Enjoy Sweet Potato Overseas


Sweet Potato But Not Sweet


Finns vs. Sweet potato

Many things have the same name, but are quite different between Japan and Finland. One such example is the sweet potato. Japanese sweet potato is purple skin and yellow inside. We have many spices, and all of them are so sweet. In Finland, sweet potatoes are surprisingly orange inside. They have more water than Japanese one, and, they are not sweet. Most of them are imported from the United States.

Why? The reason is simple: the variety is different from the Japanese sweet potatoes. So, I had thought that Finnish sweet potatoes are not sweet.
But the other day, Juhani ordered sweet potato fries at Burger King. Of course, they were not sweet at all. Even though their name is sweet potato! I didn’t order it, I just shared only one piece with him, but it was still enough to motivate me.
So, I transform the Finnish sweet potatoes into “sweet” potatoes, as the name implies.

How a Baked Sweet Potato Becomes Sweet

Sweet potatoes change their sweetness depending on how they are cooked. Baking them is sweeter than steaming them. In winter in Japan, sweet potatoes baked on a stone are sold in trucks. Even though they are expensive, we tend to buy not only because of the sound of their calling, but also because the taste is much sweeter than making it yourself.

Why is baked sweet potato sweeter than steamed sweet potato? The answer lies in the heating temperature.

Chewing Rice Makes It Sweeter

To begin with, sweet potatoes contain a lot of starch. Starch is called polysaccharide, and as the name suggests, it is made up of many sugars connected together. If you cut these connections and take out the sugars one by one, it becomes sweet.
For example, when you put rice in your mouth and chew it well, it gradually becomes sweet. This is because an enzyme called amylase, which is contained in saliva, cuts the connection of starch in the rice and produces sugar.

For the same reason, if you put a sweet potato in your mouth and chew it carefully, it will eventually become sweet. But that is not what we want. We want the sweet potato to be sweet from the start. In other words, the starch needs to be cut into pieces and the sugar needs to be produced before it enters the mouth.

Sweetening Sweet Potatoes Without Chewing

How can we chop the starch into pieces before we put it in our mouth? Actually, sweet potatoes contain amylase. It is a little different from the amylase in our saliva, but we will skip that for now.
When this amylase works to cut the starch, the sweet potatoes become sweeter without chewing.
サツマイモが甘くなる加熱のコツ : Z-SQUARE | Z会
Amylase is an enzyme. Each enzyme has its own optimum temperature at which it can actively work. According to a study, the optimal temperature for amylase in sweet potatoes is 70-80℃. If the sweet potato is kept at this temperature, the amylase will cut the starch into pieces and make it sweet.

However, this temperature seems to vary depending on the variety, so it may be necessary to consider that as well.

Also Used in Finnish Cooking

Experiment to Sweeten Sweet Potatoes

Experimental Conditions

I bought two sweet potatoes, 7 cm in diameter at the thickest point and about 20 cm long. To make sure that the difference in sweetness is not due to individual differences, I divided each of the two sweet potatoes into four. The vines and the tips of the potatoes were also mixed.

All of them were heated in a microwave oven to make them soft enough to be combed. To compare with the original taste, one set was not heated additionally. The others were wrapped in aluminum foil and cooked in the oven at 70°C for 0.5, 1, and 2 hours respectively.

In hindsight, heating them in the microwave would have caused the enzymes to become inactive due to the high temperature; it would have made more sense to keep them at 70°C and then heat them at a higher temperature to soften them, but I already did it.

Experimental Results

From left to right: microwaved only, heated at 70℃ for 0.5 hour, 1 hour, and 2 hours. There is not much change in appearance.
The microwaved ones were not as sweet as usual, while the ones cooked for 0.5 and 1 hour were a bit sweeter. The one heated for 2 hours was surprisingly sweet. I let Juhani try it without saying anything, because I didn’t want personal subjectivity to come into play, and he was amazed to find that the 2 hour version was the sweetest of all.

Additional Experiment

I cooked it for another 3 hours and 4 hours.
The 3-hour ones were a little sweeter than the 2-hour ones, and the 4-hour ones had sweet
syrup on the cross-section, but there was not much difference in the sweetness of the potatoes.

In short, the sweetness was fully extracted after 2-3 hours of heating.

Comparison Experiment

This time, I heated potato in the microwave before heating it at 70℃. After heating at 70℃ for 2 hours, I cooked it in the microwave or in the oven at 200℃ until it became soft.

In conclusion, it is still important not to heat them first and to use an oven. I was skeptical that it would make that much difference, but it did.

Look at the amount of syrup. I was surprised at how the sweet potatoes changed. The power of enzymes is truly amazing.

Baked Sweet Potato Only in the Oven

As I said, there is a difference in the variety of sweet potatoes, and the baked sweet potatoes did not turn out as sweet and sticky as the Annou sweet potatoes, one of the sweetest spieces in Japan. Still, I think it’s quite convenient to be able to make sweet baked sweet potatoes with almost no effort, just by putting them in the oven.


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