How much puss can milk have in it and still be sold legally? ie what
is the maximum percentage of puss in milk that I could drink?

Thirstybadger, 

Well that’s quite the unappetizing question! But a question’s a
question and you need an answer, so I’m going to jump in with one.

Having spent some time working on a dairy farm, I’m sure by “puss” you
are talking about the udder infection that dairy cows get which is
commonly referred to “mastitis.” This infection is caused by a number
of different bacteria and is ever dairy farmer’s nightmare. Absolute
vigilance must be paid to the cleanliness and the health of the cows
in order to prevent this infection. Infected cows must be removed from
the milking schedule until the infection is cleared up.

First, what is Pus?

Pus is formed by the collection of large numbers of white cells called
polymorphonuclear cells in a localised area of the body in response to
the presence of bacterial infection. These cells break down and
release chemicals that kill the bacteria as well as cause enlargement
of the blood vessels (inflammation) and attract more white  cells to
the "fight."

So pus is just white blood cells. 

Now, as for the “pus” finding its way into the milk ---

The milk collected from the cows is stored on the farm in a bulk tank.
The dairy farmer and inspectors routinely check this milk for
contamination before it is shipped out to be pasteurized for sale. If
it does not pass the test, it does not go out!

The contamination of the milk is measured in terms of the SCC –

“The SCC amount is the Somatic Cell Counts (number of white blood
cells …Leukocytes) per milliliter of milk (SCC/ml). A dairy industry
rule of thumb states that cows measuring less than 200,000 SCC/ml of
milk are considered healthy or not significantly infected with
mastitis. Cows measuring over 200,000 SCC/ml are considered infected
cows.”

That was the PER COW level. The BULK MILK level regulation is: 

“Beginning July 1, 1993, the SCC level in milk quality regulations
must be less than 750,000 SCC to comply with the State and Federal
Pasteurized Milk Ordinance. A violation of the PMO makes the milk
non-marketable.”

http://www.ianr.unl.edu/pubs/dairy/g1151.htm

“If your bulk tank SCC is 750,000 or greater, you are in danger of
losing your milk market. Since mastitis can be caused by man, machine,
and the cows' environment, all items must be checked to determine its
cause.”

http://www.ext.nodak.edu/extpubs/ansci/dairy/coping/mastitis.htm

Now understand, this is not level of bacteria in the milk, but the
level of the cows white blood cells in the milk IN A GIVEN BULK TANK
that is being measured. This is still not what comes to you.

The next thing that happens is that all of the milk is mixed with milk
of other farms as it is picked up and transported. This means the SCC
will vary depending on the SCC levels at all the all the different
farms. But, it will never be greater than 750,000.

Finally, the milk is pasteurized

“When you pasteurize a food (almost always a liquid), what you are
doing is heating it to a high enough temperature to kill certain (but
not all) bacteria and to disable certain enzymes, and in return you
are minimizing the effects on taste as much as you can.”

http://www.howstuffworks.com/food-preservation6.htm

And after this process the milk is tested once again and must meet the
following standards: (NOTE LETTER h)

MILK REQUIREMENTS

Article 203.- Milk characteristics shall be the following:

a) Normal organoleptic characteristics;

b) Free of foreign substances;

c) Specific weight: 1.028 to 1.034 at 20ºC

d) Cryoscopic index: -0.53 to -0.57 "Horvet" or -0.512 to -0.550ºC;

e) pH: 6.6 to 6.8;

f) Acidity: 12 to 21 ml of sodium hydroxide 0.1 N/100 ml of milk;

g) Nonfat solids: 82.5 grams per liter, as a minimum;

h) Free of blood and pus;

i) Free of antiseptics, antibiotics and neutralizers. Pesticide
residues and other harmful to health substances must not exceed the
limits established by the Ministry of Health;

j) Its microbiological requirements and fat content, will be those
determined by this Regulation in each case.

http://www.usembassy.cl/agriculture/fas8_9e.htm

So, after all that, your answer is NONE. 

Milk goes through so many inspections and processes that it is free of
all contaminants by the time it gets to your table. So, unless you
drink straight from the cow, you (and nronronronro-ga) are free to
drink up!

Enjoy!

-K~

search terms:

mastitis levels milk
SCC levels milk
pus definition
pasteurization

Absolutely fabulous, Thank you very much I certainly didn't expect
such a full answer. A very reassuring answer too. Thank you I'll be
using this service again. Very good. And I learnt how to spell pus...
oops

Never would have guessed milk has puss in it.
I'll pay someone $5 to not answer this question.

;-)

nronronronro-ga,

Thanks for the laff.  Yours is possibly the best comment in the whole
Google Answer archives.

;-)

:-)

Milk contains pus prior to any processing, as stated.

It is NOT removed by any of the processes detailed above.
Pasturisation is a partial cooking process, it does not filter out
pus.

Even if the US Embassy in Chile did have a statement on a web page
stating that milk characteristics shall be "Free of blood and pus" it
is NOT.

=========================================================

Additional info.

An indicator of the quality of milk is the somatic cell count (SCC). Somatic
cells are body cells, including leucocytes (white blood cells). The number
of somatic cells increases in response to pus producing bacterium like
Staphylococcus aureus, a cause of the painful disease mastitis.

The SCC is quantified as cells per ml.

General agreement rests on the values of less than 100,000 cells/ml for
uninfected cows and greater than 300,000 for cows infected with significant
pathogens.

The average SCC during 2000 for the U.S. was 316 000.

The U.S. has the highest upper limit for SCC (750 000 cells/ml) of the
entire major developed dairy producing countries of the world. The E.U., New
Zealand, Australia, Switzerland, and Norway all accept 400 000 SCC as the
upper limit and New Zealand may consider adopting 300 000 cells/ml in the
future. Canada has now agreed on 500 000 SCC throughout all of the provinces
and is already investigating the possibility of going to 400 000 cells/ml.

Bulk tank SCC are also used as an indicator of hygienic conditions of milk
production and particularly in the E.U.. In general, the hygienic conditions
on farms producing low SCC milk are more desirable than conditions on farms
producing high SCC milk.

Increased SCC increases the risk of residues and potential pathogens and
their toxic products in the milk supply.

29.5% of the herd test days taken for the U.S. are over the 400 000 E.U.,
New Zealand, Australia, Switzerland, and Norway SCC limit for food use. 

In Florida, Louisiana, Puerto Rico, Mississippi, Alabama and Georgia over
half the herd test days are over this limit.

References and further reading:

http://www.gov.on.ca/OMAFRA/english/livestock/dairy/facts/84-012.htm

http://www.nmconline.org/articles/keynote98.htm