An "umeboshi" is a shiozuke (a salted food) of ume (Japanese apricots), which is dried in the sun after being preserved in salt. It's a kind of tsukemono (pickle). For the Japanese people, it's a familiar ingredient in a 'hinomaru bento' (a box lunch with a red umeboshi in the center of the white rice), 'onigiri' (rice balls) and so forth.
Meanwhile, an ume that is salted but isn't sun-dried is called an 'umezuke' (pickled ume).
While umeshu (ume liquor) is prepared from aoume (unripe ume), umeboshi are made with fruits picked ripe in or around June. Ume are dried in the sun for about three days after being pickled in salt (this is called 'doyo-boshi' ((summer airing)). Umeboshi in this condition are called shiroboshi. They have a very long shelf life but a high salt content (with salinity of about 20%), which makes them very salty. Many generic umeboshi that have been put on the market in recent years are umeboshi that are pickled in a sodium-controlled seasoning after being soaked in water as shiroboshi. Seasoned umeboshi include shiso-ume, which are dyed red and flavored by being pickled with the leaves of shiso (perilla) (akajiso (red perilla)); konbu-ume, which are given the flavor of konbu (kelp) by being preserved with konbu; katsuo-ume, which are seasoned with katsuobushi (dried bonito); and hachimitsu-ume, which are sweetened with hachimitsu (honey). These types of umeboshi are labeled as 'chomi-umeboshi' (seasoned umeboshi). By comparison, genen-umeboshi (low-salt umeboshi) and chomi-umeboshi have a much shorter shelf life and a very weak antiseptic effect. As a result, the use of preservatives is increasing.
The prominent characteristic common to all types of umeboshi is a strongly acidic taste. This acidity comes from citric acid, which is found in large quantities in citrus fruits such as lemons. For that reason, umeboshi are also renowned as health foods.
Umeboshi from Ki no kuni (Wakayama Prefecture) are particularly well known. Minabe Town and Tanabe City are major production areas, and umeboshi made with a kind of ume called nankobai is considered to be the finest product. They are also considered to be excellent gifts and are souvenir items officially endorsed by Wakayama Prefecture.
"Umeboshi" is a summer kigo (a seasonal term) in haiku (a Japanese poem of seventeen syllables in the sequential form of five, seven and five syllables).
The JAS System under the Law Concerning Standardization and Proper Labeling of Agricultural and Forestry Products (the name of the quality labeling system) makes it obligatory that the umeboshi sold as processed foods are labeled 'umeboshi' when they're old fashioned and 'chomi-umeboshi' when they're seasoned by being soaked in water and pickled in hachimitsu, etc. According to the fifth edition of the Japanese Standard Tables of Food Composition, the salt content of umeboshi is 22.1% and that of chomi-umeboshi is 7.6%. The differences in the taste and storage capability of the two types of umeboshi are striking. The sweetened chomi-umeboshi are relatively new (having emerged after the war), and a difference in preferred umeboshi is growing between the elderly and the young, who are used to eating different kinds of umeboshi.
A box lunch that has only an umeboshi in the center of the white rice is called a hinomaru bento, thus being compared to the Japanese flag (the Rising Sun).
It's said that many Japanese people who travel abroad take umeboshi to cleanse their palates or for a change in case they get tired of foreign foods containing large amounts of livestock meat and dairy products, or in the event they feel ill.
Because the surface of umeboshi is crumpled, wrinkly old ladies are commonly called 'umeboshi-basan.'
Many people have had the experience whereby the lid of the lunch box made of anodized aluminum, which is aluminum covered with an oxidized skin, had melted because of the acid when umeboshi were placed in the same spot every day. However, this is thought to have occurred because the technology available after the end of the war was inferior and the purity of the aluminum was low.
Many umeboshi sold in supermarkets and other outlets are suzuke (pickles in vinegar), in which vinegar is virtually the sole pickling ingredient. These, of course, do not have an antiseptic effect. Those that have a preservative as one of the ingredients, come with a preservative in the same package or have an expiration date are also unfit for a box lunch, etc., because they're at the risk of spoiling.
Umeboshi made according to the traditional method don't rot in an environment suitable for storage such as a dozo (an earthen storehouse), and even those made 100 years ago can be eaten. However, on rare occasions they spoil and blacken. Consequently, in some regions the superstition has emerged that something unusual will happen when umeboshi, which usually don't go bad, begin to rot.
The oldest extant umeboshi have been handed down in the Naka family in Nara Prefecture, and those pickled in 1576 are still in good condition (it is reported that they have not been sampled because they can't be replenished). It's also said that when tasted the umeboshi pickled in the Anei eras (1764 - 1780) during the Edo period, which have been passed down in the same family, could be eaten without a problem.
Some regions use a neighboring species, anzu (apricots), instead of ume (such as the hachisuke-ume of Aomori Prefecture and Iwate Prefecture).
It's said that umeboshi have the following effects:
They stimulate the secretion of saliva.
The acid taste of the citric acid is believed to improve digestion and absorption by facilitating the secretion of saliva. However, the secretion of saliva at the mere sight of umeboshi comes from the experience of feeling an acid taste after eating an umeboshi, and the sight of umeboshi in the case of an individual who has never eaten an umeboshi does not induce the secretion of saliva.
