Elizabeth L. Andress, Ph.D.
National Center for Home Food Preservation
Tomatoes can be preserved by canning, drying, freezing, or pickling. They can also be used in creating fruit spreads like jams, jellies and marmalades. Raw tomatoes or raw tomato products can be kept refrigerated (below 40 degrees Fahrenheit), but will spoil over time due to bacteria, yeasts, and molds. Preserving tomatoes in oil is currently not recommended. Oil may protect botulism organisms trapped in a water droplet. Furthermore, oil may have a deleterious effect on lid gaskets and the at least one manufacturer of home canning lids recommends against it.
Only boiling water or pressure canning methods are recommended for canning foods. Older methods, such as oven canning and open-kettle canning, have been discredited and can be hazardous (Equipment and Methods Not Recommended from the USDA Complete Guide to Canning 2009). The risk of botulism poisoning determines the choice of either boiling water or pressure canners for canning foods. In foods that are acid (pH 4.6 or lower) the microorganism that causes botulism cannot grow. Therefore it is safe to use a boiling water bath canner. All other foods must be canned using tested pressure canning processes (Ensuring Safe Canned Foods).
Tomatoes for many years were considered high acid. However, new varieties, over-mature fruits, and tomatoes from dead or frost-killed vines may have a pH greater than 4.6. The USDA and University-based researchers have determined that to ensure a safe acid level for boiling water canning of whole, crushed, or juiced tomatoes, add 2 tablespoons of bottled lemon juice or ½ teaspoon of citric acid per quart of tomatoes. For pints, use 1 tablespoon of bottled lemon juice or ¼ teaspoon of citric acid. Acid can be added directly to the jars before filling the jars with product. Add sugar to offset acid taste, if desired. Four tablespoons of 5-percent-acidity vinegar per quart may be used instead of lemon juice or citric acid. However, vinegar may cause undesirable flavor changes. (Canning Tomatoes, Introduction). Here are some tested recipes from USDA and Cooperative Extension Service sources:
Frozen tomatoes will have a mushy texture when thawed and are suitable only for cooking, i.e. in soups, stews, spaghetti sauces, etc. Directions for freezing raw, juice, or stewed tomatoes are found here: http://www.uga.edu/nchfp/how/freeze/tomato.html. Tomato products, such as sauce, puree, catsup, and chili sauce, can be frozen. Prepare as usual, cool rapidly, pack into rigid containers leaving headspace, and freeze.
Tomatoes are an excellent food to dry. They do not need blanching and are dried to a crisp. For more details please consult this Colorado State University Cooperative Extension Service publication: http://www.uga.edu/nchfp/how/dry/csu_dry_vegetables.pdf
The safety concerns of pickled tomatoes are the same as those for canning tomatoes. A pH below 4.6 must be achieved before a food can be safely boiling water canner. When pickling, commercially prepared vinegar is typically added to achieve the necessary acidity. Do not alter vinegar, food, or water proportions in a recipe or use vinegar with unknown acidity. Use only recipes with tested proportions of ingredients. There must be a minimum, uniform level of acid throughout the mixed product to prevent the growth of botulinum bacteria. Here are some tested recipes from USDA and Cooperative Extension sources:
Once again the safety of a tomato preserve is dependent on it's acid level. Here are two recipes that have been tested by University of Georgia Cooperative Extension Service:
Think creatively when planning to preserve tomatoes, but also think "safety". There are many tested procedures and recipes available to preserve your tomatoes. Creating your own procedures and recipes could result in a hazardous product, since the pH range of tomatoes is on the borderline between acid and low-acid foods.
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