A Quick Look at Sour Cream

• Several bacteria, most notably in the Lactobacillus family, have a distinct affinity for milk. Some cause it to spoil, quickly becoming unpalatable or even dangerous. Others digest the milk's natural sugars, turning them into alcohol and eventually lactic or acetic acid. These acids give the milk or cream a distinct tang, and usually make it thicker by firming up its proteins until they form a gel. Commercial buttermilk is made this way, from low-fat milk. Yogurt is typically made from whole milk, sometimes with added proteins. Sour cream is richer than either, containing about 20 percent milk fat. Like its lower-fat counterparts, it's a long-lasting kitchen ingredient.

Refrigerator Time

• In the days before refrigeration, sour cream might only remain usable for a day or two in the cool of a cellar. With modern refrigeration, that storage life is extended dramatically. Most brands are acidic enough to remain usable for up to two weeks, though seven to 10 days is a more typical recommendation. The opened life of your sour cream is difficult to pin down, because it depends on a number of factors. If you've diligently kept it cold at all times, and it was far from its freshness date when you opened it, the sour cream will last longer.

Taking Appropriate Care

• Although sour cream isn't as perishable as most dairy products, it should be handled with close attention to temperature. Take it from the cooler immediately before checking out, and get it into your refrigerator as soon as possible once you've gotten home. In hot weather, it's helpful to transport sour cream and other perishables in a picnic cooler. Once it's open, make a practice of returning your sour cream to the fridge immediately rather than leaving it on the table. Only dip into it with scrupulously clean utensils, and don't use the inside of the lid as a spoon rest. Otherwise, you run the risk of contaminating your sour cream.

Taking its Measure

• Most brands of sour cream are labeled with a consumer-friendly freshness date, which tells you how long the product should keep its best flavor. In practice, the cream usually tastes good for up to two weeks past that date if it's unopened. Once opened, a tub of cream that was near its freshness date usually won't last as well. Try to use it within the first week, as a precaution. If the sour cream develops an unpleasant smell, an off color or visible mold, discard it immediately. Those are all signs it's no longer safe to eat.