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I make yogurt with a yogurt maker and the yogurt is nice and thick, but it is very sour at the top only and the rest is almost sweet, like a sweet fresh cheese. Why isn't it sour all over?

This is how I prepare it:

  • heat 1l (goat or sheep) milk slowly (1h) to 85° Celsius,
  • keep it at 85° Celsius for 10 minutes,
  • let it cool slowly to 45° Celsius,
  • mix well with 50g simple yogurt,
  • put it in the yogurt maker at 45° Celsius for 12h.
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    is it separating? maybe try mixing it? – SnakeDoc Jun 1 '18 at 21:43
  • Some whey is separating, but the yogurt is pretty thick. Some water is evaporating while heating and cooling the milk. Mixing it would certainly distribute the sour part, but I want to understand why this is happening. – aleb Jun 2 '18 at 7:17
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    Sounds like you're getting aerobic fermentation (lactate) at the top, and anaerobic fermentation at the bottom (ethanol). Should not happen. How deep are your yogurt pots? Maybe your "mix well" isn't getting enough oxygen to the bottom? – Wayfaring Stranger Jun 2 '18 at 23:15
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    I did not pay attention at "oxygen". According to en.wikipedia.org/wiki/Lactic_acid_fermentation#Yogurt "The main method of producing yogurt is through the lactic acid fermentation of milk", which "is an anaerobic fermentation". So IIUC the less oxygen the better. – aleb Jun 3 '18 at 22:03
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    Does this happen with all types of yogurt you have tried, or only this type? Do you trust the yogurt maker to be of good quality (e.g. not the cheapest brand on the market)? Do you have a way to measure the temperature in your yogurt maker at different positions, to see if it is maybe just blasting heat on the top? Have you tried using something else to keep the yogurt warm during fermentation? – rumtscho Jun 6 '18 at 11:04
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There could be a few things that are going on here.

My best guess from what you have said would be the temperature or mixing is a little off.

Let the mixture cool a little more before combining with the yoghurt. Also, do this gradually, adding a small part of the cooled milk to the yoghurt each time, mixing until smooth.

Hopefully, this helps.

  • You are right, the temperature was not good. I retried with 42° Celsius instead of 45° Celsius and now it's sour all over, not only at the top. The heating part of the yogurt maker is at the bottom, so the yogurt at the very top, being cooler, got sour, while the rest not so, because of the too-high temperature. – aleb Jun 20 '18 at 16:12
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Sourness depends on:

  • Temperature. The highest the temperature while fermenting, the sourer it becomes.
  • Sourness of added yogurt. If you add sour yogurt to the warm milk, the produced yogurt will be sour too.

Why isn't it sour all over?

Maybe your yogurt maker is warming the milk on the top and not much in the bottom and that might explain the difference in sourness. Try wrapping it in a thick, warm blanket (the one that you use in your bed for very cold winter nights) right after mixing the yogurt, in order to keep it evenly warm until it thickens (no machine needed).

Some more tips:

  1. The milk should be 38-43° Celcius when you add the yogurt, NOT higher.
  2. 12h might be too much. Try leaving it for about 5 hours or overnight. If it hasn't thickened till then, just leave it in the blanket until it does (that's what have worked for me so far).
  • You are right, the temperature was not good, but the other way. :) Not so sure how much the starter sourness matters. – aleb Jun 20 '18 at 16:12

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