What is it that naturally thickens a stock-made sauce? I've made a lot of classical stock-made sauces and they always thicken well when cooked down. Though, the other day i made a boef bouerguignonne, but without the flour. Instead, I wanted to remove the meat once cooked, sieve the sauce, and then reduce it down to thick consistency. The thing is, not matter how much I reduced it down, it never got thick. Just more and more muddy from the small particles that had fallen of the meat (chuck). How does a Boef Bourguignoone without flower, differ from a stock? Well, it is made with wine and so on, but the key difference is it's made with meat, not bones. So what is the reason a stock has natural thickening in it? These are the arguments I've heard. 1. Some people say the gelatin from the collagen in the meat. But gelatin is not active in temperatures of say, 70-80 degrees celcius, is it? Also, there is some connective tissue in chuck, and that didn't thicken it while hot. 2. Some people say the sugars and starch in the vegetables the stock is cooked with thickens the sauce when concentrated. That would also have thickened my sauce since i had a lot of carrot's and onions to cook it with. 3. The concentration of all the ingredients will thicken the sauce. This didn't apply to my boef bourguignonne either. All i got was something with the viscosity of water, but with small beef particles creating a non-pleasent muddy look. 4. There is meat/fish - glue present in the bones/carcasses of your selected stock. This stickens the stock. I don't know. Might this be true? Wouldn't transglutaminase then be used to thicken sauces without changing flavour? Lastly, can anybody comment on how these "new" hydrocolloids, e.g. xanthan gum etc. do the trick at thickening sauces?