A Medley of Potpourri

Tuesday, August 26, 2025

History of thermodynamics

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/History_of_thermodynamics

The history of thermodynamics is a fundamental strand in the history of physics, the history of chemistry, and the history of science in general. Due to the relevance of thermodynamics in much of science and technology, its history is finely woven with the developments of classical mechanics, quantum mechanics, magnetism, and chemical kinetics, to more distant applied fields such as meteorology, information theory, and biology (physiology), and to technological developments such as the steam engine, internal combustion engine, cryogenics and electricity generation. The development of thermodynamics both drove and was driven by atomic theory. It also, albeit in a subtle manner, motivated new directions in probability and statistics; see, for example, the timeline of thermodynamics.

Antiquity

The ancients viewed heat as that related to fire. In 3000 BC, the ancient Egyptians viewed heat as related to origin mythologies. The ancient Indian philosophy including Vedic philosophy believed that five classical elements (or pancha mahā bhūta) are the basis of all cosmic creations. In the Western philosophical tradition, after much debate about the primal element among earlier pre-Socratic philosophers, Empedocles proposed a four-element theory, in which all substances derive from earth, water, air, and fire. The Empedoclean element of fire is perhaps the principal ancestor of later concepts such as phlogiston and caloric. Around 500 BC, the Greek philosopher Heraclitus became famous as the "flux and fire" philosopher for his proverbial utterance: "All things are flowing." Heraclitus argued that the three principal elements in nature were fire, earth, and water.

Vacuum-abhorrence

The 5th century BC Greek philosopher Parmenides, in his only known work, a poem conventionally titled On Nature, uses verbal reasoning to postulate that a void, essentially what is now known as a vacuum, in nature could not occur. This view was supported by the arguments of Aristotle, but was criticized by Leucippus and Hero of Alexandria. From antiquity to the Middle Ages various arguments were put forward to prove or disapprove the existence of a vacuum and several attempts were made to construct a vacuum but all proved unsuccessful.

Atomism

Atomism is a central part of today's relationship between thermodynamics and statistical mechanics. Ancient thinkers such as Leucippus and Democritus, and later the Epicureans, by advancing atomism, laid the foundations for the later atomic theory. Until experimental proof of atoms was later provided in the 20th century, the atomic theory was driven largely by philosophical considerations and scientific intuition.

17th century

Early thermometers

The European scientists Cornelius Drebbel, Robert Fludd, Galileo Galilei and Santorio Santorio in the 16th and 17th centuries were able to gauge the relative "coldness" or "hotness" of air, using a rudimentary air thermometer (or thermoscope). This may have been influenced by an earlier device which could expand and contract the air constructed by Philo of Byzantium and Hero of Alexandria.

"Heat is motion" (Francis Bacon)

The idea that heat is a form of motion is perhaps an ancient one and is certainly discussed by the English philosopher and scientist Francis Bacon in 1620 in his Novum Organum. Bacon surmised: "Heat itself, its essence and quiddity is motion and nothing else."^[3] "not ... of the whole, but of the small particles of the body."

René Descartes

Precursor to work

In 1637, in a letter to the Dutch scientist Christiaan Huygens, the French philosopher René Descartes wrote:

Lifting 100 lb one foot twice over is the same as lifting 200 lb one foot, or 100 lb two feet.

In 1686, the German philosopher Gottfried Leibniz wrote essentially the same thing: The same force ["work" in modern terms] is necessary to raise body A of 1 pound (libra) to a height of 4 yards (ulnae), as is necessary to raise body B of 4 pounds to a height of 1 yard.^[6]

Quantity of motion

In Principles of Philosophy (Principia Philosophiae) from 1644, Descartes defined "quantity of motion" (Latin: quantitas motus) as the product of size and speed, and claimed that the total quantity of motion in the universe is conserved.

If x is twice the size of y, and is moving half as fast, then there's the same amount of motion in each.

[God] created matter, along with its motion ... merely by letting things run their course, he preserves the same amount of motion ... as he put there in the beginning.

He claimed that merely by letting things run their course, God preserves the same amount of motion as He created, and that thus the total quantity of motion in the universe is conserved.

Boyle's law

Irish physicist and chemist Robert Boyle in 1656, in coordination with English scientist Robert Hooke, built an air pump. Using this pump, Boyle and Hooke noticed the pressure-volume correlation: PV=constant. In that time, air was assumed to be a system of motionless particles, and not interpreted as a system of moving molecules. The concept of thermal motion came two centuries later. Therefore, Boyle's publication in 1660 speaks about a mechanical concept: the air spring. Later, after the invention of the thermometer, the property temperature could be quantified. This tool gave Gay-Lussac the opportunity to derive his law, which led shortly later to the ideal gas law.

