Oort cloud
Distant planetesimals in the Solar System
The Oort cloud (pronounced or ), sometimes called the Öpik–Oort cloud, is theorized to be a vast cloud of icy planetesimals surrounding the Sun at distances ranging from 2,000 to 200,000 AU (0.03 to 3.2 light-years). The concept of such a cloud was proposed in 1950 by the Dutch astronomer Jan Oort, in whose honor the idea was named. Oort proposed that the bodies in this cloud replenish and keep constant the number of long-period comets entering the inner Solar System—where they are eventually consumed and destroyed during close approaches to the Sun.
The cloud is thought to encompass two regions: a disc-shaped inner Oort cloud aligned with the solar ecliptic (also called its Hills cloud) and a spherical outer Oort cloud enclosing the entire Solar System. Both regions lie well beyond the heliosphere and are in interstellar space. The innermost portion of the Oort cloud is more than a thousand times as distant from the Sun as the Kuiper belt, the scattered disc and the detached objects—three nearer reservoirs of trans-Neptunian objects.
The outer limit of the Oort cloud defines the cosmographic boundary of the Solar System. This area is defined by the Sun's Hill sphere, and hence lies at the interface between solar and galactic gravitational dominion. The outer Oort cloud is only loosely bound to the Solar System and its constituents are easily affected by the gravitational pulls of passing stars, the Milky Way itself and the cloud's own microgravity. These forces served to moderate and render more circular the highly eccentric orbits of material ejected from the inner Solar System during its early phases of development. The circular orbits of material in the Oort disc are largely thanks to this galactic gravitational torquing. By the same token, galactic interference in the motion of Oort bodies occasionally dislodges comets from their orbits within the cloud, sending them into the inner Solar System. Based on their orbits, most but not all of the short-period comets appear to have come from the Oort disc. Other short-period comets may have originated from the far larger spherical cloud.
Astronomers hypothesize that the material presently in the Oort cloud formed much closer to the Sun, in the protoplanetary disc, and was then scattered far into space through the gravitational influence of the giant planets. No direct observation of the Oort cloud is possible with present imaging technology. Nevertheless, the cloud is thought to be the source that replenishes most long-period and Halley-type comets, which are eventually consumed by their close approaches to the Sun after entering the inner Solar System. The cloud may also serve the same function for many of the centaurs and .
Development of theory.
By the early 20th century, astronomers had identified two main types of comets: short-period comets (also called "ecliptic" comets) and long-period comets (also called "nearly isotropic" comets). Ecliptic comets have relatively small orbits aligned near the ecliptic plane and are not found much farther than the Kuiper cliff around 50 AU from the Sun (the orbit of Neptune averages about 30 AU and 177P/Barnard has aphelion around 48 AU). Long-period comets, on the other hand, travel in very large orbits thousands of AU from the Sun and are isotropically distributed. This means long-period comets appear from every direction in the sky, both above and below the ecliptic plane. The origin of these comets was not well understood, and many long-period comets were initially assumed to be on parabolic trajectories, making them one-time visitors to the Sun from interstellar space.
In 1907, Armin Otto Leuschner suggested that many of the comets then thought to have parabolic orbits in fact moved along extremely large elliptical orbits that would return them to the inner Solar System after long intervals during which they were invisible to Earth-based astronomy. In 1932, the Estonian astronomer Ernst Öpik proposed a reservoir of long-period comets in the form of an orbiting cloud at the outermost edge of the Solar System. Dutch astronomer Jan Oort revived this basic idea in 1950 to resolve a paradox about the origin of comets. The following facts are not easily reconcilable with the highly elliptical orbits in which long-period comets are always found:
Oort reasoned that comets with orbits that closely approach the Sun cannot have been doing so since the condensation of the protoplanetary disc, more than 4.5 billion years ago. Hence long-period comets could not have formed in the current orbits in which they are always discovered and must have been held in an outer reservoir for nearly all of their existence.
Oort also studied tables of ephemerides for long-period comets and discovered that there is a curious concentration of long-period comets whose farthest retreat from the Sun (their aphelia) cluster around 20,000 AU. This suggested a reservoir at that distance with a spherical, isotropic distribution. He also proposed that the relatively rare comets with orbits of about 10,000 AU probably went through one or more orbits into the inner Solar System and there had their orbits drawn inward by the gravity of the planets.
Structure and composition.
The Oort cloud is thought to occupy a vast space somewhere between from the Sun to as far out as or even . The region can be subdivided into a spherical outer Oort cloud with a radius of some and a torus-shaped inner Oort cloud with a radius of .
The inner Oort cloud is sometimes known as the Hills cloud, named for Jack G. Hills, who proposed its existence in 1981. Models predict the inner cloud to be the much denser of the two, having tens or hundreds of times as many cometary nuclei as the outer cloud. The Hills cloud is thought to be necessary to explain the continued existence of the Oort cloud after billions of years.
Because it lies at the interface between the dominion of Solar and galactic gravitation, the objects comprising the outer Oort cloud are only weakly bound to the Sun. This in turn allows small perturbations from nearby stars or the Milky Way itself to inject long-period (and possibly Halley-type) comets inside the orbit of Neptune. This process ought to have depleted the sparser, outer cloud and yet long-period comets with orbits well above or below the ecliptic continue to be observed. The Hills cloud is thought to be a secondary reservoir of cometary nuclei and the source of replenishment for the tenuous outer cloud as the latter's numbers are gradually depleted through losses to the inner Solar System.
The outer Oort cloud may have trillions of objects larger than , and billions with diameters of . This corresponds to an absolute magnitude of more than 11. On this analysis, "neighboring" objects in the outer cloud are separated by a significant fraction of 1 AU, tens of millions of kilometres. The outer cloud's total mass is not known, but assuming that Halley's Comet is a suitable proxy for the nuclei composing the outer Oort cloud, their combined mass would be roughly , or five Earth masses.
Formerly the outer cloud was thought to be more massive by two orders of magnitude, containing up to 380 Earth masses,
but improved knowledge of the size distribution of long-period comets has led to lower estimates. No estimates of the mass of the inner Oort cloud have been published as of 2023.
If analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist of ices such as water, methane, ethane, carbon monoxide and hydrogen cyanide.
However, the discovery of the object 1996 PW, an object whose appearance was consistent with a D-type asteroid in an orbit typical of a long-period comet, prompted theoretical research that suggests that the Oort cloud population consists of roughly one to two percent asteroids. Analysis of the carbon and nitrogen isotope ratios in both the long-period and Jupiter-family comets shows little difference between the two, despite their presumably vastly separate regions of origin. This suggests that both originated from the original protosolar cloud, a conclusion also supported by studies of granular size in Oort-cloud comets and by the recent impact study of Jupiter-family comet Tempel 1.
In 2020 researchers discovered an unexpected spiral-shaped pattern in the inner part of the Oort Cloud. This discovery sheds light on how gravity shapes the outer edges of the Solar System.
Origin.
