Vol. XXII · Stocks, Soups and Sauces Plate 8 of 11reduction, reducing, concentration, glace de viande, demi-glace, gelatin, salt, syrup, balsamic reduction, nappe, volatile aromas, 收汁, 浓缩MF

Reduction.

Ten litres of veal stock become one litre of glace — and a pinch of salt in the stock becomes a spoonful in the glace.

Boiling a liquid removes water and leaves everything else behind. Reduce to half and the salt, sugars, acids, gelatin and non-volatile flavour compounds are all twice as concentrated; to a tenth, ten times. That is how cooks make intense sauces from mild liquids: a gelatin-rich stock reduced to a tenth is glace de viande, which sets like rubber when cold and melts into a glossy, coating sauce (X.8); balsamic vinegar and fruit juices reduce to syrups; 收汁 at the end of a Chinese braise turns thin liquid into a sticky glaze that coats the meat (XVI.5).

Concentration is indiscriminate. A stock that tasted right at 0.8 % salt is at 4 % when reduced to a fifth, and nobody can eat that. Delicate, volatile aromas — fresh herbs, wine's fruitiness — boil off with the water, while bitter and sour notes concentrate. And as gelatin and sugar concentrate, the liquid thickens steeply, then catches and burns on the pan's edges. So: reduce unsalted stocks, add herbs and acid at the end, watch the last minutes, and season after. The workshop reduces a stock.

Below: the salt and gelatin in a stock reduced to the share you choose, from the salt and gelatin you start with (exact concentration by mass balance), against the 0.8 % at which a sauce tastes well salted and the gelatin levels of a coating sauce and a glace.

What you will learn
Workshop
Mass balance, exact: concentration after reducing = concentration before ÷ the share remaining, for anything that does not evaporate (water only is removed). Gelatin bands (teaching): a sauce coats a spoon above about 4 %; glace sets firm above about 15 %.

Concentrate first, season last

A stock already at 0.8 % salt, reduced to a fifth, is at 4.0 % — four times too salty; a 1.5 % gelatin stock reduced to a tenth reaches 15 % gelatin, a true glace. Reduction is the most powerful flavour tool in the kitchen and the most common way to ruin a sauce with salt. Make stocks unsalted (XXII.1), buy low-sodium ones (XXII.4), and taste only at the end.

Practical reductions: reduce in a wide pan for speed (but the burner still sets the pace, XXII.7); skim as it reduces; move to a smaller pan as it gets low so it does not scorch; stop when it coats a spoon (nappe) — it thickens further as it cools. Add herbs, citrus zest, a splash of fresh wine or a few drops of vinegar at the end to bring back the aromas that boiled off. In Chinese braises, 收汁: uncover and boil hard at the end, spooning the thickening liquid over the meat until it glazes; turn the heat down as it gets sticky, because sugar and soy burn suddenly (VIII.8).

At the stove

Reduce a cup of lightly salted stock to a quarter and taste it beside the original. Then reduce a cup of unsalted stock to a quarter and season it to taste.

Professional kitchens once kept glace de viande as a solid block, shaving pieces into sauces; Liebig's nineteenth-century meat extract was an industrial glace, and the stock cube is its descendant — mostly salt and flavour enhancers, with little of the gelatin that made the original.

Terms

Reduction
Boiling a liquid to concentrate it.
Glace
Stock reduced until it sets firm when cold.
收汁
Boiling a braise's liquid hard at the end until it glazes the food.
Nappe
Just thick enough to coat a spoon.
Reduction concentrates everything that does not evaporate — salt, sugar, acid, gelatin — by 1 ÷ the share left. Reduce unsalted liquids, season last, and add delicate aromas at the end.

Sources

  1. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  2. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.