Promote purple sweet potato

By Prof. Kirthi Tennakone

The sweet potato is probably the oldest cultivated crop. Native to tropical America, Peruvians grew it 8,000 years ago. Colombus introduced sweet potatoes to Europe; later the Spanish and Portuguese dispersed the species in Asia and Africa. Today, sweet potato is one of the staple foods that alleviate starvation and malnutrition in many parts of the globe.

In the tropics, sweet potatoes produce more food per hectare than any other cultivation. Being drought – resistant and requiring minimal irrigation, fertilizer and pesticide inputs, the crop would be a promise for the world confronted by climate change.

There are highly nutritious varieties of sweet potatoes, of which sweet potato with bright purple flesh; rich in disease-fighting agents, vitamins, minerals and fiber is emerging as a superfood – chosen as the vegetable of the year 2024 by Veganuary (the organization that promotes vegetarianism by abstaining from eating animal-based food in the month of January, every year)

The sweet potato stands out as the first genetically modified vegetable people have unknowingly consumed for millennia. The modification was done not by humans, but by a bacterium found in soil. The ancestor of sweet potatoes was a creeper belonging to the Morning Glory family (Convalesce). Thousands of years ago microbe named Agrobacterium transferred a portion of its DNA (a gene) into the genome of a plant of the original kind in Ecuador, favourably altering its characteristics. In the modified plant, roots were enlarged to yield giant tubers, storing starch, proteins, minerals, water and a host of other beneficial substances. Naturally, the genetically engineered variety withstood harsh conditions and survived better. Noting the exceptional food value, South American farmers domesticated the transgenic species. The rise of Mayan and Aztec civilizations owes much to the sweet potato. Transgenic crops can do wonders!

Today, thousands of cultivars of sweet potato exist, generally described by the colour of the skin and flesh of the yam. The skin colours of white, brown, pink, yellow and purple are common, and the flesh colours vary from white, yellow and orange to purple. These colours originate from two types of pigments known as carotenes and anthocyanins.

Apart from chlorophyll, which imparts green colour to leaves, plants carry two other pigment types: carotenes yellowish orange in colour and anthocyanins displaying hues ranging from red to blue. As we all know, in leaves chlorophyll harvests sunlight and utilizes trapped energy to fix carbon dioxide synthesizing carbohydrates. Carotene and anthocyanins, present in leaves and other plant tissue play an important role in photosynthesis and other plant biochemical processes and give conspicuous colours to flowers, fruits and yams.

Nowadays, people are told to eat coloured vegetables and fruits, because carotenes and anthocyanins have numerous health benefits, including the prevention of cancer, heart disease, diabetes and dementia. What is the secret of coloured produce? A condition that greatly contributes to the above complications is oxidative stress, a side effect of the indispensable oxygen we continuously breathe. Oxygen burns carbohydrates and fat in our bodies, liberating energy and releasing water and carbon dioxide. However, this chemical reaction creates highly reactive molecular species, known as free radicals. Free radicals damage DNA and attack cellular tissue, causing disease and aging. Fortunately, there are agents called antioxidants that scavenge free radicals before they do any harm. Carotenes and anthocyanins act as superb antioxidants.

Current research reveals anthocyanins, the red and blue pigments in vegetables and fruits have a myriad of health benefits because they are water soluble and transmissible throughout the body after absorption from the gut. They effectively quench free radicals, reducing DNA and cellular damage – a primary cause of cancer and inflammatory diseases. Anthocyanins cross the blood-brain barrier and possess a protective action against neurodegenerative cognitive impairments such as Parkinsonism and Alzheimer’s disease. With the increase in human longevity, these illnesses pose a major health issue for which no curative treatments are available. Recent investigations provide evidence for the beneficial effect of consuming anthocyanins rich vegetables in reducing the risk of developing these debilitating conditions.

The brilliantly coloured purple sweet potatoes are gaining recognition as a superfood because of the exceptionally high anthocyanin content, and the remarkable stability of the pigment and its resistance to cooking. The Spanish seemed to have collected this special variety of sweet potatoes from the Aztecs in Peru and planted it in the Philippines around the 1400s and from there, it spread to China and Japan. The climate on the island of Okinawa was ideal for sweet potatoes, a staple food for the islanders, who preferred the purple variety. Okinawans obtain more than 50% of their calorie requirements from sweet potatoes. The incidence of cancer, heart disease and dementia stand remarkably low in this land and people live relatively long compared to other regions of the world. Are purple sweet potatoes the secret of Okinawan longevity and healthiness?

Of course, other factors such as lifestyle and assortments in the diet would contribute to the good health and longevity of the people of Okinawa. As more and more studies confirm the consumption of purple sweet potatoes is indeed an important cause, the world is rushing to cultivate this crop and promote its consumption.

Purple-fleshed sweet potatoes are rare in Sri Lanka. The farmers and consumers are largely unaware of its value.

The author was fascinatingly attracted to purple sweet potato, because of his interest in anthocyanin pigments, which he used to demonstrate a novel solar cell concept for the first time now pursued worldwide. He successfully grew this plant in his home garden, Hantana, Kandy, and obtained a good yield; some tubers weighed more than a kilo. Purple sweet prefers colder night temperatures. Our Central Highlands are probably best suited to cultivating this crop. He wishes Sri Lankans will cultivate this crop and promote its consumption.

(The author is a theoretical physicist, formerly working at the Institute of Fundamental Studies, Sri Lanka. He can be reached via email: ktenna@yahoo.co.uk)