The Oort cloud is thought to have developed after the formation of planets from the primordial protoplanetary disc approximately 4.6 billion years ago. The most widely accepted hypothesis is that the Oort cloud's objects initially coalesced much closer to the Sun as part of the same process that formed the planets and minor planets. After formation, strong gravitational interactions with young gas giants, such as Jupiter, scattered the objects into extremely wide elliptical or parabolic orbits that were subsequently modified by perturbations from passing stars and giant molecular clouds into long-lived orbits detached from the gas giant region.
Recent research has been cited by NASA hypothesizing that a large number of Oort cloud objects are the product of an exchange of materials between the Sun and its sibling stars as they formed and drifted apart and it is suggested that many—possibly the majority—of Oort cloud objects did not form in close proximity to the Sun. Simulations of the evolution of the Oort cloud from the beginnings of the Solar System to the present suggest that the cloud's mass peaked around 800 million years after formation, as the pace of accretion and collision slowed and depletion began to overtake supply.
Models by Julio Ángel Fernández suggest that the scattered disc, which is the main source for periodic comets in the Solar System, might also be the primary source for Oort cloud objects. According to the models, about half of the objects scattered travel outward toward the Oort cloud, whereas a quarter are shifted inward to Jupiter's orbit, and a quarter are ejected on hyperbolic orbits. The scattered disc might still be supplying the Oort cloud with material. A third of the scattered disc's population is likely to end up in the Oort cloud after 2.5 billion years.
Computer models suggest that collisions of cometary debris during the formation period play a far greater role than was previously thought. According to these models, the number of collisions early in the Solar System's history was so great that most comets were destroyed before they reached the Oort cloud. Therefore, the current cumulative mass of the Oort cloud is far less than was once suspected. The estimated mass of the cloud is only a small part of the 50–100 Earth masses of ejected material.
Gravitational interaction with nearby stars and galactic tides modified cometary orbits to make them more circular. This explains the nearly spherical shape of the outer Oort cloud. On the other hand, the Hills cloud, which is bound more strongly to the Sun, has not acquired a spherical shape. Recent studies have shown that the formation of the Oort cloud is broadly compatible with the hypothesis that the Solar System formed as part of an embedded cluster of 200–400 stars. These early stars likely played a role in the cloud's formation, since the number of close stellar passages within the cluster was much higher than today, leading to far more frequent perturbations.
In June 2010 Harold F. Levison and others suggested on the basis of enhanced computer simulations that the Sun "captured comets from other stars while it was in its birth cluster." Their results imply that "a substantial fraction of the Oort cloud comets, perhaps exceeding 90%, are from the protoplanetary discs of other stars." In July 2020 Amir Siraj and Avi Loeb found that a captured origin for the Oort Cloud in the Sun's birth cluster could address the theoretical tension in explaining the observed ratio of outer Oort cloud to scattered disc objects, and in addition could increase the chances of a captured Planet Nine.
Comets.
Comets are thought to have two separate points of origin in the Solar System. Short-period comets (those with orbits of up to 200 years) are generally accepted to have emerged from either the Kuiper belt or the scattered disc, which are two linked flat discs of icy debris beyond Neptune's orbit at 30 AU and jointly extending out beyond 100 AU. Very long-period comets, such as C/1999 F1 (Catalina), whose orbits last for millions of years, are thought to originate directly from the outer Oort cloud. Other comets modeled to have come directly from the outer Oort cloud include C/2006 P1 (McNaught), C/2010 X1 (Elenin), Comet ISON, C/2013 A1 (Siding Spring), C/2017 K2, and C/2017 T2 (PANSTARRS). The orbits within the Kuiper belt are relatively stable, so very few comets are thought to originate there. The scattered disc, however, is dynamically active and is far more likely to be the place of origin for comets. Comets pass from the scattered disc into the realm of the outer planets, becoming what are known as centaurs. These centaurs are then sent farther inward to become the short-period comets.
There are two main types of short-period comets: Jupiter-family comets (with orbits smaller than 5 AU) and Halley-family comets. Halley-family comets, named after Halley's Comet, are distinct because, even though they are short-period comets, they are thought to come from the Oort Cloud rather than the scattered disc. Based on their orbits, it is suggested they were long-period comets that were captured by the gravity of the giant planets and sent into the inner Solar System. This process may have also created the present orbits of a significant fraction of the Jupiter-family comets, although the majority of such comets are thought to have originated in the scattered disc.
Oort noted that the number of returning comets was far less than his model predicted, and this issue, known as "cometary fading", has yet to be resolved. No dynamical process is known to explain the smaller number of observed comets than Oort estimated. Hypotheses for this discrepancy include the destruction of comets due to tidal stresses, impact or heating; the loss of all volatiles, rendering some comets invisible, or the formation of a non-volatile crust on the surface. Dynamical studies of hypothetical Oort cloud comets have estimated that their occurrence in the outer-planet region would be several times higher than in the inner-planet region. This discrepancy may be due to the gravitational attraction of Jupiter, which acts as a kind of barrier, trapping incoming comets and causing them to collide with it, just as it did with Comet Shoemaker–Levy 9 in 1994. An example of a typical dynamically old comet with an origin in the Oort cloud could be C/2018 F4.
Tidal effects.
Most of the comets seen close to the Sun seem to have reached their current positions through gravitational perturbation of the Oort cloud by the tidal force exerted by the Milky Way. Just as the Moon's tidal force deforms Earth's oceans, causing the tides to rise and fall, the galactic tide also distorts the orbits of bodies in the outer Solar System. In the charted regions of the Solar System, these effects are negligible compared to the gravity of the Sun, but in the outer reaches of the system, the Sun's gravity is weaker and the gradient of the Milky Way's gravitational Galactic Center compresses it along the other two axes; these small perturbations can shift orbits in the Oort cloud to bring objects close to the Sun. The point at which the Sun's gravity concedes its influence to the galactic tide is called the tidal truncation radius. It lies at a radius of 100,000 to 200,000 au, and marks the outer boundary of the Oort cloud.
Some scholars theorize that the galactic tide may have contributed to the formation of the Oort cloud by increasing the perihelia (smallest distances to the Sun) of planetesimals with large aphelia (largest distances to the Sun). The effects of the galactic tide are quite complex, and depend heavily on the behaviour of individual objects within a planetary system. Cumulatively, however, the effect can be quite significant: up to 90% of all comets originating from the Oort cloud may be the result of the galactic tide. Statistical models of the observed orbits of long-period comets argue that the galactic tide is the principal means by which their orbits are perturbed toward the inner Solar System.
Stellar perturbations and stellar companion hypotheses.
Besides the galactic tide, the main trigger for sending comets into the inner Solar System is thought to be interaction between the Sun's Oort cloud and the gravitational fields of nearby stars or giant molecular clouds. The orbit of the Sun through the plane of the Milky Way sometimes brings it in relatively close proximity to other stellar systems. For example, it is hypothesized that 70,000 years ago Scholz's Star passed through the outer Oort cloud (although its low mass and high relative velocity limited its effect). During the next 10 million years the known star with the greatest possibility of perturbing the Oort cloud is Gliese 710. This process could also scatter Oort cloud objects out of the ecliptic plane, potentially also explaining its spherical distribution.
