Tenkasu (bits of deep-fried dough) (天かす)
Tenkasu (天かす, also written as てんかす, 天カス, 天滓) are small pieces of deep-fried dough produced during cooking tenpura. They are also referred to as Age-dama. With its deep-fried flavor, rich taste of the cooking oil, and special crispy texture, tenkasu is used as foodstuff for trimmings or garnishes of various dishes.
When frying tempura, the fluid batter of soft flour and beaten egg is removed from tempura ingredients by bits. Tenkasu are these deep-fried bits. To continue frying with tenkasu left in the cooking oil causes deterioration of the oil by the burnt tenkasu. Therefore after taking cooked tempura out of the oil, tenkasu should be frequently collected with a strainer or the like. Tempura restaurants generate large quantities of tenkasu and some restaurants dispose of them as food wastes.
At soba (Japanese buckwheat noodle) restaurants and udon (Japanese wheat flour noodle) restaurants, besides tenkasu generated from tempura, only batter dough is deep fried to be used as a food. Plain udon or soba noodle with tenkasu on it turns tanuki noodle (in the Kanto region) or haikara noodle (in the Kansai region) that enables addition of inexpensive rich, oily taste that plain shoyu noodle soup doesn't have sufficiently. At some udon restarurants mainly in the Kansai region, tenkasu in the container is put on the table so that customers can freely use it as garnish.
The usage of tenkasu has expanded to other dishes such as okonomiyaki (Japanese pancake)
Tenkasu is easily-oxidizable, and therefore doesn't have good keeping quality. On the other hand, cooking just a small amount of tenkasu for home use can be considered time-consuming. Then, because of the constant demand, tenkasu is made and sold at supermarkets, tenpura or soba restaurants, and delicatessens as foodstuff. Some tenkasu contain other ingredients such as shrimp to improve the product, since tenkasu is originally just a residue.
Tenkasu' and 'Age-dama'
In 2003, according to a survey by The NHK Broadcasting Culture Research Institute, 68% of those asked called that deep-fried dough bit 'tenkasu' and 16% called it 'age-dama'. In western Japan, including the Tokai region, the result showed that the number of people who called it 'tenkasu' exceeded the national average, while in the Kanoto, Koshinetsu, and Hokkaido region, the number of people who called it 'age-dama' are higher than the national average.
A mass of deep-fried tenkasu with the remaining heat left in the dumpster can be the cause of sudden fire in one or more hours. If nobody is around, this leads directly to fire, and this is known as tenkasu fire. Since tenkasu has a large surface area exposed to the air (oxygen), it is easily-oxidizable. Also, since the reaction heat hardly escapes from the piece, the trapped heat accelerates the reaction and takes fire when exceeding the fire point of the oil. The mass of warm tenkasu needs to be spread in a flat container or the like and watered to cool down before disposal.