What Are the Dangers of Unrefrigerated Milk?

A cold glass of milk is a refreshing addition to quench your thirst. But when you get up in the morning and see the carton on the counter, it should give you pause: Is drinking milk left out overnight safe? The short answer is, unless your kitchen is refrigerator-cold at night, probably not. While most milk and milk products are pasteurized, bacteria can still grow at temperatures above 40 degrees Fahrenheit.

Pasteurization, developed by Louis Pasteur in 1862, heats liquids to reduce bacterial contamination and growth. Pasteurizing milk was first considered in 1886, and the practice started in the United States in the 1890s. Today, most pasteurized milk is heated by the high temperature short time (HTST method). This method uses hot water and metal plates to bring the milk to 161 degrees Fahrenheit for 15 seconds or more.

Ultra pasteurized (UP) milk is heated to 280 degrees Fahrenheit or higher for two seconds. It must be refrigerated to stay safe. Ultra high temperature (UHT) milk is heated in sterile vats and hermetically sealed, making it shelf-stable. However, even UHT milk needs refrigeration once the package is opened.

In addition to pasteurization, milk is also homogenized, or forced through tiny holes to break up the fat so it doesn't separate and rise to the top. All of these processes, plus the quality tests at the processing plant, are meant to reduce the number of pathogens in milk and make it safe for consumers. Raw milk is still available to consumers, but it and the products made from raw milk may contain bacteria, such as Brucella, E. coli, Listeria, Salmonella and other microorganisms.

Milk that has been left out for an hour or two, depending on the room temperature, should be viewed with caution. More than three hours or overnight, throw it out — pasteurization does reduce the number of possible pathogens, but they can revive and begin replicating once the milk warms above 40 degrees Fahrenheit.

The pathogens in milk can cause serious gastrointestinal issues in healthy individuals, but are particularly dangerous to infants and young children, immunocompromised individuals and adults over age 65. Drinking unrefrigerated milk may lead to diarrhea, stomach cramps and a fever for up to a week after infection. In a worst-case scenario, it can lead to hospitalization and death.

Milk and other products are marked with a "sell-by" date. This date is not a "use-by" deadline. When kept on the bottom shelf of the refrigerator between 38 and 40 degrees Fahrenheit, milk can be used for up to one week after the "sell-by" date or after being opened. The "once opened" deadline includes all types of pasteurized milk and remember: "When in doubt, throw it out."

If you have a power outage, you don't necessarily have to throw out the milk. Your refrigerator will stay cold for at least four hours and longer if you keep the door closed. Put a bag or block of ice in a pan to help keep your milk and other perishable items cold. A refrigerator thermometer will help you keep track of the temperature to ensure the refrigerator stays cold until the power comes back on.