In 1984, physicist Richard A. Muller postulated that the Sun has an as-yet undetected companion, either a brown dwarf or a red dwarf, in an elliptical orbit within the Oort cloud. This object, known as Nemesis, was hypothesized to pass through a portion of the Oort cloud approximately every 26 million years, bombarding the inner Solar System with comets. However, to date no evidence of Nemesis has been found, and many lines of evidence (such as crater counts), have thrown its existence into doubt. Recent scientific analysis no longer supports the idea that extinctions on Earth happen at regular, repeating intervals. Thus, the Nemesis hypothesis is no longer needed to explain current assumptions.
A somewhat similar hypothesis was advanced by astronomer John J. Matese of the University of Louisiana at Lafayette in 2002. He contends that more comets are arriving in the inner Solar System from a particular region of the postulated Oort cloud than can be explained by the galactic tide or stellar perturbations alone, and that the most likely cause would be a Jupiter-mass object in a distant orbit. This hypothetical gas giant was nicknamed Tyche. The WISE mission, an all-sky survey using parallax measurements in order to clarify local star distances, was capable of proving or disproving the Tyche hypothesis. In 2014, NASA announced that the WISE survey had ruled out any object as they had defined it.
Future exploration.
Space probes have yet to reach the area of the Oort cloud. "Voyager 1", the fastest and farthest of the interplanetary space probes currently leaving the Solar System, will reach the Oort cloud in about 300 years and would take about 30,000 years to pass through it. However, around 2025, the radioisotope thermoelectric generators on "Voyager 1" will no longer supply enough power to operate any of its scientific instruments, preventing any further exploration by "Voyager 1." The currently escaping the Solar System have either already stopped functioning ("Pioneer 10", "Pioneer 11") or are predicted to also stop functioning before they reach the Oort cloud ("Voyager 2", "New Horizons").
In the 1980s, there was a concept for a probe that could reach 1,000 AU in 50 years, called "TAU"; among its missions would be to look for the Oort cloud.
In the 2014 Announcement of Opportunity for the Discovery program, an observatory to detect the objects in the Oort cloud (and Kuiper belt) called the "Whipple Mission" was proposed. It would monitor distant stars with a photometer, looking for transits up to 10,000 AU away. The observatory was proposed for halo orbiting around L2 with a suggested 5-year mission. It was also suggested that the Kepler space telescope could have been capable of detecting objects in the Oort cloud.
See also.
<templatestyles src="Div col/styles.css"/>
References.
<templatestyles src="Reflist/styles.css" />
Explanatory notes.
<templatestyles src="Reflist/styles.css" />

Comet
Natural object in space that releases gas
A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or coma surrounding the nucleus, and sometimes a tail of gas and dust gas blown out from the coma. These phenomena are due to the effects of solar radiation and the outstreaming solar wind plasma acting upon the nucleus of the comet. Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times Earth's diameter, while the tail may stretch beyond one astronomical unit. If sufficiently close and bright, a comet may be seen from Earth without the aid of a telescope and can subtend an arc of up to 30° (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures and religions.
Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. Long-period comets are set in motion towards the Sun by gravitational perturbations from passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition.
Extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids. Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System. However, the discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets. In the early 21st century, the discovery of some minor bodies with long-period comet orbits, but characteristics of inner solar system asteroids, were called Manx comets. They are still classified as comets, such as C/2014 S3 (PANSTARRS). Twenty-seven Manx comets were found from 2013 to 2017.
As of  2021[ [update]], there are 4,584 known comets. However, this represents a very small fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (in the Oort cloud) is about one trillion. Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular. Particularly bright examples are called "great comets". Comets have been visited by uncrewed probes such as NASA's "Deep Impact", which blasted a crater on Comet Tempel 1 to study its interior, and the European Space Agency's "Rosetta", which became the first to land a robotic spacecraft on a comet.
<templatestyles src="Template:TOC limit/styles.css" />
Etymology.
The word "comet" derives from the Old English from the Latin or . That, in turn, is a romanization of the Greek 'wearing long hair', and the "Oxford English Dictionary" notes that the term () already meant 'long-haired star, comet' in Greek. was derived from () 'to wear the hair long', which was itself derived from () 'the hair of the head' and was used to mean 'the tail of a comet'.
The astronomical symbol for comets (represented in Unicode) is , consisting of a small disc with three hairlike extensions.
Physical characteristics.
Nucleus.
The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen carbon dioxide, carbon monoxide, methane, and ammonia. As such, they are popularly described as "dirty snowballs" after Fred Whipple's model. Comets with a higher dust content have been called "icy dirtballs". The term "icy dirtballs" arose after observation of Comet 9P/Tempel 1 collision with an "impactor" probe sent by NASA Deep Impact mission in July 2005. Research conducted in 2014 suggests that comets are like "deep fried ice cream", in that their surfaces are formed of dense crystalline ice mixed with organic compounds, while the interior ice is colder and less dense.
The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. Nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, ethane, and perhaps more complex molecules such as long-chain hydrocarbons and amino acids. In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA's Stardust mission. In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine, and related organic molecules) may have been formed on asteroids and comets.
The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley's Comet (1P/Halley) reflects about four percent of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects less than 3.0%; by comparison, asphalt reflects seven percent. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like tar or crude oil. The low reflectivity of cometary surfaces causes them to absorb the heat that drives their outgassing processes.
Comet nuclei with radii of up to have been observed, but ascertaining their exact size is difficult. The nucleus of 322P/SOHO is probably only in diameter. A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than across. Known comets have been estimated to have an average density of . Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.
Roughly six percent of the near-Earth asteroids are thought to be the extinct nuclei of comets that no longer experience outgassing, including 14827 Hypnos and 3552 Don Quixote.
Results from the "Rosetta" and "Philae" spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals. Further, the ALICE spectrograph on "Rosetta" determined that electrons (within above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma. Instruments on the "Philae" lander found at least sixteen organic compounds at the comet's surface, four of which (acetamide, acetone, methyl isocyanate and propionaldehyde) have been detected for the first time on a comet.
Coma.
The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the "coma". The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous "tail" to form pointing away from the Sun.
The coma is generally made of water and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units (450,000,000 to 600,000,000 km; 280,000,000 to 370,000,000 mi) of the Sun. The parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry. Larger dust particles are left along the comet's orbital path whereas smaller particles are pushed away from the Sun into the comet's tail by light pressure.
Although the solid nucleus of comets is generally less than across, the coma may be thousands or millions of kilometers across, sometimes becoming larger than the Sun. For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun. The Great Comet of 1811 had a coma roughly the diameter of the Sun. Even though the coma can become quite large, its size can decrease about the time it crosses the orbit of Mars around from the Sun. At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, and in doing so enlarging the tail. Ion tails have been observed to extend one astronomical unit (150 million km) or more.
Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflects sunlight directly while the gases glow from ionisation. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye. Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to Comet Holmes.
In 1996, comets were found to emit X-rays. This greatly surprised astronomers because X-ray emission is usually associated with very high-temperature bodies. The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "stealing" one or more electrons from the atom in a process called "charge exchange". This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion by the emission of X-rays and far ultraviolet photons.
