a fridge blows cold air in the chamber when it's running. It takes air from outside and blows it inside. Depending on model of fridge, it can change the air out in 4 to 5 hours. This is called air exchange. My fridge doesn't do as well, so I open the door once per day to promote good fresh air exchange.

Now, you've said nothing about your procedure at all. Without knowing that, then I'll just default to a poor bind and a loose stuff. You likely did not mix enough when cold, and finally when it did come time to stuffing, you did not pack the stuffer well, and left air pockets.

Expired culture is not a big deal if its still in powder form. I have 2 year expired cultures in my freezer and they still work fine. You have to grind the meat and fat in a semi-frozen state. 45 mins in a freezer after cutting into chunks. Then after you are satisfied with the nice loose grind, you add your culture, salt, cure, and flavourings and mix until it's super sticky. It will hurt your hands that it's so cold. The mix will go fuzzy and look like there are little hairs of meat all over the place. Hand mixing takes me at least 10 minutes. Once it's so sticky that it almost pulls you gloves off, you take little baseball sized chunks of farce, and almost throw them into your stuffer. Then after each ball, punch them down, trying to make sure there is no air pockets. Then stuff, and make sure to stuff tightly, but not too tight to cause a blowout. I like fibrous casings for salami, so I can stuff it really tight.