When I became sous vide enabled (controlled water bath, anyway, a proper vacuum chamber is on the to-buy list), it immediately dawned on me that a tightly temperature controlled water bath would be perfect for purposefully growing microorganisms as much as for preventing the growth while achieving perfectly done foods.
Roger has long been a fan of yogurt and yogurt is nothing more than milk fermented by a lactose consuming bacteria of, most commonly, the Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus varieties.
And, yes, it has proven incredibly easy, cost effective, and exceptionally delicious to cultivate these bacteria in the milk of your choice using a sous vide rig. Note that you really don’t need a water bath; a cooler, some hot bottles and towels can work well enough.
However, you do need the precision of a water bath to achieve consistent results of the highest quality!
1. Divide a gallon of milk amongst 5 1-quart Bell canning jars (which aren’t actually 1 quart in many cases — go figure — just divide the milk evenly between them). Lids on loosely!
We use generic organic whole milk, but low-fat works, too. It cannot be ultra-pasteurized (as ultra-pasteurization breaks down the proteins to the point that the bacteria can’t survive. The implications on the healthiness of said milk are quite bad). Raw milk works beautifully, but is expensive. I’ve been meaning to try a mix of sheep and/or goat with the cow’s milk.
2. Place in an 80℃ water bath for ~2 hours (long enough to heat the milk thoroughly to 80℃ for at least an hour).
3. Cool milk to 50℃. I find it easiest to scoop out the hot water and replace with cold water to bring the temperature down rapidly.
I usually set the target temp of the water bath to 48℃. By doing this, the residual heat from the milk will leach out into the water bath and you can tell when it is stable when it holds steady. You can inoculate and bring the temperature back to 50℃ simultaneously (48 would be a fine temperature to incubate the cultures). The key is to make sure the milk is solidly below 55℃ before inoculation, otherwise you’ll run the risk of killing off the live cultures!
4. Stir in one tablespoon of the plain flavor of your favorite brand of yogurt into each jar. Place lids back onto jar loosely.
You can use previous batches to inoculate new batches, but it is generally recommended that you refresh with new yogurt every few batches. Honestly, I’m not sure how much this matters. It’d seem that as long as you store your live culture yogurt properly, you shouldn’t have any more of an issue than one might with, say, a sour dough starter. Mostly, I end up “starting over” with store bought plain yogurt simply because we eat the previous batch so quickly and we forget to preserve enough to start the next batch!
5. Leave in the water bath for at least 12 hours and up to 24 hours (maybe even longer). The longer they are in the bath, the more greek-like sourness to the yogurt. I usually target 18 hours, maybe 22, and everyone in the family eats the stuff like crazy.
6. Tighten the lid and submerge in ice water to stop the bacteria from continuing reproduction as the temperature falls (more below). You should hear a “pop” as each jar seals itself.
7. Refrigerate. Keeps for at least three weeks.
You’ll get a bit of whey on some jars, some more than others, I typically collect the whey and use it in bread making.
Obviously, 80/50 refers to the two temperatures used. The first temperature isn’t that critical — anything over 65℃ will work.
That 50℃ number, though, is quite key. When researching yogurt recipes, I ran across the weblog of David B. Fankhauser, Ph.D, Professor of Biology and Chemistry. My recipe is based on his, but optimized for the water bath method.
Professor Frankhauser makes a very interesting observation. The yogurt making bacteria are thermophilic and, thus, prefer an elevated temperature for ideal growth. They require temperatures above 37℃ (body temperature, effectively), but won’t grow above 130℃. On the other hand, various pathogenic bacteria are also thermophilic and grow in a similar range, with their growth tapering off rapidly (but still dangerous) above ~40℃.
Thus, by incubating the yogurt in a water bath at 50℃, which is slightly higher than what most recipes call for (but most recipes assume the typical kitchen temperature control of +/- 10℃), we can inhibit the growth of the pathogenic bacteria while still maintaining healthy growth of the yogurt producing bacteria.
Not only is this safer, but it yields a far more consistent product in both flavor and texture!