Bow shock.
Bow shocks form as a result of the interaction between the solar wind and the cometary ionosphere, which is created by the ionization of gases in the coma. As the comet approaches the Sun, increasing outgassing rates cause the coma to expand, and the sunlight ionizes gases in the coma. When the solar wind passes through this ion coma, the bow shock appears.
The first observations were made in the 1980s and 1990s as several spacecraft flew by comets 21P/Giacobini–Zinner, 1P/Halley, and 26P/Grigg–Skjellerup. It was then found that the bow shocks at comets are wider and more gradual than the sharp planetary bow shocks seen at, for example, Earth. These observations were all made near perihelion when the bow shocks already were fully developed.
The "Rosetta" spacecraft observed the bow shock at comet 67P/Churyumov–Gerasimenko at an early stage of bow shock development when the outgassing increased during the comet's journey toward the Sun. This young bow shock was called the "infant bow shock". The infant bow shock is asymmetric and, relative to the distance to the nucleus, wider than fully developed bow shocks.
Tails.
In the outer Solar System, comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from observations by the Hubble Space Telescope but these detections have been questioned. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.
The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the type II or dust tail. At the same time, the ion or type I tail, made of gases, always points directly away from the Sun because this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. On occasions—such as when Earth passes through a comet's orbital plane, the antitail, pointing in the opposite direction to the ion and dust tails, may be seen.
The observation of antitails contributed significantly to the discovery of solar wind. The ion tail is formed as a result of the ionization by solar ultra-violet radiation of particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an "induced magnetosphere" around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. Because the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called "pick-up ions") congregate and act to "load" the solar magnetic field with plasma, such that the field lines "drape" around the comet forming the ion tail.
If the ion tail loading is sufficient, the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a "tail disconnection event". This has been observed on a number of occasions, one notable event being recorded on 20 April 2007, when the ion tail of Encke's Comet was completely severed while the comet passed through a coronal mass ejection. This event was observed by the STEREO space probe.
In 2013, ESA scientists reported that the ionosphere of the planet Venus streams outwards in a manner similar to the ion tail seen streaming from a comet under similar conditions."
Jets.
Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet's nucleus, like a geyser. These streams of gas and dust can cause the nucleus to spin, and even split apart. In 2010 it was revealed that sublimation of dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus. Infrared imaging of Hartley 2 shows such jets exiting and carrying with it dust grains into the coma.
Orbital characteristics.
Most comets are small Solar System bodies with elongated elliptical orbits that take them close to the Sun for a part of their orbit and then out into the further reaches of the Solar System for the remainder. Comets are often classified according to the length of their orbital periods: The longer the period the more elongated the ellipse.
Short period.
Periodic comets or short-period comets are generally defined as those having orbital periods of less than 200 years. They usually orbit more-or-less in the ecliptic plane in the same direction as the planets. Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, the aphelion of Halley's Comet is a little beyond the orbit of Neptune. Comets whose aphelia are near a major planet's orbit are called its "family". Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.
At the shorter orbital period extreme, Encke's Comet has an orbit that does not reach the orbit of Jupiter, and is known as an Encke-type comet. Short-period comets with orbital periods less than 20 years and low inclinations (up to 30 degrees) to the ecliptic are called traditional Jupiter-family comets (JFCs). Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets (HTCs). As of January 2025[ [update]] there are 73 known Encke-type comets (six of which are classified as Near-earth objects (


Devil's food cake
Moist, airy, rich chocolate layer cake
Devil's food cake is a moist, rich chocolate layer cake.
Because of differing recipes and changing ingredient availability over the 20th century, it is difficult to precisely qualify what distinguishes devil's food from the more standard chocolate cake. However, it traditionally has more chocolate than a regular chocolate cake, making it darker in color and with a heavier texture.
Devil's food cake was invented in the United States in the early 20th century.
Ingredients.
Devil's food cake traditionally uses unsweetened chocolate baking squares in lieu of unsweetened cocoa powder. However, contemporary recipes typically use cocoa powder for its convenience over the more traditional chocolate baking squares. Also, because of its reduced amount of cocoa butter, cocoa powder has a more intense chocolate flavor than unsweetened chocolate. Moreover, coffee is frequently added as a liquid to enhance the chocolate flavor. Some recipes use hot or boiling water as the cake's main liquid, rather than milk. Its antithetical counterpart, the angel food cake, is a very light white cake that uses stiffly beaten egg whites and no dairy.
Devil's food cake is sometimes distinguished from other chocolate cakes by the use of additional baking soda (sodium bicarbonate), which raises the pH level and makes the cake a deeper and darker mahogany color. Devil's food cake incorporates butter (or a substitute), flour, and less egg than other chocolate cakes.
Presentation.
The cake is often paired with a rich chocolate frosting.
History.
Recipes for devil's food cakes appeared in two cookbooks published in 1902. Chocolate cake had become popular in the late 19th century, eventually leading toward development of devil's food cake.
The name probably had several sources of inspiration, including the culinary term "deviled" to describe flavorful foods like deviled eggs and the contrast of this dark, dense, flavorful cake with the light and airy angel food cake. The name has inspired humorous comments; one of the first printed recipes declares it to be "Fit for Angels", and another early recipe recommends topping it with divinity frosting.
The first commercial cake mix for devil's food cake was marketed by P. Duff and Sons, Inc. of Pennsylvania in the 1930s.
References.
<templatestyles src="Reflist/styles.css" />

Dark chocolate
Chocolate with high cocoa solid content
Dark chocolate is a form of chocolate made from cocoa solids, cocoa butter and sugar. It has a higher cocoa percentage than white chocolate, milk chocolate, and semisweet chocolate. Dark chocolate is valued for claimed—though unsupported—health benefits, and for being a sophisticated choice of chocolate. Similar to milk and white chocolate, dark chocolate is used to make chocolate bars and as a coating for confectionery.
Dark chocolate gained much of its reputation from an effort by French chocolatiers in the late 20th century to assert a preference for dark chocolate in the French national palate, over milk chocolate. As this palate was exported to other countries, such as the United States, the associated values of terroir, bean-to-bar chocolate making and gourmet chocolates followed. Due to the high cocoa percentage, dark chocolate can contain particularly high amounts of heavy metals such as lead and cadmium. As of 2024, the dark chocolate market was projected to grow by over 9% per annum for the next four years, particularly driven by growth in the European market.
Dark chocolate has a more bitter and intense flavor than other types of chocolate, and compared to other types of chocolate, is reliant on the quality of the cocoa beans and cocoa butter used as ingredients. Making dark chocolate involves a process of mixing, refining, conching, and standardizing. Government and industry standards of what products may be labeled "dark chocolate" vary by country and market.
Terminology.
In Britain, dark chocolate is known as plain chocolate. In North America and continental Europe, dark chocolates are categorized into sweet, semi-sweet, and bittersweet chocolate, the last used as a general term in North America for dark chocolate. Dark chocolate without added sweetener is known as bitter chocolate, unsweetened chocolate, or 100% chocolate.
History.
