TUBERS – ‘The Good, The Bad & The Ugly’

By Janet Colston

The potato was first domesticated in Peru and Bolivia between 8000 and 5000BC [1]. Andean farmers found they grew well in higher altitudes around the lakes of Titicaca where the ancestors of the Inca settled [2].

Rich in starch, potatoes contain higher vitamin C than oranges, more potassium than bananas, and more fibre than apples [3]. The Spanish introduced potatoes to Europe in the 15th century having realised the food security significance [1]. Almost a century later the governor of Bermuda sent plants to Virginia in the U.S. [4]. It would take another century and a half to become an established crop in the U.S. In the 19th century, potatoes became a staple food, mostly to immigrants from Ireland and Scotland.  During the Klondike gold rush, prospectors prized potatoes as much as the gold they extracted. Although vitamin C was yet to be discovered, the gold diggers knew potatoes contained vital nutrients that helped them to survive in the wild [5].

Western diets have literally been consumed by the potato and all its derivatives but we have forgotten the benefits of other South American/African tubers, namely yacón, sweet potato and yam. Popular in South America and Africa, these are still relatively uncommon SUPERFOODS that grow and adapt well to controlled environments.

Strengthening the Immune System to protect against disease – ‘The Good’

YACóN (Smallanthus sonchifolius)

Yacón is a relative of the sunflower and Jerusalem artichoke. It produces a large tuber with a sweet taste and texture like an apple or watermelon [6].  The edible tuber can weigh from a few hundred grams up to a kilogram. Yacón roots contain inulin, oligosccharides and fructooligosaccharides (FOS), indigestible polysaccharides that pass through the human digestive tract without being absorbed. Yacón has been used to make syrups suitable for diabetic patients, highly valued in Japan for antihyperglycemic properties [7]. However the percentage of FOS versus monomer type sugars can vary with variety, particularly post harvest (personal communications Beotanics Ltd). They showed that monomer sugars increased dramatically within 4 months of storage. Proper variety selection, growing culture, storage temperature and processing are essential to maximise this vegetables unique traits. They have a prebiotic effect, promoting  beneficial gut bacteria to improve gastric health [7]. Yacón leaves also display medicinal properties with protocatechuic, chlorogenic, caffeic, and ferulic acids, which help to lower cholesterol [8]. 

Growing Yacón – Protected Controlled Environment

Yacón products including flour, syrup, juice, concentrate, tea extracts, both fresh and organic are predominantly exported to U..S markets [9]. If you want to grow yacón,  space is a prime consideration as they can grow to over 2m in height. They produce small yellow flowers at the end of the growing season. Although yacón is not photoperiod sensitive, it still requires several months to establish and tubers tend to be late forming with most development happening 5 to 8 months after planting. Yacón does not do well in dry hot summers as it’s unsuitable for maximising high FOS in tubers. The hot climate also makes it susceptible to thrips and whitefly so there is a definite advantage to cooler coastal conditions such as the Pacific Northwest.

SWEET POTATO (Ipomoea batatas)

Sweet potatoes are a member of the morning glory family. They are gathering popularity as a healthy alternative to the starchy potato. Sweet potatoes produce more edible energy per hectare per day than wheat, rice or cassava [10] and are a good source of carbohydrates, fiber, and micronutrients. The edible leaves and shoots contain vitamins A, C and B (Riboflavin) [11]. Orange sweet potato contains high levels of beta-carotene which is converted by the body to vitamin A. They offer a cost-effective, sustainable way to supplement diets with only 125g needed to provide a RDA of vitamin A [12]. 

Purple sweet potato contains 4-7 times higher levels of anthocyanins compared to other sweet potatoes [13]. There is a wide range of genetic variability which leads to some interesting opportunities in the intensity of beta-carotene and anthocyanin, making this crop suitable for future food ingredients and nutraceutical exploration in both leaf and root. Beotanics Ltd are at an advanced stage exploring commercial opportunities in protected and vertical farming possibilities with specially selected genetics for high value applications.

A search for sweet potatoes in the U.S. will take you to California Sweetpotates, family owned farms in Merced, Stanislaus and Kern County. These sweet farmers practise sustainable controlled fertilization and drip irrigation to produce bountiful harvests. They have one of the best sites for learning how to cook with sweet potatoes. So why not make some amazing healthy dishes, we love the grilled sweetpotato tacos with queso fresco and cucumber-tomato salad.

YAM  (Dioscorea)

Some people confuse sweet potato with yam but it bears no relation to yam which are sometimes called true yams so as to make the genus clear.

