Prepare 2 ounces (50 grams) mirepoix and gently cook it in butter until it softens. Moisten with 5 fluid ounces (150 milliliters) Rhine wine. Reduce the mixture by half, and add 1 tablespoon (15 milliliters) chopped truffle peelings or ¼ cup (60 milliliters) truffle juice or essence. Add 1¾ cups (450 milliliters) demi-glace to the infusion and bring to a simmer. Skim off any scum that floats to the top. Strain through a fine chinois.