Can soured raw milk be used to make yogurt?

Question

I have a large quantity of raw milk at my disposal, and am trying to find creative ways to use it.

Various web sites (here, here, and many others) have suggested that raw milk, even (perhaps even especially) after it begins to sour, can be used to make yogurt.

I've also heard the claim that sour milk is too acidic for yogurt cultures to grow.

Which is it?

Please note this question is specifically about soured raw milk, not spoiled milk, which is not edible. And please assume that the sour raw milk being used is free from harmful bacteria and pathogens and is safe to consume. I'm not asking about food safety here, I'm asking: Is it possible to make yogurt from sour raw milk?

Asked by Flimzy

Answer

Based on your edit to your question, and with some additional thought, I'm going to answer this differently.

Soured milk differs from what you called "spoiled" milk in only one way- what wild bacteria reproduced faster: bacteria with tasty waste products or bacteria with disgusting waste products. With that in mind the major potential problems with using this milk to make yogurt are:

1. If the wild bacteria crowd out the yogurt culture
2. If the yogurt culture is not able to work because the milk is too sour.

In making homemade yogurt neither of these should be a problem.

In making all yogurt- including yogurt made from raw milk in commercial settings- the milk is heated to 190F for some time to denature albumin proteins and to partially sterilize the milk so that the yogurt cultures will have the upper hand. This means that all yogurt has been pasteurized and the fact that the milk was purchased raw is a red herring. This will make problem #1 above unlikely. Not heating the milk will leave you with @rumtscho's answer- you can only guess what you might get by incubating it.

As for problem #2. Milk proteins begin to coagulate at a Ph of about 4.6. The target Ph for yogurt is usually 4.5. When making yogurt, Streptococcus thermophilus brings the milk down to about 5.0 and Lactobacillus Acidophilus takes over and brings it down to 4.5. Lactobacillus Acidophilus is active well below this Ph. Therefore, if your milk is not already thickened then it is, by definition, not too acidic for the yogurt culture to work.

Unfortunately, as the commenter from your first link indicated, heat combined with acid will cause the milk to prematurely denature. If you milk is too sour already then heating it to 190F may cause the protein to precipitate out and you would have paneer (sort of). I looked but was unable to find at what Ph milk proteins will denature at 190F.

http://www.foodsci.uoguelph.ca/dairyedu/yogurt.html
http://food.oregonstate.edu/learn/milk.html

If you can strictly verify the handling of your raw milk to have confidence that it has not been contaminated with harmful bacteria, and if the milk has only slightly soured so that it can be heated without breaking, you should be able to successfully make yogurt out of it knowing that the texture might vary from that of your starter.

Answered by Sobachatina

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