Patulin is a secondary metabolite of certain Penicillium, Aspergillus, and Byssochlamys species but, more specifically, P. expansum appears to be the fungus responsible for patulin production. It is a common contaminant of apples and the patulin quantity detected in apple products is generally taken as a measured value for the quality of the apples used for the food production. Patulin can be contained in apple juice, apples sauce, apple puree, baby food and apple cider.