Lactic acid & milk?

Lactic acid is not normally in milk. If it is it will cause the milk to coagulate. Lactose in milk is broken down by bacteria to give lactic acid which is sour milk. Lactic acid denatures the protein casein and causes the coagulation so the curds are formed. is pasteurized it kills the bacteria so that the milk lasts for about 10 days under refrigeration. Bacteria naturally present, Strep Lactis are the ones esponsible for milk spoiling, unless it got contaminated otherwise. Of course if you are making yogurt you want the coagulation to occur.

Lactic Acid In Milk

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.

systematic name: 2-hydroxypropanoic acid), also known as milk acid, is a chemical compound that plays a role in several biochemical processes. Lactic acid is a carboxylic acid with a chemical formula of C3H6O3. It has a hydroxyl group adjacent to the carboxyl group, making it an alpha hydroxy acid (AHA). In solution, it can lose a proton from the acidic group, producing the lactate ion CH3CH(OH)COO−.

Lactic acid is chiral and has two optical isomers. One is known as L-(+)-lactic acid or (S)-lactic acid and the other, its mirror image, is D-(-)-lactic acid or (R)-lactic acid. L-(+)-Lactic acid is the biologically important isomer.

In animals, L-lactate is constantly produced from pyruvate via the enzyme lactate dehydrogenase (LDH) in a process of fermentation during normal metabolism and exercise. It does not increase in concentration until the rate of lactate production exceeds the rate of lactate removal which is governed by a number of factors including: monocarboxylate transporters, concentration and isoform of LDH and oxidative capacity of tissues. The concentration of blood lactate is usually 1-2 mmol/L at rest, but can rise to over 20 mmol/L during intense exertion.

Lactic acid fermentation is also performed by Lactobacillus bacteria. These bacteria can operate in the mouth; the acid they produce is responsible for the tooth decay known as caries.

In medicine, lactate is one of the main components of Ringer's lactate or lactated Ringer's solution. This intravenous fluid consists of sodium, chloride, potassium, and lactate in solution with distilled water in concentration so as to be isotonic compared to human blood. It is most commonly used for fluid resuscitation after blood loss due to trauma, surgery or a burn injury.

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I think u need to run some tests on both. Such as pH, number of microbial cultures (yes there are microbes in milk! if dont belive me ask FDA) percentage of vitamin D and how much calcium is really in it, it also depends on the brand of milk.