There are several reasons why bitterness may develop in cheese. With a few exceptions bitterness is generally associated with proteolysis. Protein breakdown is critical to texture and flavour development in many cheeses. However, the process can generate hydrophobic peptides that if present at a sufficiently high concentrations can give rise to bitterness. Because these peptides are soluble in fat bitterness tends to be more of a problem in reduced fat varieties. There has been considerable research into the causes of bitterness in Cheddar and Gouda cheeses and there is now a good understanding of 1) the mechanisms responsible for bitterness and 2) how to prevent or control bitterness. The following is an attempt at summarising and simplifying how bitterness is produced. The coagulant, rennet, hydrolyses casein to produce polypeptides. These are then degraded to peptides and amino acids by peptidases associated with the membrane proteins of the starter cultures, lactococci, used in the