By Harold McGee
Slowly simmered until it’s reduced to a tenth its original volume, stock becomes glace de viande, literally “meat ice” or “meat glass,” which cools to a stiff, clear jelly. Glace has a thick, syrupy, sticky consistency thanks to its high gelatin content, about 25%, an intensely savory taste thanks to the concentrated amino acids, and a rounded, mellow, but somewhat flat aroma thanks to the long hours during which volatile molecules have been boiled off or reacted with each other. Meat glace is used in small quantities to lend flavor and body to sauces. Intermediate between stock and glace is demi-glace or “half-glace,” which is stock simmered down to 25–40% of its original volume, often with some tomato puree or paste to add flavor and color, and with some flour or starch to supplement its lower gelatin content (10–15%). The tomato particles and flour gluten proteins cloud the stock and are removed by skimming the stock as it reduces, and then by a final straining. The starch in demi-glace, around 3–5% of its final weight, is largely an economy measure—it gives a greater thickness with less stock reduction and loss of volume to evaporation—but it also has the advantage of sparing some of the stock’s flavor from being boiled off, and avoiding the sticky consistency of very concentrated gelatin.