Temperature Tantrums

• Fluctuations in temperature – such as leaving the milk out of the refrigerator -- accelerate spoilage, causing bacteria to grow and milk proteins to break down. Milk also curdles when exposed to high heat, even if no bacteria are present and the milk is fresh. To avoid curdling your milk when making cream soups and other recipes, add your milk near the end of cooking and don’t bring it to a boil. When making scalloped potatoes with ham, the high levels of salt in the cured meat can also curdle your milk; to avoid this, bake the ham separately. Milk stays fresh for up to seven days once opened, when kept at or below 40 degrees Fahrenheit.

Good and Bad Bacteria

• To kill potentially harmful bacteria such as salmonella, E. coli and listeria, milk undergoes a process known as pasteurization. When refrigerated and pasteurized milk turns sour, it means that psychrophilic -- cold-loving -- bacteria are beginning to grow, and the milk is turning bad. This is not the same thing as the beneficial bacteria used to make yogurt and cheese. Fermented dairy products rely on live microorganisms and lactic acid bacteria to produce tangy dairy products such as yogurt and cheese. These cultured dairy products, such as cottage cheese, can look like curdled milk, but they have a longer shelf life and contain beneficial enzymes.

Going Bad and Tasting Bad

• When fresh milk curdles due to exposure to high heat or acidic ingredients, the milk hasn’t turned bad. There are no dangerous bacteria present and the milk won’t taste sour. Milk curdled this way has an undesirable texture. Lumps and clumps form in the milk due to the proteins binding together, destroying the silky smooth texture desired when cooking or making sauces with milk. Sour milk, on the other hand, does have an unpleasant odor and taste, and harmful bacteria are present. Discard sour milk for health reasons; discard curdled milk for the sake of appearance, taste and texture, unless the milk has curdled in the carton, in which case it has spoiled.

Storing Milk

• Store perishable dairy items such as milk in the coldest part of your refrigerator -- not in the door. During the summer months, milk sours faster if left at warm temperatures. Refrigerate milk as soon as possible after purchasing; for longer trips, pack in an insulated cooler or purchase bags of ice. Use ice packs to keep milk chilled for lunches, and always throw away milk that tastes bad. Milk can be frozen for up to three months; thaw by placing the milk in the refrigerator and not at room temperature.