ABSTRACT
Molds and yeasts form a very large group of microorganisms, with most coming from the air or soil. Yeasts are more demanding than molds. Most species are unable to assimilate nitrate and complex carbohydrates; some require vitamins, while others, such as Zygosaccharomyces bailii, for example, cannot use sucrose as sole source of carbon. Molds and yeasts are also quite resistant to adverse conditions, such as acidic pH and low water activity. As a general rule, yeasts predominate in liquid foods because they are unicellular microorganisms that disperse more easily in liquids. Food spoilage molds, like nearly most other filamentous fungi, require oxygen for their growth, and for that reason have been strictly aerobic. Some strains of yeasts and molds are also psychrotrophic, although they have been much less studied than bacterial psychrotrophic strains. The heat-resistant molds are commonly associated with the deterioration of fruits and heat-processed fruit-based products.