Gas laws in brief

Boyle's law (1662)
Charles's law was first published by Joseph Louis Gay-Lussac in 1802, but he referenced unpublished work by Jacques Charles from around 1787. The relationship had been anticipated by the work of Guillaume Amontons in 1702.
Gay-Lussac's law (1802)

Steam digester

Denis Papin, an associate of Boyle's, built in 1679 a bone digester, which is a closed vessel with a tightly fitting lid that confines steam until a high pressure is generated. Later designs implemented a steam release valve to keep the machine from exploding. By watching the valve rhythmically move up and down, Papin conceived of the idea of a piston and cylinder engine. He did not however follow through with his design. Nevertheless, in 1697, based on Papin's designs, Thomas Newcomen greatly improved upon engineer Thomas Savery's earlier "fire engine" by incorporating a piston. This made it suitable for mechanical work in addition to pumping to heights beyond 30 feet, and is thus often considered the first true steam engine.

Heat transfer (Halley and Newton)

The phenomenon of heat conduction is immediately grasped in everyday life. The fact that warm air rises and the importance of the phenomenon to meteorology was first realised by Edmond Halley in 1686.

In 1701, Sir Isaac Newton published his law of cooling.

18th century

Phlogiston theory

The theory of phlogiston arose in the 17th century, late in the period of alchemy. Its replacement by caloric theory in the 18th century is one of the historical markers of the transition from alchemy to chemistry. Phlogiston was a hypothetical substance that was presumed to be liberated from combustible substances during burning, and from metals during the process of rusting.

The world's first **ice-calorimeter**, used in the winter of 1782–83, by Antoine Lavoisier and Pierre-Simon Laplace, to determine the heat evolved in various chemical changes; calculations which were based on Joseph Black's prior discovery of latent heat. These experiments mark the foundation of thermochemistry.

Limit to the "degree of cold"

In 1702 Guillaume Amontons introduced the concept of absolute zero based on observations of gases.

Kinetic theory (18th century)

An early scientific reflection on the microscopic and kinetic nature of matter and heat is found in a work by Mikhail Lomonosov, in which he wrote: "Movement should not be denied based on the fact it is not seen. ... leaves of trees move when rustled by a wind, despite it being unobservable from large distances. Just as in this case motion ... remains hidden in warm bodies due to the extremely small sizes of the moving particles."

During the same years, Daniel Bernoulli published his book Hydrodynamics (1738), in which he derived an equation for the pressure of a gas considering the collisions of its atoms with the walls of a container. He proved that this pressure is two thirds the average kinetic energy of the gas in a unit volume. Bernoulli's ideas, however, made little impact on the dominant caloric culture. Bernoulli made a connection with Gottfried Leibniz's vis viva principle, an early formulation of the principle of conservation of energy, and the two theories became intimately entwined throughout their history.

Thermochemistry and steam engines

Heat capacity

Bodies were capable of holding a certain amount of this fluid, leading to the term heat capacity, named and first investigated by Scottish chemist Joseph Black in the 1750s.

In the mid- to late 19th century, heat became understood as a manifestation of a system's internal energy. Today heat is seen as the transfer of disordered thermal energy. Nevertheless, at least in English, the term heat capacity survives. In some other languages, the term thermal capacity is preferred, and it is also sometimes used in English.

Steam engines

Prior to 1698 and the invention of the Savery engine, horses were used to power pulleys, attached to buckets, which lifted water out of flooded salt mines in England. In the years to follow, more variations of steam engines were built, such as the Newcomen engine, and later the Watt engine. In time, these early engines would eventually be utilized in place of horses. Thus, each engine began to be associated with a certain amount of "horse power" depending upon how many horses it had replaced. The main problem with these first engines was that they were slow and clumsy, converting less than 2% of the input fuel into useful work. In other words, large quantities of coal (or wood) had to be burned to yield only a small fraction of work output. Hence the need for a new science of engine dynamics was born.

Caloric theory

In the mid- to late 18th century, heat was thought to be a measurement of an invisible fluid, known as the caloric. Like phlogiston, caloric was presumed to be the "substance" of heat that would flow from a hotter body to a cooler body, thus warming it. The utility and explanatory power of kinetic theory, however, soon started to displace the caloric theory. Nevertheless, William Thomson, for example, was still trying to explain James Joule's observations within a caloric framework as late as 1850. The caloric theory was largely obsolete by the end of the 19th century.

