Arguably one of the oldest forms of cheese, callu de cabrettu is made by taking the stomach of a freshly slaughtered young goat while it is still full of its mother’s milk, tying it closed, and hanging it to age until it naturally becomes cheese from the acids and rennet present in the stomach. It needs to be hung somewhere not in direct sunlight, but warm enough for the cultures to be active and with good ventilation so it will lose some moisture to evaporation.
It can range from creamy when young to firm and crumbly if it has been hanging for a while. All varieties have a distinctly gamey flavor from the goat rennet but also tend to retain a lot of the characteristics of the original milk. Since they are most often made in the spring, this means lots of green grasses and wild flowers and herbs. The older varieties also have a spicy tang similar to Spanish Valdeón.
Some producers have taken to removing the stomach contents and adding in fresh milk in order to tone down the gaminess a bit, but even with this method, the rennet, acid, and cultures are provided by what is naturally present in the stomach lining.