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.
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References.
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Explanatory notes.
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Jan Oort
Dutch astronomer (1900–1992)
Jan Hendrik Oort ( or ; 28 April 1900 – 5 November 1992) was a Dutch astronomer who made significant contributions to the understanding of the Milky Way and who was a pioneer in the field of radio astronomy. "The New York Times" called him "one of the century's foremost explorers of the universe"; the European Space Agency website describes him as "one of the greatest astronomers of the 20th century" and states that he "revolutionised astronomy through his ground-breaking discoveries." In 1955, Oort's name appeared in "Life" magazine's list of the 100 most famous living people. He has been described as "putting the Netherlands in the forefront of postwar astronomy".
Oort determined that the Milky Way rotates and overturned the idea that the Sun was at its center. He also postulated the existence of the mysterious invisible dark matter in 1932, which is believed to make up roughly 84.5% of the total mass in the Universe and whose gravitational pull causes "the clustering of stars into galaxies and galaxies into connecting strings of galaxies". He discovered the galactic halo, a group of stars orbiting the Milky Way but outside the main disk. Additionally Oort is responsible for a number of important insights about comets, including the realization that their orbits "implied there was a lot more solar system than the region occupied by the planets."
The Oort cloud, the Oort constants, Oort Limit, an impact crater on Pluto (Oort), and the asteroid 1691 Oort were all named after him.
Early life and education.
Oort was born in Franeker, a small town in the Dutch province of Friesland, on April 28, 1900. He was the second son of Abraham Hermanus Oort, a physician, who died on May 12, 1941, and Ruth Hannah Faber, who was the daughter of Jan Faber and Henrietta Sophia Susanna Schaaii, and who died on November 20, 1957. Both of his parents came from families of clergymen, with his paternal grandfather, a Protestant clergyman with liberal ideas, who "was one of the founders of the more liberal Church in Holland" and who "was one of the three people who made a new translation of the Bible into Dutch." The reference is to Henricus Oort (1836–1927), who was the grandson of a famous Rotterdam preacher and, through his mother, Dina Maria Blom, the grandson of theologian Abraham Hermanus Blom, a "pioneer of modern biblical research". Several of Oort's uncles were pastors, as was his maternal grandfather. "My mother kept up her interests in that, at least in the early years of her marriage", he recalled. "But my father was less interested in Church matters."
In 1903 Oort's parents moved to Oegstgeest, near Leiden, where his father took charge of the Endegeest Psychiatric Clinic. Oort's father, "was a medical director in a sanitorium for nervous illnesses. We lived in the director's house of the sanitorium, in a small forest which was very nice for the children, of course, to grow up in." Oort's younger brother, John, became a professor of plant diseases at the University of Wageningen. In addition to John, Oort had two younger sisters and an elder brother who died of diabetes when he was a student.
Oort attended primary school in Oegstgeest and secondary school in Leiden, and in 1917 went to Groningen University to study physics. He later said that he had become interested in science and astronomy during his high-school years, and conjectured that his interest was stimulated by reading Jules Verne. His one hesitation about studying pure science was the concern that it "might alienate one a bit from people in general", as a result of which "one might not develop the human factor sufficiently." But he overcame this concern and ended up discovering that his later academic positions, which involved considerable administrative responsibilities, afforded a good deal of opportunity for social contact.
Oort chose Groningen partly because a well known astronomer, Jacobus Cornelius Kapteyn, was teaching there, although Oort was unsure whether he wanted to specialize in physics or astronomy. After studying with Kapteyn, Oort decided on astronomy. "It was the personality of Professor Kapteyn which decided me entirely", he later recalled. "He was quite an inspiring teacher and especially his elementary astronomy lectures were fascinating." Oort began working on research with Kapteyn early in his third year. According to Oort one professor at Groningen who had considerable influence on his education was physicist Frits Zernike.
After taking his final exam in 1921, Oort was appointed assistant at Groningen, but in September 1922, he went to the United States to do graduate work at Yale and to serve as an assistant to Frank Schlesinger of the Yale Observatory.
