Source: Based Cooking (community recipes)
Demi-glace is a reduction built on the principle of concentrating gelatin and umami through sustained heat. You're doing two things at once: caramelising tomato paste to deepen flavour, then collapsing stock into a glossy, rich glaze that sets firm when cold.
Dry your pot well — moisture interferes with browning. Set heat to medium and add the tomato paste. You're not cooking it gently; you're encouraging it to darken. Spread it across the bottom and let it sit for 30 seconds, then stir. You'll see it shift from bright red to rust-brown at the edges within two to three minutes. That's browning — the Maillard reaction building depth. Stop before it blackens, which tastes acrid.
Add the broth all at once and scrape the bottom hard with a wooden spoon. The paste will dissolve; you're performing a deglazing that captures the flavour compounds stuck to the pan. Stir in the soy sauce and onion powder now, before the boil. Gelatin goes in last — sprinkle it over the surface and let it bloom for one minute, then stir it through. Blooming prevents clumping; dry gelatin absorbs liquid unevenly and can seize.
Bring the pot to a rolling boil over high heat. Don't cover it. You need a vigorous surface to track evaporation. The target is one-sixth of the original volume. At 1 litre, that's about 160 millilitres — roughly the thickness of thin double cream when hot, coating a spoon with a light tremor as you lift it. This takes 45 minutes to an hour, depending on surface area and whether your stove runs hot. The stock reduces, collagen and gelatin concentrate, and the liquid becomes visibly darker.
Cool to room temperature, then pour into ice cube trays and freeze solid. Each cube is roughly 15 millilitres. Keep them in a sealed container for up to three months. To use: drop one or two into a warm sauce to thicken and gloss it, or melt gently with a splash of water to make a quick pan glaze.
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