Let me see what I can help you with. First I make my own yogurt SCD style. I like the sour yogurt since I was brought up on European style and the short fermentation cycle used by most commercials is too sweet for me.
I like the Casei Bifidus Acidophilus yogurt starter from http://www.lyo-san.ca/english/yogourmet.html#casei
What makes this one my favorite is the simplicity in temperatures.
- Heat the milk
- Cool to room temperature
- Add the starter
- Bring it backup (slowly) to 44°C in 12-15 hours
If you are making it strictly for pro-biotic you must stop at 12 hours and leave some lactose or somehow restrict the PH change. Once the sourness gets too high you end up loosing some strength via die off.
Now there may be a way to play with this by using Skim Milk, it would be the exact opposite of commercial yogurt where extra milk powder is added for richness and thickness but the PH actually stalls out the fermentation by killing off the bacteria before all the lactose has been digested.
Now that I thought about it I may try this next...
As for fermentation equipment, here is what I did:
- Get a small cooler
- Drill a hole through the lid, to feed the electrical cable
- Screw a square electrical box on the inside, above the hole
- Mount a 30-40w appliance light-bulb on the electrical box
- Optionally buy a thermostat from eBay and mount that set to 44°C
- Run a test with water to make sure it takes the temp up slowly (change wattage as required)
- Mine is setup to ferment 4L so if I only make two I need to add two water to keep the temperature rising slowly
Of course you can buy a bigger cooler and add a higher wattage bulb, or use your oven.