find yeast and copy it to wherever your code is then remove the bit about the temperature i.e if(parts[i].temp=293.0f) or whatever it is then change the number after the bit with (rand>()200%) or whatever and change the percent number, the higher the slower, then at the create part change YEST to GAS. This is not exact code as I don't have visual studio up at the moment, but basically that is what you do. If you don't understand I'll post the entire code tommorow afternoon.

put this:
if(rand()%n < 1)
create_part(-1, x + rand()%5 - 3, y + rand()%5 -3, PT_GAS);

replace n with any number greater than 1, the higher it is, the less GAS it forms.

As an explanation of the code above, % is modulus in C. If I read the code above correctly, making n 99 would get you a one in a hundred chans of it happening.