Fresystem, an Italian company leader in the deep-frozen pastry and croissants, studies its yeast, guarded for over 40 years in the " Mother House Cupiello." The study has been assigned and edited by Danilo Ercolini, Professor of Microbiology - Agriculture Department of the University of Naples Federico II. The aim of this study was to investigate the microbial ecology of the sourdough Cupiello, trace an "identity card" and learn more about the scientific reasons of the benefits and the organoleptic characteristics always referred to the use of natural yeast. The study was conducted using metagenomic analysis, that is to say advanced approaches based on the use of modern genomic techniques for the study of microbial communities directly in their natural environment, thus avoiding the problem of the cultivation in laboratory. The yeast Cupiello has been therefore studied in its initial composition, in the next 3 refreshments and then in the final dough. Sourdough is an acid mixture composed of water and flour, in which occur fermentation phenomena due to lactic acid bacteria and yeasts: it is therefore a complex environment and the investigation showed the dominant species and dominant sub-species. The study showed that the dominant species in Cupiello’s yeast is Lactobacillus sanfranciscensis, a species identified in 1971 and frequently present in sourdough. Scientific Literature informs us that Lactobacillus sanfranciscensis is an heterofermentative bacteria, characterized by the production of flavourings and flavour precursors; that may carry antimicrobial activity against molds and performs very well in terms of technological positive impact on the structure of the product. Its predominance, in fact , makes the dough more workable and stable in freezing. From the nutritional point of view Lactobacillus sanfranciscensis may result in a greater bioavailability of nutrients and gives products a richer and complex aromatic bouquet.