Why does eating petai make your urine stink?
18 Jan 2013
Petai are flat edible beans borne on a huge tree Parkia speciosa that can grow to about 30 meters. The bright green seeds are carried in a cluster of 7 to 8 long flat pods. When these pods ripen, the petai beans within are removed and best cooked with onions, garlic, chilli and dried shrimp to make a very popular culinary delight called “sambal petai.”
Why does eating petai make your urine stinks?
If you are one of those petai lovers, people around you find it hard to ignore your presence, especially after a hearty meal of rice where sambal petai as one of the side dishes. This is because your body starts emitting a strong repulsive odour that is so pervasive and characteristic of petai. The stink can start as soon as 30 minutes after consumption of petai and it can remain a source of embarrassment for as long as two days.
The smell is especially telling while you are urinating or when you give out a belch after a petai meal. But sweat, burp and flatulence all tend to also give the unmistakable aroma of petai, aptly known also as ‘stink beans’ in some cultures. So, if you are out on a date, petai is a definite ‘no-no’
The effect of eating petai on the eater’s urine has long been a mystery and many had asked what precisely is responsible for the distinctive smell. Some studies of the seeds suggest that the sulphur-containing compounds may be the contributing reason. Cystein and its derivatives such as glutathione, djenkolic acid and thiazolidine-4- carboxylic acid are some of these sulphur-rich compounds known to produce distinctive aroma similar to that emitted by petai eaters.
Another study analysed Parkia speciosa seeds using sophisticated methods including Gas-Chromatography (GC) and GC-Mass Spectroscopy (GC-MS). The study found a total of 21 components that are highly volatile which include four major constituents; namely hydrogen sulfide (41.30%), ethanol (39.15%), 1,2,4-trithiolane (4.75%) and acetaldehyde (3.59%). Of these four major components, the characteristic smell of petai was believed to be from 1,2,4-trithiolane.
Interestingly, in the Western countries, it is also common knowledge that eating the vegetable asparagus can give urine a distinctive offensive smell. Unlike petai urine, the asparagus urine phenomenon has attracted many scientific studies since 1891. Much has been known about it. It is known for instance that certain compounds in asparagus are metabolised; giving rise to a distinctive smell in the urine of asparagus eaters. These degraded products of asparagus include a number of sulphur-containing thiols, thioesters and ammonia.
A host of volatile organic compounds responsible for the smell was also detected. These include methanethiol, dimethyl sulphide, dimethyl disulfide, bis(methylthio)methane, dimethyl sulfoxide and dimethyl sulfone. Of these compounds, methanethiol and dimethyl sulfide were the most pungent, while the last two dimethyl sulfoxide and dimethyl sulfone produced a sweet aroma.
However, only when all these volatile compounds were mixed together, were scientists then able to form a reconstituted mixture that produced the unmistakable odor of asparagus urine.
Puzzlingly, with petai, scientists are still not entirely sure which set of chemical compounds do the smelly urine actually originate from. Nonetheless, at this point, we are only able to speculate and put up a hypothesis on what could be the culprit.
Before being eaten, the pods and seeds of petai themselves do not have a similar odor as the urine of petai eaters. So whatever is responsible for the urine smell most likely happens after ingestion. Inside our body, a set of digestive enzymes may be produced to break down the petai into various chemical compounds. One of those compounds could be methyl mercaptan, a compound notoriously known for its stench. It is in fact the same chemical which gives a skunk its defensive smell.
One plausible theory is that the petai gets quickly broken down in the body and is acted upon by an enzyme that releases methyl mercaptan as one of the products. This methyl mercaptan eventually goes through the kidneys and is excreted as a waste product in the urine.
Many would wonder if studying the chemistry of petai can lead us to any economic applications? Well, for one, I would say the petai can be a good source of chemical for the production of stink bombs. What is a stink bomb? A stink bomb is manufactured as a device to emit a horrible smell. As in any other types of bombs, they are for use in stopping the advancing of enemies or pests. The smell produced by a stink bomb is so obnoxious that enemy targets are forced to flee the scene as they simply are not able to tolerate the reeking odour.
Stink bombs are popularly used in confrontations where you don’t want to cause bodily harm to the targets they are meant to get rid of. Hence the use of stink bombs is usually limited to controlling street riots or flushing out people hiding or barricading in buildings. Stink bombs have a clear advantage over traditional riot control agents like pepper spray and tear gas as they cause no stinging pain to the eyes, but is still very effective at low concentrations.
Today, two standard mixtures of chemicals are used for the manufacture of stink bombs. One is called ‘Bathroom Malodor’ which constitutes a mixture of eight chemicals that strongly stinks like human faeces. Another distinctive one is called, “Who me?” which constitutes a mixture of five sulphur-containing chemicals that smells like rotting food and carcasses.
The mixtures used in stink bombs are relatively harmless and often contain ammonium sulfide, which smells like rotten eggs. When exposed to air, the ammonium sulphide reacts with the moisture in the air to release hydrogen sulphide and ammonia. Apart from that, another group of substances commonly used in the manufacture of stink bombs are the low molecular weight thiols. These include methyl mercaptan and ethyl mercaptan; the same chemicals that skunks produce to drive away their intruders or potential predators.
Nonetheless, whether the petai contains much of this potent stuff for use in the manufacture of our home-grown stink bomb needs to be determined by our chemists. With the frequency of demonstrations and riots hitting our Malaysian streets recently, making stink bombs out of our own petai could become a good research and development project for our military.
Barring the smell, does petai has any health benefits?
It may well make your urine smell terrible, but some people believe the health benefits of Parkia speciosa beans far outweigh its minor anti-social aspects.
To some people, eating petai offers far-reaching medical and nutritional benefits. The beans are purportedly effective in treating a number of medical disorders including depression, premenstrual syndrome (PMS), blood pressure, diabetes, obesity and constipation.
In Malaysia, the health benefit of petai has long been associated with diabetic sufferers. However, conclusive studies to verify this claim is still forthcoming. Moreover, no unique constituent of petai has been identified as specifically effective for treatment of diabetes.
The petai seed, nonetheless, is rich in complex carbohydrates. This is also generally true for other vegetables such as green beans, broccoli and spinach which contain less starch but with more fibre. Fibre is divided into two categories, namely: soluble and insoluble. Whereas insoluble fibre passes through the digestive tract relatively unchanged, and the soluble fibre dissolves to form a soft gel. Did you know that eating extra fibre in our diet slows down the absorption of the carbohydrates in the digestive system. As a result, we will feel full longer and be less likely to over eat throughout the day.
Furthermore, high dietary fibre intake has also been proven to benefit especially diabetics. For instance, when they consume food with high soluble fibre content, the absorption of digested food occurs at a slower rate. This prevents the rapid rise of blood glucose, which in turn does not result in overproduction of insulin by the pancreas.
Apart from that, scientists have shown that soluble fibres such as konjac glucomannan do indeed slow down insulin response and the rise in blood glucose levels after a meal. Dietary sugar like carbohydrates and starch become trapped inside the gel formed by soluble fibre in our intestines. Consequently, the sugar is absorbed into the bloodstream more slowly preventing the sharp rise in blood glucose typically experienced by diabetic patients after a meal.
Diabetic, diabetic patients have to be on the guard at all times to avoid high peaks and low valleys in their blood glucose levels. The failure to control this rapid fluctuation in blood glucose can cause much stress on their pancreas, hence leading to worsening of the many symptoms and complications associated with the disease.