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FAQ About Tomatoes


By Joan Hegerfeld-Baker

Published on September 2nd in Healthy FamiliesFood Safety

 Can ripe or semi ripe tomatoes on the vine be safely canned if they were exposed to a frost?

Do not can tomatoes from frost killed vines. Tomatoes have the potential to become a low acid food if from a frost killed vine. Tomatoes are very close to the category of a low-acid food to begin with.

The tomatoes should be made into a tomato sauce, spaghetti sauce or some other mixture that can be frozen.


 What is the best method of ripening tomatoes that were picked before they were fully ripened?

To ripen tomatoes, keep them out of direct sunlight. Room temperature should be between 60 and 70 degrees F. if the temperature falls below 55 degrees F., flavor will suffer. Fruit should be checked daily for decay and rotting. To prevent the spread of decay, the fruit may be wrapped individually in tissue paper. The damaged fruit should be disposed of as soon as it is noticed.

Green tomatoes will usually ripen if they are removed from the vine and wrapped individually in newspaper. Place wrapped tomatoes at room temperature or slightly cooler. If at all possible green tomatoes should be picked before the first frost. If there is a light frost, treat the tomatoes as above. If the frost is heavy, use the tomatoes immediately in recipes calling for green tomatoes, such as pickles, relishes, chow-chow, marmalade or preserves.


 What causes flat sour spoilage in canned tomatoes?

Flat-sour spoilage is caused by microorganisms that survive boiling water bath processing heat treatment. They are classified as thermophilic, heat loving. This type of spoilage is not a health hazard but the product is inedible. To minimize the chance of spoilage be sure tomatoes are well washed. Also, do not let jars remain in the boiling water bath or pressure canner after processing, remove them to cool on the counter. Store canned tomatoes in a cool dark location.


 Can tomatoes be canned unpeeled?

Fruits and tomatoes may be canned without peeling. Be sure to wash the product well. New potatoes should be peeled because the skin contains a high bacteria count increasing the chance of botulinum toxin formation.


 Can I can my tomato juice in a ½ gallon jar?

The only food for which USDA processing times are given for half gallon containers is fruit juice. No other product should be canned in half gallon containers.


 Black deposits have formed on the underside of the lid – is the product safe to eat?

Black deposits on the underside of lids of properly processed products do not mean spoilage. Naturally occurring compounds in food react with the metal in the lid to produce the black deposits. This is especially common in tomato products. the products are safe to eat.


 Tomatoes canned with white deposits around top.

Sometimes white dots appear around the top of the jar in canned tomato products, especially those that have added ingredients such a onion and green pepper. It occurs after the tomato product as been stored for a period of time. There are several explanations. First, it may be a mineral deposit such as calcium nitrate. If the deposit is grainy then this may be its origin.

Another explanation for a grainy deposit is the result of a reaction between the acid in the food and calcium carbonate in the sealing composition of the lid. Calcium acetate crystals form.

If the deposit is soapy feeling it may be soap. There is a small amount of fat in the tomato seeds and this combines with an alkaline substance to form soap.

In all cases the products are safe to eat as long as they were processed correctly. As with any other canning question, be sure to ask if the product was processed by recommended procedures.


 Preserving overripe or blemished tomatoes

Overripe tomatoes should not be canned because they lose acidity as they mature. Tomatoes with soft spots or decayed areas should not be canned.

Use overripe tomatoes in products that will be frozen. They may also be used in products that contain added vinegar: bbq sauces, ketchup, etc.


 What causes tomato juice to separate?

Home canned tomato juice may separate after it has been stored on the shelf for several days. The top layer will be a clear amber colored liquid. The bottom layer is a pulp containing red liquid.

If the consumer does not mind that the juice separated there is no problem with it. Just shake to blen the layers before using.

If you want a tomato juice that does not separate, the key is in heating the tomatoes quickly and not letting them sit unheated after they are cut into pieces. If allowed to sit, enzymes, pectinases, begin to break down cell structures. These enzymes are not inactivated until the product reaches about 180F. so, as soon as you begin quartering the tomatoes in preparation to juicing them, place a layer in your cooking pot and start heating them. Add other tomatoes as soon as they are cut. Stir ot distribute the heat throughout.


 How can I prevent the spoilage of my tomato sauce?

If tomato sauce products like tomato sauce, chili sauce, ketchup or even applesauce, spoil despite processing for the recommended time, the product was probably not hot enough when poured into the jars. Because these products are thick, heat penetration is slow during processing. For the processing times indicated the products must be boiling hot when placed into the jars.
Products will have a tendency to splatter near the boiling point so be sure to protect your hands, arms and any other exposed skin area.


 Can I can cherry tomatoes?

They can be canned just like any other tomatoes. Since peeling them may be a tedious task, using them for juice may be the best use of them. If they are canned unpeeled the peeling will probably separate during canning and give a less than desirable product.


 What are the white spots under the skin of the tomato?

White spots, about the size of a pinhead, beneath the skin of tomatoes are caused by insects sucking juice out of the tomato in that area. The white spots are scar tissue. This should not affect the acidity of the product but does affect the aesthetics. The scar may be removed by peeling deeper in those areas. The tomato is safe to eat.

For more information, contact Joan Hegerfeld-Baker at South Dakota State University at 605.688.6233