Eco-friendly selenium nanoparticles (SeNPs) were prepared by biological-methods using three individual or mixed strains of lactobacilli, which were evaluated for yield, selenium content, and particle size. The prepared SeNPs were used to fortify yoghurt, and the product was evaluated for its chemical composition, rheological properties, and microbiological quality. The highest SeNPs yield was obtained using Lactobacillus casei (26.87 g/L), whereas L. acidophilus produced nanoparticles with a significantly high selenium content. The lowest particle size (131.0 nm) was found in eco-friendly SeNPs produced using mixed strains followed by those produced using L. plantarum (146.2 nm), L. acidophilus (202.8 nm), and L. casei (271.3 nm). Adding eco-friendly SeNPs had no notable effect on the chemical composition and texture of functional yoghurt. Eco-friendly SeNPs powder prepared with L. plantarum had the highest viable count, whereas the lowest was obtained using the mixed culture. No cytotoxicity was observed in a tested colon cancer cell line for all prepared eco-friendly SeNPs. Practical applications The methods used in the preparation of selenium nanoparticles using lactic acid bacteria are safe, cheap, and easy to apply. It can be applied on an industrial scale as one of the ingredients that can be used to safely support different foods. Selenium nanoparticle prepared in this way can be added in the fortification of milk, children's food, and also bakery products to meet their daily needs of the important selenium element. They can also be produced in tablets containing the daily dose of selenium and also supported by probiotic bacteria. Therefore, the method used to prepare selenium nanoparticle in this study is one of the most promising ways to be applied in different food products.
