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The image of fast food in Japan is that of standing soba noodles, but in ancient times it was ochazuke.

The original ochazuke was simply hot or cold water poured over rice, called yuzuke or mizuzuke, and was a common way of eating rice among the upper classes by the Asuka period, according to the "Nihon Shoki."

In the summer, when appetite is low, the rice is soaked in hot water after being rinsed to prevent spoilage from the heat. In the winter, the cold rice is heated to re-pasteurize the starch and to replenish moisture for a tasty meal.

It was used as a diet food in the Heian period, a regular food for samurai from the Kamakura period to the Warring States period, and a fast food for the common people in the Genroku period.

It is said that chazuke first appeared in the middle of the Edo period, when bancha and sencha became popular tastes among the general public. Sencha contains a slight amount of monosodium glutamate, an umami ingredient, which makes it delicious along with the aroma of the tea.

In the days without rice cookers, cooked rice was stored in an ohitsu. When rice is kept warm for a long time in a modern rice cooker, it becomes less tasty because of the Maillard reaction, which causes the rice to turn yellow like roasted rice, and the Bacillus bacteria, which are naturally present in rice and do not die even at high temperatures, causing the rice to decay and produce an unpleasant odor. Ohitsu, on the other hand, removes rough heat and excess moisture from freshly cooked rice, making it taste better, and the sterilizing effect of the wood makes the rice less prone to damage. Ohitsu, filled with the wisdom of our ancestors, continues to be used today.

Another thing is that cold rice is said to be healthy these days. This is based on research that shows that hot rice is an easily digestible starch, but when it cools down, it turns into an indigestible starch called "resistant starch," which burns fat better due to its function in the intestines.

In Yamagata Prefecture, there is a local dish called "mizumama," which is made by pouring cold water over washed cold rice and eating it with preserved side dishes from the Tohoku region. Ochazuke may be incorporated into daily life as a modern fast food for healthy eating.

Kurikyu's Magewappa Ohitsu
https://www.shokunin.com/en/kurikyu/ohitsu.html 
Seiryugama's Rice Bowl
https://www.shokunin.com/en/seiryu/chawan.html 
Nambu Bunshudo's Iron Teapot
https://www.shokunin.com/en/bunshudo/kyusu.html 
Yamagata Casting's Cast Iron Pot Stand
https://www.shokunin.com/en/chushin/ 

References
https://ja.wikipedia.org/wiki/茶漬け
https://ja.wikipedia.org/wiki/湯漬け