After the domestication of the cacao tree over 5000 years ago, indigenous Americans began producing cacao beverages. The earliest people known to have used the cacao plant are the ancient Mayo-Chinchipe people of what’s now Ecuador in circa 3300 BC. While it is unclear when non-alcoholic cacao beverages were first consumed, some academics claim this occurred by 1650BC. Other academics claim this occurred by circa 1800 BC by ancient Olmec people of what’s now Mexico and Central America. When the Spanish conquistadors first tasted chocolate around 1520, there were many different variations of cacao beverages, of which one, made in the Guatemala, was referred to as chocolate. After this area became a dominant producer of cacao in the later 16th century, chocolate became the word for all cacao drinks. Europeans adapted chocolate with Old World ingredients, including sugar, and after bringing it to Europe, it spread and became popular among the elite. Over the following centuries it became simpler and less spiced.
In 1828, Coenraad Johannes van Houten received a patent for the manufacturing process for making Dutch cocoa, removing cocoa butter from cocoa liquor, and creating the potential for mass production. In 1847, the first modern chocolate bar was created by the British chocolate maker Fry's, and over the next century it would be improved with a series of new techniques including conching and tempering. With the invention of the modern milk chocolate in 1875, the term "dark chocolate" was coined to distinguish the traditional chocolate from the new form. By 1899 in the United States, dark chocolate was considered masculine and inappropriate for children. For much of the following century in the US, dark chocolate was not consumed by itself, but together with other foods such as ice cream, nuts and coconut. During the World Wars, dark chocolate was fortified with vitamins A, B1, B2, C, D, niacin and sometimes calcium to prevent malnutrition. Beginning in the 1930s and 1960s respectively, British chocolate makers Rowntree's and Cadbury began associating their dark chocolate products with upper-class women in their advertising. This was a response to market research of the time claiming dark chocolate was favored by the higher social classes, compared to a working class who preferred milk chocolate.
During the late 1970s, when the price of cocoa was very low, the palate of French connoisseurs moved strongly in favor of dark chocolates and against milk chocolate after advocacy from chocolatier Robert Linxe. At the same time, some American consumers began to take interest in dark chocolates for the first time, often sourcing product from France and Belgium. The following decade, a nationwide campaign in France aimed to move the public to appreciate locally produced dark chocolates with sophisticated flavor. Taste makers and producers worked together to create flavor standards using concepts borrowed from wine connoisseurship. This was motivated by foreign firms capturing swathes of the French confectionary market at the expense of local chocolatiers. Simultaneously, high-quality dark chocolate began to be attributed psychoactive and possibly aphrodisiac qualities due to its theobromine content. In America, broader recognition of dark chocolate as distinct from milk chocolate, and of the concepts of bean origin and the percentage of cocoa arose in the years following 1984, when French chocolate manufacturer Valrhona entered the US market, selling initially to pastry and chocolate manufacturers, and later to the general public.
In the 1990s, French flavor standards, having gained mass uptake domestically, were intentionally exported to the United States. These standards, laid out in published guides and chocolate tastings, evoked terroir, bean varietals and estate growths. Chocolates with high cocoa content and novel flavors (such as pepper, ginger and fennel) were promoted. These chocolates were significantly more expensive than the chocolates that had previously consumed. By the late 2000s, a preference for dark chocolate was seen as the sign of a "discriminating palate" in the United States.
Characteristics.
Dark chocolate is harder than milk chocolate, due to its particle size and fat and lecithin content. Dark chocolate has a more bitter and intense flavor than milk chocolate, due to a higher proportion of cocoa mass, which contains theobromine, caffeine, l-leucine, and catechin flavonoids. Compared to other types of chocolate, the quality of cocoa beans is more important for flavor, and the most highly flavored cocoa butters are reserved for dark chocolates. Dark chocolate can exhibit burnt, chocolate, smoky, nutty and sour flavors, among others. These variations are particularly noticeable among dark chocolates using single-origin cocoa beans. While the appeal of the taste of sugary and milky milk chocolate is often immediately apparent, dark chocolate's more complex flavors can lead to it being more of an acquired taste.
Dark chocolate contains 60 compounds contributing to its flavor, with 33 of these considered particularly important. Basic and neutral compounds are responsible for a "chocolate" flavor, while acidic compounds contributed more sweet flavors. Some of these compounds are the product of Maillard reactions. Some manufacturers age dark chocolate to improve flavor. This is done at a minimum for a few weeks, and it is debated whether ageing for more time is desirable. The antioxidants in cocoa solids are responsible for preserving chocolate; dark chocolate has a shelf life of about two years, longer than milk chocolate. Dark chocolate can range in color from mahogany to black.
Health effects.
Nutrition.
Nutrients in dark chocolate include 46% carbohydrates, 43% fats, 8% protein, and 1% water (table). In a reference serving, dark chocolate provides of food energy, and is a rich source (defined as more than 20% of the Daily Value, DV) of several dietary minerals, including copper, iron, magnesium, manganese, phosphorus, potassium, and zinc (table).
Research.
While dark chocolate is popularly considered a health food, only limited high-quality clinical research has been conducted to evaluate the effects of compounds found in cocoa on physiological outcomes, such as blood pressure, for which only small (1–2 mmHg) changes resulted from short-term, high consumption of chocolate up to 105 grams and 670 milligrams of flavonols per day. A recent 2024 study though showed that those who consumed more than five servings a week of dark chocolate had about 20% lower risk of developing type 2 diabetes.
Flavanols found in dark chocolate include the monomers catechin and epicatechin, and (to a lesser extent) the polymeric procyanidins, which remain under laboratory research. Flavanols break down as cocoa beans are fermented, roasted and treated with the Dutch process. To consume enough cocoa flavanols for the presumed effects claimed in some limited studies requires eating at least around of dark chocolate a day, which also involves intake of significant amounts of sugar and saturated fats. The chocolate industry, and in particular Mars, Inc., has funded research to promote chocolate as a health food. As of 2018, Mars had funded more than 150 studies into cocoa flavanols since the 1980s. That year, they said they would no longer try to imply chocolate is a health food.
A 2021 systematic review of the health effects of chocolate and cocoa found high-quality research had still not been performed to evaluate physiological outcomes. The only health effects observed were improvements in lipid profiles; control subjects showed no significant differences in terms of skin, cardiovascular, anthropometric, cognitive and quality of life outcomes.
Metal content.
Chocolate, particularly dark chocolate, may contain appreciable levels of toxic heavy metals, such as cadmium, which may be present naturally in the soil of cocoa plantations, particularly in Latin America. For products containing over 50% cocoa, the European Commission has set a limit for cadmium of 0.8mg/kg, while for chocolate containing between 30%–50% cocoa, the limit is 0.3mg/kg. After these limits came into enforcement in 2019, similar regulations were enforced in other jurisdictions across the world. The state of California recommends a maximum daily intake of 4.1 micrograms of cadmium.
A 2024 report analyzing dark chocolate and cocoa samples in the United States from 2014 to 2022 found that multiple samples exceeded Prop 65 levels for heavy metals: 43% of samples exceeding levels for lead, and 35% exceeding levels for cadmium. However, a 2024 study published in Food Research International found that high metal content in dark chocolate may pose health concerns mainly only for children.
