Umeboshi are pickled ume fruits common in Japan. Ume is a species of fruit-bearing tree in the genus Prunus, which is often called a plum but is actually more closely related to the apricot. Umeboshi are a popular kind of tsukemono (pickles) and are extremely sour and salty. They are usually served as side dishes for rice or stuffed inside of rice balls (sometimes without removing their seeds inside) for breakfast and lunch. They are occasionally served boiled or seasoned for dinner.
Umeboshi are usually round, and vary from unwrinkled to very wrinkled. They taste salty, and are extremely sour due to high citric acid content. Umeboshi were notorious for their ability to eat their way through the plain drawn aluminum lunch boxes commonly used in the 1960s. The combination of organic acids and salt in the umeboshi were the cause of this phenomenon.
The central area of Wakayama prefecture is known throughout Japan for the number and quality of its ume and umeboshi. The town of Minabe, Wakayama, in particular, grows more ume and produces more umeboshi than any other town in all of Japan.
Umeboshi are traditionally made by harvesting ume fruit when they ripen around June and packing them in barrels with salt. A weight is placed on top and the fruit gradually exude juices, which accumulate at the bottom of the barrel. This salty, sour liquid is marketed as umezu (often translated as "ume vinegar"), although it is not a true vinegar.
Most modern umeboshi are made by using less salt and by pickling the ume in a seasoned liquid or vinegar. They are typically dyed red using purple perilla herbs (called akajiso), or flavoured with katsuobushi, kombu or even sweetened with honey. Because modern methods of preservation use less salt, they usually contain an artificial preservative to extend shelf life.