Career.
At Yale, Oort was responsible for making observations with the Observatory's zenith telescope. "I worked on the problem of latitude variation", he later recalled, "which is quite far away from the subjects I had so far been studying." He later considered his experience at Yale useful as he became interested in "problems of fundamental astronomy that [he] felt was capitalized on later, and which certainly influenced [his] future lectures in Leiden." Personally, he "felt somewhat lonesome in Yale", but also said that "some of my very best friends were made in these years in New Haven."
Early discoveries.
In 1924, Oort returned to the Netherlands to work at Leiden University, where he served as a research assistant, becoming Conservator in 1926, Lecturer in 1930, and Professor Extraordinary in 1935. In 1926, he received his doctorate from Groningen with a thesis on the properties of high-velocity stars. The next year, Swedish astronomer Bertil Lindblad proposed that the rate of rotation of stars in the outer part of the galaxy decreased with distance from the galactic core, and Oort, who later said that he believed it was his colleague Willem de Sitter who had first drawn his attention to Lindblad's work, realized that Lindblad was correct and that the truth of his proposition could be demonstrated observationally. Oort provided two formulae that described galactic rotation; the two constants that figured in these formulae are now known as "Oort's constants". Oort "argued that just as the outer planets appear to us to be overtaken and passed by the less distant ones in the solar system, so too with the stars if the Galaxy really rotated", according to the "Oxford Dictionary of Scientists". He "was finally able to calculate, on the basis of the various stellar motions, that the Sun was some 30,000 light-years from the center of the Galaxy and took about 225 million years to complete its orbit. He also showed that stars lying in the outer regions of the galactic disk rotated more slowly than those nearer the center. The Galaxy does not therefore rotate as a uniform whole but exhibits what is known as 'differential rotation'."
These early discoveries by Oort about the Milky Way overthrew the Kapteyn system, named after his mentor, which had envisioned a galaxy that was symmetrical around the Sun. As Oort later noted, "Kapteyn and his co-workers had not realized that the absorption in the galactic plane was as bad as it turned out to be." Until Oort began his work, he later recalled, "the Leiden Observatory had been concentrating entirely on positional astronomy, meridian circle work and some proper motion work. But no astrophysics or anything that looked like that. No structure of the galaxy, no dynamics of the galaxy. There was no one else in Leiden who was interested in these problems in which I was principally interested, so the first years I worked more or less by myself in these projects. De Sitter was interested, but his main line of research was celestial mechanics; at that time the expanding universe had moved away from his direct interest." As the European Space Agency states, Oort "sh[ook] the scientific world by demonstrating that the Milky Way rotates like a giant 'Catherine Wheel'." He showed that all the stars in the galaxy were "travelling independently through space, with those nearer the center rotating much faster than those further away."
This breakthrough made Oort famous in the world of astronomy. In the early 1930s he received job offers from Harvard and Columbia University, but chose to stay at Leiden, although he did spend half of 1932 at the Perkins Observatory, in Delaware, Ohio.
In 1934, Oort became assistant to the director of Leiden Observatory; the next year he became General Secretary of the International Astronomical Union (IAU), a post he held until 1948; in 1937 he was elected to the Royal Academy. In 1939, he spent half a year in the U.S., and became interested in the Crab Nebula, concluding in a paper, written with American astronomer Nicholas Mayall, that it was the result of a supernova explosion.
Nazi invasion of Netherlands.
In 1940, Nazi Germany invaded the Netherlands. Soon after, the occupying regime dismissed all Jewish professors from Leiden University and other universities. "Among the professors who were dismissed", Oort later recalled, "was a very famous … professor of law by the name of Meyers. On the day when he got the letter from the authorities that he could no longer teach his classes, the dean of the faculty of law went into his class … and delivered a speech in which he started by saying, 'I won't talk about his dismissal and I shall leave the people who did this, below us, but will concentrate on the greatness of the man dismissed by our aggressors.'"
