The use of alginate oligosaccharides (AOS) in fermented dairy products and tested against common spoilage organisms was investigated. In a broth model, yeast growth was measured by optical density, while hyphae propagation of mould was assessed with a live-cell imaging system. Reduction of the growth rate of Candida parapsilosis, Debaryomyces hansenii and Meyerozyma guilliermodii was observed in broth supplemented with 2% (w/v) AOS. Colony forming units (cfu) were enumerated to quantify yeast growth in fresh yoghurt prepared with AOS. Direct observation and pictures were taken to assess mould growth. The time to reach the spoilage limit (106 cfu g−1) with 2% AOS was significantly increased for the yeasts C. parapsilosis and M. guilliermodii, by 11.8% and 22.5% respectively, while no effect of AOS was observed for all mould species. AOS could potentially be used as a food preservative against some yeasts, although a relatively high concentration is required.