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Writer's picturedrbeckyandrews

Decoding Sweet Potatoes and Yams

Yes, it is time for another super random food related post! I think it is nice to dilute all the delicious science with something light and fun, but still educational and health related. Now that it is officially autumn, this post is extra timely.


variety of sweet potatoes
Spoiler Alert: These are all sweet potatoes

Ever since watching the 2023 Netflix docuseries about Blue Zones, I have been semi-obsessed with the purple sweet potatoes they were eating in Okinawa. These dark purple sweet potatoes are a staple of the diet in rural Okinawa where people live remarkably long lives. The hosts suspected that these sweet potatoes were part of the longevity equation for this group of people, and they assumed it was due to the abundance of anthocyanins (the purple pigment in blueberries). As I have been helping my friends and family with garden planing and harvesting this year, I ended up going down a research rabbit hole (shocking, I know) looking for unique, extra healthy foods to grow. I really wanted to see if I could find these purple sweet potatoes and finally learn the difference between sweet potatoes and yams.



Sweet Potato vs Yam

I've always been curious and confused. At the grocery store there are always big piles of brown tubers (in various shades) that look and taste pretty much the same, but some are labeled yams and some are labeled sweet potatoes. Some have redder skin, some browner, and some are cream colored inside and others orange. Was it that the orange ones were yams and the white ones sweet potatoes? What about the orange sweet potatoes in sweet potato pie?


As an herbalist, I have studied quite a bit of botany and have always been interested in the common traits that related plants will have. Before starting this research I knew a few things: There are several medicinal plants (from different continents) that are called "wild yam" and they are all in the genus Dioscorea. I also knew that white potatoes were in the nightshade family (Solanum) along with tomatoes, and that sweet potatoes were not related to those.


The host of the "Blue Zones" documentary said that the Okinawa purple sweet potato was in the morning glory family (which is not Dioscorea).  Was it possible that some of the yams in my grocery store were related to medicinal "wild yams"? It turns out, NO. Everything in your U.S. grocery store is a sweet potato, regardless of color or label.



Botany of the Tubers

A quick note on botanical nomenclature (how botanists name plants):

There is family, then genus, then species. Many plants also have cultivars that are variants of the same species. Plant names are usually given as Genus species (Genus name is capitalized and species name is in lower case).


Regular potatoes

  • Latin Name: Solanum tuberosa

  • Common name for genus: the Nightshade family

baskets of white potatoes
Plain 'Ol Potatoes
  • Many cultivars:

    • different colors / textures are different cultivars of the same genus and species

    • Idaho potatoes, small red, white and blue potatoes, as well as fingerling potatoes are all Solanum tuberosa

  • Myth confirmed: Don't eat the green potatoes!

    • Potatoes that have been exposed to sunlight and have developed a green tint under the skin really are toxic.

    • The green pigment indicates a large increase in the alkaloid solanine which can cause severe abdominal pain. It may also cause nausea, diarrhea and vomiting. Solanine is not well absorbed, so it takes a huge intake (2-3 full sized baked potatoes) to consume enough to have systemic symptoms. These symptoms affect the heart and nervous system, so are pretty dangerous. In even small amounts, Solanine can cause significant joint pain in addition to the abdominal pain.

    • If you peel all the green pigment away before cooking, you will reduce the excess solanine significantly.

    • You should also remove any sprouting eyes as they also contain solanine.

    • Take avoiding solanine seriously.

    • Really, don't ask how I know this...



Sweet potatoes

  • Latin Name: Ipomoea batatas

    • I love that name so much! pronounced "eye-poe-moe-ee-ya ba-ta-ta"

  • Common name for genus: the Morning Glory family (Convolvulacea)

Japanese and US sweet potatoes
Satsuma imo, Beni Imo, US sweet, US Garnet Left is raw, right is cooked

  • Many cultivars

    • Different skin colors and inside (flesh) colors

    • All are fairly sweet, but the amount of starch and sugar varies - which gives each cultivar a slighly different taste and texture

      • Brown skinned with white flesh (traditional sweet potato)

      • Red skin with orange flesh (like Garnets)

      • White skin with purple flesh (Ben imo)

      • Red skin with white flesh (Japanese purple or Satsuma imo)

      • Purple skin with purple flesh (Stokes purple)

      • are all sweet potatoes (Ipomoea batatas)

    • While these come in many colors, and slightly different sizes, they all

      • have thin, smooth flesh that you can easily bite through and eat.

