Lemon juice has a long shelf life because it is highly acidic, but once you open a container of it, you must refrigerate it to prevent it from spoiling within hours. Unopened canned or bottled lemon juice can stay good for as long as 18 months in the pantry. Once it's opened, however, use it within about a week, unless it's made from concentrate, which lasts much longer.

Lemon juice made from concentrate that is sold unrefrigerated is safe to drink up to its expiration date. But once you open that bottle, contact with air will reduce its shelf life to about nine months. Refrigeration at just below 40 degrees Fahrenheit slows down the growth of harmful spoilage microorganisms, such as mold and bacteria, that will ruin the juice. These microorganisms thrive in temperatures between 40 and 140 F, which is why refrigeration is necessary to preserve the freshness of lemon juice. Freezing the juice in an airtight container at 0 F helps it last even longer, for up to 12 months.

Lemon juice does eventually go bad. Spoiled lemon juice smells unusually sour, more so than is normal for lemon juice, and it may appear discolored. Mold is a definite sign of spoilage. Other microorganisms, such as yeast and bacteria, can cause the lemon juice to ferment, making it taste or smell like vinegar. A sign that your juice has fermented is unusual bloating of the container from the carbon dioxide produced by the microorganisms feeding on the juice itself.

Freshly squeezed lemon juice requires refrigeration after you make it to keep it from going bad. Fresh-squeezed lemon juice lasts two to three days when refrigerated. Freeze it in an ice cube tray and it will keep for as long as four months. Defrost the cubes one at a time as needed for cooking or to make lemonade. Before squeezing the lemons, wash them thoroughly to remove any harmful bacteria that can make their way into the juice. While lemon juice is highly acidic, which discourages many types of bacteria from thriving in it, some microorganisms, like yeasts, mold and lactic acid bacteria, can still thrive in this environment.

While lemon juice is commonly added as a preservative to some canned goods, the juice itself does begin to spoil rapidly if not refrigerated within two to three hours of opening it. Commercially-prepared juices tend to last longer than freshly-squeezed ones because they contain preservatives and have been pasteurized to kill any bacteria and other microorganisms in them. This process involves briefly heating the juice to around 185 F. Unpasteurized lemon juice may spoil much more quickly than pasteurized juice, and you should avoid it in food and lemonade if you have a compromised immune system.