Chapter 11 379 involves the conformation by which two proteins fit together and when this structure is changed the protein is unable to carry out its specific function. This may involve either partial or total unraveling of the protein via changes to the hydrogen bonding which define the higher-order native structure of the protein. This process is called denaturation and it can be either partial or total: meaning that the process may not necessarily complete for a specific condition, and it can also be reversible or irreversible. Denaturation does not involve breaking of individual covalent bonds between atoms in the polypeptide backbone of the protein molecule. The denaturation process can be accompanied by aggregation, coagulation, and gelation. Aggregation is a general term referring to protein–protein interactions with formation of complexes with higher molecular weight. Coagulation is the random aggregation of already denatured protein molecules and is usually a thermally irreversible process. Finally, gelation is an orderly aggregation of proteins, which may or may not be denatured, and forms a three-dimensional network that may be thermally reversible. The thermally irreversible loss Figure 5. Yellowing of hair fibers subjected to heat for various lengths of time a-b-c indicates how many times (a) how many seconds each time (b) temperature of appliance (c) e.g. BU_40-1-200 means this particular hair strand was subjected to test conditions forty times at 1 sec/time at a constant temperature of 200°C.