Not All Potatoes are Equal

Potatoes are made up of cells that contain starch and water. However, not all potatoes are created equal, and this is why the choice of potato for a specific recipe is critical.

Russet potatoes can contain as little as 22 percent starch and as much as 25 percent starch. As the potatoes cook, the starch granules absorb water and swell, which creates a lot of pressure and causes them to burst. This breaks down the structure of the potato because so many cells have exploded. You end up with a mealy texture, which is ideal for dishes like mashed potatoes.

Waxy potatoes can contain as little as 15 percent starch and as much as 17 percent starch. In a waxy potato—because there is less starch—there is less pressure on the granules, so fewer cells burst.

But not all potato starch is created equal either. There are two types of starch molecules in potatoes: amylopectin and amylose. Waxy potatoes contain only one type, amylopectin, which is stickier than amylose. When cells in waxy potatoes burst during cooking, the amylopectin is released and acts like a glue, holding together the remaining intact cells.

So waxy potatoes have two things going for them that make them the choice for recipes that require longer cooking times, like our Braised Red Potatoes with Lemon and Chives: Most of the cells stay intact, and the amylopectin is able to repair the limited damage, keeping the cells together. As a result, you get a smoother and creamier—not overcooked or blown-out—potato.

Glycemic index and the factors that can affect it.¹ Whenever I hear people discussing Glycemic index numbers are not absolute.  Below is a good example of how easy a 'bad' food can become 'good.'

Different varieties of potatoes and different cooking methods could affect the Glycemic index.³ In the first part of the study, the meals compared russet potatoes that were baked, refrigerated at least 24 hours, and then reheated in a microwave with russet potatoes that were baked that day. In the second part they compared russet potatoes that were precooked in a microwave oven, refrigerated at least one day, and then reheated in a microwave oven with russet potatoes that were microwaved that day. They also compared white potatoes that had been boiled, refrigerated, and reheated in a microwave oven with white potatoes that were boiled that day. The results showed there was little difference between baking and microwaving a russet potato just prior to consumption. The precooked russet potatoes by microwave had an 18% lower Glycemic response than the same-day microwaved russet potatoes, although the authors said this was not statistically significant due to the small sample size of subjects (10). The russet potatoes that had been oven baked, refrigerated, and reheated in a microwave had a 30% lower response than those that were oven baked and consumed fresh. Finally, the Glycemic index of white potatoes that were boiled, refrigerated, and reheated did not differ from white potatoes that were freshly boiled.

In a second phase of the study, the authors tested 7 meals that each provided 50 gm of available carbohydrate. The following were compared:

1 Russet potatoes cooked in a microwave oven

2 Instant mashed potatoes

3 White potatoes cubed and oven roasted in a baking dish

4 White potatoes cooked in a microwave oven

5 Red potatoes cubed and boiled

6 Red potatoes cubed, boiled, refrigerated at least 18 hours, and eaten cold

7 Frozen french fries baked in a conventional oven

Table 1 - Results

Test Meal Glycemic Index

1. Microwaved russet potatoes 76 ± 8.7

2. Instant mashed potatoes 87.7 ± 8

3. Oven-roasted white potatoes 73 ± 8.2

4. Microwaved white potatoes 72 ± 4.5

5. Boiled red potatoes 89 ± 7.2

6. Boiled red potatoes, refrigerated, and consumed cold 56 ± 5.2

7. French fries 63 ± 5.5

It is clear that both the method of cooking and variety of potato can affect Glycemic index. What was most interesting was when a red potato was boiled, refrigerated, and consumed cold the next day, the Glycemic index plummeted 37% from the upper end of a high Glycemic index food (89) to one point away from a classification of a low Glycemic index food (56)(see Table 2).

Table 2

Glycemic Index:

Glucose Scale

High 70-100

Medium 55-69

Low 54 or less

When potatoes are cooked, the starch granules absorb water. This is called gelatinization and tends to change the structure of the starch, making it more susceptible to the digestive enzymes. When the cooked potato starch is cooled, the molecules bond in an irregular fashion, making it more difficult to be hydrolyzed by enzymes. The authors mention that repeating the cooking-cooling cycle will continue to result in a more resistant starch. The more resistant a starch is, the longer it will take the body to break it down, digest, and absorb it.  Hence a lower Glycemic index.

Please note this was only one small study. But the next time you read a GI chart the published values are far from absolute. In this case of red potatoes, eating them cold the next day makes a huge difference.

References

¹Andersen, G.D. The Glycemic Index. Dynamic Chiropractic. November 30, 2002; 20(25): 24.

²Andersen, G.D. Clinical Nutrition in Principles and Practices of Chiropractic, 3rd Ed. Haldeman, S., Ed. 2005. McGraw-Hill.

³Frandes, G., Velangi, A., Wolever, T. J. Glycemic Index of Potatoes Commonly Consumed in North America. Amer. Diet. Assoc. 2005; 105(4): 557-562.