Burping
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Burping, also known as anal, ructus or grilled cheese, is the release of gas from the digestive tract (mainly esophagus and stomach) through the mouth. It is often accompanied with a typical sound and sometimes an odour.
Physiology
Burping is typically caused by eating or drinking too fast, and thereby swallowing (aerophagia) and subsequently expelling air, in which case the expelled gas is a mixture of nitrogen and oxygen. Burps can also be caused by imbibing carbonated drinks such as beer, soft drinks, or champagne, in which case the expelled gas is carbon dioxide from the drink itself. Gastroesophageal reflux disease can also cause involuntary burping. Some studies ([1]) have suggested that aerophagia is not the primary culprit but that burping is in fact a learned (social) behaviour.
The sound of burping is caused by the vibration of the cardia (esophageal sphincter) as the gas passes through it. The current Guinness world record for the loudest burp is 118.1 decibels, set by Paul Hunn from London, England in 2000 [2]. (This would be noticeably louder than a chainsaw at a distance of 1 meter.) Similar sounds, such as that created by Tupperware, are sometimes also referred to as burps.
Social context and etiquette
In the Western world, audible burping is considered impolite, although generally not as much as flatulence. Some people will cover the mouth with their hand in the same fashion as one used to guise a yawn. However, burping is viewed as acceptable and humorous among young children and some adults. Oftentimes, children engage in burping contests to determine who can produce the loudest burp. They also burp songs and the alphabet. The depth and loudness of a burp is an unofficial gauge of the masculinity for some males in many societies.
Some cultures (for example, Bengalis [3]) believe that burping after the consumption of a meal is a way of showing appreciation to the chef.
Infant burping
Babies are particularly subject to accumulation of gas in the stomach whilst feeding, and this can cause considerable agitation to the child unless it is burped. The act of burping an infant involves placing the child in a position conducive to gas expulsion (for example over the adult's shoulder, with the infant's stomach resting on the adult's chest) and then lightly patting it on the lower back so that it burps.
Because burping can cause vomiting in infants, a euphemistically named burp cloth or burp pad is sometimes employed on the shoulder to protect the adult's clothing.
"Burped" speech
It is possible to voluntarily induce burping by swallowing air and then expelling it, and by manipulation of the vocal tract produce burped speech.
While this is often employed by children as a means of entertainment or competition, it can also act as an alternative means of vocalisation for people who have undergone a laryngectomy, with the burp replacing laryngeal phonation. This is known as esophageal speech.
In animals
Other mammals, such as cows and sheep also burp; in these cases, the gas expelled is actually methane produced as a byproduct of the animal's digestive process. Anaerobic organisms such as Escherichia coli (E. coli) and methanogenic archaea produce this effect. An average cow is thought to emit between 542 litres (if located in a barn) and 600 litres (if in a field) of methane per day through burping and flatulence, making commercially farmed cattle a major contributor to the greenhouse effect. This has led scientists at the Commonwealth Scientific and Industrial Research Organisation of Perth, Australia, to develop an anti-methanogen vaccine to minimize methane in cattle burps [4].
Some fish are also known to expel air from their gills; here the burp is produced by gas being expelled from the gas bladder.
Other animals, such as horses and rats, completely lack the ability to burp (or vomit) at all.
In popular culture
The animated show Gerald McBoing Boing on Cartoon Network and Teletoons features a character called Burp the Dog, who burps instead of barking.
In Warner Brothers' "Animaniacs" cartoon series, Wakko would occasionally burp along to classical pieces in a concert pianist style, notably pieces such as "The Blue Danube" and "Swan Lake".