Amazing how threads spawn off topics that are way out in left field from the original. Back in the mid 1950’s my Grandfather who was in his mid 80’s lived with us. Mom or Dad would always give him a bottle of whole milk when it arrived fresh and Grandpa would set it aside for three or four days on his dresser until it was curdled, sour and nasty, at which point he drank the whey and ate the curds. He considered it a delicacy. I’ve since seen reference to the fact that this was common among east European farmers.
Yes. Feel kind of odd discussing this here. I would guess that there were different bacteria involved given what went into the counter milk jar was the leftover milk from our glasses.
Kefir is fermented at room temperature by a gummy matrix of bacteria and yeast called “kefir grains”- although commercial operations use a powder instead. Yogurt is fermented at a warmer temperature by bacteria alone, innoculating the milk from a previous batch of yogurt or with powdered bacteria. Lactobacillus acidophilus would be a bacteria common to both.
Pasteurized milk has to be inoculated with the bacteria needed to produce kefir and yogurt- and the yeast in the case of kefir. These will crowd out any pathogenic bacteria that would try to feed on the milk- the good bacteria are the probiotics that you are seeking to create.
Apparently raw milk often has lactobacillus bacteria in it so it is possible to set it out and let it ferment on its own- but you may well be risking food poisoning that way versus innoculating it with a known probiotic.