Yam is a tuber of the Dioscoreacea family, commonly grown throughout African countries predominantly grown for starch. The most common cultivars include white yam (D. rotundata), water yam (D. alata), yellow yam (D.cayenensis), aerial yam (D.bulbifera), Chinese yam (image inset) (D.esculenta) and trifoliate yam (D.dumentorum) [14]. Bulbifera also known as ‘air potato’ is a highly invasive species [15]. The USDA has a citizen science programme to monitor Bulbifera vines and the biological beetles introduced to control their spread in Florida [16]. One of the most under-rated medicinal varieties is the water yam, cultivated in Nigeria for its large roots and edible white and purple flesh [17]. 

Medicinal Properties Water of Yam

Water Yam is a natural source of vitamin C, beta carotene and vitamin B6 with important cardiovascular protectant properties [18]. Traditionally hard to grow, water yam has been difficult to commercialise in comparison to other cultivars. Farmers in Nigeria have concentrated on the more lucrative white yam and mixed crop farming of maize, cassava, cocoyam and sweet potato to ensure food security and protect against crop failure. But water yam is very high in total dietary fiber and has a high amylase content [19], which helps to break down starch. Water yam is also low in sodium and high in potassium, manganese and calcium which has implications in food fortification supporting bone health, metabolism and heart function [20].

Yam has been shown to contain powerful antioxidants with anticancer properties. It has preventative effects against many other ailments including arthritis and gastrointestinal disorders [20]. In turn these antioxidants display anti-inflammatory effects which help control diabetes and obesity as well as related heart disease [20]. A study examining people that ate yam extract for 12 weeks showed improvements in brain function. The effect is thought to be due to diosgenin which promotes neuronal growth [21].  

Circulatory levels of two estrogen hormones, estrone and estradiol typically decrease during menopause, but both hormones can be increased through daily consumption of yam which has been demonstrated to increase levels by more than a quarter [22]. 

Why Yacon, Sweet Potato and Yam need to start clean – ‘The Bad’

Despite being relatively easy to grow, potato tubers have a bad past history and are susceptible to more than 75 diseases [22]. The origins of some of the more serious diseases including blight have since been traced back to Mexico [24]. Early potato blight most often seen in North America is caused by the fungal disease of Alternaria [25], but it is late potato blight caused by Phytophthora infestans that resulted in the devastating mid 18th century European and Irish famines [24]. This also affects other members of the solanacea family including tomatoes. Around the world potato blight causes around $6 billion of damage to crops each year [26].

There needs to be a clear path for viral and fungal free starting material and that starts with plant tissue culture and clean potato seeds.

The solution is to source the best plants for the field from plant micropropagation that guarantees the plants are free from diseases. The International Potato Centre, CIP in Lima is the central genebank for all things potato related in the world, preserving every known tuber germplasm for future biodiversity. We don’t know of many commercial growers of yacón and sweet potato using protective environments but one Irish company, Beotanics in Kilkenny, Ireland are proving it can be done. They hold a wide range of virus free stock  in their laboratories to support over 1000 ha of client crops.

Protected cropping is essential for seed root production planting materials due to sweet potato being very prone to virus. In more temperate regions protected cropping of planting stock is essential as it’s a slip raised crop and huge amounts are needed for a very short planting window. 

Sweet potatoes can be grown using a dripper system which may circumvent contaminated fields allowing time for regeneration of agricultural fields. In Nigeria Professor Morufat Balogun oversees the work done by the Institute of Tropical Agriculture (IITA) on yam seed production from meristem tip culture followed by aeroponic growing in tanks within polytunnels before finally planting out in the field. Commercial crops are subsequently harvested after 5 months with tubers as large as 5Kg. Funded by the Bill and Melinda Gates Foundation hear first hand from Professor Balogun on the impressive work being done in Nigeria in the film below.

Potato Piracy – ‘The Ugly’ 

If you are planning to grow any of these crops, be aware that some have protected status. Cultivar piracy is a real threat to the South American economy who are reliant on the income from native crops.

Feeding future world populations

The potato tuber is the third most eaten crop in the world, but who would have thought the potato could sustain life beyond earth in years to come? Proof of concept experiments are currently taking place to grow potatoes in simulated Martian conditions [27]. So the humble spud is indeed a SUPERFOOD that has survived famines, disasters, maybe even become a sustainable food source for settlers on Mars but for now and more importantly could boost personal immunity to fight disease.

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Janet Colston PhD is a pharmacologist with an interest in growing ‘functional’ foods that have additional phytonutrients and display medicinal qualities that are beneficial to human health. She grows these using a range of techniques including plant tissue micropropagation and controlled environmental agriculture to ensure the highest quality control.