The single most significant technical shift in French professional cooking over the last 50 years is the replacement of flour-thickened sauces with jus-based sauces — a revolution that began with nouvelle cuisine's rejection of the roux and has now become the universal standard in serious French kitchens. The classical system (Escoffier): five mother sauces, most thickened with roux (butter + flour) or starch, built from long-simmered stocks. The modern system: intense, concentrated jus (meat cooking juices reduced to syrup), essences (vegetable or herb infusions reduced to concentrate), and emulsions (butter or oil whisked into a warm liquid) that coat food through their natural body rather than through added starch. The key technique: the modern jus begins with roasted bones and mirepoix (as with classical stock), but is reduced much further — a classical brown stock reduces 4:1; a modern jus reduces 10:1 or even 20:1, producing a liquid of extraordinary intensity that sets to a firm gel when cold (the gelatin concentration is the thickening mechanism, not flour). This jus is then 'stretched' at service with a splash of wine, vinegar, or citrus and finished with a small amount of cold butter (monter au beurre) to give sheen and body. The result: sauces that are transparent, intensely flavored, light on the palate, and that allow the protein's own flavor to dominate rather than being masked by a flour-and-butter coating. The beurre blanc family (emulsified butter sauces thinned with acid) has similarly displaced béchamel-family sauces for fish and vegetables. Cream, where used, is reduced into the jus rather than thickened with flour first. The roux survives only in bistro cooking (béchamel for croque-monsieur, velouté for blanquette), in home cooking, and as a deliberate classical reference. In professional kitchens, it has essentially vanished — a shift as fundamental as the replacement of lard with butter in the 19th century.
Roux-based sauces replaced by jus, essences, emulsions. Modern jus: 10:1 or 20:1 reduction (vs. classical 4:1). Gelatin = thickening mechanism, not flour. Stretched at service with wine/acid, finished with butter (monter au beurre). Transparent, intense, light. Beurre blanc family displaces béchamel for fish. Cream reduced into jus, not flour-thickened. Roux survives only in bistro/home cooking.
For a restaurant-quality chicken jus: roast 2kg chicken wings at 220°C until deeply golden (45 minutes), transfer to a pot with a mirepoix, deglaze the roasting pan with white wine, add to the pot with 3L cold water, simmer 4 hours, strain, reduce to 400ml (a 7.5:1 reduction) — this should gel firmly when cold. At service: warm 50ml jus per portion, add a splash of sherry vinegar, mount with 10g cold butter, season. For a modern vegetable essence: simmer 500g mushroom trimmings in 1L water for 1 hour, strain, reduce to 100ml — a 10:1 mushroom essence that replaces cream-of-mushroom sauce with pure, intense mushroom flavor. The test of a great modern sauce: it should taste like the most intense version of itself possible, be transparent enough to see through, and coat a spoon without any sense of starchiness.
Reducing jus too fast over high heat (produces bitter, acrid flavors — reduce over medium heat for clean concentration). Not skimming during reduction (impurities concentrate along with flavor — skim constantly). Finishing with too much butter (a modern jus needs just 15-20g butter per 200ml — more and it becomes a beurre sauce). Using jus that hasn't set when cold (if your jus doesn't gel in the fridge, it lacks gelatin — add more bones or reduce further). Attempting modern jus with storebought stock (commercial stock has no gelatin — the technique requires homemade stock from bones). Abandoning the roux entirely for home cooking (béchamel and velouté remain useful for gratins and comfort food — the shift is professional, not universal).
Sauces — James Peterson; Le Guide Culinaire — Escoffier; The French Laundry Cookbook — Thomas Keller