Lactic Acid in milk.....?

Basically, the process involves titrating the milk to phenolpthalein indicator with 0.1 normal (N) of alkali solution and then expressing the result either as % of lactic acid or the number of cc of alkali required to neutralize 100cc of milk. I haven't read into it, but I hope that the following link should help you with specifics regarding the procedure....

http://www.jbc.org/cgi/reprint/35/1/147.pdf

When milk goes sour, it is from the formation of lactic acid. Lactose (a sugar naturally found in milk) will turn to lactic acid if the bacteria (which feed on the lactose) count is sufficiently high. The longer you keep milk and the higher (within reason) the temperature, the more the bacteria multiply. I don't know the answer to your percentage question, but my guess is you could smell/see milk that's gone sour with 3% or more lactic acid.

Lactic acid is used in a variety of food stuffs to act as

AN ACIDITY REGULATOR.

Although it can be fermented from lactose (milk sugar),

most commercially used lactic acid is DERIVED FROM BACTERIA

such as Bacillus acidilacti, Lactobacillus delbueckii or L.

bulgaricuswhey to ferment carbohydrates

from sources such as:

cornstarch

potatoes or

molasses.

Thus, although it is

commonly known as "milk acid", products claiming to be vegan

do sometimes feature lactic acid as an INGREDIANT.