This speech (26 November 1940) made such an impression on all his students that on leaving the auditorium they defiantly sang the anthem of the Netherlands and went on strike. Oort was present for the lecture and was greatly impressed. This occasion formed the beginning of the active resistance in Holland. The speech by Rudolph Cleveringa, the dean of the faculty of Law and former graduate student of professor Meijers, was widely circulated during the rest of the war by the resistance groups. Oort was in a little group of professors in Leiden who came together regularly and discussed the problems the university faced in view of the German occupation. Most of the members of this group were put in hostage camps soon after the speech by Cleveringa. Oort refused to collaborate with the occupiers, "and so we went down to live in the country for the rest of the war." Resigning from the Royal Academy, from his professorial post at Leiden, and from his position at the Observatory, Oort took his family to Hulshorst, a quiet village in the province of Gelderland, where they sat out the war. In Hulshorst, he began writing a book on stellar dynamics.
Oort's radio astronomy.
Before the war was over, he initiated, in collaboration with a Utrecht University student, Hendrik van de Hulst, a project that eventually succeeded, in 1951, in detecting the 21-centimeter radio emission from interstellar hydrogen spectral line at radio frequencies. Oort and his colleagues also made the first investigation of the central region of the Galaxy, and discovered that "the 21-centimeter radio emission passed un-absorbed through the gas clouds that had hidden the center from optical observation. They found a huge concentration of mass there, later identified as mainly stars, and also discovered that much of the gas in the region was moving rapidly outward away from the center." In June 1945, after the end of the war, Oort returned to Leiden, took over as director of the Observatory, and became Full Professor of Astronomy. During this immediate postwar period, he led the Dutch group that built radio telescopes at Radio Kootwijk, Dwingeloo, and Westerbork and used the 21-centimeter line to map the Milky Way, including the large-scale spiral structure, the Galactic Center, and gas cloud motions. Oort was helped in this project by the Dutch telecommunications company, PTT, which, he later explained, "had under their care all the radar equipment that was left behind by the Germans on the coast of Holland. This radar equipment consisted in part of reflecting telescopes of 7 1/2 meter aperture... Our radio astronomy was really started with the aid of one of these instruments… it was in Kootwijk that the first map of the Galaxy was made." For a brief period, before the completion of the Jodrell Bank telescope, the Dwingeloo instrument was the largest of its kind on Earth.
It has been written that "Oort was probably the first astronomer to realize the importance" of radio astronomy. "In the days before radio telescopes," one source notes, "Oort was one of the few scientists to realise the potential significance of using radio waves to search the heavens. His theoretical research suggested that vast clouds of hydrogen lingered in the spiral arms of the Galaxy. These molecular clouds, he predicted, were the birthplaces of stars." These predictions were confirmed by measurements made at the new radio observatories at Dwingeloo and Westerbork. Oort later said that "it was Grote Reber's work which first impressed me and convinced me of the unique importance of radio observations for surveying the galaxy." Just before the war, Reber had published a study of galactic radio emissions. Oort later commented, "The work of Grote Reber made it quite clear [radio astronomy] would be a very important tool for investigating the Galaxy, just because it could investigate the whole disc of the galactic system unimpeded by absorption." Oort's work in radio astronomy is credited by colleagues with putting the Netherlands in the forefront of postwar astronomy. Oort also investigated the source of the light from the Crab Nebula, finding that it was polarized, and probably produced by synchrotron radiation, confirming a hypothesis by Iosif Shklovsky.
Comet studies.
Oort went on to study comets, which he formulated a number of revolutionary hypotheses. He hypothesized that the Solar System is surrounded by a massive cloud consisting of billions of comets, many of them "long-period" comets that originate in a cloud far beyond the orbits of Neptune and Pluto. This cloud is now known as the Oort Cloud. He also realized that these external comets, from beyond Pluto, can "become trapped into tighter orbits by Jupiter, and become periodic comets, like Halley's comet." According to one source, "Oort was one of the few people to have seen Comet Halley on two separate apparitions. At the age of 10, he was with his father on the shore at Noordwijk, Netherlands, when he first saw the comet. In 1986, 76 years later, he went up in a plane and was able to see the famous comet once more."
