Foie gras is the liver of force (gavage) fed ducks or geese. The force feeding system makes use of the birds’ natural propensity to store extra energy as fat in their livers (as they would before migrating for the winter) to produce a liver that is so full of fat that it becomes creamy and soft.
Despite the unique flavor and texture of foie, it is still the liver. Like other offal, (in fact more so than a lot of offal) the liver is extremely perishable and will start to lose quality very quickly, which is why freezing is necessary.
As with everything else, the faster you freeze something, the smaller the ice crystals will be. With meats, produce, seafood, etc – the problem is that ice crystals poke holes in the product if they’re allowed to get too big. This is particularly a problem with foie, as the holes allow the fat to escape faster than it should during cooking, rendering out the goodness that you’re hoping to serve to your customers.
This is why commercially frozen product is inherently better than the same item frozen from fresh in a home or restaurant freezer – commercial freezers work much more quickly, producing smaller crystals and preserving quality. Commercially freezing is fine for most foods, but to preserve foie’s unique texture and succulence, some companies do something even better: flash freezing.
The ultimate freezing method used in the culinary world is flash freezing, often through the application of liquid nitrogen. Flash freezing happens so fast that the crystals are absolutely tiny. Because foie is something best purchased frozen anyway, it thus follows that flash freezing produces the best finished product.
For more information about the specifics of why flash frozen foie is the best, we recommend checking out Modernist Cuisine, Volume 3, pages 138 & 140.