Creamy Mashed Potatoes with Shio Koji マッシュドポテト塩麹入り

Buttery, creamy, and umami-packed, these Mashed Potatoes with Shio Koji add a Japanese flair to a classic recipe. They’ll blow you away! Follow my tips for making the fluffiest, silkiest, and creamiest mashed potatoes.

There’s nothing quite like a warm bowl of creamy, comforting mashed potatoes. In fact, it ranks at the top of my family’s Thanksgiving and Christmas menu. Today, I’m sharing a truly special recipe: Japanese-inspired Creamy Mashed Potatoes with Shio Koji (マッシュドポテト塩麹入り).

If you’re looking to elevate the classic side dish, this recipe will become your instant favorite. At first glance, it may seem familiar, but after the first bite, you’ll be blown away by the deep, savory flavor. That’s the magic of Shio Koji!

What’s Shio Koji and What Does It Do?

Made with salt, water, and rice koji, Shio Koji is a traditional natural seasoning used in Japanese cooking. We use it to marinate, tenderize, and enhance the umami, or richness in foods.

Shio Koji is a live food rich in enzymes, making it ideal for dishes like mashed potatoes. Similar to miso, it works wonders in bringing out the umami and natural sweetness of your ingredients. With Shio Koji, you can keep the ingredients simple, as this all-purpose seasoning alone adds incredible depth to any dish.

For these mashed potatoes, I use Shio Koji All-Purpose Seasoning from Hikari Miso. It’s a simple seasoning, but absolutely game-changing!

Where to Find Shio Koji?

You can find Hikari Miso Shio Koji in Japanese and Asian grocery stores such as Nijiya and H-Mart. You can also buy it from Amazon.

Shio Koji Substitute

If you prefer to stick with classic mashed potatoes without shio koji (still delicious — follow my tips!), simply add salt in the following two steps:

1. Add salt to your water when boiling the potatoes, as the starch will absorb it, enhancing the flavor for mashing.
2. Season with salt after tasting (it’s important to taste before seasoning!).

7 Tips to Make the Fluffiest Mashed Potatoes

1. Use Both Russet Potatoes and Yukon Gold Potatoes

There are plenty of articles online debating which potatoes are best for mashed potatoes. Russets, with their higher starch content, are a classic choice. They are soft and can be mashed into a light, fluffy texture. Since they break down easily, they release less starch during the process.

Yukon Golds, on the other hand, have beautiful yellow flesh, a creamy texture, and a buttery flavor. They fall into the medium-starch category, which means they require more effort to break down and release more starch.

After experimenting with various potato varieties, we firmly believe that a combination of Russets and Yukon Golds delivers the best results for mashed potatoes. Period. This combination provides optimal textural satisfaction. Give it a try!

2. Cut Potatoes into 2-Inch Uniform Shape

My mom believes potatoes should be boiled with the skin on, then peeled while still hot, using your hands. I can never manage it properly (my fingers are too sensitive to the heat!), so I opt to peel them beforehand. Personally, I don’t think it makes a huge difference (but don’t tell her).

However, it’s important to cut the potatoes into uniform sizes so they cook evenly. I typically cut each Russet potato into 6 equal pieces and each Yukon Gold into 4.

3. Cook Potatoes from Water

Potatoes take time to cook until tender inside. It’s important to start cooking them in room-temperature water and gradually bring it to a boil. It typically takes about 10-15 minutes for the water to reach a boil, and another 10-15 minutes to cook the potatoes through. Adding salt to the water allows the starch in the potatoes to absorb it, resulting in more flavorful potatoes (similar to boiling pasta).

4. Double Rinse, Please

I used to make Japanese croquettes with my mom when I was growing up, so I knew we had to soak and rinse the potatoes with cold water before cooking to remove excess starch.

What I didn’t know until recently is that you can also rinse the potatoes AFTER cooking. As J. Kenji López-Alt explains in his book The Food Lab (which Mr. JOC and I love for its cooking science references), after draining the water, quickly rinsing boiled potatoes with hot water removes even more starch. To my surprise, I found that this trick really improved the texture of my mashed potatoes.

5. Dry Up Potatoes

Russets are higher in starch than Yukon Golds, which means they absorb water more easily. To avoid excess starch and moisture, you can do the following steps:

1. Rinse the potatoes before and after boiling.
2. Dry the potatoes properly to let moisture evaporate before mashing.

I’ll show you how to do it in the recipe card—it’s super easy! No one wants soggy mashed potatoes!

6. Use Good Old Potato Masher or Ricer

A ricer is probably the best tool for making mashed potatoes, but not everyone has one. The next best option is a good old potato masher (I use this brand). Don’t use a blender, food processor, or electric mixer.

7. Warm the Butter and Cream

Heat the butter, cream, and Shio Koji in a saucepan before adding them to the mashed potatoes. This helps the potatoes absorb the ingredients more easily, especially while they’re warm. Plus, you don’t want to lower the temperature of the mashed potatoes by adding cold ingredients!

How to Store Mashed Potatoes

Mashed potatoes are freezer-friendly, so you can definitely make them ahead of time or save your leftovers for later. Be sure to coat the potatoes with butter and cream; the dairy and fat help keep them fresh and ensure they heat up nicely and remain creamy.

To store, you can either:

1. Portion out the mashed potatoes using a cookie scoop onto a parchment-lined baking sheet. Freeze for at least a few hours or until solid. Then, transfer the individual servings to a large freezer bag or container and store them in the freezer.
2. Transfer the cooled mashed potatoes directly to a container or large freezer bag and store them in the freezer.

