We also faced the same problem and found that when the quantity of water is more, the problem persits. The solution is very simple. Boil the milk as long as you wish. (Make sure you dont evaporate the liquid part). Depending on the consistency of the thickness of the milk, stop the heating and proceed with subsequent steps.
Also, make sure that the extracts that you add, should have low water content.
Don't get frustrated. There really aren't that many variables to keep track of.
1- I don't know the age of store bought yogurt but I have never had just-purchased yogurt not work as a starter. I have had month old yogurt from my fridge not turn out. If you don't plan on making yogurt often enough to keep you starter viable then consider freezing some in ice cube trays.
Whatever starter you use- make sure it lists "live, active cultures". Personally I wouldn't use a flavored yogurt as a starter.
2- I don't think that is enough starter. I will use 1/4 cup of starter for a quart of milk. If you are using 1 cup of milk that would be one Tbs of starter (12X the amount you used).
3- It depends. The goal is to denature the albumin proteins in the milk. 180F is hot enough if the milk is held at that temperature for at least half an hour. It needs to be held at 190F for closer to 10 minutes.
4- You need to get better control of your temperature. 130F will kill your starter but erratic temperature fluctuations will also produce very poor yogurt. Without a temperature controller or manual intervention your slow cooker will get much too hot- even with a water bath.
More successful approaches are to put it in a draft free place in an insulated container, or in your switched off oven.
5- The humidity is not going to play a role unless it is condensing into your container enough to dilute things.
Good luck. I hope you get it working. Fermenting milk is fun.
Best Answer
Quicker freezing = smaller ice crystals. One hour is a long time, that likely contributed to your grainy texture. If the Breville ice maker is your only option, make sure your base is as cold as possible before using the ice cream maker. That means refrigerating it for several hours in advance. You also might want to strain your base through a fine sieve first. This would help to ensure that the graininess is not coming from your ingredients.