A dairy near me was recently licensed to bottle on-farm and sell to the public. So I am able to get local, grass-fed milk from which to make yogurt. Plus, it is milked the day that I buy it. I couldn't wait to get home and make some yogurt with it. Much to my disappointment, it came out really thin, with lots of curds and whey. It was delicious, and I ate it. But it was more like drinkable yogurt.
A Cultural Thing?
I was using a frozen culture from a previous batch, and figured that it must have gone bad. So I started anew with a fresh yogurt culture and another half gallon of the milk. Same result. WTF? Well it turns out that the milk is non-homogenized. I knew this before I bought it, and I shook it up really well before making the yogurt. Silly me thought that the yogurt making processes would somehow cancel out the milk's natural tendency to separate into fat and liquid. It doesn't.
It's Purely Physical
If you are not familiar with how homogenization works, the milk is forced through a series of ever finer sieves, under higher and higher pressure, until the fat is broken down into such fine particles that it will no longer separate. Despite what you might read, there is nothing added to milk in the homogenization process. It's purely a physical process, but one that results in a permanent change. Besides keeping it from separating, it also makes the milk appear whiter, and many people feel it changes the flavor. I would add to that list that you cannot make a thick yogurt that does not separate without it.
If you are opposed to homogenization, the process of yogurt making does not change for making it with non-homogenized milk. But like non-homogenized milk, your yogurt will separate into two parts. And unlike the milk, there really is no way to shake the yogurt to get it to go back together. That only makes it thinner yet. I for one am going back to the homogenized milk I was using before. It's not local, but it is a grass-fed product from a responsible group of farmers. And I like eat my yogurt with a spoon and not a straw.