I watch "Tameshite Gatten" on NHK TV programme sometimes.
The show on October 18th ran "Cooking methods of chestnut". At the beginning, the show said "Jomon pot cooking made chesnuts sweeter".
In fact, Jomon pot cooking keeps the best temperature to activate amylase, and this takes a sweet taste from chestnuts. The show also said "Jomon people ate sweeter chestnuts".
Very recently, I just read papers by Michiko Niimi*, and I got some ideas about ancient food from this programme.
*Niimi, M. and Miyamoto, Y. 2006. Taste of chestnuts in the Jomon period. Zooarchaeology 23: 103-111. (Japanese)
Niimi, M. 2005. Did Jomon people gather chestnuts every day? Zooarchaeology 22: 65-72. (Japanese)