The “popiah” or the spring roll is a popular food item that most Singaporeans should not not familiar with. The filling that is used to make this dish is commonly called via its Hokkien name “Bang Kuang” which essentially the cooked tuber shreds of the yam bean tuber.
Those of us who have been to the local wet market or supermarket would have seen the elliptical shaped yam bean tuber on sale in the vegetable section but I reckon most of us may not have seen the actual plant. Some months back, one of my community gardeners, Mrs Yap planted some seeds of the yam bean which have grown into several vines climbing up and tying themselves around the bamboo supports that had been put up.
Botanically known as Pachyrhizus erosus, the yam bean is a member of the bean family, Fabaceae and is native to Central America. Unlike the vegetable beans we are familiar with, the edible portion of the yam bean is its swollen tap root which is prepared by first peeling away the fibrous brown skin. The white flesh that is revealed can be eaten cooked or raw and has a crispy and moist texture.
This is one of the first times I have seen the real yam bean plant. The seeds can be quite hard to obtain and locally in Singapore, only one seed supplier sells them. The plant produces the familiar trifoliate leaves we often encounter with other legume vines. Instead of being roughly oval in shape, the leaves of the yam bean are lobed with several pointed edges. They are also velvety to touch!
The yam bean plant surprisingly produces rather attractive lilac-coloured flowers. They are borne on a tall and erect inflorescence. As such, I find yam bean flowers are more ornamental than those of the winged bean (Psophocarpus tetragonolobus) because they are more conspicuous and do not get hidden by the plant’s foliage.
Like the winged bean plant, the yam bean plant behaves somewhat like a perennial in the garden. It takes about half a year from seed to harvest a sizeable tuber and while waiting, one can actually admire the beautiful flowers that are produced.
After the flowers fade, velvety bean pods are produced and do remember that they are not suitable for consumption! So do not harvest any for the cooking pot! They are not edible due to the numerous hairs on the young pods which can cause irritation. The rest of the plant is highly toxic and the seeds are well known to contain rotenone, a widely used chemical to kill fish and insects. It is a component in some pesticides used in organic gardening.