I have no idea what it is really. I do know that is was quite good and that you end up with 4 started bags . You can freeze it but it's always pliable, doesn't freeze really hard and it's not supposed to go bad . I'm going to give a bag to one of my friends today to see what happens - she should end up with 4 bags also. It has yeast in it so I don't think it's sourdough, that doesn't have yeast does it?