While called "Yam Cake", this product is not made from a yam, but from a large root corm of a tropical Arum plant, not related to yams even at the Order level. It is much used in the cuisines of Japan and Korea. It is made up into blocks as shown in the photo, but also into noodles, called shirataki. A flour made from the dried corms is also an important product.
The Konjac plant is native to tropical and warm temperate zones of East Asia, as far north as Japan, and as far south as Indonesia. It has been planted in Hawaii.
The photo to the left is what this stuff is made from, the root corm of an Arum related to the famous Corpse Flower. The Konnyaku jelly has no color and very little flavor, being valued mainly for its texture. The dark version, more common than the white, is colored with ground Hijiki seaweed. The konjac jelly consists mostly of dietary fiber, so a 1 ounce serving provides less than 1 calorie and 0 carbs. Ingredients: Yam Flour, Water, Calcium Hydroxide, color from Seaweed Powder. Photo by Silke Baron distributed under license Creative Commons Attribution v2.0 Generic.
Konnyaku noodles are made in many shapes and sizes, both white and
colored with powdered seaweed. The photo specimen, about 3-3/4 inches
long, is sold as "Konnyaku Knots". I have also seen white rolls scored
to look like curls of scored squid, for use in vegetarian "seafood"
dishes. Unlike rice noodles, which look almost the same when cooked,
Shirataki noodles do not disintegrate. Rice noodles break into short
pieces with even moderate cooking. Ingredients: Yam Flour, Water,
Calcium Hydroxide.
This product, the raw material from which Konnyaku and Shirataki are made, is extracted from ground konjac corms, which are 40% Glucomannan, a soluble fiber. The extracted flour is almost entirely Glucomannan. This flour has a number of food industry uses as a gelling agent, thickener, film former, emulsifier, and stabilizer. Glucomannan will jell cold, a very useful characteristic, but this can also be dangerous.
Glucomannan is important to vegans as a substitute for gelatin, though its properties are rather different. It is similarly used by diabetics as a thickener containing no carbs and almost no calories. It is being tested and used as a weight control potion, but must be used with care, as there are dangers from clogging and choking. Weight control uses are not currently FDA approved.
Certain Japanese candies made with konjac flour are banned from the
United States and Europe, because people unfamiliar with the
characteristics of these candies, particularly children and elderly,
have died from choking.