I have 10 liters of raw cow milk and my fridge has stopped working for last 12 hours. Basically the milk has soured or spoiled, but I'm not sure. There are small culture particles in the milk and it smells a little sour. What can I use this for now? Can I still make cheese, say feta or ricotta? Would love some advice.
Actually, you might.
Depending on the strain(s) of bacteria in your milk, you might be well on the first steps for making "Handkäs", a speciality from the Frankfurt area in Germany. It uses no curdling agent like lemon juce (for fromage blanc) or rennet.
Try this recipe, (english version at the bottom): Handcheese with music
If you're not sure if your milk has soured or spoiled, then you are playing with fire. Either way, I'm sticking with the science on this one: doing anything without first pasteurizing what you have would be at the risk of acquiring a foodborne illness. To make any other recommendation on this site I believe would be irresponsible.
The fact that that quantity of milk would turn in only 12 hours without refrigeration seems a bit suspect to begin with. At the very least, you should think about it this way - would you be willing to drink the soured/spoiled milk you have as-is? If not then why would you want to eat something made from milk you wouldn't want to drink?
I understand that 10 liters is a LOT of milk, but is it worth risking your well being?
Is milk sour or did it spoil? Its a big difference! While soured milk is perfectly edible, spoiled milk can be toxic!
If it doesn't have foul (not rotten) odor, yellowish and slimy appearance (trust your nose and your eyes!), it is soured milk and you can make cheese with it. Harzer for example, is cheese made from sour milk.
Or you can make cottage cheese if you prefer. Check this link.