You can choose to thaw the potatoes in the refrigerator a day before reheating to shorten the heating time, or you can reheat them directly from the freezer.

How to Reheat Mashed Potatoes

To reheat, there are a few choices, but below are my preferred methods:

• Microwave: Place a portion in a microwave-safe bowl, cover it, and microwave at 50 percent power for about 5 minutes, stirring occasionally, until heated through. The time may vary based on the amount of potatoes and your microwave wattage. Once heated through, stir well and add butter or seasoning as needed.

• Oven: Place the potatoes, thawed or straight from the freezer, in a covered casserole dish and bake at 350°F (180°C) for about 30 minutes, or until heated all the way through. Fluff and taste the potatoes. Add more butter, cream, and seasoning as needed.

What to Do with Leftover Mashed Potatoes?

If you’re tired of eating mashed potatoes, use them for these recipes!

• Mashed Potato Teriyaki Pork Rolls
• Korokke (Japanese Croquettes)

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Creamy Mashed Potatoes with Shio Koji

4.50 from 22 votes

Buttery, creamy, and umami-packed, these Mashed Potatoes with Shio Koji add a Japanese flair to the classic recipe. They'll blow you away! Follow my tips on how to make the fluffiest, silkiest, and creamiest mashed potatoes.

Print Pin

Prep Time: 20 minutes mins

Cook Time: 30 minutes mins

Total Time: 50 minutes mins

Servings: 4

Ingredients

 

US CustomaryMetric

 

1x2x3x

• ▢ 1½ lb russet and Yukon Gold potatoes (2 large potatoes should be about 1½ lb)
• ▢ 1 tsp Diamond Crystal kosher salt (for boiling the potatoes; use it if you do not use Shio Koji)

For the Seasonings

• ▢ 2 Tbsp unsalted butter
• ▢ ¼ cup heavy (whipping) cream
• ▢ 2 Tbsp shio koji (you can replace it with ½ tsp kosher/sea salt)
• ▢ freshly ground black pepper

For the Garnish

• ▢ 1 Tbsp unsalted butter
• ▢ a few stalks chives
• ▢ freshly ground black pepper

Japanese Ingredient Substitution: If you want substitutes for Japanese condiments and ingredients, click here.

Cook ModePrevent your screen from going dark

Instructions

 

• Gather all the ingredients.
  
• Peel 1½ lb russet and Yukon Gold potatoes and remove the eyes.
  
• Cut each potato in half lengthwise and then cut into rough chunks, about 2-inch cubes.
  
• Transfer to a pot of cold water to soak for 2–3 minutes.
  
• Drain water and rinse the potatoes under cold running water. Use the lid of the pot to hold back the potatoes when draining.
  
• Add cold water about 1 inch above the potatoes. If you are not using shio koji, sprinkle 1 tsp Diamond Crystal kosher salt into the water to season the potatoes. Put the lid on, leaving a small gap, and bring to a boil over medium-high heat (it’ll take about 10–15 minutes to boil).
  
• Once boiling, reduce the heat to medium (or even lower heat to maintain the boil). Cook the potatoes until tender, about 15 minutes (I usually cover the lid, slightly ajar, so water won‘t evaporate too fast.
  
• Meanwhile, in a saucepan, heat 2 Tbsp unsalted butter, ¼ cup heavy (whipping) cream, and 2 Tbsp shio koji over medium-low heat. Turn off the heat and set aside to keep warm (you may need to reheat before adding to the potatoes).
  
• When a skewer goes through the potato easily, turn off the heat. Drain the water completely.
  
• Rinse under hot running water and drain. Repeat 1–2 times to wash away the excess starch. Drain really well.
  
• Put the pot back on the stove and let them dry for a few minutes on low heat. With a stainless steel potato masher, mash the potatoes until you have a creamy and even mash.
  
• Pour the warm butter mixture over the mash and fold gently with a silicone spatula to combine.
  
• Season to taste with freshly ground black pepper and salt (if you are not using shio koji). Transfer to a serving bowl.
  
• Using a spoon to create grooves and top with 1 Tbsp unsalted butter, a few stalks chives (chopped), and more freshly ground black pepper. Enjoy!
  

To Store

• Mashed potatoes keep for 3 days in the refrigerator. You can also freeze them for a month in an airtight container or freezer bag.

Notes

Recipe by Namiko Chen of Just One Cookbook. All images and content on this site are copyright protected. Please do not use my images without my permission. If you’d like to share this recipe on your site, please re-write the recipe in your own words and link to this post as the original source. Thank you.

Nutrition

Calories: 274 kcal · Carbohydrates: 32 g · Protein: 4 g · Fat: 14 g · Saturated Fat: 9 g · Trans Fat: 1 g · Cholesterol: 43 mg · Sodium: 550 mg · Potassium: 725 mg · Fiber: 2 g · Sugar: 3 g · Vitamin A: 515 IU · Vitamin C: 10 mg · Calcium: 35 mg · Iron: 1 mg

Author: Namiko Hirasawa Chen

Course: Side Dish

Cuisine: American

Keyword: potato

©JustOneCookbook.com Content and photographs are copyright protected. Sharing of this recipe is both encouraged and appreciated. Copying and/or pasting full recipes to any website or social media is strictly prohibited. Please view my photo use policy here.

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