Exploring the Chemistry of Sweet Potatoes

A study of purple-fleshed sweet potatoes grown in northern latitudes reveals the large structural diversity of anthocyanins and flavonoids in this popular crop.

Anthocyanins are blue, red and purple phytochemical pigments of significant health and economic value, which belong to a class of compounds called flavonoids.

Found in a variety of fruits and vegetables, flavonoids possess antioxidant activities of benefit in managing ageing, stress, cancer and other health conditions, making them desirable for cosmetic, nutritional and health industry applications. Anthocyanins are also of interest as natural food colourings. The global market for flavonoids is expected to exceed $1 billion by 2026.

The major sources of industrial production of anthocyanins include grape skins, berries, black carrots, red cabbage and purple-fleshed sweet potatoes. But the anthocyanin yield and profiles can vary between sources and growing conditions.

A Spotlight on Purple Potatoes

While the major cultivated varieties of sweet potato (Ipomoea batatas) are orange-fleshed, there are also purple-fleshed varieties that may offer attractive opportunities for anthocyanin extraction. Unlike fruits, sweet potato roots produce high biomass, store better, and can be cultivated on large scale. They also primarily produce more stable, acylated anthocyanins, unlike many non–acetylated fruit compounds.

Many growers in northern, cooler latitudes are becoming increasingly interested in cultivating this nutritious crop – both for general consumption and anthocyanin extraction. Evaluating the anthocyanin content of purple-fleshed sweet potatoes grown in cooler climates is an important step in understanding their economic potential.

Rich in Phytochemicals

In a new study, published in Scientific Reports, researchers examined the anthocyanin content of three purple-fleshed varieties of sweet potato grown in northern latitudes.¹ The researchers extracted ~800 mg average anthocyanins per 100 g dry weight from three purple-fleshed cultivars – ‘Kotobuki’, ‘All Purple Sweet Potato’ and ‘Purple Passion’ – using acidified ethanol extraction. They went on to identify 18 high–confidence compounds across all three varieties using ultra–high–performance liquid chromatography–diode array (UHPLC–DAD), mostly acetylated peonidin and cyanidin derivatives.

The researchers used ultrapure water generated from an ELGA PURELAB® laboratory water purification system to perform these experiments, minimising the risk of introducing contaminants that may affect their results. The team also performed a more comprehensive assessment of the flavonoid content by applying machine learning and molecular networking tools to their untargeted metabolomics data. This revealed all three purple-fleshed cultivars are also rich sources of flavonoids, as evidenced by the inclusion of over 350 LC-MS/MS peaks in the molecular network.

Tapping into the Potential Benefits

This study indicates that purple-fleshed sweet potatoes grown in northern latitudes produce substantial amounts of anthocyanins and flavonoids that can provide potential health benefits to consumers and economic benefits to farmers.

While the slender build of the tested purple sweet potatoes may reduce their marketability in grocery stores, the high levels of anthocyanins and flavonoids make them attractive for other commercial applications – such as health food, specialty chemicals, food processing and cosmetics industries.

Further research is now needed to determine the optimal varieties and growing conditions suited for northern soils and climates.

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Reference:

1. Bennett, A. A. et al. Untargeted metabolomics of purple and orange-fleshed sweet potatoes reveals a large structural diversity of anthocyanins and flavonoids. Sci Rep. 2021;11:16408

Dr Alison Halliday

After completing an undergraduate degree in Biochemistry & Genetics at Sheffield University, Alison was awarded a PhD in Human Molecular Genetics at the University of Newcastle. She carried out five years as a Senior Postdoctoral Research Fellow at UCL, investigating the genes involved in childhood obesity syndrome. Moving into science communications, she spent ten years at Cancer Research UK engaging the public about the charity’s work. She now specialises in writing about research across the life sciences, medicine and health.