Calorimetry

Joseph Black and Antoine Lavoisier made important contributions in the precise measurement of heat changes using the calorimeter, a subject which became known as thermochemistry. The development of the steam engine focused attention on calorimetry and the amount of heat produced from different types of coal. The first quantitative research on the heat changes during chemical reactions was initiated by Lavoisier using an ice calorimeter following research by Joseph Black on the latent heat of water.

Thermal conduction and thermal radiation

Carl Wilhelm Scheele distinguished heat transfer by thermal radiation (radiant heat) from that by convection and conduction in 1777.

In the 17th century, it came to be believed that all materials had an identical conductivity and that differences in sensation arose from their different heat capacities. Suggestions that this might not be the case came from the new science of electricity in which it was easily apparent that some materials were good electrical conductors while others were effective insulators. Jan Ingen-Housz in 1785-9 made some of the earliest measurements, as did Benjamin Thompson during the same period.

In 1791, Pierre Prévost showed that all bodies radiate heat, no matter how hot or cold they are. In 1804, Sir John Leslie observed that a matte black surface radiates heat more effectively than a polished surface, suggesting the importance of black-body radiation.

Heat and friction (Rumford)

In the 19th century, scientists abandoned the idea of a physical caloric. The first substantial experimental challenges to the caloric theory arose in a work by Benjamin Thompson's (Count Rumford) from 1798, in which he showed that boring cast iron cannons produced great amounts of heat which he ascribed to friction. His work was among the first to undermine the caloric theory.

As a result of his experiments in 1798, Thompson suggested that heat was a form of motion, though no attempt was made to reconcile theoretical and experimental approaches, and it is unlikely that he was thinking of the vis viva principle.

Early 19th century

Modern thermodynamics (Carnot)

Although early steam engines were crude and inefficient, they attracted the attention of the leading scientists of the time. One such scientist was Sadi Carnot, the "father of thermodynamics", who in 1824 published Reflections on the Motive Power of Fire, a discourse on heat, power, and engine efficiency. Most cite this book as the starting point for thermodynamics as a modern science. (The name "thermodynamics", however, did not arrive until 1854, when the British mathematician and physicist William Thomson (Lord Kelvin) coined the term thermo-dynamics in his paper On the Dynamical Theory of Heat.)

Carnot defined "motive power" to be the expression of the useful effect that a motor is capable of producing. Herein, Carnot introduced us to the first modern day definition of "work": weight lifted through a height. The desire to understand, via formulation, this useful effect in relation to "work" is at the core of all modern day thermodynamics.

Even though he was working with the caloric theory, Carnot in 1824 suggested that some of the caloric available for generating useful work is lost in any real process.

Reflection, refraction, and polarisation of radiant heat

Though it had come to be suspected from Scheele's work, in 1831 Macedonio Melloni demonstrated that radiant heat could be reflected, refracted and polarised in the same way as light.

Kinetic theory (early 19th century)

John Herapath independently formulated a kinetic theory in 1820, but mistakenly associated temperature with momentum rather than vis viva or kinetic energy. His work ultimately failed peer review, even from someone as well-disposed to the kinetic principle as Humphry Davy, and was neglected.

John James Waterston in 1843 provided a largely accurate account, again independently, but his work received the same reception, failing peer review.

Further progress in kinetic theory started only in the middle of the 19th century, with the works of Rudolf Clausius, James Clerk Maxwell, and Ludwig Boltzmann.

Mechanical equivalent of heat

Quantitative studies by Joule from 1843 onwards provided soundly reproducible phenomena, and helped to place the subject of thermodynamics on a solid footing. In 1843, Joule experimentally found the mechanical equivalent of heat. In 1845, Joule reported his best-known experiment, involving the use of a falling weight to spin a paddle-wheel in a barrel of water, which allowed him to estimate a mechanical equivalent of heat of 819 ft·lbf/Btu (4.41 J/cal). This led to the theory of conservation of energy and explained why heat can do work.

Absolute zero and the Kelvin scale

The idea of absolute zero was generalised in 1848 by Lord Kelvin.

Late 19th century

Entropy and the second law of thermodynamics

Lord Kelvin

In March 1851, while grappling to come to terms with the work of Joule, Lord Kelvin started to speculate that there was an inevitable loss of useful heat in all processes. The idea was framed even more dramatically by Hermann von Helmholtz in 1854, giving birth to the spectre of the heat death of the universe.

