Curing (food preservation)
Curing is any of various food preservation and flavoring processes of foods such as meat, fish and vegetables, by the addition of salt, with the aim of drawing moisture out of the food by the process of osmosis. Because curing increases the solute concentration in the food and hence decreases its water potential, the food becomes inhospitable for the microbe growth that causes food spoilage. Curing can be traced back to antiquity, and was the primary method of preserving meat and fish until the late 19th century. Dehydration was the earliest form of food curing. Many curing processes also involve smoking, spicing, cooking, or the addition of combinations of sugar, nitrate, and nitrite.
Meat preservation in general comprises the set of all treatment processes for preserving the properties, taste, texture, and color of raw, partially cooked, or cooked meats while keeping them edible and safe to consume. Curing has been the dominant method of meat preservation for thousands of years, although modern developments like refrigeration and synthetic preservatives have begun to complement and supplant it.
While meat-preservation processes like curing were mainly developed in order to prevent disease and to increase food security, the advent of modern preservation methods mean that in most developed countries, curing is instead mainly practiced for its cultural value and desirable impact on the texture and taste of food. For less-developed countries, curing remains a key process in the production, transport and availability of meat.
File:Prague powder No 1.jpg|thumb|186x186px|Curing salt, also known as "Prague powder" or "pink salt", is typically a combination of sodium chloride and sodium nitrite that is dyed pink to distinguish it from table salt.
Some traditional cured meat is cured with salt alone. Today, potassium nitrate and sodium nitrite are the most common agents in curing meat, because they bond to the myoglobin and act as a substitute for oxygen, thus turning myoglobin red. More recent evidence shows that these chemicals also inhibit the growth of the bacteria that cause the disease botulism.
The combination of table salt with nitrates or nitrites, called curing salt, is often dyed pink to distinguish it from table salt. Neither table salt nor any of the nitrites or nitrates commonly used in curing is naturally pink.
Reasons for curing
Meat decomposes rapidly if it is not preserved. The speed of decomposition depends on several factors, including ambient humidity, temperature, and the presence of pathogens. Most types of untreated meat cannot be kept at room temperature for lengthy periods before spoiling.Spoiled meat changes color and exudes a foul odor. Ingestion can cause serious food poisoning. Salt-curing processes were developed in antiquity in order to ensure food safety without relying on then unknown anti-bacterial agents.
The short shelf life of fresh meat does not pose significant problems when access to it is easy and supply is abundant. But in times of scarcity and famine, or when the meat must be transported over long distances, food preservation is necessary.
Curing significantly increases the length of time meat remains edible, by making it inhospitable to the growth of microbes.
Chemical actions
Salt
Salt is the primary ingredient used in meat curing. Removal of water and addition of salt to meat creates a solute-rich environment where osmotic pressure draws water out of microorganisms, slowing down their growth. Doing this requires a concentration of salt of nearly 20%.In sausage production, salt causes the soluble proteins to come to the surface of the meat that was used to make the sausages. These proteins coagulate when the sausage is heated, helping to hold the sausage together.
Sugar
The sugar added to meat for curing purposes comes in many forms, including honey, corn syrup solids, and maple syrup. However, with the exception of bacon, it does not contribute much to the flavor, but it does alleviate the harsh flavor of the salt. Sugar also contributes to the growth of beneficial bacteria such as Lactobacillus by feeding them.Nitrates and nitrites
Nitrates and nitrites extend shelf life, help kill bacteria, produce a characteristic flavor and give meat a pink or red color. Nitrite is generally supplied by sodium nitrite or by potassium nitrate. Nitrite salts are most often used to accelerate curing and impart a pink colour. Nitrate is specifically used only in a few curing conditions and products where nitrite must be generated in the product over long periods of time.Nitrite further breaks down in the meat into nitric oxide, which then binds to the iron atom in the center of myoglobin's heme group, reducing oxidation and causing a reddish-brown color when raw and the characteristic cooked-ham pink color when cooked. The addition of ascorbate to cured meat reduces formation of nitrosamines, but increases the nitrosylation of iron.
