Casu Marzu

Casu marzu (casu martzu in Sardinian, and also called casu modde, casu becciu or simply formaggio marcio) is a wheel of Sardinian sheep's cheese in which fermentation is continued by the larvae of Piophila casei, the cheese fly. The digestive enzymes of the larvae break down fats and proteins far past the point where ordinary ageing stops. The paste loses structure, turns soft and spreadable, and releases a liquid known locally as lagrima.

The larvae are translucent white, up to eight millimetres long, and alive in the cheese. Disturbed, they flex and jump, sometimes fifteen centimetres, which accounts for most of the folklore attached to the product.

Technically this is arthropod-assisted ripening, the same principle behind German Milbenkäse, matured by mites, and aged French Mimolette, whose rind is worked by mites as well. It is an old technique of pushing preservation past its limit rather than an accident.

The key point is that casu marzu is a description of a state, not a specification. There is no codified recipe, no controls, no standard, and no two wheels are alike.

The reference points are the strongest cheeses you already know, pushed further. It is intensely pungent and ammoniacal, saltier and sharper than a well-aged pecorino, with the burn at the back of the throat that very ripe blue cheeses give. The texture is soft and spreading rather than firm, and there is a persistent bitterness in the finish that a normal cheese does not have.

Sardinians who eat it describe a flavour continuous with mature Fiore Sardo, only much further along the same line: not a separate flavour, an extreme of one.

If you want to understand where the taste lives without any of the complications, a two-year Fiore Sardo DOP or a very ripe Gorgonzola piccante will put you in the same register, legally and safely.

In traditional practice the cheese is spread on pane carasau, often lightly moistened, and eaten in small quantities at the end of a meal with a robust red wine. It is never cooked, never grated and never used as an ingredient.

Those who eat it follow habits passed down locally: sealing a portion in a paper bag for a few minutes so the larvae, deprived of air, move away or become still; chilling the wheel to slow them; or separating the larvae mechanically before eating the paste.

None of these is a validated safety measure. Medical literature documents cases of intestinal pseudomyiasis: larvae surviving temporarily in the digestive tract, with gastrointestinal symptoms associated with eating foods that contain them. This is rare but real, and it is part of why the product is not admitted to sale.

This page describes a practice that exists; it is not a recommendation to try it. Sardinia's dairy tradition offers dozens of cheeses that can be bought, carried home and eaten with none of this attached, and the section above lists the ones that get closest to the same flavour.

There is no storage advice to give for a product that cannot be bought, shipped or brought through customs. In Sardinia it is made and eaten inside a local window rather than kept: the wheel is worked on where it sits, in a cool dark room, and the paste changes continuously rather than holding a state. Nothing about it carries a pack, a date or a producer contact, which is exactly the information that would make storage guidance possible. What follows concerns the Sardinian cheeses that do come with all three.

A wedge of Fiore Sardo or aged Pecorino Sardo keeps a month in the fridge, wrapped in waxed cheese paper or parchment inside a loose box, never cling film against the paste, which traps moisture and makes the surface slick. Over that month the paste dries at the cut face, hardens, throws tyrosine crystals and concentrates in flavour; the rind may darken and the oils in a smoked Fiore Sardo slowly turn flatter. Vacuum sealed wedges hold longer and carry a date. For anything you are unsure about, the shop that sold it is the place to ask.

Outside Sardinia there is nothing to recognise, because there is nothing on legal sale. No shop stocks it, no importer handles it, and any website offering to ship casu marzu is selling something else. What travellers describe finding is a wheel of sheep milk pecorino with the top cut away like a lid, the paste beneath collapsed into a soft, pale, translucent cream rather than a solid, and a liquid called lagrima weeping through it. That description is the whole of it.

What you can recognise, and buy, is the family it comes from. Fiore Sardo DOP carries a dark, smoke burnished rind, a hard amber paste and a producer stamp; Pecorino Sardo DOP maturo is paler, firmer and sharply salty; Pecorino Romano, despite the name, is largely made in Sardinia and comes stamped with a sheep head mark. For the sharp, almost volatile end of the spectrum, ask a Sardinian counter for a long aged pecorino or one rubbed with oil and vinegar. Prices run roughly $20 to $35 a pound, and the label will say Sardegna.

Nowhere, and that is the complete answer. There is no legal market for casu marzu in Italy, the EU, the UK or the US. It cannot be sold in shops, listed on menus, shipped, exported or carried across a border as a food product.

Under EU food hygiene rules (Regulations 852/2004 and 853/2004) food offered for sale must be free of parasites, and Italian food hygiene law provides administrative and criminal penalties for selling non-compliant products. Reported enforcement cases have involved fines running into tens of thousands of euros.

Any website, market stall or intermediary offering casu marzu for sale is operating outside the law, and the product on offer has no traceability of any kind: no known base cheese, no known conditions, no inspection.

What you can buy, from the same island and the same shepherds, is excellent and unrestricted: Fiore Sardo DOP at roughly 25 to 40 dollars or 20 to 32 pounds per kilo abroad, mature Pecorino Sardo DOP a little less, and pane carasau to eat them on. Italian delicatessens and online specialists in both the US and the UK carry all three.

It does not appear on any menu, because no restaurant can serve it legally. The occasions on which it is eaten are private ones (family lunches, village feasts, exchanges between shepherds) concentrated in the Barbagia, the province of Nuoro and the Ogliastra.

What is worth travelling for is the dairy culture the cheese comes out of, and that is entirely accessible. Fiore Sardo DOP, smoked over a wood fire and aged for months in the inland villages, is the cheese casu marzu starts from and a remarkable thing in its own right. Gavoi, in Nuoro province, holds an annual event dedicated to it.

The area around Ozieri, Bitti, Dorgali and Orgosolo is dense with dairies working Sardinian sheep's milk by traditional methods, and many receive visitors.

In Cagliari and Sassari, the town cheesemongers carry the full range of island pecorini from fresh to two years old. Tasting them side by side explains more about Sardinian shepherding than any single extreme product does.

Asking a Sardinian shepherd directly about casu marzu is a fast way to get a vague answer, for reasons that should by now be obvious.