Authentic Yaki Imo (Baked Japanese Sweet Potatoes)

Yaki Imo is a Japanese snack made from baked Japanese sweet potatoes, typically enjoyed from autumn through winter. When you split open a freshly baked yaki imo, steam rises, and its rich, natural sweetness fills your mouth with each bite.

Yaki Imo is a sweet snack made by baking Japanese sweet potatoes. "Yaki" means baked, and "Imo" refers to potatoes. The potatoes used in this snack are Japanese sweet potatoes, also known as satsumaimo. When baked slowly, their flavor concentrates inside the skin, resulting in a rich sweetness.

In Japan, it has been enjoyed for centuries and is considered a staple snack, commonly sold in grocery stores and from food trucks during the fall and winter. Especially on chilly days, many people look forward to its sweet aroma and comforting warmth.

Japanese sweet potatoes vary significantly in taste and texture depending on the variety, so if you have the option, I highly recommend trying different types to experience their distinct flavors.

In Japan, there are around 60 different varieties of sweet potatoes currently being cultivated. Below, I have listed some of the most popular ones.

• Beni-Azuma (紅あずま): Known for its mild sweetness, this is one of the traditional sweet potato varieties most widely consumed in eastern Japan.
• Naruto-Kintoki (鳴門金時): This variety also has a mild sweetness and is most commonly found in western Japan as a traditional sweet potato.
• Anno-Imo (安納芋): With its slightly orange flesh and intensely sweet, creamy texture, it is considered one of the representative sweet potatoes with high sugar content.
• Beni-Haruka (紅はるか): Characterized by its intensely sweet and creamy texture, it has been a leading variety since its introduction in 2010 and has contributed to the yaki imo craze in Japan.
• Silk Sweet (シルクスイート): This variety is also renowned for its sweet, creamy texture; however, it tends to be slightly less sweet and creamy compared to Anno-Imo and Beni-Haruka.

While you can make yaki imo with any type of Japanese sweet potato, I highly recommend trying Anno-Imo or Beni-Haruka if possible. When properly cooked, yaki imo made from these varieties is said to reach a sugar content of up to 50-60° Brix (more than twice as sweet as most fresh fruits).

When purchasing Japanese sweet potatoes, choose the best ones available. You can assess their quality based on the following visual cues:

• Look for dark, soil-like streaks or spots on the surface.

These markings are caused by a white substance called “yarapin,” which seeps out of the sweet potato and then oxidizes.

Yarapin tends to appear on sweet potatoes with a high sugar content, indicating a greater potential for sweetness. The markings should have a glossy appearance, feel firm, and be slightly sticky to the touch. Be careful not to confuse these marks with mold or black spot disease.

• Avoid ones that are overly large.

Sweet potatoes that are too large are usually less sweet and harder to cook through. A weight of 7-9 ounces (200-250 g) is ideal for yaki imo. Smaller ones can also taste good. To ensure even cooking, it is best to choose sweet potatoes with a uniform thickness.

Even if two sweet potatoes look the same size, their weights might differ. In that case, choose the heavier one, as lighter sweet potatoes are likely to have less dense flesh.

In Japan, sweet potatoes are typically harvested from September to November. However, they become sweeter and more flavorful after aging for about 2-3 months post-harvest. Therefore, those available from December to February are generally the best for eating.

To create delicious yaki imo, there are four key factors to keep in mind: the gelatinization and saccharification of starch, the softening of pectin, and the concentration of flavor achieved through high-temperature cooking.

Japanese sweet potatoes become sweet when baked because their components undergo transformations. The starch in sweet potatoes softens when heated (gelatinization), and β-amylase, an enzyme naturally present in sweet potatoes, converts the softened starch into maltose (saccharification).

Depending on the variety of Japanese sweet potato, starch generally begins to soften at 158-176℉ (70-80℃), and β-amylase typically becomes active at 149-167℉ (65-75℃). Based on these facts, sweet potatoes become sweeter when heated slowly over a long period within a temperature range of 158-167℉ (70-75℃).

However, there is a drawback to heating them at 158-167℉ (70-75℃): yaki imo can end up with a dry texture. This is because the pectin in sweet potatoes doesn't soften at these temperatures. Pectin acts as a binder that holds cells together and can either soften or harden when heated. Since pectin hardens between 140-176℉ (60-80℃), sweet potatoes must reach at least 176℉ (80℃) to achieve a moist and creamy texture.

In other words, simply heating sweet potatoes won’t simultaneously achieve both sweetness through the gelatinization and saccharification of starch and a creamy texture through the softening of pectin.

Therefore, I recommend initially baking the sweet potatoes at 482℉ (250℃) for 30 minutes and then letting them rest in the residual heat for 90 minutes. Baking them at 482℉ (250℃) ensures that the internal temperature rises well above 176℉ (80℃), which softens the pectin during the baking process, resulting in a moist, creamy texture. As the sweet potatoes cool down during the residual-heat phase, their natural sweetness develops.

Finally, bake them once more at 482℉ (250℃) for another 30 minutes. This process allows excess moisture to evaporate from the gelatinized sweet potatoes, concentrating their flavor and enhancing their sweetness.

