The purpose of canning is obviously to preserve food for later consumption. In terms of safety, the same things that cause foods to spoil (such as molds or bacteria) are also responsible for making us sick. The heat and/or pressure in the cooking process is able to destroy the largest expected number of heat-resistant microorganism in the canned foods. Then, the sealed jars keep any more from getting in.
Just cleaning food is not enough. First of all, bacteria, yeasts, and molds are difficult to remove from food surfaces. E.g washing does not have much effect. Peeling certain fruits and vegetables can help reduce the count of microorganisms, but it will not prevent spoilage. In our article on Canning and Nutrition, we discussed how sugar or salt help preserve food by reducing the water microbes have available to reproduce. However, the most important safety step by far is cooking the food hot enough and long enough to destroy any lurking microbes.
Botulism, a rare but deadly form of food poisoning, is a concern that many people have with home-canned food. Botulism is caused by rapid growth of the bacterium Clostridium botulinum. Although botulism spores are found on most fresh food surfaces, they do not normally present a danger because they only grow in the absence of air. The conditions of a can give the spores a chance to grow and produce a potent toxin.
The best way to prevent botulism or another form of food poisoning is to follow a tested recipe. Most importantly, the recipe needs to follow the current canning guidelines. Outdated cookbooks or recipes handed down by friends or relatives can be dangerous, as many do not follow up-to-date scientific guidelines. However, you can easily find the current guidelines on the National Center for Home Food Preservation’s website and make the necessary changes (the website is funded by the USDA).
Another important factor in canning is food acidity. Acidic foods, like many fruits or pickles (due to the added vinegar) aid in preventing bacteria growth. Acidity in certain foods is often increased by adding lemon juice, citric acid, or vinegar. In canning, high acid foods are considered to be foods with a PH of 4.6 or lower. These foods include fruits, pickles, sauerkraut, jams, jellies, and fruit butters. Tomatoes are often considered high acid, however select varieties are known to be less acidic. To be safe, experts recommend adding 2 tablespoons of lemon juice per quart of tomatoes canned.
High acid foods can be canned using a simple water bath technique. Low acid foods must be canned using a pressure canner, which is a slightly longer and more complex process. The pressure canner allows the food to reach a core temperature of 240 °F, which is needed for low acid foods. The water bath method is only capable of reaching 212 °F. For food safety, it is very important that you use a pressure canner with low acid foods. We go into both processes in detail in upcoming articles on Water Bath canning and Pressure canning.
Some people are wary of using the pressure canning method because they have heard horror stories from an older relative about canners exploding or spraying hot steam all over the place. In truth, poor quality canners or an improper following of instructions likely caused the majority of these accidents. Modern pressure canners are very safe, provided they are used according to the manufacturers operating instructions. We can’t emphasize this enough, read the manual and follow the instructions. It is also important to inspect the canner before each use to make sure all the parts are not damaged and working properly.
One final note about pressure canning: there is a difference between pressure canners and pressure cookers. While pressure canners are, obviously, designed for canning, not all pressure cookers are safe for canning use. If your pressure cooker can be safely used for canning, the manufacturers instructions will say so.
Another thing people may not realize is that boiling times change with altitude. At higher altitudes, atmospheric pressure is reduced. In turn, the reduced atmospheric pressure causes water to boil at lower temperatures than normal. To compensate for this temperature difference, canning time or pressure must be increased. Canning books should all include altitude adjustment charts, or they can be easily found online.
In summary, canning is a very safe process. Provided you follow a safe, tested recipe and other precautions, you should not have any problems canning. Furthermore, it is a great learning experience to involve your children or other family members in. We hope you’ll be able to start canning soon! Our next article is on essential canning supplies, don’t miss it.