By Harold McGee
The basic preparation for many refined fish mixtures is the mousseline, from the French mousse, or “foam,” a term that describes the airy, delicate consistency aimed for. Chilled raw fish is very finely chopped or pureed (with care to avoid overheating in high-speed processors), then whisked with one or more of several binding and enriching ingredients. The whisking also incorporates air, which lightens the mixture. If the fish is very fresh, then it can be enriched and tenderized with cream and bound simply with salt, which extracts some myosin protein from the muscle fibers to help them stick together. With less pristine fish—weeks in the freezer can cause premature protein aggregation and a wet, crumbly puree— egg whites help hold the particles of fish muscle together, as do various starchy materials, including bread crumbs, flour-based béchamel and velouté sauces, pastry doughs, and mashed rice or potatoes. The mousseline mixture is firmed by refrigeration, then shaped into dumpling-like quenelles, or wrapped inside thin fish fillets (paupiettes), and gently poached; or it’s put in ramekins or a pan and cooked in a water bath to make pâtés and terrines. The target temperature at the center is 140–150°F/60–65°C; higher temperatures give a harder, heavier result.