Yoghurt - Making it in Bulk
I use either empty Dannon yogurt containers (since I use regular Dannon plain yogurt as starter) and/or large plastic containers, those translucent plastic containers with tops, that one can buy in various sizes in the supermarket. I do put the tops on, notwithstanding Harry at Customprobiotics suggesting that one not have the tops on to minimize the collection of water on the top... I might try the latter approach. I use no water (for the incubation that is). I simply place the containers in the oven and let the oven air heat/maintain the milk temperature. One could of course incubate the milk/yogurt for more than 24 h to take into account the several hours it probably takes to heat up the room temperature milk/starter mix to the proper incubation temp.
I realize that in an oven, the temperature may vary from the top to the bottom, but I measure it with a couple different thermometers and try for around 105 F or so because I tend to use about twice the recommended amount of starter for a given quantity of milk, I figure that this will create SCD™-legal yogurt in less than 24 h, but that it may take longer than 24 h for the milk in the containers to warm up to the oven temperature from the room temp that the cooled milk and yogurt starter is mixed to... so I figure 24 hr averages out. Just my own tried and true method.
The one thing one has to be careful with using the oven temp is the fact that there is no thermostat to regulate heat, but since the oven is so big, temp won't change that fast (certainly not within the milk) either. I wouldn't do the oven method unless one was supervising a little bit, but generally you can get the temp. to stabilize pretty good and just leave it to itself for hours at a time, periodically checking on it every 8 h or so. One can adjust the oven temp using different strengths of light bulbs and cracking the oven door open various amounts or not at all. I find a 60 or 75 watt bulb with a partially propped open oven does the job for me. For some the standard 40 watt bulb or whatever that's in an oven, with a closed door, may yield a 105 F oven.
As for heating large quantities of milk at a time, I use two very large stock pots, one a little smaller than the other, the outer one filled with water i.e. I create a double boiler, which prevents the milk boiling over and also prevents the milk container from scorching (which is hard to clean), and assures even heating of the milk. You can do the same with two small pots.
Hope this helps. I have made as little as half a quart or as much as 15 quarts of yogurt at a time this way.