The Oxford English Dictionary has this to say about the etymology of horse chestnuts:
late 16th century: translating (now obsolete) botanical Latin Castanea equina; its fruit is said to have been an Eastern remedy for chest diseases in horses
Like its European counterparts, the fruit of the Japanese horse chestnut (Aesculus turbinata) or tochinoki (栃の木) contains toxic alkaloids. While deer are able to digest these quite safely, they are poisonous to humans and, despite the origins of the English name, most likely to horses. They have, nevertheless, a long history in Japanese food culture – archaelogical evidence shows that horse chestnuts were utilized by the pre-rice Jomon people.
From medieval times down to the not-so-distant past, the failure of rice crops could result in starvation, particularly in the mountains where large quantities of rice could not be grown. Horse chestnuts were gathered as a reserve food supply and stored in the roofs of houses, where they could be accessed if the family got snowed in.
A long process of soaking and boiling renders the kernel edible. Freshly gathered nuts are soaked in water for a couple of days to kill any weevils or other insects, and then sun dried for up to a month, before being put into storage. To leach the toxins out, the nuts are soaked for a few days, the shells removed, then leached in running water for between a week and ten days. The nuts are then boiled and left to soak for a few days in a water and ash mixture. After the leaching process, the kernels are generally pounded with glutinous rice into mochi.
Today, mochi made with horse chestnut flour – tochimochi (栃餅) – are sold as local cuisine in some areas. This website shows some of the process of making tochimochi.
In addition to its “food value”, the horse chestnut is prized by apiarists for its nectar-rich flowers. The timber is valued similar to walnut, and is traditionally the material used for making mochi mortars (usu).
There is a Japanese horse chestnut tree in the park near my house. Unfortunately, it was heavily pruned and didn’t produce any conkers this year. I’ll have to get my photos from somewhere else…
Contemporary Japanese tend to be obsessed with two things: food; and their country’s unique (and often considered uniquely unique) culture. White rice is ubiquitous; it still remains the stable food – so much that following WWII, huge efforts were made to ensure 100% self- sufficiency in rice – despite 70% of food being imported. In feudal days, warlords measured their wealth in rice production, and samurai were paid stipends in bushels of rice. The Japanese word for “meal” (gohan) is identical for the word for “cooked rice”. I (and I’m sure thousands of other Westerners too) frequently get asked the question, “Which do you eat, bread or rice?” (The assumption is that if rice is the basis of the Japanese diet, then bread must be the basis of European-based societies)
So I am not usually thanked for pointing out that for at least half of Japan’s history, rice was not the staple. Most people are blissfully unaware that rice is not native to the island chain at all.
So, what did people eat before rice was brought from the mainland? They were hunter-gatherers. Deer, wild pigs and fish provided a large portion of their protein intake, but the staple was acorns.
Acorns – known locally as donguri (団栗) – come from three sources: evergreen chinkapins (or chinquapin) – sudajii (すだ椎) or shii (椎); evergreen oaks- kashi (樫); and deciduous oaks – nara (楢), all members of the beech family. Each type contains a certain amount of tannin and saponin, which effects how the acorn can be eaten.
Some of the chinkapins can be eaten raw, and the acorns of certain deciduous oak require no more than a simple roasting. On the other end of the scale, the nuts from the evergreen oaks contain a significant amount of toxins and need to be treated in a manner similar to (but not as extensive as) the horse chestnut.
Acorns are a vital part of the food chain. Most people associate them with squirrels, but rats, tanuki, deer, pigs and bears feed extensively on them too. In fact, recent cases of Japanese black bears making their way onto farms and into villages have been attributed to a lack of acorns in the mountains, forcing the animals for forage elsewhere.