Manufacturing.
Ingredients.
Dark chocolate is mainly a combination of chocolate liquor, cocoa butter and sugar. Milk fats are often added to dark chocolate to delay the formation of chocolate bloom; dark chocolate containing 1–2% milk fat experiences a delay in blooming. Milk fats are also added by some manufacturers to slightly soften dark chocolate and allow for more flavor to be released. During the final step of standardizing, emulsifiers such as lecithin or PGPR are added to improve texture. Some manufacturers add vanilla or vanillin as a flavoring.
Many dark chocolate products sold label the cocoa percentage. This percentage refers to the percent of the chocolate that is chocolate liquor and cocoa butter, with almost all of the rest being sugar. As what part is chocolate liquor and what part is cocoa butter is not identified, chocolates with an identical cocoa percentage can have vastly different compositions and characteristics. Chocolates containing more chocolate liquor are more intensely flavored and more viscous. Variability in the quality of cocoa beans mean chocolates with lower cocoa percentages can have more desirable flavors than those with higher percentages.
Process.
The basic process of making dark chocolate involves mixing, refining, conching and standardizing. With its relatively low viscosity, dark chocolate is the easiest chocolate to handle in manufacturing. In the mixing stage, chocolate liquor is combined in a melanger with sugar and some cocoa butter, mixing until a paste is formed. As long as the cocoa mass has been correctly milled, the refining process for dark chocolate is primarily about grinding down sugar. Refining involves putting chocolate through a refiner machine, which carries the cocoa mass through large steel rollers set to varying widths, enveloping them in fat until the chocolate particles are as small as is desired, and the cocoa mass is a fine powder. Refining dark chocolate to be finer makes the cocoa flavor more intense, and the ideal particle size for dark chocolate has been identified as 35μm. Dark chocolates with a higher cocoa mass require special accommodations here, as, with high portions of cocoa butter, there can be issues with the roller refining process. These accommodations can involve beginning the refining process with less cocoa mass, and only integrating the rest during the conching stage. Other alternatives include using cocoa powder or processing in a ball mill.
Conching machines mix and knead a cocoa mass, changing the flavor and texture. More than for other types of chocolate, conching dark chocolate serves to remove undesirable flavors. Dark chocolate is conched at higher temperatures than other chocolates, between . If dark chocolate takes on moisture during the conching process, for example by being conched in the same room as milk chocolate with neither enclosed, the dark chocolate can become undesirably thick and develop unpleasant flavors. During the final step of standardizing, emulsifiers and sometimes vanilla flavorings are added. If needed to reach a desired yield or viscosity, more cocoa butter can be added before the mixture undergoes laboratory testing for particle size and food safety. If these standards are met, it is pumped through fine screens to remove any agglomerates. Tempering is performed at different temperatures than for milk chocolate because of how milk fats impact the formation of crystal lattices.
Legal requirements.
According to a 2000 European Union directive, dark chocolate must contain at minimum 18% cocoa butter, not less than 35% total dry cocoa solids and a minimum of 14% dry non-fat cocoa solids. As of 2017, the United States Food and Drug Administration did not regulate a standard identity for dark chocolate, which had led to concerns that some products were made from vegetable fats and customers had been misled by health claims.
Market.
During the 2010s, demand for high-cocoa dark chocolate increased in light of research linking the antioxidant content with a reduced risk of cancer and cardiovascular health. As of 2019, demand for dark chocolate containing more than 70% cocoa, particularly using beans from a single-source, was increasing.
France is considered the "home of dark chocolate", and the industry there uses the term Grand Cru to refer to particularly valued chocolates. As of 2018, retailers and dark chocolate manufacturers there received an equal portion of over two thirds of the margins generated across the cocoa supply chain for dark chocolate tablets. As of 2016, the dark chocolate market was concentrated more in Continental Europe than the United States and England. 31% of chocolate produced was dark chocolate. From 2005 to 2011, dark chocolate consumption in the US increased by 9% per annum. Beyond beliefs around the healthiness of dark chocolate, increased demand for dark chocolate in the United States has also been attributed to a trend of consumers expanding preferences beyond traditional mass-produced chocolate. In the US, this growth has occurred at the expense of milk chocolate.
A large percentage of chocolates sold by the Swiss chocolate maker Lindt are dark chocolate of varying cocoa solid percentages, including 70%, 85% and 90%. In a 2024 market research report entitled "Dark Chocolate Market Research", the authors identified various forms dark chocolate was sold in: bars, blocks, pieces, chunks, bonbons and as other confectionary items. Across these forms, it was consumed as a snack, and used by confectioners and in recipes. The report identified Europe as one of the largest producers and the market overall as having grown by over 8 percent since the previous year. About a third of sales were identified as from organic chocolate, and the market as influenced by the volatile cost of cocoa beans.
Variants.
Low-sugar dark chocolate is made by replacing sugar with maltitol, a sugar alcohol, which may be replaced with a fiber blend and stevia. According to "Dark Chocolate Market Analysis", the markets for dark chocolates marketed as "gluten-free" and "gourmet" were growing. Those dark chocolates marketed as "gourmet" were identified as made using flavor cocoa, dried fruits, and using sugars such as coconut sugar. Other varieties of dark chocolate identified included artisanal, organic, milk and vegan chocolates. Nuts, cereals, creams, liqueurs and syrups can be added. Dark chocolates range in the percentage of cocoa solids they contain, from products being called dark chocolate with percentages from 40% to 100%.
Flavor cocoas, cocoas purchased for a premium that are valued for their flavor or other qualities are mostly used for dark chocolate. These include single source chocolates. Single source dark chocolates are often from countries such as Ecuador and Venezuela. Fruity, astringent and acidic flavors are highlighted in these chocolates, while flavors of smoke and mold flavors are avoided as they cannot be removed by further processing.
References.
<templatestyles src="Reflist/styles.css" />
Sources.
Books.
<templatestyles src="Refbegin/styles.css" />
Journal articles and reports.
<templatestyles src="Refbegin/styles.css" />

Milk chocolate
Solid chocolate containing added milk
Milk chocolate is a form of solid chocolate containing cocoa, sugar and milk. It is the most consumed type of chocolate, and is used in a wide diversity of bars, tablets and other confectionery products. Milk chocolate contains smaller amounts of cocoa solids than dark chocolates do, and (as with white chocolate) contains milk solids. While its taste (akin to chocolate milk) has been key to its popularity, milk chocolate was historically promoted as a healthy food, particularly for children.
Major milk chocolate producers include Ferrero, Hershey, Mondelez, Mars and Nestlé; collectively these supply over half of the world's chocolate. Four-fifths of all milk chocolate is sold in the United States and Europe, and increasing amounts are consumed in both China and Latin America.
