Moderately yours: The story of ‘Shankhalu’

It is not easy to find shankhalu in the fruit and vegetable markets of Delhi unless one waits patiently for February to taste its earthy sweetness. The root vegetable, with white and crunchy flesh and slightly sweet taste, is primarily grown in West Bengal, Odisha, Bihar and several northeastern states, and is available around Vasant Panchami, a Hindu festival that marks the beginning of spring. In some states, particularly in West Bengal, where goddess Saraswati is worshipped on the occasion of Vasant Panchami, the tuber is one of the fruits offered to the deity. The turnip-shaped root vegetable with striations on its light brown papery skin resembles a conch or shankh that the goddess holds.

The plant, also known as yam bean, jicama and Mexican turnip, is not native to India. Rather, it is native to tropical America; the first yam bean plant to be described by Carl Linnaeus, the father of taxonomy, in 1753 was a species from Mexico and is known as Pachyrhizus erosus in scientific lexicon. So far, taxonomists have identified five species of the yam bean, of which three—P erosus, P tuberosus and P ahipa—are cultivated for their tuberous roots in Central America, China, India, Southeast Asia, Bangladesh, the Caribbean, French Guiana, Brazil and Central and West Africa.

The genus Pachyrhizus has several characteristics that establish it as a sustainable crop for the tropics and sub-tropics. For instance, the plant, a perennial vine, grows easily and is drought resistant. Unlike other yams such as the elephant foot yam and colocasia, yam bean plants can fix atmospheric nitrogen and enhance soil fertility.

The tuber is also known for its health benefits. Jicama might look like a potato, but it is not as starchy and has fewer carbohydrates. Its texture is crunchy and juicy, similar to that of a pear or water chestnut, and it is slightly sweet. According to the US Department of Agriculture, 100 g of the yam bean contains around 90 per cent of water and 4.9 g of fibre. The root is also rich in micronutrients; 100 g of the yam bean contains as much as 150 mg of potassium. A study published in the journal Antioxidants in November 2021 states that more than 50 phytochemicals have been reported from the plant, which belong to different chemical classes such as triterpenoids, organic acid, flavonoids and fatty acids.

These chemicals are known for antioxidant, immune modulation, anticancer, anti-diabetes, anti-osteoporosis, antiviral, and anti-ageing properties.

Raw yam bean is also a rich source of fructooligosaccharides and inulin, which provide the sweet taste to the tuber. Fructooligosaccharides and inulin are inert carbohydrates and do not turn into simple sugars when broken down by digestion. This is good news for those with diabetes; eating this root will not lead to fluctuations in blood sugar levels. Several studies involving animal models have found that the root crop has been effective in controlling metabolic syndrome and preventing the development of diabetes and obesity caused by a high-sugar diet. In a study published in Journal of Advanced Veterinary and Animal Research in April 2019, mice were fed with fibres isolated from the root for eight weeks. The researchers found that supplementation of 25 per cent of the diet with the fibres significantly prevented the blood glucose increase, excessive body weight gain, and glucose intolerance caused by high sugar diet. Another study, published in Cosmetics in December 2023, shows that the root extracts protect from obesity. The study results indicate that the yam bean extract has excellent carbohydrate-digestive enzyme inhibitory effects, suggesting its potential use as a functional food ingredient for diabetes and obesity prevention.

In almost all the countries where the vine grows, yam bean is part of the street food. In Mexico, the roots are sold roasted, baked and even grilled. In India, the tuber, also known as misrikand, kesaru and sankesh, is eaten raw or mixed with spices (see recipes).

Despite multiple benefits, farmers in India still rely on traditional landraces as breeding efforts have not received much attention from researchers. The Central Tuber Crops Research Institute, Thiruvananthapuram, Kerala, and centres under the All India Coordinated Research Project on Tuber Crops maintain over 60 germplasm accessions from different parts of India.

Though the roots of the plant are edible, the seeds, leaves and stems are poisonous. They contain a chemical rotenone that is moderately toxic to humans and can even lead to death, as per the World Health Organization. Rotenone is used as an insecticide to control aphids. While this does suggest caution in case people accidentally consume the seeds, it also means that yam bean provides two products that can keep humans and plants healthy.

This was first published in the 1-15 March, 2024 print edition of Down To Earth

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