Recipe: Buttermilk Mashed Potatoes

Ideally, buttermilk mashed potatoes should be simultaneously tart and rich, like a baked potato drenched in butter and dolloped with sour cream. But when I simply stirred buttermilk instead of the usual cream into boiled and mashed Yukon gold potatoes (their naturally creamy texture lends itself to a mash), the buttermilk flavor never showed up. The obvious fix — more buttermilk — turned the mash into soup. For other recipes, the test kitchen has had success cooking potatoes in cream rather than boiling them in water. The potatoes drink up the cream, and you can imagine how fabulous that tastes. I tried the same method with buttermilk.

Following our recipe ratios, I peeled and cubed 2 pounds of potatoes and cooked them with butter, salt, and 1 1/2 cups of buttermilk. But when I uncovered the pot 25 minutes later, I discovered things had gone drastically wrong. The buttermilk had separated, the potato cubes hadn't softened, and the mash was so sour, tasters wondered if I'd checked the date on the buttermilk carton. Happily, the separated buttermilk turned out to be a nonissue — once I mashed the potatoes, the problem disappeared. Buttermilk is acidic, and I knew acid slows cooking. I figured that's why the potatoes weren't cooking all the way through.

To balance the buttermilk, I added a pinch of baking soda — an old test kitchen trick; now, the potatoes cooked through expediently. At the same time, I gradually cut the buttermilk back, landing at 1 cup thinned with 6 tablespoons of water. This kept the sourness in check, but now that the buttermilk was simmering for a half-hour, its flavor dulled. The next time I made the mash, I held back 1/4 cup of the buttermilk and folded it in at the end.

(Serves 4)

Don't be alarmed if the buttermilk looks separated. Once you mash in the potatoes, the puree comes together.

2 pounds Yukon gold potatoes, peeled, quartered and cut into 1/2-inch pieces

6 tablespoons unsalted butter, cut into pieces

1 cup buttermilk

6 tablespoons water

1/8 teaspoon baking soda

Salt and pepper

1. Add potatoes, 2 tablespoons butter, 3/4 cup buttermilk, water, baking soda, and 1/2 teaspoon salt to a Dutch oven and stir to combine. Bring to boil, cover, and reduce heat to low. Simmer, stirring occasionally, until potatoes are nearly tender, 20 to 25 minutes. Remove lid and cook over medium heat until liquid has nearly evaporated, about 3 minutes.

2. Off heat, add remaining butter to pot and mash with potato masher until smooth. Using rubber spatula, fold in remaining buttermilk until absorbed and potatoes are creamy. Season with salt and pepper. Serve.

Excerpted from The Cook's Illustrated Magazine. Excerpted by permission of America's Test Kitchen.

http://www.npr.org/blogs/thesalt/2014/10/23/358101692/test-kitchen-how-to-buy-the-safest-meat-and-make-the-juiciest-steaks