To understand why mashed potatoes can look like glue, you need to know a little bit more about the potato. Its flesh is composed of millions of small cells attached to each other with a sort of vegetal cement.
These cells contain water and grains of starch. Keep in mind that starch is found inside the cells. During cooking, the vegetal cement softens and this allows cells to slide over one another (as if a brick wall suddenly turned into jello). The potato becomes tender and is ready to be mashed.
Everything is perfect so far. You bring out the potato masher and get to work. The purée looks great when you first start making it but the more you mash, the more it seems to stick. You add milk and keep mashing but it gets even stickier. What’s going on? The mashing is causing cells to separate from one other and this gives the mix its puréed consistency (the brick wall collapses). But if you mash for too long, the cell walls break and release their starch into the purée. The more mechanical the mashing motion, the more starch will be released. Culinary translation: do not use a robotic arm to make mashed potatoes.
What difference does it make if starch is now on the outside instead of the inside of cells? Well, it’s as if you added corn starch or flour to your mashed potatoes. The mixture thickens and sticks together because starch grains swell and turn into gel after coming into contact with hot liquid. Sticky, say it ain’t so! What can be done to avoid disaster? Here are some tips.
The choice of potato
There are basically two types of potato: waxy and floury. Each has its own characteristics and cooking uses. As a general rule, floury potatoes are rather long and brown. The classic example is the russet potato (also known as the Idaho). Potatoes with the word ‘oven’ on the bag are also floury potatoes. Their flesh is soft and crumbly, I would even say fluffy, after cooking. They come apart easily which makes them an excellent candidate for mashing. You don’t need to work too hard to make a smooth purée so starch is less likely to escape.
Waxy potatoes usually have a round shape and are red or brown. You’ll often find the word ‘boil’ on a bag of waxy potatoes. The flesh is thin, smooth and creamy (like wax) and it does not fall apart during cooking. It’s the ideal choice for salads, stew and boiled potatoes. This type also makes a good purée, but you have to make sure you don’t mash it too much. The yellow-fleshed Yukon Gold variety is known for producing an excellent mashed potato.
The mashing technique
Reminder: you want to separate cells, not cause them to break and release starch grains into the purée which makes it sticky. The ideal tool for this is a pestle or potato masher. An electric mixer would also do the trick but don’t overdo it. What I do: first I mash the potatoes with a pestle to break down the big pieces. Then I finish off with an electric mixer to give the purée a lighter texture. Another trick: do not overcook (the cells will be less damaged), drain the potatoes well and let them sit and cool a few minutes before mashing (there is less risk of cell breakage when they are warm).
Liquid, usually milk, is added to make the purée creamier. But if it is moisture you want, why not simply add water? You do have taste and nutritional value to consider but, according to French researchers, it is possible that milk contributes to a successful mash. In fact, milk contains large protein molecules (casein) which insert themselves among starch grains and prevent them from swelling too much or turning the purée into a sticky mass. There’s a scientific explanation for one of the most common culinary practices.