Food that is preserved in jars or cans can start to spoil in about a year, maybe earlier if not stored correctly. The best want to ensure no spoiled food is consumed is to purchase canning jar labels, or at least use a marker to write basic information on the top of each jar. This information should at least include the date of processing, the contents of the jar, and the exact method and time used to process each jar.
According to the U.S. Department of Agriculture, most preserved food should be canned at 250°F (about 121°C) for at least 15 minutes to produce a lasting, sterile environment, free of the potentially deadly Clostridium botulinim bacterial spores, known as botulism. Low-acid foods like meat and vegetables may require more time, while high-acid foods like fruits may need less time and are the only foods for which a water bath canning method is acceptable. A pressure canner is advised for all other types of preserving. Those new to the process should utilize an online canning guide to determine the exact cooking and canning time suitable for the types of food being preserved.
The main information needed on canning jar labels is the date that each jar underwent the canning process. Otherwise, guesswork will be needed after several months when you wish to eat the canned food. Though the canner may know exactly when his or her foods were canned, a customer or neighbor wanting to eat the contents would not.
The date is not the only important information that should be written on canning jar labels. Since some foods look alike, a basic description also should be given for what is in each jar, as in "peaches in syrup" or "chicken chunks/salt." If other ingredients made their way into the jars, such as a pickling agent like vinegar or certain herbs, these can be written down too, to give the eventual consumer an idea of exactly what is in the jar.
Whether the jar includes raw or cooked food is also an important piece of data, since foods like meat can be either cooked or raw before the pressure canning process. Also important for canning jar labels is the time that the contents underwent its water bath or pressure canning. This is particularly pertinent for those who will be cooking the food after opening the jars.
Aside from visible damage to the jars, lids or rings, signs of spoilage are discoloration of the food and a putrid aroma, green, white or black spores growing anywhere on the surface. Even a date on the canning jar labels that is well past a year could indicate spoilage, though this is not a definite sign. Some foods can last several years if properly canned and stored in a cool and dark place.