Chocolate was originally sold and consumed as a beverage in pre-Columbian times, and upon its introduction to Western Europe. The word "chocolate" arrived in the English language about 1600, but initially described dark chocolate. The first use of the term "milk chocolate" was for a beverage brought to London from Jamaica in 1687, but it was not until the Swiss inventor Daniel Peter successfully combined cocoa and condensed milk in 1875 that the milk chocolate bar was invented. Switzerland developed as the centre of milk chocolate production, particularly after the development of the conche by Rodolphe Lindt, and was increasingly exporting to an international market. Milk chocolate became mainstream at the beginning of the twentieth century following the launch of Milka, Cadbury Dairy Milk and the Hershey bar, inducing a dramatic increase in world cocoa consumption.
To provide ethical assurances on cocoa harvesting for consumers, Fair Trade and UTZ Certified chocolate was established in the 21st century.
History.
The word "chocolate" was first used in English in 1604. The first instance of "milk chocolate" appeared soon after, referring to a drink of chocolate combined with milk. In 1687, Hans Sloane, an Irish physician and collector, introduced the beverage to London after seeing the people of Jamaica enjoying the drink. The preparation was promoted for its medicinal properties, and was manufactured by Nicholas Sanders and William White, and was joined by other milk chocolates around the city. From there, milk chocolate spread, first to France, where the pharmacist to Louis XVI, Sulpice Debauve, introduced the drink to the Court, and then further afield, reaching as far as the United States by 1834.
Early attempts to make eating milk chocolate were foiled by the substantial water content of milk which did not willingly mix with cocoa butter (a fat); efforts at combining the two produced chocolates that were an "oily and milky mess". In Dresden in the German Confederation, Jordan & Timaeus were developing a mechanism to produce hard chocolate using steam power. On 23 May 1839, they advertised a solid chocolate containing fresh milk, calling it "steam chocolate" (). However, that version of milk chocolate did not become successful and when major companies like Fry's of Bristol and Lindt of Zürich started producing eating chocolate in the 1840s, they only made dark chocolate.
In 1875, the Swiss entrepreneur and chocolatier Daniel Peter, based in Vevey and related to the Cailler family, first successfully combined cocoa mass, cocoa butter, and sugar with condensed milk (recently created by his neighbour and friend Henri Nestlé) to produce solidified milk chocolate. However, this was not meant to be eaten, but it was meant to be used for chocolate milk drinks; milk chocolate as we know it today was only created a decade later. In 1887, the first eating milk chocolate brand, Gala Peter, was finally launched. Daniel Peter called his product 'Gala' after the Greek word meaning 'milk'. Milk chocolate also benefited from another recent Swiss invention, Rodolphe Lindt's conche, which allowed the creation of a smoother chocolate.
Not only did milk soften the bitterness of chocolate and refined its taste, but it also lowered its production cost due to a lower cocoa content. As a consequence, Peter's recipe leaked to other nearby manufacturers: Cailler and Kohler. In 1898, Cailler opened its new factory at Broc, where milk chocolate began to be produced on a large scale. Peter also opened a larger factory at Orbe in 1901, before merging with Kohler. The same year, Suchard of Neuchâtel launched the Milka brand; Carl Russ-Suchard had previously developed a first milk bar in 1896. The Swiss chocolate industry also expanded in the late nineteenth century with the establishment of new companies, such as Frey and Tobler. From these developments, Switzerland soon dominated the chocolate market. Production increased dramatically, and by 1905, the country was producing of chocolate, a vast proportion of it exported.
Meanwhile, there were other developments outside Switzerland. Swiss dominance was challenged in 1905 by a product from England, Cadbury Dairy Milk. Although there had been other milk chocolates produced outside Switzerland before – Cadbury themselves had produced one in 1897 – they suffered from low sales. In contrast, Dairy Milk quickly rose in prominence and, by the 1920s, was the bestselling chocolate in the UK. Simultaneously, in 1900, Milton Hershey had introduced the first Hershey bar, which revolutionised the popularity of milk chocolate in the United States. Although initially only available in Pennsylvania, by 1906 it was sold across the country. Popularity blossomed, particularly following World War I, when the United States Army issued chocolate bars to troops, for many their first taste of milk chocolate. By 1911, Peter's milk chocolate recipe represented half of the world's chocolate consumption. Milk chocolate became the standard of what the public thought chocolate should be.
As a result of the increasing popularity of chocolate, especially among the working and middle-class, cocoa consumption began to grow extraordinarily; global demand grew 800 percent between 1880 and 1900. To meet these demands, cocoa production expanded, notably in West Africa, where the Forastero variety began to be mass cultivated in the early twentieth century. Although considered inferior to the Criollo variety, the Forastero type bean is more suited for the manufacture of milk chocolate and is cheaper to produce owing to its higher yields. Countries in West Africa eventually dominated world production of cocoa. Conversely, milk became the critical ingredient. Contrary to cocoa and sugar, milk spoils quickly, therefore it cannot be stored for long periods of time. This favored the implantation of large factories (as well as new populations of workers) in the countryside, where abundant fresh milk supplies are readily available. The Cailler factory of Broc and the Hershey factory of Derry Township are typical examples. The popularity of milk chocolate and the wide availability of milk also favoured the creation in the 1930s of a new variety of chocolate containing even more milk: white chocolate.
Over the decades, milk chocolate manufacture spread worldwide and new brands appeared. In 1910, Arthur and George Ensor created the first milk chocolate in Canada, using milk from Jersey cows. At the same time, Belgian chocolate production also expanded rapidly. From small beginnings in the 1870s, by the 1920s, there were ninety chocolate manufacturers around Brussels alone. In 1926, Meiji brought out their bar, the first example to be made in Japan. Milk chocolate swiftly dominated chocolate sales in most markets. It even found a place during World War II, when US troops carried D Ration chocolate, nicknamed Logan Bars after Quartermaster Paul Logan, as an emergency supply. At the same time, new ways of presenting chocolate appeared, from different shapes, like Cadbury Buttons, to the profusion of boxed varieties that became a feature of Belgian chocolate.
At the same time, the number of independent manufacturers declined sharply. The first consolidations in the industry were in Switzerland, where the takeover of Lindt by Sprüngli took place in 1899, and Nestlé had already emerged as the largest manufacture in the country by 1929. However, pace quickened in the latter half of the century. During the last three decades of the twentieth century, there were over 200 takeovers in the industry. By 2001, over half the global chocolate market was held by 17 companies. By 2013, the top four manufacturers, Mondelez, Mars, Nestlé and Ferrero, comprised 49 percent of the sales.
In 2018, the global market for milk chocolate was worth $63.2 billion, and is expected to approach $73 billion by 2024. Consumption is dominated by the United States and Europe, which between them consumed over 80 percent of global production. However, the new century saw expansion in different markets. For example, between 2000 and 2013, the areas that saw the highest growth included the Middle East and Africa (where retail value rose 239 percent), Latin America (up 228 percent). Even in China and Japan, which traditionally are places of very low milk consumption, milk chocolate sales increased at the start of the twenty-first century. Between 1999 and 2003, Chinese chocolate imports rose from $17.7 million to $50 million. By 2007, over 38 percent of chocolate sales in China were milk chocolate. By 2018, the value of sales by Japanese chocolatier Meiji was approaching that of the top producers in Europe, and the total sales by the group had surpassed the total for all confectionery sales by Hershey, putting the American company outside a top five ranking.
