Response surface design for accumulation of selenium by different lactic acid bacteria

The accumulation of selenium (Se) by Lactobacillus delbrueckii ssp. bulgaricus (Lb) and Streptococcus thermophilus (St) at the different cultivation conditions, including initial pH, inoculum dose (%), and temperature (°C), was investigated in this work. Se enrichment efficiency was optimized using the Design-Expert software for response surface methodology on a basis of single-factor experiment. The antioxidant activities of Se-enriched Lactic acid bacteria (LAB) were also investigated. The qualitative analysis of Se-enriched LAB was performed by FT-IR spectra. The cell morphology and chemical element components were measured by a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy. The results indicated that the optimum initial pH, inoculum doses, and temperatures of Lb and St were 5.96, 6.73%, 33.24 °C, and 6.37, 6%, 40 °C, respectively. Under the optimal conditions, the ratios of Se enrichment reached 94.34% for Lb and 97.05% for St. Furthermore, Se-enriched LAB enhanced scavenging rates on DPPH, ABTS free radical, and also heightened reducing activity. The FT-IR results showed that the two Se-enriched strains had similar characteristic absorption peaks, which were further demonstrated that both Se biomasses had the same carbonyl, carboxyl, and hydroxyl groups. Elemental selenium nanoparticles were verified around cell surfaces of Se-enriched LAB, which implied that both strains had detoxification ability when grown in liquid media containing selenite.