In 1951 Oort and his wife spent several months in Princeton and Pasadena, an interlude that led to a paper by Oort and Lyman Spitzer on the acceleration of interstellar clouds by O-type stars. He went on to study high-velocity clouds. Oort served as director of the Leiden Observatory until 1970. After his retirement, he wrote comprehensive articles on the galactic center and on superclusters and published several papers on the quasar absorption lines, supporting Yakov Zel'dovich's pancake model of the universe. He also continued researching the Milky Way and other galaxies and their distribution until shortly before his death at 92.
One of Oort's strengths, according to one source, was his ability to "translate abstruse mathematical papers into physical terms," as exemplified by his translation of the difficult mathematical terms of Lindblad's theory of differential galactic rotation into a physical model. Similarly, he "derived the existence of the comet cloud on the outskirts of the Solar System from the observations, using the mathematics needed in dynamics, but then deduced the origin of this cloud using general physical arguments and a minimum of mathematics."
Personal life.
In 1927, Oort married Johanna Maria (Mieke) Graadt van Roggen (190


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.
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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 market with sales surpassing $140 billion in 2018. As of 2023, milk chocolate was most commonly paired globally with caramel flavours and almonds.
Ethical issues.
Ethical issues have been intrinsically linked to chocolate in general since the early days. Many of the early chocolatiers, including Cadbury, Fry's, Rowntree's and Terry's, were founded by Quakers who saw the wellbeing of their workers part of their business ethic. The companies were pioneers in social welfare, providing a safe working environment, high quality housing and other benefits to employees that were ahead of many of the industrial norms. Cadbury, for example, provided paid holidays, insurance and night schools for workers, as well as constructing Bournville in Birmingham, UK. However, the working conditions of many in the wider chocolate supply chain remained poor. Slavery, and later bonded labour, was often employed on the plantations that provided the sugar used to make chocolate. Even after the abolition of slavery, the working conditions in many plantations was still poor, with child labour being frequent and unreported. In 1975, the first in a series of International Cocoa Agreements tried to set what were termed "fair labour conditions" and eliminate child labour.
Rising consumer awareness, as well as greater corporate and employee interest, led to increasing voluntary action to address human rights issues. Fundamental to this has been the rise of Fair Trade and UTZ Certified chocolate. Initially launched by Stichting Max Havelaar in the Netherlands in 1988, the Fair Trade movement expanded into the mainstream in the following decades, with cocoa second to coffee in terms of sales and volume by 2011. Much of this is driven by the use of Fair Trade ingredients by major brands. For example, in Germany, major supermarket Lidl commenced promoting their own brand milk chocolate with their own Fair Trade label in 2006. Similarly, in the UK, two of the best selling milk chocolate bars, Cadburys Dairy Milk and Nestlé's Kit Kat were marketed with a Fair Trade label starting in 2009 and 2010 respectively.
References.
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Chocolate
Food produced from cacao seeds
Chocolate is a food made from roasted and ground cocoa beans that can be a liquid, solid, or paste, either on its own or as a flavoring in other foods. The cacao tree has been used as a source of food for at least 5,300 years, starting with the Mayo-Chinchipe culture in what is present-day Ecuador. Later, Mesoamerican civilizations consumed cacao beverages, of which one, chocolate, was introduced to Europe in the 16th century.
The seeds of the cacao tree ("Theobroma cacao") have an intense bitter taste and must be fermented to develop the flavor. After fermentation, the seeds are dried, cleaned, and roasted. The shell is removed to produce nibs, which are then ground to cocoa mass, unadulterated chocolate in rough form. Once the cocoa mass is liquefied by heating, it is called chocolate liquor. The liquor may also be cooled and processed into its two components: cocoa solids and cocoa butter. Baking chocolate, also called bitter chocolate, contains cocoa solids and cocoa butter in varying proportions without any added sugar. Powdered baking cocoa, which contains more fiber than cocoa butter, can be processed with alkali to produce Dutch cocoa. Much of the chocolate consumed today is in the form of sweet chocolate, a combination of cocoa solids, cocoa butter, and added vegetable oils and sugar. Milk chocolate is sweet chocolate that additionally contains milk powder. White chocolate contains cocoa butter, sugar, and milk, but no cocoa solids.
