Naturally occurring strains of yeasts living on fruit skins, in the air, and on the surfaces of wineries, bakeries, and orchards, as opposed to pure cultured strains. They drive spontaneous fermentations in lambic beers, natural wines, traditional ciders, and some sourdoughs, creating complex, distinctive flavours that cultured yeasts cannot fully replicate. The trade-off is unpredictability: alongside desirable strains come organisms that produce off-flavours, excessive acidity, or stuck fermentations, which is why most commercial fermentations use selected pure cultures for reliability. In food safety they are mostly benign, though some species spoil sweetened and acidic products. Understanding the resident microflora of a fermentation environment is part of the craft of traditional producers.