If you have a bad habit of leaving milk sitting out on the counter, you could risk picking up a nasty bacterial infection. Even pasteurized milk still contains bacteria, although they normally grow slowly as long as you keep milk cold. Many factors affect the speed at which bacteria multiply, including the length of time you leave milk out, the room temperature, the age of the milk and the way you stored the milk before leaving it out.
Quicker Spoilage
Pasteurization kills fast-growing pathogens in milk but doesn't kill slow-growing pathogens or spores. When you leave milk out, bacteria multiply. Refrigerate milk below 40 degrees Fahrenheit to slow pathogen growth, the Dairy Council of California recommends. Storing milk at temperatures even 5 degrees over the recommended limit decreases the shelf life of milk by 50 percent, according to the Wisconsin Dairy Council. Discard milk that sits out above temperatures of 40 degrees for more than two hours, the United States Department of Agriculture advises.
Possible Illness
Milk can contain a number of bacteria that cause human illness, including salmonella. While pasteurization kills salmonella bacteria, poor handling processes can re-introduce the bacteria during manufacturing. Salmonella grows at temperatures between 41 and 113 Fahrenheit, the Colorado State University Extension explains. Symptoms of salmonella poisoning normally start between 12 and 26 hours after exposure and include nausea, diarrhea, vomiting, headache, fever chills and muscle aches. Death can occur, especially in small children, the elderly and people with compromised immune systems. Pasteurization also kills E. coli, listeria and staph bacteria, but poor handling can introduce these bacteria into milk.
Bad Taste
Even if you haven't left the milk out long enough for bacteria to grow, warm milk might not taste as good. Milk tastes best at a temperature of 35 to 40 degrees Fahrenheit, according to the Wisconsin Dairy Council.
Considerations
Milk will spoil more quickly when you leave it out if it already contains a large number of bacteria. Do not drink milk directly from the container; this introduces multiple bacteria from your mouth. Look for resealable caps rather than cardboard tops that require you to handle the surface of the container to open it. Pasteurization won't prevent new bacterial growth; it only kills bacteria present at the time of pasteurization. Store milk at the back of refrigerator, where opening the refrigerator door has less effect on temperatures.
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