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When I make yoghurt at home the result is different every time even though I follow exactly the same procedure:

  1. Bring a gallon of milk to 85°C
  2. Cool to 43°C
  3. Add yoghurt culture from the previous time
  4. Keep it at 43°C for 2-3 hours or until ready
  5. Cool it to refrigerator temperature

Milk is always of the same kind from the same store (Costco). I use thermometer and keep the temperature with 1°C precision.

Sometimes it is extremely creamy, soft, homogeneous, and almost sweet, and sometimes a lot of whey forms on top and it becomes very sour.

Obviously I like the the former outcome rather than latter one.

I can't figure out what is wrong, why it is different every time?

I also noticed that when I start with the store-purchased culture the first batch is likely good (creamy).

  • How often do you reuse your culture? - How many batches/how long? – Catija Mar 29 '16 at 18:55
  • @Catija I make one gallon every 4-5 days. So the culture sits in refrigerator this time. – Grammar Addict Mar 29 '16 at 19:07
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The explanation is easy, the correction isn't.

Bacterial growth is an exponential process, but with all the environmental variables, you get even more trouble than a simple exponential. Tiny changes in the initial conditions can result in completely different end states. What you are seeing in the "sour" state is either too much bacterial activity for your taste, or activity from the wrong kind of bacteria - thermophilic cultures produce a more sour yogurt than mesophilic ones.

Note that the low-sourness state is not obviously the preferable one, and your recipe may well have been geared towards producing the sour state.

The way to getting to the exact result every time is to be more consistent in the initial conditions. A laboratory works with much higher precision than what you describe, not only in temperature but also in selecting the culture type and the culture density and reinoculating with pure culture.

Your best chance here would be to try using lower temperatures and see if it gets better. If you can find a temperature in which you can get the creamy style yogurt consistently, great. If not, you might need to find other ways to ensure precision, such as using a commercial culture and not reusing your own.

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    But exponentiality doesn't necessarily imply unpredictability. It's maybe that there are several strains of culture and some grow faster than others under different conditions, so some of them would outgrow others. – Grammar Addict Mar 29 '16 at 23:30
  • That's one of the explanations I had in mind, yes. The other would be that you always have one strain of bacteria "win", but under some conditions you end up with density D and some amount of lactic acid secreted by the colony, and under infinitesmally different conditions you end up with density 10D and ten times the lactic acid, because a tenth of a degree was sufficient to change the growth rate. Take a few interactions into that, and it becomes unpredictable both in the practical and theoretical sense. – rumtscho Apr 19 '16 at 18:38

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