Potatoes

INTRODUCTION

Russet (Idaho) potatoes are “high-starch” potatoes. Red potatoes are waxy or “low-starch” potatoes. The starch granules of a Russet potato are also larger than other types of potatoes. High-starch potatoes contain starch granules that are densely packed with amylose and amylopectin (most potatoes have between 20-30% amylose. Russet potatoes have more amylose that waxy potatoes). High-starch potatoes  make fluffy baked potatoes, creamy mashed potatoes and french fries with a flaky interior, why is this?

When a Russet potato is cooked, the starch granules absorb water (this could come from inside the potato if you are making a baked potato), the heat and water cause gelation of amylose and amylopectin molecules making the granules swell and separate from one another. This results in a dry, mealy  texture.
When we cook whole potatoes, the starch granules are inside of cells. The image below is a potato cell (the square-ish looking box) with starch granules inside (the brownish oval particles) as visualized using a very powerful microscope.
image from: visualphotos.com

These images below are of potato cells before and after cooking. Notice the small starch granules in the uncooked image and the swollen starch granules of the cooked image.

Potato, uncooked. Compact starch granules. Potato, cooked, swollen starch granules full of gelled starch. 
http://www.ifrn.bbsrc.ac.uk/fb/tex/2_5.html http://www.ifrn.bbsrc.ac.uk/fb/tex/2_6.html
The potato is a plant after all…and it is made of cells. Therefore, “gelling” of the starch is occurring within this matrix of cells – this is why a whole potato doesn’t turn to goey gel when cooked (remember what pure cornstarch turned into when heated? pure cornstarch or potato starch is just the starch granules…no cells). However, if you were to smash and break open the potato cells (using a mixer or food processor) then you will also break open the swollen starch granules, releasing the gelantinous starch within.
Ever make gluey mashed potatoes? They are the result of smashed open potato cells and broken starch granules – all the gelled starch leaked out…
In this lab, we will steam the potato slices to gel the starch while it is still inside the potato cells, then we will bake the potato slices to brown the surface to a crunch – then we’ll have oven fries with a soft center and crispy exterior.

INGREDIENTS

  • 1-3 large (0.75-1 pound) Russet Burbank (Idaho) baking potatoes. Weigh potatoes to be sure you have between 0.75-1 lb)
  • Nonstick cooking spray
  • 1/2 tablespoon olive oil
  • 1 teaspoons salt
  • 1/8 teaspoon cayenne (add the cayenne according to your preference for spicy foods)
  • 1 teaspoons finely minced fresh rosemary
  • 1/8 cup grated Parmesan

MATERIALS

  • Sharp knife
  • baking sheet
  • 2-3 qt saucepan with lid
  • wire mesh strainer or steaming basket that fits inside the saucepan but rests above the bottom
  • clean kitchen towels

INSTRUCTIONS

When you see text in boldface blue font. You should document that step with a photo on your blog.

1.        Place the potatoes, unpeeled, in the refrigerator for a day or two. When ready to cook, scrub but do not peel, and cut into fat “French-fry” sticks. Make sure your potato sticks are uniform in thickness.  Rinse the cut “sticks” well under running water.

Letting the potatoes sit for a few days in the refrigerator converts some of the starch to free sugar molecules (glucose monomers) and makes the potatoes brown better in the oven. Although we won’t discuss sugar “browning” chemistry until unit 3, free sugars (like glucose monomers) are able to undergo chemical “browning” reactions while starch molecules (polymers of glucose) cannot undergo this chemistry.

The conversion of starch to sugar in a potato is a process catalyzed by enzymes and (paradoxically) actually occurs FASTER at colder temperatures – this is thought to be a strategy the potato uses to protect itself from frost damage.

Rinsing the potato under running water removes excess starch (that white filmy liquid that sticks to your knife and your hands). We want crispy fries, so we don’t want excess starch gelling on the outside of our potato sticks.

2.       Preheat the oven to 450˚F.  Spray a baking sheet with nonstick cooking spray.
3.      Heat 2 cups of water to boiling in your saucepan. Place the potato sticks in a wire mesh strainer and sit the strainer over the pot of boiling water. Cover with aluminum foil and/or the pot lid. Steam the potato sticks for 8-10 minutes. Remove the wire mesh strainer from over the boiling water. Gently remove potato sticks and pat dry with a clean kitchen towel. Can you see the change in the potato flesh as the starch gels? Take a closeup photo of the steamed potato stick. 
4. Stir together the olive oil, salt, cayenne, and rosemary in a medium bowl.  Add the steamed potato sticks and toss well to coat.  Arrange in a single layer on the baking sheet.  Bake until lightly browned, about 40-60 min.
5.       Sprinkle the Parmesan over the potatoes and return to the oven just long enough to melt the cheese, about 4 minutes.  Taste and add salt as needed.  Serve immediately.

On your Blog

Take photos

  • Of your finished oven fries that have been broken in half to show the interior.

Post a video in which…

  • You taste test the oven fries. Is the interior soft and the exterior crunchy?
  • Describe what went really well during your recipe.
  • What would you change if you repeated this process again.

Answer the following questions:

  1. At what point during the recipe did the starch in the potato gel? What conditions were required?
  2. How might you expect this recipe to change if you used a “low-starch” (a.k.a. waxier) potato?
  3. How could you adapt what you learned here to making mashed potatoes?