The use of nitrite and nitrate salts for meat in the US has been formally used since 1925. Because of the relatively high toxicity of nitrite, the maximum allowed nitrite concentration in US meat products is 200 ppm. Plasma nitrite is reduced in persons with endothelial dysfunction.
Nitrite-containing processed meat is associated with increased risk of developing colorectal cancer. Adding nitrites to meat has been shown to generate known carcinogens such as nitrosamines, N-nitrosamides and nitrosyl-heme, resulting from nitrosylation reactions; the World Health Organization advises that each of "processed meats" eaten a day would raise the risk of getting bowel cancer by 18% over a lifetime; "processed meat" refers to meat that has been transformed through salting, curing, fermentation, smoking, or other processes to enhance flavour or improve preservation. The World Health Organization's review of more than 400 studies concluded, in 2015, that there was sufficient evidence that "processed meats" caused cancer, particularly colon cancer; the WHO's International Agency for Research on Cancer classified "processed meats" as carcinogenic to humans.
The use of nitrites in food preservation is highly controversial due to the potential for the formation of nitroso-compounds such as nitrosamines, N-nitrosamides and nitrosyl-heme. When the meat is cooked at high temperatures, nitrite-cured meat products can also lead to the formation of nitrosamines. The effect is seen for red processed meat, but not for white meat or fish. Nitrates and nitrites may cause cancer and the production of carcinogenic nitrosamines can be potently inhibited by the use of the antioxidants vitamin C and the alpha-tocopherol form of vitamin E during curing. Under simulated gastric conditions, nitrosothiols rather than nitrosamines are the main nitroso species being formed. The use of either compound is therefore regulated; for example, in the United States, the concentration of nitrates and nitrites is generally limited to 200 ppm or lower.
The meat industry considers nitrites irreplaceable because they speed up curing and improve color while retarding the growth of Clostridium botulinum, the bacteria that causes botulism. Botulism, however, is an extremely rare disease and is almost always associated with home preparations of preserved food. For example, all Parma ham has been made without nitrites since 1993, but was reported in 2018 to have caused no cases of botulism.
Furthermore, while the FDA has set a limit of 200 ppm of nitrates for cured meat, they are not allowed and not recognized as safe by the FDA in most other foods, even foods that are not cooked at high temperatures, such as cheese.
Nitrites from celery
Processed meats without "added nitrites" may be misleading as they may be using naturally occurring nitrites from celery instead.A 2019 report from Consumer Reports found that using celery as a curing agent introduced naturally occurring nitrates and nitrites. The USDA allows the term "uncured" or "no nitrates or nitrites added" on products using these natural sources of nitrites, which provides the consumer a false sense of making a healthier choice.
The Consumer Reports investigation also provides the average level of sodium, nitrates and nitrites found per gram of meat in their report.
Consumer Reports and the Center for Science in the Public Interest filed a formal request to the USDA to change the labeling requirements in 2019.
Smoke
Meat can also be preserved by "smoking". If the smoke is hot enough to slow-cook the meat, this will also keep it tender. One method of smoking calls for a smokehouse with damp wood chips or sawdust. In North America, hardwoods such as hickory, mesquite, and maple are commonly used for smoking, as are the wood from fruit trees such as apple, cherry, and plum, and even corncobs.Smoking helps seal the outer layer of the food being cured, making it more difficult for bacteria to enter. It can be done in combination with other curing methods such as salting. Common smoking styles include hot smoking, smoke roasting and cold smoking. Smoke roasting and hot smoking cook the meat while cold smoking does not. If the meat is cold smoked, it should be dried quickly to limit bacterial growth during the critical period where the meat is not yet dry. This can be achieved, as with jerky, by slicing the meat thinly.
The smoking of food directly with wood smoke is known to contaminate the food with carcinogenic polycyclic aromatic hydrocarbons.