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Background
For centuries, meat has been preserved with salt. At certain levels, salt prevents growth of some types of bacteria that are responsible for meat spoilage. Salt prevents bacterial growth either because of its direct inhibitory effect or because of the drying effect it has on meat (most bacteria require substantial amounts of moisture to live and grow).
As use of salt as a meat preservative spread, a preference developed for certain salts that produced a pink color and special flavor in meat. This is the effect we see in cured meats today. Near the turn of the century it was determined that nitrate, present in some salt, was responsible for this special color and flavor. Still later it was determined that nitrate actually is changed to nitrite by bacterial action during processing and storage and that nitrate itself has no effect on meat color. Today the nitrite used in meat curing is produced commercially as sodium nitrite.
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What Nitrite Does in Meat
Nitrite in meat greatly delays development of botulinal toxin (botulism), develops cured meat flavor and color, retards development of rancidity and off-odors and off-flavors during storage, inhibits development of warmed-over flavor, and preserves flavors of spices, smoke, etc.
Adding nitrite to meat is only part of the curing process. Ordinary table salt (sodium chloride) is added because of its effect on flavor. Sugar is added to reduce the harshness of salt. Spices and other flavorings often are added to achieve a characteristic "brand" flavor. Most, but not all, cured meat products are smoked after the curing process to impart a smoked meat flavor.
Sodium nitrite, rather than sodium nitrate, is most commonly used for curing (although in some products, such as country ham, sodium nitrate is used because of the long aging period). In a series of normal reactions, nitrite is converted to nitric oxide. Nitric oxide combines with myoglobin, the pigment responsible for the natural red color of uncured meat. They form nitric oxide myoglobin, which is a deep red color (as in uncooked dry sausage) that changes to the characteristic bright pink normally associated with cured and smoked meat (such as wieners and ham) when heated during the smoking process.
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Nitrosamines
In the 1970s, newspaper articles discussed the safety of meat products cured with nitrite. Under certain conditions not yet fully understood, the natural breakdown products of proteins known as amines can combine with nitrites to form compounds known as nitrosamines. There are many different types of nitrosamines, most of which are known carcinogens in test animals.
Not all cured meat products contain nitrosamines; when present, they usually are in very minute amounts. According to S.R. Tannenbaum and T.Y. Fan in "Uncertainties about Nitrosamine Formation in and from Foods," proceedings from the Meat Industry Research Conference, University of Chicago, 1973, many variables influence nitrosamine levels: amount of nitrite added during processing, concentrations of amines in meat, type and amounts of other ingredients used in processing, actual processing conditions, length of storage, storage temperatures, method of cooking, and degree of doneness. For example, the USDA now requires adding ascorbic acid (vitamin C) or erythorbic acid to bacon cure, a practice that greatly reduces the formation of nitrosamines.
The effects of heating meat products cured with nitrite have been investigated. The previously cited study, "Effect of Frying and Other Cooking Conditions on Nitrosopyrrolidine Formation in Bacon," by J.W. Pensabene, et al., indicated that when bacon was fried at 210 degrees F for 10 minutes (raw), 210 degrees F for 105 minutes (medium well), 275 degrees F for 10 minutes (very light), or 275 degrees F for 30 minutes (medium well), no conclusive evidence of nitrosopyrrolidine could be found. But when bacon was fried at 350 degrees F for 6 minutes (medium well), 400 degrees F for 4 minutes (medium well), or 400 degrees F for 10 minutes (burned), nitrosopyrrolidine formation was conclusively found at 10, 17, and 19 parts per billion. Thus, well done or burned bacon probably is potentially more hazardous than less well done bacon. Bacon cooked by microwave has less nitrosamine than fried bacon. Consumers should cook bacon properly.
The same study and one by W. Fiddler, et al. (J. Food Sci., 39:1070, 1974) have shown that fat cook-out or drippings usually contain more nitrosopyrrolidine than the bacon contains.
It is unknown at what levels, if any, nitrosamines are formed in humans after they eat cured meat products, or what constitutes a dangerous level in meat or in humans. Nitrosamines are found very infrequently in all cured products except overcooked bacon, as discussed above.
Feeding studies documented in the "GRAS" report using meats containing high levels of nitrite showed no evidence of carcinogenesis. However, nitrosamines still are considered a definite potential hazard to human health.
Bacon manufacturers are under a USDA surveillance program whereby bacon is sampled, cooked, and tested for nitrosamines. Levels above a certain maximum amount are not permitted.
Although nitrite is a controversial food additive, recent studies indicate that nitrite can inhibit the production of malonaldehyde, which may be toxic to living cells. In small quantities (yet at 1,000 times the levels of nitrosamines), malonaldehyde frequently is found in food products that turn rancid. Wieners, ham, bacon, and corned beef resist the accumulation of malonaldehyde due to their nitrite content.
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