      • are around 6-10 inches long and 1-2 inches in diameter

      • have ends that usually taper to a point

    • The flesh of the raw sweet potatoes (all cultivars) will ooze white latex when cut

      • This is super gummy.

      • If you get it on your pots, knives or hands, it requires alcohol to remove

    • Sweet potato cultivars grow in a wide variety of climates, some are tropical but many are temperate.



Yams

  • All yams are in the Dioscorea genus, and there are several different species in use

whole and sliced African Yam
African yam
  • Dioscorea rotunda (Africa)

  • Dioscorea alata (Asia)

    • aka Ube purple yams

    • not to be confused with Bene Imo, the Okinawa purple sweet potato, a cultivar of Impoema batatas

    • there are MANY purple colored Asian deserts based on Ube yams

  • Dioscorea trifida (S. America)

  • Dioscorea polystachya (China)

  • there are more... PLUS there are several species of medicinal "wild yams" that are also Dioscorea...

  • Medicinal species

    • Dioscorea villosa (American "wild yam")

    • Dioscorea polystachya (Chinese "wild yam", Shan Yao)

    • Dioscorea opposita Thunb. (Chinese "wild yam", also called Shan Yao)


To make things more confusing - there are several other, completely unrelated plants that are locally called "yams" in different indigenous cultures around the world.  

Yam is also a generic term for "big starchy root"

  • Taro roots  (Colocasia esculenta) are sometimes called yams in Malaysia

  • Yam daisy roots (Microseris lancelata) are sometimes called yams in Australia

  • Oca (Oxalis tuberosa) is sometimes called yam in New Zealand

  • Elephant foot yam (Amorphophallus paeonifolius) is sometimes called yam in Africa and the South Pacific


True yams only grow in tropical West Africa, South America, tropical Asia / S.E. Asia, and the Caribbean. They have a woody, bark-like skin, often with lots of hair like roots in addition to the bark.  They are often much bigger than a sweet potato. Many are foot or more in length, and 3-10 inches in diameter, really big species can be the size of a small child. The flesh is bland, starchy and not particularly sweet. They look nothing like the smooth, thin skinned sweet potatoes (sometimes called Yams) that we get the the U.S. grocery stores.  ALL of the "sweet roots" we can buy in American grocery stores are sweet potatoes.  In the U.S. you have to go to international, specialty groceries to find true yams.


Why sweet potatoes are called yams

Calling orange sweet potatoes "yams" is a marketing thing to try to distinguish the ones with orange flesh from the ones with yellow/ white flesh.  It dates back to slavery, when the slaves brought up from the Caribbean called the sweet potatoes yams.. and it stuck. 


Jenne Claiborne of Sweet Potato Soul does an amazing job explaining the difference between yams and sweet potatoes. https://www.youtube.com/watch?v=3FpFGqEVnW4



Health benefits of sweet potatoes over regular potatoes and yams


Nutrient Values (1 medium potato - what size that actually is? unknown, so these numbers are a little wiggly)

White Potato


Sweet Potato

159

Calories

103

70-100+ (baked) 80 (boiled) 55-60 (boiled and cooled)

Glycemic Index

85+ (baked) 45 (boiled) 45- (boiled and cooled)

37g

Total Carbs

24g

4g

Fiber

4g

3g

Sugar

7g

30g

Starch

13g

4g

Protein

2g


Vitamins


17IU

Vitamin A

21,000IU

22mg

Vitamin C

22mg

0.4mg

B6 (Pyroxidine)

0.3mg

30mcg

Folate

7mcg


Minerals


17mg

Sodium

41mg

941mg

Potassium

542mg

17mg

Calcium

43mg

47mg

Magnesium

31mg

1mg

Iron

0.8mg

0.6mg

Copper

0.15mg

0.3mg

Manganese

0.6mg


Special


Fiber is mostly in the skins


Orange = carotenoids --> Vitamin A precursor & antioxidant

Nutrients mostly in skins


Purple = anthocyanins --> powerful antioxidant


Why are sweet potatoes supposed to be better?


Glycemic Index

Sweet potatoes are supposed to be lower in carbs, with a lower glycemic index than regular potatoes. It turns out that this is very much dependent on how you cook them, and to some extent, the varieties. The biggest difference is the cooking method.


Baked or roased both potatoes and sweet potatoes are significantly higher in simple starch and both will have a fairly high glycemic index. Boiling sweet potatoes and regular white potatoes significantly lowers the sugar, simple starch and glycemic index of both.