Although dark chocolate regained some popularity in the late twentieth century, milk chocolate remains the most preferred and consumed type of chocolate. Some chocolate consumers, who perceive it as saccharine and lacking in chocolate flavor, hold it in poor regard; chocolate maker John Scharffenberger for instance characterized the cocoa content as only being "food coloring".
Nutrition.
Milk chocolate is 51% carbohydrates, 38% fat, 8% protein, and 2% water (table). In a reference amount of , milk chocolate supplies 565 calories of food energy, and is a rich source (20% or more of the Daily Value, DV) of vitamin B12 (31% DV), riboflavin (25% DV), and dietary minerals, such as phosphorus (30% DV) (table). Milk chocolate has moderate content (10–19% DV) of thiamine and several minerals.
Manufacturing and marketing.
While all milk chocolate contains cocoa, milk and sugar, the proportion of these ingredients varies between countries and brands, which in turn affects its taste. For example, Belgian chocolate is known for its mild milky flavor, while some Russian brands have a strong cocoa taste. Cost is the main reason for the introduction of cocoa butter replacements like coconut and palm oil. However, there are also regulatory reasons. In 1973, for example, the European Union decreed that chocolate must have a minimum of 35 percent dry cocoa solids. China has also introduced legislation to require locally produced milk chocolate to contain 25 percent cocoa butter.
Milk chocolate has been presented as a health food since Cadbury first advertised Sloane's Milk Chocolate for its medicinal properties in the nineteenth century. In the 1920s, the Baby Ruth bar was touted as a health food for children by Allan Roy Dafoe. Advertisements pronounced that chocolate bars combined both a source of essential energy and the "perfectly balanced food" of milk.
Cocoa butter was claimed to reduce tooth decay. Chocolate was claimed to produce calming effects, reducing stress, and producing a similar feeling to falling in love.
Processing.
Milk chocolate is manufactured from cocoa, milk and sugar. It is the manufacturing process, rather than the raw ingredients, which is most responsible for each brand's flavor.
The ingredient which defines the product as chocolate, cocoa bean, is mainly grown in Southeast Asia, South America, and West Africa, particularly the Ivory Coast, which supplies 40 percent of the total global cocoa market. Once the cocoa pods are harvested, the seeds, known as "beans", are removed and fermented, then dried. They are then taken to a processing plant where they are cleaned and roasted. The beans are then ground, usually in a two-stage process, first with an impact mill to liquify the cocoa, and then a ball mill. Milk chocolate usually contains a much larger proportion of cocoa butter than the one that is naturally present in cocoa liquor; unlike dark chocolate, a large part of non-fat cocoa solids is going to be replaced by milk solids. Therefore, cocoa butter has to be produced in parallel by separating cocoa liquor into cocoa butter and cocoa powder. Milk chocolate has a minimum cacao content of 10% in the US, and has been produced with as much as 70% cacao.
At this stage, the two other key ingredients come into the process: milk and sugar. Milk ingredients are complex and critical in delivering the properties and taste to milk chocolate. Milk-origin (terroir) and associated farming have become an important marketing topic. Milk substitutes like rice milk are also used to create lactose-free milk-like chocolate. Milk is often added in powdered form, particularly in German, French, and Belgian milk chocolate, as excess water would damage the flowing properties of the liquid chocolate. Spray dried full-fat milk powder is normally used, but alternatives include anhydrous full fat or skimmed milk powders, and the choice affects the overall flavor. Condensed milk is preferred by some manufacturers, particularly where milk production is seasonal. In most of Europe, milk chocolate must contain at least 3.5% milkfat.
Sugar, the last major ingredient, is added at the same time as the milk powder, either in a roll refiner or conche. Sugar is an international commodity, with production of sugar cane led by Brazil, India, Thailand, China and Australia. Sugar beet is also used. Sometimes the milk and sugar are mixed separately before being added to the liquid cocoa mass and cocoa butter. About 45 to 50% of most milk chocolate is sugar, by weight.
The liquid chocolate is then poured into moulds and formed into bars or any other shape.
Chocolate crumb.
This is the original method developed by Daniel Peter to make milk chocolate. It consists of mixing cocoa liquor with sweetened condensed milk and drying it into a hard, dry, brittle powder resembling bread crumbs. The powder is then refined with the additional cocoa butter.
British milk chocolate derives its characteristic, slightly cooked flavor by using a dehydrated blend of milk, sugar, and cocoa called chocolate crumb. Originally developed because milk production was high during the summer but chocolate demand was highest during the Christmas shopping season, the cocoa and sugar preserve the milk fats better than full-cream milk powder. The process of making chocolate crumb usually produces a Maillard reaction, resulting in a subtle "cooked", caramel flavor.
The Hershey process.
The actual Hershey process is a trade secret, but experts speculate that the milk is partially lipolyzed, producing butyric acid, and then the milk is pasteurized, stabilizing it for use. The resulting milk chocolate has been described by experts as "tangy", "sour", and "acidified".
National preferences.
Milk chocolate developed in different places, using different processes and locally available technology, and the end result is that milk chocolate produced in different countries has different characteristic flavor profiles. For example, British milk chocolate tastes slightly cooked or baked, American milk chocolate tastes more acidic, Swiss milk chocolate has a fresh milk flavor, and Belgian milk chocolate has more cocoa flavor and less milk flavor than the Swiss milk chocolate. This is primarily due to the different approaches to preparing and incorporating milk into the chocolate.
No matter what the flavor is, consumers prefer the style that they are accustomed to and dislike less familiar flavors. Multinational chocolate producers adapt their products to the style preferred locally. When that hasn't been done, the product generally sells poorly. For example, the Hershey process gives that brand's milk chocolate a particular taste, which is common and expected in the US, so some rival manufacturers now add butyric acid to their milk chocolates. Cadbury's attempted to introduce their Cadbury Dairy Milk recipe, using the chocolate crumb process, to the former East Germany, which was accustomed to the flavor profile of milk chocolate made from powdered milk, and to the US, which was accustomed to the flavor profile of milk chocolate made through the Hershey's process, and in both cases the unfamiliar flavor proved less popular than they expected.
Milk chocolate combination bars.
At the beginning of the twentieth century, bars which combined milk chocolate with other sweet ingredients appeared on both sides of the Atlantic. In 1904, Cailler launched its Branche, a praline-filled and branch-looking bar. Other Swiss chocolatiers, Theodor Tobler and Emil Baumann, invented Toblerone in 1908 which contained almonds, honey and nougat. In the United States, the Goo Goo Cluster was first introduced in 1912 which added caramel, marshmallow and peanuts to the list of ingredients. These were soon followed, in 1914, by Fry's Turkish Delight in the UK. Shortly afterwards, Clark introduced the Clark Bar, which has been called the first combination bar, in 1917. In 1920, Otto Schnering of the Curtiss Candy Company created the Baby Ruth bar. By 1925, it was the most popular bar in the US. Soon afterwards, two other brands that would become global giants followed, the Mars Bar in 1932 and, three years later, Rowntree's introduced the Kit Kat. By 2014, 650 Kit Kat bars were being consumed each second. Combination bars came to dominate the confectionery