on the gastronomical quality of foie gras

In addition to eliminating toxins, storing energy in the form of glycogen and synthesizing many of the proteins in blood, the livers of fish and birds serve an important function: storing oils to supply calories during these animals’ long migrations. This phenomenon of storing fat has a technical name, hepatic steatosis, and can be analyzed in great detail through a microscope—characteristic clear vacuoles form in the interior of the liver cells. In humans, steatosis is always pathological and has various causes, though the most common is nutrition (obesity) or toxicity (alcohol or drugs). Steatosis is always accompanied by enlargement of the liver, which takes on a yellowish colored carotene build-up and a soft, greasy appearance. In the case of duck foie gras, in which the steatosis is caused by force-feeding for 15 or 20 days, the liver grows to 6 to 10 times the size that of a non-fattened animal.
From a microscopic point of view, two types of steatosis are observed:

a)Microvesicular or microvacuolar steatosis characterized by marked accumulation inside the hepatocytes of small fatty vacuolas.

b)Macrovesicular or macrovacuolar steatosis when the hepatocytes are occupied by one large fat vacuola.

We have had the opportunity to study numerous examples of raw foie gras under the microscope, as well as ones cooked by the standard grilling method developed by Andoni Luiz Aduriz at the Mugaritz de Guipúzcoa restaurant (Basque Country, Spain). This technique involves browning the foie gras in the pan or on the grill in whole lobes until they reach 60ºC in the center; after a short rest in the oven, the assembly and serving of each specific recipe is continued.
Through the microscope, we have observed great heterogeneity in the steatosis of the studied livers, such that practically all of the industrially produced ducks showed a preponderance of macrovacuolar steatosis (figure 1). This was in contrast to the morphology of animals raised artisanally, where the microvacuolar component was much more significant, in some cases even predominating clearly (figure 2).
After cooking the livers, we observed that in the foie gras with macrovacuolar degeneration, the temperature produced a patent retraction of the proteins of the hepatic trabecula (figure 3). This caused a progressive alveolization of the tissue with rupturing of the membranes that surround the large fat vacuolas. In this way, they release their fat and leave large empty spaces or easily visible macrovacuolas, a characteristic of low-quality foie gras. The fact that the process is instigated by the protein contraction induced by the cooking heat explains why the quality of foie gras cannot be simply evaluated through a chef’s observation of fresh liver.
On the other hand, when microvacuolar steatosis predominates, the post-cooking phenomenon described is considerably reduced—although the protein retraction is similar to that described, the microvacuola membranes are more resistant to rupturing and the fat remains partially trapped in the still intact hepatocytes (figure 4)—the foie retains its internal structure and macro queen cells do not form, making the release of fat much more difficult. In some livers we have observed sizeable alternating sections of macro and microvacuolar degeneration. When these heterogeneous foies were cooked, observation with the naked eye as well as with a microscope corroborated the different behaviors of each section (figure 5). This explains why the same foie gras will yield cuts that, once cooked in the pan, behave completely heterogeneously—one may be more or less acceptable, the other poor. With respect to their origin, the existence of macro or microvacuolar steatosis in foie gras is probably related to the type of feed given to the animals; purely artisanal raising produced the best livers—that is, the livers with the most microvacuolar steatosis.