If you want to get really serious about lowering glycemic index, boil and then cool potatoes. This increases the amount of resistant starch. This works with other starches like rice and sweet potatoes. Surprisingly, this actually works BETTER in white potatoes. White potatoes have a type of starch that forms a gell like substance after boiling and cooling. Resistant starch 'resists' being broken down, and therefore the carbs enter your system more gradually (giving you a lower glycemic index).


Nutrient Content

As you can see above, white potatoes and sweet potatoes actually have surprisingly similar macronutrient profiles. Both have very similar amount of calories, carbs, fiber, and protein. They even have similar amounts of vitamins and minerals in the skins. The real difference turns out to be in the "special" nutrients that give sweet potatoes their pigment. The orange and red colors in the flesh (and skin) of some sweet potatoes comes from pigments called carotenoids. Beta caroteine is an orange carotenoid (yes, it is what makes carrots orange, thus the name). It is antioxidant, and also is the water-soluble version of vitamin A. It benefits the retina of the eye and vision. The purple colors in the skins and flesh of the purple sweet potatoes comes from a pigment called anthocyanins (also found in blueberries and blackberries). This pigment is a powerful antioxidant that helps protect and repair all of our tissues, and is associated with anti-aging action.


references for nutrition:



The Great Sweet Potato Comparison


variety of sweet potatoes raw and cooked
Japanese sweet, Beni Imo, American Sweet, Garnet

I found this video well made, fun and informative - showing lots of different types of sweet potatoes




Beni Imo vs Stokes Purple is ON!


Yes! The organic Stokes Purple Sweet potatoes are back in stock! I finally get to do a side by side comparison with the Beni Imo from Okinawa. Beni Imo is touted for it's health benefits. This is attributed to the extreme density of the antioxidant molecules called anthocyanins (the same purple pigment in blueberries and blackberries). Beni Imo is presumed to have fairly low glycemic index while tasting very sweet.


I had tried Beni Imo with the other sweet potatoes earlier in the summer and they were my absolute favorite for flavor, texture and the sheer novelty of eating something that purple (and that I saw on Netflix!). I love to garden, and try to grow unusual edibles and medicinals. Bene Imo is strictly a tropical plant, so can't be grown in Kentucky where I live. The Stokes are grown in the US, so I was very curious if it would be similar.


Stokes purple and Okinawa sweet potato
Stokes Purple vs Beni Imo

Because the Beni Imo is traditionally just chucked in a pot and boiled until fork tender (about 20 minutes), I decided to do that with both of these purple sweet potatoes. I think the Stokes is traditionally baked or roasted, and I'm sure you can cook the Beni Imo that way as well. However, the glycemic index of sweet potatoes goes up with roasting, and goes down with boiling. Since part of this experiment was to compare sweetness, starchiness and purpleness, I went with the healthier boiling.


In the picture above, the Stokes Purple is on the left, and the Beni Imo is on the right. In each grouping, the sliced example to the left is raw, the sliced sample on the right is cooked. You can see that the Stokes Purple has deep red skin and flesh when raw, and the purple flesh gets darker and a little more blue with cooking but retains its vibrant red tone. The skin also did not really survive the boiling time.


The Beni Imo (Okinawa purple sweet potato) has pale tan, almost white skin when raw with lavender flesh inside. It is fascinating that there is no hint at all that the inside is purple. These are REALLY deceptive when you are looking at them in the store. In the picture at the very top of this post, the 4 tubers on the right are all Beni Imo. Once cooked, the skin becomes much darker, sort of a dusty grey / brown, and as you can see the inside becomes so dark purple it is almost black. The skin is a little heartier, so stays intact during the boiling. That keeps some of the sugar and pigment locked inside (though the water does turn purple).


I discussed the Beni Imo flavor and texture above (starchy and a little dry while still being weirdly silky, sweet, with floral fruity notes similar to lychee). Compared to that the Stokes was a little less exciting. The texture is just a little less silky, the flavor is just sort of "plain sweet potato" flavor. It is yummy, and the deep red-purple is gorgeous, but the flavor is not as complex as the Beni Imo. Based on the nearly black-purple color, I am going to assume that the Okinawa potato has a higher anthocyanin content and would therefore win our contest for the healthiest of all potatoes.


So, for me the Beni Imo wins again. I have to admit looking at the two together in the same bowl is REALLY pretty.


OK, that is all I have to say about sweet potatoes. I learned a lot and I hope you did too!

Boil and cool your potatoes, and see if you can find purple sweets!

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