How does the milky flavor of young Parmigiano Reggiano become nutty, spicy, or broth-like as the cheese ages?

Answer: Proteolysis breaks proteins into savory peptides and free amino acids.

Proteolysis breaks proteins into savory peptides and free amino acids. ✓ — Correct: proteolysis breaks proteins into peptides and free amino acids, and those products carry savory, complex notes. Tasting descriptions move from milk and yogurt in younger wheels to nuts, spices, and meat stock in older ones, while dairy-science work tracks the peptide changes behind that shift. That makes protein breakdown the leading explanation here.

Repeated osmotic salting during aging draws water from the curd and sharpens the taste. — Salt can make food taste stronger, so this guess gets the salt-flavor link almost right. But Parmigiano Reggiano's recipe is not built around repeated seasoning during aging; tasters instead describe a shift in texture and aroma. Saltiness may change, but it does not turn milk notes into nuts, spice, and broth.

Volatile compounds from wooden shelves migrate into the cheese and add aroma. — Storage environment can matter in many aged foods, so the wooden-shelf guess is understandable. For this cheese, tasting descriptions and peptide research point more directly to internal ripening chemistry. A surface-aroma story fits less well than protein breakdown does.