Growing popularity of the "locavore" movement, a struggling economy and a victory garden on the South Lawn of the White House have contributed to a nationwide stampede to the vegetable patch. Farmers markets have swelled in size and number, while backyard and community gardens are popping up in urban and suburban yards. One challenge? Even well-planned harvests can yield an abundance of food that seems to come at once. Canning is one way to make the fruits (and vegetables) of your labors last beyond the growing season, but without proper handling and know-how, some food preserving projects can turn into a hotbed for bacteria and foodborne illness. Here is how to preserve the harvest safely.
The Pleasures and Perils of Home Canning
Once common in American households, canning has gradually become a relative novelty. But home canning appears to be making a robust comeback. Canning is a fun and relatively inexpensive way to preserve the bounty of the harvest, but it is not without risk. As anyone who has ever spent a steamy, late-summer weekend processing endless jars of tomato sauce knows, canning can be a time- and labor-intensive project. It's bad enough to lose the results of this considerable effort to spoilage. Even worse, improperly-processed canned goods can harbor harmful bacteria and molds that can sicken eaters.
The biggest concern is C. botulinum, a bacterium that produces a toxin so lethal that even a single taste of contaminated food can be deadly. While botulism poisoning is fairly rare (the Centers for Disease Control and Prevention states that an average of 145 cases are reported each year), 15 percent of yearly cases are foodborne.
Botulinum spores are prevalent in the environment but they only become dangerous when conditions favor their growth. The ideal environment for the botulinum bacteria is a moist, room temperature, airless, low-acid environment — precisely the conditions created in many canned goods. Not only is C. botulinum the most dangerous organism, it is also the hardest to kill — requiring processing times and temperatures well beyond what's required to eliminate other potential pathogens.
The Basics: Canning and Nutrition
There are two methods for canning foods. In the "raw pack" method, jars are filled with raw produce and covered with a hot liquid or syrup before being processed. In the "hot-pack" method, foods are cooked before being ladled into jars. Both methods require heat processing.
The raw-pack method might seem like it would yield a more nutritious product because the food is exposed to less heat, but this is not necessarily the case. In canning, heat processing preserves by killing pathogens and denaturing enzymes that would otherwise break down the foods and by creating a vacuum that protects the food from air. Because the hot-pack method is more effective in removing air, it actually preserves the nutritional quality, appearance, flavor and texture of foods better and longer than the raw-pack method. This is particularly true of foods that are processed in a boiling-water canner as opposed to a pressure canner. Home canning offers another nutritional advantage: Canned goods can be made with less salt and sugar than commercially canned products.
Safe Canning by the Book
Ensuring the safety of home canned goods is not difficult, but it does require attention to detail.
Acid versus Low-Acid Foods
With canning, it is vitally important to understand the distinction between so-called "acid" and "low-acid" foods. Botulinum spores flourish in low-acid environments, so controlling the pH of canned goods is a big deal. Foods with a pH of 4.6 or lower are considered acid foods; anything higher is a low-acid food and requires special processing.
Most fruits (including tomatoes) are acid foods; most vegetables, meats and other foods are low-acid and can be acidified with vinegar, citric acid or lemon juice. Exact adherence to recipes and ingredient specifications (such as the type of vinegar used) is essential to ensure that the proper pH is achieved for safety.
Using the Correct Processing Equipment
The acidity of the foods will also determine what type of equipment is used to process the jars. Acid foods can be safely processed in a boiling water canner because the low pH blocks the growth of botulinum spores. However, low-acid foods (if they are not acidified with vinegar or another low pH solution) must be processed in a pressure canner to ensure that C. botulinum is destroyed (repeat: a boiling water canner cannot be used when a recipe calls for a pressure canner) and exact adherence to processing times is essential to ensure that the food has been effectively sterilized before storage.
Processing at Altitude
Because water boils at lower temperatures at higher altitudes, those living at altitudes of 1,000 feet or higher need to increase processing time or pressure to effectively sterilize the product. When processing foods above 1,000 feet, stick with recipes that have altitude-specific procedures.
Storing Canned Goods
As long as the seals remain intact, canned goods will keep indefinitely, although appearance, texture and nutritional quality may deteriorate over time. For best quality, consume acid foods such as fruit, pickles and tomato sauce within 18 months. Other vegetables are best consumed within two to three years. Canned goods retain their quality longer if stored between 50°F and 70°F.
The Do's and Don'ts of Canning
To get started, and help your clients do the same, follow these guidelines to begin your own home canning.
Do start with the freshest possible produce. Although some nutrients are lost in processing, you can maximize the nutritional value of canned goods by harvesting foods at the peak of ripeness and preserving them as quickly as possible.
Don't improvise. With canning, it is crucial to follow food selection, preparation, filling and processing instructions to the letter, and only use recipes from trusted sources that have been properly developed and tested.
Do prepare your equipment as carefully as you prepare your foods. Jars and lids should be sanitized and sterilized as outlined in the procedures. Note: Running jars through the dishwasher is not sufficient to sterilize.
Do use kettles designed for canning. If using a pressure canner, understand how to lock, vent and adjust the pressure. Check gauges for accuracy and examine gaskets for cracks or nicks that can prevent a tight seal.
Do know your altitude. Proper procedures depend on your altitude. If you are not sure of your altitude, check with your local extension service or weather station.
Don't reuse canning lids. Although glass jars can be reused, only new, unused canning lids will ensure a proper seal. Unused lids should be used within five years of manufacture date.
Do store canned foods properly. Canned foods retain their quality and nutritional value best when stored between 50°F and 70°F. Try to can only as much as you can consume within one to two years, or give away extra cans as gifts.
Do inspect canned goods for signs of spoilage. Before opening a canned food, look for signs of leaks and check that the lid is firmly sealed and concave. After opening, look for visible signs of spoilage, such as mold, changes in color or unexpected odors.
Don't eat foods you're not sure about. If you have any doubts about the integrity or safety of canned foods, discard them. Be extra vigilant about safety with low-acid foods.
Do learn more. Before starting your canning project, consult the USDA's Complete Guide to Home Canning or another reliable source on safe canning procedures.