Dehydrated Yam Processing
The species is Dioscorea polystachya Turcz., the Chinese yam, known as shanyao in Chinese and nagaimo in Japanese, a twining vine of the yam family (Dioscoreaceae). The name has long been used interchangeably with Dioscorea opposita, and the two names refer to the same plant in most treatments. The tropical food yams, Dioscorea rotundata and Dioscorea alata, are different species with a coarser, drier tuber, and the sweet potato, Ipomoea batatas, is an unrelated plant sometimes called yam in North America.
The raw material is fresh yam tuber. Raw yam flesh is crisp and sticky with mucilage, and some people experience skin irritation when handling it because of fine needle-like crystals. Tubers are peeled, cooked or steamed, cut and dried in warm air, then ground into powder. Cooking removes the irritation and gelatinizes the starch, which gives a powder that hydrates smoothly.
Tubers are harvested in late autumn when the vine dies back, and they store well in cool, dark rooms. Cultivar types vary from long, thin tubers to short, thick ones, and the sticky texture is strongest in some varieties. Cultivars are not named for this product, which is described by its mild flavor and pale shade.
The slime of Chinese yam comes from a glycoprotein and polysaccharide mix, and it survives drying and cooking in part. That is why the powder, unlike that of potato or taro, gives a distinctly slippery body when stirred into hot water, comparable in feel to a light porridge.