William Rankine

In 1854, William John Macquorn Rankine started to make use of what he called thermodynamic function in calculations. This has subsequently been shown to be identical to the concept of entropy formulated by the famed mathematical physicist Rudolf Clausius.^[14]

Rudolf Clausius

In 1865, Clausius coined the term "entropy" (das Wärmegewicht, symbolized S) to denote heat lost or turned into waste.("Wärmegewicht" translates literally as "heat-weight"; the corresponding English term stems from the Greek τρέπω, "I turn".) Clausius used the concept to develop his classic statement of the second law of thermodynamics the same year.

Statistical thermodynamics

Temperature is average kinetic energy of molecules

In his 1857 work On the nature of the motion called heat, Clausius for the first time clearly states that heat is the average kinetic energy of molecules.

Maxwell–Boltzmann distribution

Clausius' above statement interested the Scottish mathematician and physicist James Clerk Maxwell, who in 1859 derived the momentum distribution later named after him. The Austrian physicist Ludwig Boltzmann subsequently generalized this distribution for the case of gases in external fields. In association with Clausius, in 1871, Maxwell formulated a new branch of thermodynamics called statistical thermodynamics, which functions to analyze large numbers of particles at equilibrium, i.e., systems where no changes are occurring, such that only their average properties as temperature T, pressure P, and volume V become important.

Degrees of freedom

Boltzmann is perhaps the most significant contributor to kinetic theory, as he introduced many of the fundamental concepts in the theory. Besides the Maxwell–Boltzmann distribution mentioned above, he also associated the kinetic energy of particles with their degrees of freedom. The Boltzmann equation for the distribution function of a gas in non-equilibrium states is still the most effective equation for studying transport phenomena in gases and metals. By introducing the concept of thermodynamic probability as the number of microstates corresponding to the current macrostate, he showed that its logarithm is proportional to entropy.

Definition of entropy

In 1875, the Austrian physicist Ludwig Boltzmann formulated a precise connection between entropy S and molecular motion:

S = k \log W

being defined in terms of the number of possible states W that such motion could occupy, where k is the Boltzmann constant.

Gibbs free energy

In 1876, chemical engineer Willard Gibbs published an obscure 300-page paper titled: On the Equilibrium of Heterogeneous Substances, wherein he formulated one grand equality, the Gibbs free energy equation, which suggested a measure of the amount of "useful work" attainable in reacting systems.

Enthalpy

Gibbs also originated the concept we now know as enthalpy H, calling it "a heat function for constant pressure". The modern word enthalpy would be coined many years later by Heike Kamerlingh Onnes, who based it on the Greek word enthalpein meaning to warm.

Stefan–Boltzmann law

James Clerk Maxwell's 1862 insight that both light and radiant heat were forms of electromagnetic wave led to the start of the quantitative analysis of thermal radiation. In 1879, Jožef Stefan observed that the total radiant flux from a blackbody is proportional to the fourth power of its temperature and stated the Stefan–Boltzmann law. The law was derived theoretically by Ludwig Boltzmann in 1884.

20th century

Quantum thermodynamics

In 1900 Max Planck found an accurate formula for the spectrum of black-body radiation. Fitting new data required the introduction of a new constant, known as the Planck constant, the fundamental constant of modern physics. Looking at the radiation as coming from a cavity oscillator in thermal equilibrium, the formula suggested that energy in a cavity occurs only in multiples of frequency times the constant. That is, it is quantized. This avoided a divergence to which the theory would lead without the quantization.

Third law of thermodynamics

In 1906, Walther Nernst stated the third law of thermodynamics.

Erwin Schrödinger

Building on the foundations above, Lars Onsager, Erwin Schrödinger, Ilya Prigogine and others, brought these engine "concepts" into the thoroughfare of almost every modern-day branch of science.

