I wouldn't want to use milk that sat out overnight. When I've made buttermilk in the past, I just added vinegar to regular milk. I don't remember the proportions though.

I agree that you never leave milk out over night. What is the reason you are doing that?

And if you are pouring buttermilk in, then you alreayd have buttermilk?
So, I don't get what you are doing?

I agree with Susan on making your own buttermilk and you keep it in the fridge.

Buttermilk is thick and adding stuff to it will thicken it up

1T lemon juice or vinegar to one cup milk is what I do.

Making a recipe that calls for buttermilk? Use this simple substitute, and you won't need to buy any:
Prep Time: 5 minutes
Ingredients:
Milk (just under one cup)
1 Tablespoon white vinegar or lemon juice
Preparation:
1. Place a Tablespoon of white vinegar or lemon juice in a liquid measuring cup.

2. Add enough milk to bring the liquid up to the one-cup line.

3. Let stand for five minute. Then, use as much as your recipe calls for.

You can make buttermilk by making butter. Home-made, that is. Place heavy cream (organic is best) in a bowl in your stand mixer. Turn on the beaters and wait till the cream forms. It will stick to the beaters. What liquid remains is the buttermilk.

I remember my Mom & Grandma making homemade buttermilk.I know they used the powdered milk, a glass gallon jug/jar and it was left sitting on top of the stove (close to the pilot) over night! Sure would love to have a big cold glass of it right now! Stuff you buy now couldn't hold a candle to it!

I know what she is talking about. Say you have a quart of buttemilk and you are just about to the end of the carton. Then you add the remaining buttermilk to some regular milk, and, as I remember it, let it sit out overnight and then shake it up to incorporate it all and refrigerate. I can't remember the whole procedure anymore and my instructions have disappeared in a couple moves.

To answer your question, I have no idea what would make the milk be so strange! No help with that. But I do understand how you are making the buttermilk. My mother in law used to do this all the time,and so did I.

For those who are confused by this.... you first mix up some milk using dry milk powder and water. 3 3/4 cups of water to which is added 1 1/3 cups of dry milk powder. Mix well. To this add 1/2 cup of buttermilk, which you probably got at the store. Mix well and let set out overnight to develop.

My mother in law always kept this in the fridge and used the "made" buttermilk as a starter for the next batch. Once in a while, she would use it all up and get a fresh quart from the store, then make more from it. She and her son drank it. I used it for cooking.

In order to get buttermilk from cream, you first have to let the cream clabber, or sour. My mother used to let the churn full of cream sit by the hearth overnight to clabber. Then you churn it. If you beat whipping cream too much with a mixer, you will get butter without the buttermilk.

Try googling "buttermilk from dry milk powder" and you will find several sources of information.

Was your starter covered or uncovered? If uncovered, a yeast spore of questionable origin settled in and caused the entire batch to turn bad.
It is sort of like making yogurt, I think.
Start with very clean equipment, keep at a certain temperature covered with a lid....or very clean cheesecloth.
Better luck with your next batch.

Your problem may be that the milk ingredient (from the store) had an anti-spoilage inhibitor in it that resists the growth of bacteria, or at least, the bacteria responsible for making cottage cheese. For example, when a carton of milk finally spoils in the refrigerator, it won't have clabber, as in cottage cheese, but will have sone kind of foul mold or fungus. That's because it has an anit-spoilage agent that was added by the milk supplier. Sometimes, this agent is identified on the carton.

What you need is raw milk straight from the dairy, or pastuerized milk without spoilage inhibitors. Raw milk is NOT recommended for safety reasons.

Yes I covered my starter, and 2nd jar came out the same way. This has never happen before. I threw it out again. I think I will buy it ready made again.

Sour milk is often substituted for buttermilk. Mixing vinegar or lemon juice with milk is sour milk, not buttermilk.

I too don't understand why not just buy buttermilk or even the powdered buttermilk?

Buttermilk is not always that easy to find in the stores. For instance, Aldi never carries it and I don't imagine many smaller stores do either. I am one of the few who likes to drink buttermilk, especially in the summer when it is hot. My father, who I never knew to drink buttermilk otherwise, always craved it after one of his numerous heart attacks.