I forgot to answer the most important question: Why 500 ml at a time?
Answer: Because it's delicious and I eat the rest :-) Sometimes I do 1 liter mother cultures, use whatever I need for my cheese and then drain the rest to make a lactic cheese. In fact, I just did that the other day ;-) If you use homogenised, pasteurised milk, it's easy to drain and captures basically all of the fat. Drain it for 24 hours, then pack it in a small basket, salt at between 1-1.5% salt and drain it for another 3 days or so. Then into the cheese fridge for at least a week, carefully flipping every day. Either encourage wild geotrichum on it, or spray on a bit when it goes into the fridge. Once you get good at this, it takes basically no time out of your day and so you have delicious lactic cheese as often as you want. If I had access to goat's milk, I would use it every time, of course!
You can also (surprisingly) do this with UHT milk, but it's a gigantic pain in the bum. Because the UHT milk has scrambled whey proteins, the curd retains a ton of water and it takes at *least* a week to drain (really closer to 2 weeks). It can easily get hopelessly covered in mold before it's properly drained. However, I've managed to do it a few times. It doesn't really go creamy as it ages, due to the denatured proteins, but it makes a decent cheese. If you have UHT milk lying around, it's worth a try. If it goes moldy, though, cut off the mold and eat it early -- not worth the hassle of trying to save it. Usually with UHT milk, I'll drain it for 24 hours to get this thick, unbelievably creamy, yummy goop. I'll eat that (unsalted) with bread.
When I'm in cheese making mode, I'll have at least one each of a mesophilic and thermophilic mother culture in the fridge and will make up a new 500 ml every day (rotating through my collection). That and the cheese I make are all the dairy my wife and I eat (so 250 ml each per day plus whatever cheese we eat). I did practically nothing this summer, though, so I'm really looking forward to getting back to it :-)