Branches of thermodynamics

The following list is a rough disciplinary outline of the major branches of thermodynamics and their time of inception:

Thermochemistry – 1780s
Classical thermodynamics – 1824
Chemical thermodynamics – 1876
Statistical mechanics – c. 1880s
Equilibrium thermodynamics
Engineering thermodynamics
Chemical engineering thermodynamics – c. 1940s
Non-equilibrium thermodynamics – 1941
Small systems thermodynamics – 1960s
Biological thermodynamics – 1957
Ecosystem thermodynamics – 1959
Relativistic thermodynamics – 1965
Rational thermodynamics – 1960s
Quantum thermodynamics – 1968
Black hole thermodynamics – c. 1970s
Theory of critical phenomena and use of renormalization group theory in statistical physics – 1966-1974
Geological thermodynamics – c. 1970s
Biological evolution thermodynamics – 1978
Geochemical thermodynamics – c. 1980s
Atmospheric thermodynamics – c. 1980s
Natural systems thermodynamics – 1990s
Supramolecular thermodynamics – 1990s
Earthquake thermodynamics – 2000
Drug-receptor thermodynamics – 2001
Pharmaceutical systems thermodynamics – 2002

Concepts of thermodynamics have also been applied in other fields, for example:

Thermoeconomics – c. 1970s

Curing (food preservation)

From Wikipedia, the free encyclopedia

Sea salt being added to raw ham to make prosciutto

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.

view of rolled up slices of meat in a box whose lid has been removed — Slices of beef in a can

Meat preservation in general (of meat from livestock, game, and poultry) 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 today, 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.

Some traditional cured meat (such as authentic Parma ham and some authentic Spanish chorizo and Italian salami) is cured with salt alone. Today, potassium nitrate (KNO₃) and sodium nitrite (NaNO₂) (in conjunction with salt) 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 (e.g., sodium nitrate [NaNO₃], sodium nitrite, and potassium nitrate) 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 (sodium chloride) 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 the purpose of curing it 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 (NO⁻
₂) is generally supplied by sodium nitrite or (indirectly) 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 (which may be generated from nitrate) must be generated in the product over long periods of time.

Nitrite further breaks down in the meat into nitric oxide (NO), which then binds to the iron atom in the center of myoglobin's heme group, reducing oxidation and causing a reddish-brown color (nitrosomyoglobin) when raw and the characteristic cooked-ham pink color (nitrosohemochrome or nitrosyl-heme) when cooked. The addition of ascorbate to cured meat reduces formation of nitrosamines (see below), 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 lethal dose in humans is about 22 mg/kg of body weight), 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 (WHO) advises that each 50 g (1.8 oz) 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 (Group 1).

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 (less than 1000 cases per year reported worldwide) 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 (or other natural sources) 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 (pit barbecuing) 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.

Research

Since the 20th century, with respect to the relationship between diet and human disease (e.g. cardiovascular, etc.), scientists have conducted studies on the effects of lipolysis on vacuum-packed or frozen meat. In particular, by analyzing entrecôtes of frozen beef during 270 days at −20 °C (−4 °F), scientists found an important phospholipase that accompanies the loss of some unsaturated fat n-3 and n-6, which are already low in the flesh of ruminants.

Health effects

Elevated levels of nitrites in preserved meats increase the risk of nasopharyngeal cancer.

In 2015, the International Agency for Research on Cancer of the World Health Organization classified processed meat, that is, meat that has undergone salting, curing, fermenting, or smoking, as "carcinogenic to humans".

History

A survival technique since prehistory, the preservation of meat has become, over the centuries, a topic of political, economic, and social importance worldwide.

Traditional methods

Food curing dates back to ancient times, both in the form of smoked meat and salt-cured meat.

Several sources describe the salting of meat in the ancient Mediterranean world. Diodore of Sicily in his Bibliotheca historica wrote that the Cosséens in the mountains of Persia salted the flesh of carnivorous animals. Strabo indicates that people at Borsippa were catching bats and salting them to eat. The ancient Greeks prepared tarichos (τάριχος), which was meat and fish conserved by salt or other means. The Romans called this dish salsamentum – which term later included salted fat, the sauces and spices used for its preparation. Also evidence of ancient sausage production exists. The Roman gourmet Apicius speaks of a sausage-making technique involving œnogaros (a mixture of the fermented fish sauce garum with oil or wine). Preserved meats were furthermore a part of religious traditions: resulting meat for offerings to the gods was salted before being given to priests, after which it could be picked up again by the offerer, or even sold in the butcher's.

A trade in salt meat occurred across ancient Europe. In Polybius's time (c. 200 – c.118 BCE), the Gauls exported salt pork each year to Rome in large quantities, where it was sold in different cuts: rear cuts, middle cuts, hams, and sausages. This meat, after having been salted with the greatest care, was sometimes smoked. These goods had to have been considerably important, since they fed part of the Roman people and the armies. The Belgae were celebrated above all for the care which they gave to the fattening of their pigs. Their herds of sheep and pigs were so many, they could provide skins and salt meat not only for Rome, but also for most of Italy.The Ceretani of Spain drew a large export income from their hams, which were so succulent, they were in no way inferior to those of Cantabria. These tarichos of pig became especially sought, to the point that the ancients considered this meat the most nourishing of all and the easiest to digest.

