Re: Why does milk sour?

Milk is good to grow on.  When I was a child, I drank a lot of milk, and I 
grew quite large.  Bacteria also like to grow in milk, but they are 
microscopic — instead of getting larger, they divide (one cell becomes two, 
two become four, etc.), so that a few soon become millions.  To do this, 
they need energy, and they get most of it from milk sugar, "lactose."  
Lactose is quite like the sugar in your sugar bowl (sucrose), but not quite 
as sweet.  Bacteria can't taste, so they don't know the difference.

When milk bacteria use lactose as an energy source, they change it into 
lactic acid, which makes the milk taste sour.  For what it's worth, lactic 
acid is the same substance that accumulates in our muscles during intense 
"aerobic" exercise and makes them feel like they're burning.  To produce 
lactic acid, the bacteria must first split lactose into its two component, 
simple sugars, glucose and galactose.  These can diffuse into to bacterial 
cells and be used as energy sources.  The principal protein of milk — 
casein — turns into curd in this acid environment, so another way of 
describing sour milk is to say it "curdled."  Both the lactose and the 
protein are used by the bacteria to produce additional bacterial cells.  
Eventually, other kinds of bacteria (and even molds, if air is available) 
may grow in the milk and make protein derivatives that smell really bad; 
this is called "putrefaction."

Milk souring is a natural process that happens more slowly now that most of 
our milk is pasteurized: the heat treatment kills most of the bacteria that 
are in the milk beforehand, as well as all of the disease agents that might 
be present.  Milk souring is not necessarily "bad" — this is how yogurt, 
many kinds of cheese, and various other milk products begin.  However, we 
use specific, friendly bacteria for these purposes.  

Otherwise, as souring occurs, the length of time before we can see, smell, 
or taste differences is largely a matter of how cold the milk has been 
while being stored and distributed after pasteurization.  Milk keeps best 
at temperatures just above freezing.  It is easy for the dairy industry to 
control storage temperatures at the processing facility, but not so easy to 
ensure that these same temperatures are maintained on trucks and in retail 
stores.  Finally, home refrigerators are often at far higher than ideal 
temperatures, and the milk may be stored on the door of the refrigerator, 
where temperatures are higher than inside the box.  There is no health 
threat associated with souring of milk, but the processors and some 
government jurisdictions put dates on containers that represent their guess 
as to how long the milk will be acceptable.  There are meters and paper 
test kits that could be used to measure acid as it develops in milk, but 
msot of us would rather trust our sense of taste or smell.

Here in California, milk cartons have a "sell by" date that tells the last 
day that a grocery store should sell that particular container of milk.  
Again, if the milk has not been properly refrigerated, it may be "old 
before its time."  On the other hand, it is expected that the milk will 
still be fit to drink for two or three days (usually more) if it is 
properly refrigerated after purchase.  But if people get their milk first 
at the supermarket and then wander around the store with the milk in their 
cart (and not refrigerated); put the milk in their car and do other errands 
before they take it home; and then perhaps don't put the milk in a properly 
cold refrigerator as soon as they get home, the milk may be sour by the 
time they open the carton.  So everyone has some responsibility for the 
quality of the milk, but there is essentially no threat to consumers' 
health.