Sweet potatoes and their friends

Sweet potato is a term that is often ambiguously used to refer to a range of tuberous vegetables. Recently when we wrote about Jerusalem artichoke (theory and recipes), some readers asked us if it was the same as sweet potato. It turns out that some people do refer to Jerusalem artichokes as sweet potatoes, though technically sweet potatoes are another plant entirely. 

Today we are going to introduce you to some other plants that are sometimes called sweet potatoes and share the common feature of having edible underground tubers that are reminiscent of potatoes. That being said, no one calls carrots, turnips, radishes, or beets sweet potatoes (at least we hope not!) as they have their own pronounced shapes, colors, and tastes. All the vegetables that are referred to as “sweet potatoes” are mostly prepared in similar fashions to potatoes (baked, fried, fried, or mashed). These are often eaten as stand-alone dishes rather than merely being a single ingredient in a complex soup. So, though these plants are incredibly diverse from a botanical perspective, they are very similar from a culinary standpoint. For example, potatoes are more closely related to tomatoes than they are to sweet potatoes, Jerusalem artichoke is more closely related to sunflowers, and the true sweet potato is more closely related to morning glories than any of the other plants discussed here!

Just as the term “sweet potato” causes some confusion, so too does the word “yam.” Technically each word is used for only a single species, though they have come to be broadly applied to groups of related and unrelated plants. An overview of all the plants we are discussing can be seen in the table, and below we will get into some more detail about some of these plants.

Latin nameEnglish nameBotanical familyWorld production (million tons)
Solanum tuberosumpotatoSolanaceae370
Ipomoea batatassweet potato (somevere in North Ameriki it is called yam)Convolvulaceae110
Dioscorea batatasyamDioscoreaceae70
Helianthus tuberosusjerusalem artichokeAsteraceae3
Manihot esculentacassava, manioc, yuca, macaxeira, mandioca, kappa kizhangu, aipimEuphorbiaceae280
Amorphophallus konjackonjac, konnyaku potato, devil’s tongue, voodoo lily, snake palm, elephant yamAraceae
Colocasia esculentain Xanthosoma sagittifoliumtaroAraceae10
Pachyrhizus erosusJicama, yam beanFabaceae
Oxalis tuberosaoca (in New Zeland it is called yam)Oxalidaceae
Tropaeolum tuberosummashuaTropaeolaceae

Sweet potato

If we speak technically, the term “sweet potato” refers to a single species of plant in the family Convolvulaceae with the Latin name Ipomoea batatas. The first part of this name (the genus) shows us that this plant is a close relative of garden morning glory (Ipomoea purpurea) and its wild relative, field bindweed (Convolvulus arvensis). Not only is the sweet potato closely related, but it is very similar, especially in terms of its flowers. As a result, sweet potato vines are a popular addition to annual planters in many regions. The second part of the Latin name (referring to the species) displays the structural similarity of the root to potatoes (batatas = potato). 

Sweet potatoes originated from the tropics of the Americas, where they have been growing for at least 5000 years. Today, we produce over 110 million tonnes of sweet potatoes a year in the world, much of which comes from China. To compare, common potatoes (Solanum tuberosum) has about 370 million tonnes produced annually, with China also in the lead for this production. 

Sweet potatoes contain more vitamin A than potatoes, and even a bit more than carrots! This vitamin is found in the form of beta-carotene, which is why sweet potatoes are typically orange in color. All other tubers discussed here today are much closer to white in color. One serving of sweet potato (100 g) covers our daily need for this vitamin, while only giving us about 90 calories or 5% of our daily caloric requirement. 


Yam

Yams are very far removed from their relationship to either potatoes or sweet potatoes as they are monocots rather than dicots; this means that yams are more closely related to wheat than they are to sweet potato! Several species of the genus Dioscorea (D. rotundata, D. cayennensis, D. alata, D. polystachya, D. bulbifera, D. esculenta, D. dumetorum, D. trifida) are cultivated. Though these are each species within a genus, they are about as similar to each other as cherries are to plums. Still, the name yam is used for all of them. The shape of the tubers can be rounded or elongated, and most of them are very large (over half a meter in length sometimes!). 

