The yeasts Debaryomyces hansenii and Yarrowia lipolytica are known for their proteolytic and lipolytic activity, as well as their compatibility and stimulating action when co-inoculated with lactic acid starter cultures. The effect of these two yeasts and their enzymes on lipolysis and proteolysis in Cheddar cheese was investigated, and related to sensory aspects. The main difference in lipolysis and proteolysis products was observed when both yeasts were present, with the greatest effect observed from 120 to 180 days of ripening. High amounts of free fatty acids were found, especially propionic, n-butyric, myristic, palmitic, palmitoleic, stearic and oleic acids. The proteolytic process of alpha(sl)-casein and beta-casein was found to differ, resulting in a different peptide profile of water-soluble and water-insoluble peptides and free amino acids. A chromatographic peptide fraction that was previously found in 12-month-old Cheddar cheese was observed to appear in the yeast inoculated cheese from 120 days of ripening. It consists of three peptides of MW 525.88, 594.34 and 703.84, and could originate from the caseins. High amounts of free amino acids were also observed in this cheese, of which Gly and Thr could contribute to flavour, while the others would be sensorically neutral. It is concluded that D. hansenii and Y. lipolytica could be used as adjuncts in Cheddar cheese processing to enhanced flavour development or accelerate ripening. (c) 2005 Elsevier Inc. All rights reserved.