Blue Mould Inoculation — Penicillium roqueforti in Cheese

Roquefort, the oldest documented blue cheese, has been produced in the limestone caves of Combalou in southern France since at least the 11th century, with appellation protection formalised in 1925. The spore culture historically grew on rye bread left to moulder in those caves, a method that Kindstedt documents in Cheese & Culture as one of the earliest deliberate microbial interventions in cheesemaking.

Penicillium roqueforti is a facultative anaerobe: it needs oxygen to grow, but the cheese body you are putting it into is largely anaerobic. That tension is the whole technique. Your job is to build a curd matrix open enough to allow oxygen transit once you pierce the wheel, while keeping the paste dense enough to retain structure and moisture across a 60–120 day ripening window. You inoculate in one of two ways. Spray the spore suspension onto the curd in the vat before pressing — this distributes the culture throughout the matrix — or inject a liquid suspension directly into the pressed wheel using a sterile needle array. Most serious producers do both: vat inoculation for even baseline colonisation, needle injection at day 7–10 post-pressing to activate growth along defined oxygen channels. Spore counts matter. Commercial freeze-dried P. roqueforti cultures arrive at roughly 10⁸–10⁹ spores per gram; you dilute to the manufacturer's target and verify your suspension is uniform before application. Under-inoculation means patchy mould development and the cheese reads as structurally incoherent when cut. Over-inoculation drives rapid acidification of the paste, generating a harsh, phenolic bitterness that no amount of ageing corrects. Piercing schedule determines everything after inoculation. First piercing at day 7–14 depending on curd moisture and rind development; subsequent piercings every 7–10 days until the internal vein network reads as adequately oxygenated by probe resistance and visual inspection at the rind edge. Caves or ripening rooms need to hold 8–12 °C with 90–95% RH — too dry and the rind seals before the veins establish, too wet and Mucor and other contaminants outcompete your P. roqueforti. Kindstedt's framework in Cheese & Culture is useful here: think of the cheese as a managed ecosystem, not a static product. You are steering microbial succession, not triggering a single reaction. Every piercing event is an intervention in that succession.

The characteristic flavour of P. roqueforti blues derives from two enzymatic pathways running in parallel. Lipolysis: the mould secretes lipases that cleave short- and medium-chain fatty acids — notably caproic, caprylic, and capric acids — from milk fat. These free fatty acids are then partially converted to methyl ketones (particularly 2-heptanone and 2-nonanone) via beta-oxidation, which are responsible for the peppery, camphor-edged sharpness associated with Roquefort and Gorgonzola piccante. Proteolysis: the mould's own proteases and peptidases, working alongside residual rennet and native milk enzymes, break casein into free amino acids that generate savoury depth and the slick, coating mouthfeel of aged blue paste. McGee (On Food and Cooking, 2004) identifies this fat-to-ketone conversion as the defining chemical signature of blue cheeses, separating them from all other aged styles.