Saturday, August 10, 2013

Growing My Own Sweet Potato Leaves

Sweet potato leaves is an underrated vegetable, and indeed there are sound reasons for that. Usually used as animal feed or composted at the end of the season when the much prized tubers are harvested, the leaves do have some nutritional value to humans. In Asian countries, despite being eaten (whether blanched and added to salads, in stews or stir-fried) sweet potato leaves still isn’t a very popular vegetable due to its associated with animal feed or famine food; therefore it is usually not served when other ‘better’ vegetables are available.
Sweet potato leaves - animal fodder or nutritious vegetable?

Those that do eat them can sometimes find that the leaves are chewy or stringy, and rather difficult to macerate down even with repeated mastication. Indeed, I have on occasions felt like a goat chewing on stir-fried sweet potato leaves. Also, many whom are newly introduced to this greens end up not liking the taste or the texture of sweet potato greens as it can be quite mucilaginous, and is probably a bit ‘greenish’ in taste.

Overall, the palatability of the sweet potato leaves voice down to the selection of the cultivar, the age of the leaves and finally the method of cooking them. Thus picking the right type, at the right time and with the right cooking method can make goat chow into a delicious vegetable.

Technically all sweet potato leaves are edible. Edible, but not be palatable. For example, the decorative sweet potato cultivars have been reported to be ‘chemical’ tasting by some. Some cultivars are very stringy and fibrous, even the young leaves require some effort to masticate. I prefer the Japanese-type sweet potato leaves, i.e. the leaves of the Satsumaimo type (red/purple skin with white/pale yellow flesh).
All purple sweet potato - rich in anthocyanins. They are marketed as Royal Diamond, but as far as I am aware of, it is a market branding name, like Grace Cup, and not a cultivar name.
Orange-fleshed sweet potato with rose/orange skin - full of carotenoids. These are from the supermarket and are labeled as imported from Australia. They are actually baby tubers (not quite mature ones) and the runt of the lot. I guess the good ones they keep for themselves or for some other food industry that require uniform tubers for processing.

The leaves of these types do not feel like chewing on a luffa sponge, and indeed the tubers themselves have minimal strings. Even large leaves on the lower section of the vines are still palatable. On the other hand, the orange-fleshed variety like Beauregard tends to be more fibrous, thus picking only the tender younger leaves might be advisable. 

The main reason for eating sweet potato leaves lies in its nutritional value, especially its lutein content, and this can vary from one cultivar of sweet potato to another. Lutein is a carotenoid that can help delay blindness-related macular degeneration. Leaves of sweet potato are a rich source of lutein. Khachatryan et al. (2003) identified sweet potato leaves as second in lutein content after marigold flowers, and is the number one among edible vegetables.

Kale has been known as a dark leafy vegetable with the highest amount of lutein at 0.38mg/g, but Menelaou et al. (2006) also showed that ‘Bienvillle’ and ‘Beauregard’ variety sweet potato leaves had 0.54mg/g and 0.51mg/g lutein respectively, which are very much higher than kale. In fact the lowest lutein containing cultivar that they tested had equivalent amount of lutein as kale. In this case, eating Beauregard or Bienville leaves can provide you with significant amounts of supplementary lutein.
The raw tubers cut open, revealing the vividly coloured flesh.

One note of caution, leaves of sweet potato does contain oxalates/oxalic acid. Just like amaranth leaves, the iron is less available for sorption due to the formation of insoluble oxalates. Oxalate-containing foods are also a no-no for people with renal issues.

According to Tsai et al. (2005), sweet potato leaves contain 48.6mg/100g oxalate. Amaranth (bayam -筧菜) has significantly higher oxalate content (280.62mg/100g), followed by Basella alba ‘Rubra’ or Malabar spinach (141.21mg/100g) and Amaranth tricolour - 紅筧菜 (131.4mg/100g). Even fresh starfruit has slightly lower oxalate content (263.34mg/100g) when compared to Amaranth. To reduce the amount of oxalates, the leaves can be blanched first for at least one minute, and the blanching water thrown away. 

So how do you know which cultivar of sweet potato you are eating? The Japanese type leaves are usually sold as ‘Tender / Stringless” sweet potato shoots. You can also buy Japanese sweet potato (red skin-white/pale yellow flesh) and soak it to get slips for planting.

To get Beauregard sweet potato leaves, you can easily grow a Beauregard sweet potato yourself. Okay, how do I know if it is a Beauregard sweet potato? In Malaysia it is really easy - look for orange/pale rose-skinned with golden/orange fleshed sweet potato imported from Australia. This is because the bulk of Australian sweet potato comes from the Beauregard variety!
The all-purple ones turn a deep purple when cooked, and is a little on the dry side. The orange one (imported from Australia - Beauregard?) is moist and not flaky.

The older Beerwah Gold variety, which also have similar flesh colour, has been superseded by Beauregard, Another orange type possible is Hernandez, which has distinctly darker orange flesh and deeper orange/red skin. Nevetheless, the Beauregard variety is the one that is widely marketed as it is easily peeled with smooth skin and uniformly shaped tubers.

