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Canning -- Tomatoes

What causes the white deposits around the top of canned tomatoes?

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Sometimes white spots appear around the top of the jar in canned tomato products, especially those that have added ingredients such as onion and green pepper. It occurs after the tomato product has 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.

PREPARED BY: Angela M. Fraser, Ph.D., Associate Professor/Food Safety Specialist, and Carolyn J. Lackey, Ph.D., R.D., L.D.N., Professor/Food and Nutrition Specialist, North Carolina State University (August 2004)

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