Chocolate is one of the most popular food types and flavors in the world, and many foodstuffs involving chocolate exist, particularly desserts, including cakes, pudding, mousse, brownies, and chocolate chip cookies. Many candies are filled with or coated with sweetened chocolate. Chocolate bars, either made of solid chocolate or other ingredients coated in chocolate, are eaten as snacks. Gifts of chocolate molded into different shapes (such as eggs, hearts, and coins) are traditional on certain Western holidays, including Christmas, Easter, Valentine's Day, and Hanukkah. Chocolate is also used in cold and hot beverages, such as chocolate milk and hot chocolate, and in some alcoholic drinks, such as crème de cacao.
Although cocoa originated in the Americas, West African countries, particularly Ivory Coast and Ghana, are the leading producers of cocoa in the 21st century, accounting for some 60% of the world cocoa supply. A 2020 report estimated that more than 1.5 million children are involved in the farming of cocoa in Ivory Coast and Ghana. Child slavery and trafficking associated with the cocoa trade remain major concerns. A 2018 report argued that international attempts to improve conditions for children were doomed to failure because of persistent poverty, the absence of schools, increasing world cocoa demand, more intensive farming of cocoa, and continued exploitation of child labor.
Etymology.
Cocoa is a variant of cacao, likely arising from a confusion with the word coco. Through cacao, it is ultimately derived from "kakaw(a)", but whether that word originates in Nahuatl or a Mixe-Zoquean language is the subject of substantial linguistic debate. Chocolate is a Spanish loanword, first recorded in English in 1604, and first recorded in Spanish in 1579. The word for chocolate drink in early Nahuatl texts is "cacahuatl" meaning "cacao water", which chocolate does not immediately derive from.
Despite theories that chocolate is derived from "xocoatl" meaning "bitter drink" or "chocolatl" meaning "hot water" and uncertainty around the Nahuatl origin, there is a consensus that it likely derives from "chicolatl". Whether "chicolatl" means "cacao beater", however, is contested, due to difficulty knowing what chico means.
The term "chocolatier", for a chocolate confection maker, is recorded from 1859.
History.
Evidence for the domestication of the cacao tree exists as early as 5300 BP in South America, in present-day southeast Ecuador by the Mayo-Chinchipe culture, before it was introduced to Mesoamerica. It is unknown when chocolate was first consumed as opposed to other cacao-based drinks, and there is evidence the Olmecs, the earliest known major Mesoamerican civilization, fermented the sweet pulp surrounding the cacao beans into an alcoholic beverage. Chocolate was extremely important to several Mesoamerican societies, and cacao was considered a gift from the gods by the Mayans and the Aztecs. The cocoa bean was used as a currency across civilizations and was used in ceremonies, as a tribute to leaders and gods and as a medicine. Chocolate in Mesoamerica was a bitter drink, flavored with additives such as vanilla, earflower and chili, and was capped with a dark brown foam created by pouring the liquid from a height between containers.
While Spanish conquistador Hernán Cortés may have been the first European to encounter chocolate when he observed it in the court of Moctezuma II in 1520, it proved to be an acquired taste, and it took until 1585 for the first official recording of a shipment of cocoa beans to Europe. Chocolate was believed to be an aphrodisiac and medicine, and spread across Europe in the 17th century, sweetened, served warm and flavored with familiar spices. It was initially primarily consumed by the elite, with expensive cocoa supplied by colonial plantations in the Americas. In the 18th century, it was considered southern European, aristocratic and Catholic and was still produced in a similar way to the way it had been produced by the Aztecs.Starting in the 18th century, chocolate production was improved. In the 19th century, engine-powered milling was developed, and in 1828, Coenraad Johannes van Houten received a patent for a process making Dutch cocoa. This removed cocoa butter from chocolate liquor (the product of milling), and permitted large scale production of chocolate. Other developments in the 19th century, including the "melanger" (a mixing machine), modern milk chocolate, the conching process to make chocolate smoother and change the flavor meant a worker in 1890 could produce fifty times more chocolate with the same labor than they could before the Industrial Revolution, and chocolate became a food to be eaten rather than drunk. As production moved from the Americas to Asia and Africa, mass markets in Western nations for chocolate opened up.