The Beauregard variety is also a fast tuber producing variety (90-100 days), thus you won’t have to wait long before you get the tubers ready for eating. A bit of sweet potato trivia – O’Henry, a white fleshed variety, is actually a mutant of the golden Beauregard.

I have an all-purple sweet potato here that I want to test the texture of its leaves. It is commonly sold under the market brand Royal Diamond, but is probably just the All Purple Sweet Potato, which is a Japanese type sweet potato; less moist flesh when cooked and has a more powdery texture, but still reasonably sweet.

The Japanese-type sweet potatoes are less stringy, with powdery flesh and taste like chestnuts with varying degrees of sweetness. This all-purple variety is different from the local ‘purple’ variety that has creamy white skin and white-streaked purple flesh, which is similar to the Okinawan Purple.
This little all-purple sweet potato had sprouted in April, and remained so with little stubs as shoots until the end of July when I decided to soak it in water.
One week after soaking the sprouted tuber - the roots take off like crazy, followed by the leaves. I love the red-veined leaves that pop up.
One and a half weeks later, the slips were pulled off. 

I left one small tuber lying around and it sprouted. After some time remaining the way it was (like months) the thought of let’s try and see if the leaves are okay came across my mind, did I finally decided to soak the sprouted tuber. Within a one and a half week’s time, the slips were long enough and I planted them almost horizontally, since I want to maximise shoot production.
There were a lot of roots around base of each slip, and the bases of these roots are red in colour. The tuber had turned a lighter plum red colour after sprouting.
The shoots perk up after one hour of planting into soil, despite it being a hot afternoon when this was done. Bekas kecil, so tak ada harapan nak dapat ubi.

So hopefully the leaves of this type would be reasonably palatable, as the thought of possibly getting purple tubers at the end of the growing season is a plus point should the leaves taste good and most importantly, are not stringy and tough. I am not hopeful though, as Diana has mentioned that the Murasaki (purple) type are poorer producers of tubers.

10 comments:

  1. Oh if you nak pucuknya Murasaki untuk makan atau tanam hari tu, boleh hantar hari tu sebab asyik rendam pucuknya dalam air dah akar bersimpul-simpul. The ones I tried to grow beli dekat JUSCO. Asyik berbunga so probably one of the reason not producing much murasaki tubers.
    Bought some local grown keledek Jepun and other variety at Tamu last June terlupa dalam bakul dah tumbuh pucuk, memgecut dan mengering. Will follow your way, dump them in the water.

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    1. For the time being kena hold ubi keledek dulu sebab dah tanam bulb lain, hehe. Btw, the red skin and white flesh that you plant in adelaide is probably north star sebab paling common kat sana.

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    2. Yup North Star sounds familiar.
      I tengah grow jenis 'Hawaii' can't remember correctly tapi tuber bawa dari Australia. Kulit white, flesh white and tengahnya ada pinkish sikit.
      My baby tak berapa minat yang murasaki probably too starchy for her, she prefers the orange one.

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    3. Hawaii is one of the cultivars mentioned in Northern Territory guidebook, listed as a cream skin, cream flesh variety. Kalau variety yang dah tau nama kena jaga baik-baik, next time boleh offer as named cultivar. Diana masih ada North Star lagi? Pandai baby Diana pilih. Yang murasaki tu kalau makan cepat sgt, the starch powder will make me cough sebab too dry and irritating. The orange one is probably Beauregard (especially dari down under).

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  2. Dulu pernah kirim shoot North Star when dekat OZ not sure where it goes dekat belakang tu atau dah pupus. Tikus suka gali makan tuber keledek kat tempat kami ni.

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    1. Mesti gembira tikus-tikus berparti kat sana, hehehe. Btw, I silap, it should be called Northern Star.

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    2. Oh tikus dekat belakang sebesar lengan. I think North Star tu macam nama brand kasut or something itu yang terlekat dalam kepala tu agaknya hehehe...

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    3. Hahaha, must be teringat kasut. Kasut = North Star, Keledek = Northern Star, lol. Wah sebesar lengan!!! Tikus biasa ke genetic experiment gone wrong?? Lol

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  3. I think tikus tu gemok perabihkan food dekat belakang hahaha....oh ya habranthus tu ada seed pod growing. I thought one bulb will only give me one bloom but dah 3 blooms dia keluar.

    Happy Merdeka day!

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    1. Selamat Hari Merdeka. Kalau cuba tanam Lycoris keliling petak ubi keledek, macam orang Jepun tanam keliling sawah padi sebab nak 'halang' tikus? Ke mereka langkau je Lycoris tu? That is cool, comel bunga Habranthus - bolehler tanam bijinya nanti. I ingat dulu bila I debungakan Hippeastrum dan bila podnya matang, buang je biji-bijinya dalam satu pasu, ingat tak akan tumbuh. Penuh pasu tu dengan anak-anak Hippeastrum. Tapi lama nak tunggu sampai besar. Bila pindah rumah, terus sedekahkan kat jiran sebelah, mak dengan anak-anaknya.

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