Globally, we produce about 70 million tonnes of yam annually, with as much as 96% growing in Africa. The energy value of the yam is around 115 calories per 100 g serving, which is similar to the other tubers discussed here. Such a portion covers 23% of our daily requirement for vitamin B6, 21% for vitamin C, and also has many other vitamins and minerals. The relative nutrient density (amount of nutrients in terms of calories) is over 100%! 

Of the tubers discussed here, this is perhaps the most confusing one in terms of naming… Many Americans (and some Canadians) refer to yams as sweet potatoes and sweet potatoes as yams! This can make grocery shopping very tricky… Just remember that yams are much larger, and much more of a specialty food in many parts of the world. You are much more likely to encounter a sweet potato in an average grocery store than a yam. Yams are also much larger, often being sold as pieces rather than entire tubers. That being said, both are tasty and nutritious, so it is surely not the worst mix up that could happen!


Jerusalem artichoke

Since we have already covered this tuber (theory and recipes for it) in a previous post, we will only talk about it briefly here. The flowers on the Jerusalem artichoke make it very clear that it is part of the Asteraceae or sunflower family. Its Latin name is Helianthus tuberosus, which means tuberous sunflower. This plant originated in North America, where the Native Americans used it as a food item; it was brought to Europe by the French in the 16th century. 

These tubers often have small horns and are a bit reminiscent of ginger. Very versatile, they can be eaten raw, cooked, baked, or just about any way you can imagine. When raw, they taste like hazelnuts or walnuts. In terms of vitamins and minerals, they are similar to potatoes, except that they have a higher content of iron and vitamin E but lower content of zinc and vitamin C. The main difference between the Jerusalem artichoke and the other plants described here is that its primary carbohydrate is inulin rather than starch. The inulin gives it a sweet taste, but while starch is broken down into glucose in our bodies, inulin is made of bonded fructose molecules that our digestive enzymes cannot break down. As such, inulin remains undigested and acts as dietary fiber, thus stimulating the development of beneficial intestinal bacteria in the colon. The caloric value of this tuber is about 3 times lower than in other starchy root vegetables. 


Manioc

Manioc (also known as cassava) originated in South America and today is grown in tropical regions all around the world. Manioc is a staple food for a billion people living in such areas. Not only is it eaten like a potato, but also dried and made into flour or starch for industrial purposes. Interestingly, the starch is not called manioc starch, but rather tapioca starch.

Today we produce over 280 million tonnes of cassava annually around the globe, 1/5 of which comes from Nigeria, which is then followed by Thailand, which is the largest exporter. The Latin name for this plant is Manihot esculenta. The word “esculenta” in Latin means edible. It belongs to the milkweed family (Euphorbiaceae), which means that it is related to some potted plants, for example, the poinsettias that are popular around Christmas in many countries. 

Cassava also contains toxic cyanogenic glycosides in its roots, from which hydrocyanic acid is formed. The exact content of these glycosides varies between varieties, as well as growing conditions; drought, for example, increases the content. As such, manioc needs to be properly prepared in order to remove the toxins before being used.


Konjac

This plant got its name in Japan, where it is called konnyaku. The Latin name is Amorphophallus konjac, and it belongs to the family Araceae. This makes it related to the peace lily (Spathiphyllum) and other similar plants. It develops tubers up to 25 cm in size under the ground, while above ground a single leaf up to 130 cm in size rises up along with a large, scaly inflorescence with a large sheath.

The tubers of konjac contain a great deal of glucomannan polysaccharide; this is an excellent gelling agent, which is why this tuber has become so popular in some forms of Asian cuisine. This also makes it a great vegan substitute for gelatine, which has led to its use in the food industry for this purpose. It can also be found in food products under the name E425.

In addition to gelling, there is another interesting feature of konjac glucomannan: it has no calories. In addition, it swells up while in the digestive tract giving a feeling of satiety and is thus used in weight loss products. If overconsumed, however, it can cause digestive problems. After re-examination 3 years ago by the European Food Safety Authority, they decided that less than 3 g per day should be consumed and no more than 1% of total intake should be this substance. 


Other potato-like plants are grown and used to a limited extent, though the term “sweet potato” is rarely used for them, so we will not describe them in detail; they are collected in the table at the beginning of this article.

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