In the early 20th century, British chocolate producers including Cadbury and Fry's faced controversy over the labor conditions in the Portuguese cacao industry in Africa. A 1908 report by a Cadbury agent described conditions as "de facto slavery." While conditions somewhat improved with a boycott by chocolate makers, slave labor among African cacao growers again gained public attention in the early 21st century. During the 20th century, chocolate production further developed, with development of the tempering technique to improve the snap and gloss of chocolate and the addition of lecithin to improve texture and consistency. White and couverture chocolate were developed in the 20th century and the bean-to-bar trade model began.
Types.
Several types of chocolate can be distinguished. Pure, unsweetened chocolate, often called "baking chocolate", contains primarily cocoa solids and cocoa butter in varying proportions. Much of the chocolate consumed today is in the form of sweet chocolate, which combines chocolate with sugar.
Eating chocolate.
The traditional types of chocolate are dark, milk and white. All of them contain cocoa butter, which is the ingredient defining the physical properties of chocolate (consistency and melting temperature). Plain (or dark) chocolate, as it name suggests, is a form of chocolate that is similar to pure cocoa liquor, although is usually made with a slightly higher proportion of cocoa butter. It is simply defined by its cocoa percentage. In milk chocolate, the non-fat cocoa solids are partly or mostly replaced by milk solids. In white chocolate, they are all replaced by milk solids, hence its ivory color.
Other forms of eating chocolate exist, these include raw chocolate (made with unroasted beans) and ruby chocolate. An additional popular form of eating chocolate, gianduja, is made by incorporating nut paste (typically hazelnut) to the chocolate paste.
Other types.
Other types of chocolate are used in baking and confectionery. These include baking chocolate (often unsweetened), couverture chocolate (used for coating), compound chocolate (a lower-cost alternative) and modeling chocolate. Modeling chocolate is a chocolate paste made by melting chocolate and combining it with corn syrup, glucose syrup, or golden syrup.
Cacao.
Chocolate is made from cocoa beans, the dried and fermented seeds of the cacao tree ("Theobroma cacao"), a small, 4–8 m tall (15–26 ft tall) evergreen tree native to the deep tropical region of the Americas.
Recent genetic studies suggest the most common genotype of the plant originated in the Amazon basin and was gradually transported by humans throughout South and Central America. Early forms of another genotype have also been found in what is now Venezuela. The scientific name, "Theobroma", means "food of the gods". The fruit, called a cocoa pod, is ovoid, long and wide, ripening yellow to orange, and weighing about when ripe.
Cacao trees are small, understory trees that need rich, well-drained soils. They naturally grow within 20° of either side of the equator because they need about 2000 mm of rainfall a year, and temperatures in the range of . Cacao trees cannot tolerate a temperature lower than .
Genome.
The sequencing in 2010 of the genome of the cacao tree may allow yields to be improved. Due to concerns about global warming effects on lowland climate in the narrow band of latitudes where cocoa is grown (20 degrees north and south of the equator), the commercial company Mars, Incorporated and the University of California, Berkeley, are conducting genomic research in 2017–18 to improve the survivability of cacao plants in hot climates.
Varieties.
The three main varieties of cocoa beans used in chocolate are criollo, forastero, and trinitario.
Processing.
Cocoa pods are harvested by cutting them from the tree using a machete, or by knocking them off the tree using a stick. It is important to harvest the pods when they are fully ripe, because if the pod is unripe, the beans will have a low cocoa butter content, or low sugar content, reducing the ultimate flavor.
Fermentation.
