Dioscorea alata, called water yam, winged yam, and purple yam, was first cultivated somewhere in Southeast Asia. Although it is not grown in the same quantities as the African yams it has the largest distribution world-wide of any cultivated yam, being grown in Asia, the Pacific islands, Africa, and the West Indies (Mignouna 2003). In the United States it has become an invasive species in some Southern states.
In the Philippines it is known as ube (or ubi) and is used as an ingredient in many sweet desserts. In India, it is known as ratalu or violet yam or the Moraga Surprise. In Hawaii it is known as uhi. Uhi was brought to Hawaii by the early Polynesian settlers and became a major crop in the 1800s when the tubers were sold to visiting ships as an easily stored food supply for their voyages (White 2003).
Known as khoai mỡ in Vietnam, this yam is used in both sweet and savory dishes. My encounters with khoai mỡ have been pretty limited thus far. I once ordered a cup of che thap cam at Che 278 in District 4 that featured a layer of sweetened khoai mỡ puree. However, now that I know what this purple vegetable is, I’ll be on the lookout for it on menus around town.
According to my mom, the most common preparation of khoai mỡ is in a shrimpy soup called canh khoai mỡ. Coincidentally, The Astronomer recently noticed the owners of one of his favorite bun cha joints slurping down a strange purple broth for their dinner. When he asked what it was, they ladled out an extra bowl for him and invited him to sit down. The soup turned out to be canh khoai mỡ, which he described as “plain.” My mom, on the other hand, couldn’t stop raving about the soup’s deliciousness when I spoke to her. So, I can’t wait to try the yam soup for myself!