The method described above is for creating the ideal yaki imo, but it takes time. If you prioritize saving time and don't mind sacrificing some flavor and texture, you can make yaki imo by simply baking sweet potatoes at 392℉ (200℃) for about 40 minutes. Depending on your priorities, you can adjust the cooking method accordingly.

You can also enhance the flavor and texture of yaki imo with a few extra touches beyond just the baking method.

The first step is to soak the sweet potatoes in salted water beforehand. This brings out their natural sweetness during baking by creating a contrast in flavors. There is no need to dry them before baking.

The second step is to wrap the sweet potatoes in aluminum foil while baking. Instead of wrapping them tightly, loosely wrap each one, leaving some space between the sweet potato and the foil. This helps them bake more evenly.

With just a few simple tweaks, you can fully enjoy the rich flavor and tender texture of yaki imo—so be sure to give it a try.

Let’s take a look at the equipment you will need to make yaki imo.

Yaki imo can be made in various ways, such as in an oven, toaster oven, or frying pan. However, I highly recommend using an oven or toaster oven, as they allow for better temperature control and can handle the two different temperature ranges mentioned earlier.

When using a toaster oven, keep in mind that it doesn't retain heat as well as a conventional oven. After the initial baking, you will need to maintain a temperature of 212℉ (100℃) for 90 minutes instead. The reason for using a slightly higher temperature than the ideal range for maximizing sweetness (158-167℉ / 70-75℃) is to ensure that the sweet potato reaches the proper internal temperature.

Using a frying pan can be tricky because it is difficult to control the heat precisely. That said, while the sweetness and texture may not be as pronounced, you can still make yaki imo by cooking the sweet potato over low heat for 30-40 minutes, flipping it, and then cooking for another 30-40 minutes.

You can also use a microwave, but it is not recommended. Microwaving doesn't bring out the sweetness of sweet potatoes as effectively. Additionally, if your sweet potatoes are low in moisture, there is a risk of them catching fire due to the rapid heating.

Equipment

Ingredients

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Instructions

🕒 Total: 3 hrs hrs 13 mins mins

Step 1
Wash the sweet potatoes thoroughly to remove any dirt. Mix water and salt in a container or plastic bag, then add the sweet potatoes and let them soak for about 30 minutes.

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Step 2
Remove the sweet potatoes from the container (or plastic bag) and loosely wrap each one in aluminum foil.

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Step 3 (if using an oven)
Preheat the oven to 482°F (250°C), place the sweet potatoes inside, and bake for 30 minutes. After that, turn off the oven but leave them inside to cook with the residual heat for another 90 minutes.

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Step 3 (if using a toaster oven)
Bake the sweet potatoes at 482℉ (250℃) for 30 minutes. Then, reduce the temperature to 212℉ (100℃) and continue baking for another 90 minutes.

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Step 4
Finally, bake them again at 482℉ (250℃) for an additional 30 minutes, regardless of the method used.

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To store

You can store it in the refrigerator for up to 4 days, wrapping each piece in plastic wrap after patting it dry.

When reheating, remove the plastic wrap and use one of the following methods:

• Microwave: Heat each piece on medium power (500-600W) for 2-3 minutes.
• Oven: Wrap each piece in aluminum foil and bake at 356℉ (180℃) for 10-15 minutes.
• Toaster oven: Wrap each piece in aluminum foil and bake at 356℉ (180℃) for 5-10 minutes.

If you have tried making yaki imo with this recipe and found that it wasn’t sweet enough, it may be because the Japanese sweet potatoes you used weren’t fully ripe. If you want to bring out even more sweetness, try ripening the sweet potatoes further.

First, wrap each sweet potato in newspaper to retain moisture.

When ripening a large batch, be sure to place them in a cardboard box with enough space between each one. Avoid using plastic bags instead of newspaper, as sweet potatoes need proper ventilation.

For this process, use unwashed sweet potatoes that are free of blemishes. Bruised sweet potatoes tend to deteriorate more quickly during ripening, and washed ones can’t be stored for long. Additionally, if your sweet potatoes have soil on them, let them dry in the shade for two days before storing them, to prevent direct sunlight exposure and to remove excess moisture.

Next, store the sweet potatoes in a cool, dark place.

The ideal temperature for ripening sweet potatoes is between 55-60℉ (13-16℃) with humidity levels of 85 to 90%. Sweet potatoes are prone to damage at temperatures below 50℉ (10℃) or above 68℉ (20℃), so avoid storing them in the refrigerator or in direct sunlight. Choose a storage location that can maintain a temperature of 55-60℉ (13-16℃) as consistently as possible.

The ripening duration varies: sweet potatoes purchased from grocery stores after December typically take 1 week to 2 months, while those purchased from September through November might take 1-3 months. Signs of ripening include the skin turning a deeper purple and dark syrup appearing at the ends of the sweet potatoes.

It can take time and be a bit of a hassle, so if you just want to enjoy your yaki imo casually, there is no need to go this far. However, if you are eager to enhance its sweetness, it is definitely worth trying.