This demi-glace contains no flour, starch or arrowroot for thickening or butter. It is thickened through the combined effect of long simmering and the natural gelatins contained in the bones. The best bones to use are veal knuckle bones but since it's hard to find, I use whatever beef bones I can find at the best price. Bones are first roasted for best color and taste. Once made this sauce can be frozen in portions for at least 6 months. Once you made the homemade version you will understand why homemade demi-glace is the king of all sauces.