Unfortunately, we have no first-hand experience of using lemon juice as a substitute for the coagulation enzyme rennet. Since in this case the milk must first be heated almost to boiling point before the acid is added, this is not a method that can be used with the Jona. ( By the way, lemon juice has a tendency to be slightly bitter).
If you are looking for an alternative to animal rennet, there is, for example, the so-called microbial rennet. It is also known as ‘vegetable rennet’ and is used in organic cheese dairies.
We wish you a successful result!