demonstrated favorable consumer acceptability and safety, making it a potential option for space missions. This study provides astronauts with a reliable and convenient food option that meets safety and nutritional requirements.