In some cases, however, brains outlast all other soft tissues and remain intact for hundreds or thousands of years. […] [Alexandra] Morton-Hayward's experiments suggest the brains endure through a process called molecular cross-linking, in which brain protein remnants and degraded lipids form a spongy polymer. […] The mechanism appears distinct from how some other bodily tissues are preserved after turning to adipocere, or "grave wax," which forms when body fats transform into a tallow-colored soaplike substance. "Adipocere forms in adipose tissue — that's buttocks, arms, cheeks," says Sonia O'Connor, an archaeologist and pioneering researcher of ancient brains at the University of Bradford in England. "There is no adipose tissue in the brain. It's the wrong chemistry."
Source: wiktionary