In Ethiopia, according to Pliny, and in Libya according to Saint Jerome, the Acridophages (literally, the locust-eaters) salted and smoked the crickets which arrived at their settlements in the spring in great swarms and which constituted, it was said, their sole food.

The smoking of meat was a traditional practice in North America, where Plains Indians hung their meat at the top of their tipis to increase the amount of smoke coming into contact with the food.

Middle Ages

In Europe, medieval cuisine made great use of meat and vegetables, and the guild of butchers was amongst the most powerful. During the 12th century, salt beef was consumed by all social classes. Smoked meat was called carbouclée in Romance tongues and bacon if it was pork.

The Middle Ages made pâté a masterpiece: that which is, in the 21st century, merely spiced minced meat (or fish), baked in a terrine and eaten cold, was at that time composed of a dough envelope stuffed with varied meats and superbly decorated for ceremonial feasts. The first French recipe, written in verse by Gace de La Bigne, mentions in the same pâté three great partridges, six fat quail, and a dozen larks. Le Ménagier de Paris mentions pâtés of fish, game, young rabbit, fresh venison, beef, pigeon, mutton, veal, and pork, and even pâtés of lark, turtledove, baby bird, goose, and hen. Bartolomeo Sacchi, called Platine, prefect of the Vatican Library, gives the recipe for a pâté of wild beasts: the flesh, after being boiled with salt and vinegar, was larded and placed inside an envelope of spiced fat, with a mélange of pepper, cinnamon and pounded lard; one studded the fat with cloves until it was entirely covered, then placed it inside a pâte.

In the 16th century, the most fashionable pâtés were of woodcock, au bec doré, chapon, beef tongue, cow feet, sheep feet, chicken, veal, and venison. In the same era, Pierre Belon notes that the inhabitants of Crete and Chios lightly salted then oven-dried entire hares, sheep, and roe deer cut into pieces, and that in Turkey, cattle and sheep, cut and minced rouelles, salted then dried, were eaten on voyages with onions and no other preparation.

Early modern era

During the Age of Discovery, salt meat was one of the main foods for sailors on long voyages, for instance in the merchant marine or the navy. In the 18th century, salted Irish beef, transported in barrels, were considered finest.

Scientific research on meat by chemists and pharmacists led to the creation of a new, extremely practical product: meat extract, which could appear in different forms. The need to properly feed soldiers during long campaigns outside the country, such as in the Napoleonic Wars, and to nourish a constantly growing population often living in appalling conditions drove scientific research, but a confectioner, Nicolas Appert, in 1795 developed through experimentation a method which became universal and in one language bears his name: airtight storage, called appertisation in French.

With the spread of appertisation, the 19th-century world entered the era of the "food industry", which developed new products such as canned salt meat (for example corned beef). The desire for safer food led to the creation of the US's Pure Food and Drug Act in 1906, followed by the national agencies for health security and the establishment of food traceability over the course of the 20th century.^{[citation needed]} It also led to continuing technological innovation.

In France, the summer of 1857 was so hot that most butchers refused to slaughter animals and charcutiers lost considerable amounts of meat, due to inadequate conservation methods. A member of the Academy of Medicine and his son issued a 34-page summary of works completed by 1857, which proposed some solutions: not less than 91 texts exist, of which 64 edited for only the years between 1851 and 1857.

Effects on trade

The improvement of methods of meat preservation, and of the means of transport of preserved products, has notably permitted the separation of areas of production and areas of consumption, which can now be distant without it posing a problem, permitting the exportation of meats.

For example, the appearance in the 1980s of preservation techniques under controlled atmosphere sparked a small revolution in the world's market for sheep meat: the lamb of New Zealand, one of the world's largest exporters of lamb, could henceforth be sold as fresh meat, since it could be preserved from 12 to 16 weeks, which was a sufficient duration for it to reach Europe by boat. Before, meat from New Zealand was frozen, thus had a much lower value on European shelves. With the arrival of the new "chilled" meats, New Zealand could compete even more strongly with local producers of fresh meat. The use of controlled atmosphere to avoid the depreciation which affects frozen meat is equally useful in other meat markets, such as that for pork, which now also enjoys an international trade.

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