Fermented dairy products based on ultrafiltered proteins often seem lacking in flavor particularly if the protein is washed by diafiltration. A model system based on protein collected from cottage cheese whey was used to investigate this phenomenon;The whey proteins were concentrated 10-fold by ultrafiltration through a membrane with 30,000 MW cutoff, diluted 1:1 with water and reconcentrated to a final concentration of 6-7% of the initial whey volume. This resulted in approximately 15-fold concentration of whey. The whey protein concentrate was diluted to 3.5% protein, various constituents were added, and the mixtures fermented with Leuconostoc dextranicum for 16 h at 32°C. The fermented samples were heat-coagulated in a water bath at 90°C for 10 min and cooled to room temperature. The resulting products were evaluated for flavor by a trained panel, who tasted three replicates of each sample;Retentate diluted with distilled water lacked flavor and developed a gritty texture after fermentation and coagulation. When permeate was used to dilute the whey protein before fermentation, the product had a much more intense flavor than when water was used as a diluent;Various techniques were used to isolate and identify components of whey permeate that were responsible for flavor development in the mixture. Constituents in the permeate that were important for flavor development were: ions (sodium, potassium, calcium, magnesium, phosphorus, chlorine, and citrate), dihydroxyacetone, lactose, and acetic acid. Addition of dihydroxyacetone prior to fermentation increased the production of glyoxal, methylglyoxal, and diacetyl as detected by high performance liquid chromatography.