The powder is available in both its natural state, which is black, and in a filtered, bleached version, which comes out pale grey. The black version is used to make things such as blocks of jelly that go in oden.
Both versions have has flavour to speak of.
Konnyaku Powder can be used as a thickener. A little Konnyaku Powder thickens a lot. 1 teaspoon of Konnyaku powder has the equivalent thickening ability of ten teaspoons of cornstarch. It thickens upon being heated.
Glucomannan is a diet aid made from Konnyaku Powder, purified a bit further
Konnyaku Powder is also used to make shirataki noodles (“Yam Noodles”) and Ito Konnyaku.
When using Konnyaku Powder as a thickener, mix with a cold liquid first, then add to the food you are thickening — never stir directly in
as a thickener.
Use about 1/2 teaspoon per 1/2 cup (4 oz / 125 ml) of cold liquid.
In 1885, the Bank of Japan issued its first paper money, the “Daikoku satsu” 10 yen note. Konnyaku powder was added to paper used to make the money more durable. It had the opposite effect, because it made the money more of a meal for rodents and insects. They re-issued new 10 yen notes in 1890 to replace these.
This use wasn’t unique, though: the powder has been used for centuries in Japan to size, strengthen and waterproof paper.