The beans (which are sterile within their pods) and their surrounding pulp are removed from the pods and placed in piles or bins to ferment. Micro-organisms, present naturally in the environment, ferment the pectin-containing material. Yeasts produce ethanol, lactic acid bacteria produce lactic acid, and acetic acid bacteria produce acetic acid. In some cocoa-producing regions an association between filamentous fungi and bacteria (called "cocobiota") acts to produce metabolites beneficial to human health when consumed. The fermentation process, which takes up to seven days, also produces several flavor precursors, that eventually provide the chocolate taste.
After fermentation, the beans must be dried to prevent mold growth. Climate and weather permitting, this is done by spreading the beans out in the sun from five to seven days. In some growing regions (for example, Tobago), the dried beans are then polished for sale by "dancing the cocoa": spreading the beans onto a floor, adding oil or water, and shuffling the beans against each other using bare feet.
In an alternative process known as moist incubation, the beans are dried without fermentation. The nibs are then removed and hydrated in an acidic solution. They are heated for 72 hours and dried again. Gas chromatography/mass spectrometry showed that the incubated chocolate had higher levels of Strecker aldehydes, and lower levels of pyrazines.
Grinding and blending.
The dried beans are then transported to a chocolate manufacturing facility. The beans are cleaned (removing twigs, stones, and other debris), roasted, and graded. Next, the shell of each bean is removed to extract the nib. The nibs are ground and liquefied, resulting in pure chocolate liquor. The liquor can be further processed into cocoa solids and cocoa butter.
Producers of high-quality, small-batch chocolate argue that mass production produces bad-quality chocolate. Some mass-produced chocolate contains much less cocoa (as low as 7% in many cases), and fats other than cocoa butter. Vegetable oils and artificial vanilla flavor are often used in cheaper chocolate to mask poorly fermented and/or roasted beans.
Conching and refining.
The penultimate process is called conching. A conche is a container filled with metal beads, which act as grinders. The refined and blended chocolate mass is kept in a liquid state by frictional heat. Chocolate before conching has an uneven and gritty texture. The conching process produces cocoa and sugar particles smaller than the tongue can detect (typically around 20 μm) and reduces rough edges, hence the smooth feel in the mouth. The length of the conching process determines the final smoothness and quality of the chocolate. High-quality chocolate is conched for about 72 hours, and lesser grades about four to six hours. After the process is complete, the chocolate mass is stored in tanks heated to about until final processing.
Tempering.
After conching, chocolate is tempered. This process aims to create a crystallize a small amount of fat in a particularly stable formation. Around this small amount of crystals, the rest of the fats crystallize, creating a glossy chocolate, with a crisp break.
Shaping.
Chocolate is molded in different shapes for different uses:
Storage.
Chocolate is very sensitive to temperature and humidity. Ideal storage temperatures are between , with a relative humidity of less than 50%. If refrigerated or frozen without containment, chocolate can absorb enough moisture to cause a whitish discoloration, the result of fat or sugar crystals rising to the surface. Various types of "blooming" effects can occur if chocolate is stored or served improperly.
Chocolate bloom is caused by storage temperature fluctuating or exceeding , while sugar bloom is caused by temperature below or excess humidity. To distinguish between different types of bloom, one can rub the surface of the chocolate lightly, and if the bloom disappears, it is fat bloom. Moving chocolate between temperature extremes, can result in an oily texture. Although visually unappealing, chocolate suffering from bloom is safe for consumption and taste is unaffected. Bloom can be reversed by retempering the chocolate or using it for any use that requires melting the chocolate.
Chocolate is generally stored away from other foods, as it can absorb different aromas. Ideally, chocolates are packed or wrapped, and placed in proper storage with the correct humidity and temperature. Additionally, chocolate is frequently stored in a dark place or protected from light by wrapping paper. The glossy shine, snap, aroma, texture, and taste of the chocolate can show the quality and whether it was stored well.
Health effects.
Nutrition.
One hundred grams of milk chocolate supplies 540 calories. It is 59% carbohydrates (52% as