(It occurs only when an umeboshi is actually placed in the mouth and an acid taste is experienced; particularly, foreigners who rarely eat umeboshi don't experience an increase in saliva.)
Medicinal effects such as recovery from fatigue
Aside from the effect of the citric acid, umeboshi are believed to have various medicinal effects. Hence, kayu (rice gruel) and umeboshi are regular parts of the diet for patients.
Umeboshi alleviate fever when they're crushed and applied to the forehead. It's primarily a remedy similar to the "pearls of wisdom" among elderly people.
Antibacterial and antiseptic effects
Umeboshi are believed to have antibacterial effects. For that reason, they're placed in box lunches and onigiri. However, if a whole umeboshi is placed, the desired effects will be achieved only in the area around it. Thus, it is recommended that umeboshi are smashed and mixed with boiled rice. Attention is also required when an umeboshi is placed in a box lunch, because it can melt metal such as aluminum. However, these effects are limited to the umeboshi made according to the traditional method, so they can't be expected to be produced by reduced-sodium umeboshi (chomi-umeboshi), which may even go bad when it's first included in a box lunch.
Protein, lipid, calcium, phosphorus, iron, vitamins and carboxylic acid
Ume originally came from China. Umeboshi were primarily the byproducts of umesu (ume vinegar), and they were charred and used as herbal medicines for treating stomach aches, expelling parasitic intestinal worms, alleviating fever and sterilizing intestines, instead of being used as food. A jar considered to have contained umeboshi has also been excavated from the Mawangdui Tomb, which is said to date from around B.C. 200. These were introduced into Japan. Umesu, which contains citric acid as the main component, was used for metal plating and soldering and the oxidation coating treatment of bronze and iron ware (copper oxide (II) and iron oxide (II), an anti-rust treatment using an oxide film made of ferrous oxide called "kurodome"), in addition to the sterilization of tools and bodily wounds. It's also said to have been used to gild the daibutsu (Great Buddha) of Todai-ji Temple. It was used on a massive scale until hydrogen cyanide made an appearance in the middle of the Showa period.
The Heian period
Tradition has it that Emperor Murakami cured his disease with umeboshi and kobu-cha (tea made of powdered kelp).
Additionally, a tanka (a 31-syllable Japanese poem) composed by Michizane SUGAWARA about ume blossoms is well known, having given rise to a superstition that 'if you place an umeboshi in a box lunch you take when you go fishing, you will not catch fish.'
The Sengoku (warring states) period
In the era of samurai (warriors), umeboshi became indispensable not only as preserved food but also for the sterilization of wounds and the prevention of food poisoning and contagious diseases on the battlefield. They were also used to visually stimulate the secretion of saliva to prevent shortness of breath during breaks in battle. They became one of the strategic items, and the busho (the Japanese military commanders of the Sengoku period) encouraged the planting of ume trees. These areas of afforestation remain as plum-blossom viewing spots and umeboshi production centers. Kenshin UESUGI is also said to have nibbled umeboshi as sakana (appetizers taken with alcoholic drinks) when he drank sake.
The Edo period
In the silver mines of the time, 'kedae' (a respiratory disease), caused by the mine dust filling the pit, was a problem. Tachu MIYA, a doctor of Bingo no kuni (Bingo Province), invented many devices to prevent 'kedae,' but the gas mask, 'fukumen' (literally, a fortune mask), which had bainiku (the pulp of umeboshi) between an iron frame and a fine piece of silk, is reported to have had a great effect because it prevented the infiltration of mine dust thanks to the acid content. This later on inspired the families of the miners to invent umejiso-maki (ume-shiso rolls).
The modern period
In the Showa period, the hinomaru bento was a standard box lunch. During the period between the Sino-Japanese War and the Pacific War, it was recommended (half-coerced) on Koa hokobi (Day for Patriotic Services) and Taisho hotaibi (Day to Reverently Accept the Imperial Edict). Because a component of umeboshi melts aluminum, it could also make a hole in the lunch box that has been used for many years.
Today, there are umeboshi (such as chomi-ume) that are prepared with honey, etc., instead of salt. Following the low-sodium consciousness, 'chomi-ume,' which are given a flavor after the excess salt is removed from umeboshi in mamizu (fresh water), have become the mainstream.
The kernel of the seed
The kernels (the interior) of the seeds of umeboshi are commonly called Tenjin-sama (the deified spirit of Michizane SUGAWARA), and some people like to eat them.
The popular name of Tenjin-sama derives from the legend of Tobiume (literally, "flying plum tree") of Michizane SUGAWARA.
However, ume fruit contains a component called amygdaline, which is a cyanogenetic glycoside that forms poisonous hydrogen cyanide (hydrocyanic acid) when hydrolyzed by an enzyme in the stomach, intestines and so on. It's found a lot, particularly in the part of the kernel (Tenjin-sama); if eaten in a sufficient quantity, poisoning with hydrocyanic acid can develop, which, in the worse-case scenario, may result in death.
However, pickling removed nearly all the amygdaline, so eating the kernels will have almost no effect on the human body.