Why can older Parmigiano Reggiano turn crumblier and grainier instead of simply becoming a harder block?

Answer: Proteolysis breaks the protein network and makes the paste brittle and grainy

Fat migration coats the protein network and makes the paste loose and crumbly — Not quite: fat does affect mouthfeel, so this answer points at something real. But the age-related crumbliness is not mainly a richer-fat effect. As Parmesan ages, its protein network is broken into smaller fragments, and tasting notes describe the interior as crumbly and grainy. Fat migration may make the paste feel softer or looser, but it does not explain why old wheels fracture into granules instead of just hardening.

Proteolysis breaks the protein network and makes the paste brittle and grainy ✓ — Right: aging causes proteolysis, which breaks the protein network into smaller fragments. That change weakens the internal structure, so the paste no longer fractures as one solid block. Tasting notes describe older Parmigiano as crumbly and grainy, matching a protein network that has been cut apart over time. The cheese can still be dense and dry, but its break pattern changes.

Salt crystallization wedges the protein network apart and makes the paste hard and grainy — Plausible, but incomplete: salt and crystals can affect how aged cheese feels on the tongue. Salt does not, however, drive the main texture change by wedging the protein network apart. The age-related crumbliness comes from protein breakdown, and tasting notes link older pieces with crumbliness and graininess. Salt may make the cheese taste firm or intense, but it does not explain the internal fracture pattern.