Note for those who are unable to have firm yogurt:
1 - use whole milk.
2 - remember that the pots are large (212ml instead of 125ml classic: the Germans love the great yogurt) therefore do not hesitate to put a dollop or use 2 pots of trade for the first tour.
3 - using real yoghurt, not "dairy preparations" Type 0% yogurt, bifidus, or other false Activia yogurt.
Enjoy!