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The surprisingly complex chemistry of the humble spud

26 February 2016

Not all potatoes love the deep fat fryer, and not every variety will sing in a salad. BBC Future looks at the secret properties that make certain spuds the right one for the job.

Baked, mashed, boiled, fried – in a general sense, it's hard to do potatoes wrong. There's something about the fluffiness of a well-baked potato, the crunch of a nice chip, the creaminess of mash (the best recipe I know: keep adding butter until it stops being absorbed) that warms the heart, as well as the taste buds.

But if you've ever chosen the wrong potato for the job, chances are you know it. It may not be the kind of thing explained to you in school, but anyone who's tried to fry red potatoes or make salad with russets knows, not all spuds are created equal. Some of them – to put this mildly, as my smoke detector did not – are not meant for frying.

There are hundreds of different breeds of potatoes, and it turns out that beneath that yellow or brown or purple or red skin, they have quite different chemistries. Potato breeders and food scientists divide them up into broad categories, usually mealy and waxy. Mealy (or sometimes “floury” or “starchy”) potatoes are dry, fluffy, and a little grainy when cooked; they are a relatively high 22% starch by weight, according to research by Diane McComber cited in this piece by food scientist Guy Crosby.

The poster child of the mealy potatoes, at least in the United States, is the russet. As readers of this column will know, russets are the potato of choice for making chips. Their low water content means that when their flesh hits hot oil, much of the water boils off before a skin forms on the surface, leaving just enough inside to gently steam the chip's innards. Their plentiful starch molecules help form the skin, and the fact that the flesh is quite dense means the oil never manages to seep deep enough to make the chips soggy. Mealy potatoes make the best mashed potatoes and baked potatoes as well.

Woe betide the cook who boils them for a salad, however – they will disintegrate. The right spud for that job is one of the many waxy varieties, which tend to be thin-skinned, smooth-fleshed, and moist. They are only around 16% starch by weight, and when you boil them, they keep their shape. (They also have beautifully whimsical names – the Charlotte, the Cara, the Anya.)

Comparing cooked mealy and waxy potatoes under the microscope, researchers have found that there are striking differences. In mealy potatoes, clumps of starch molecules suck water from the rest of the flesh dramatically, while waxy potatoes' starch granules leave it where it is. This may be why mealy ones feel dry and the waxy ones feel moist. Also, at the microscopic level, the cells that make up the flesh of mealy potatoes have a tendency to crumble into clumps when cooked, sticking together to create a gritty effect even while the whole potato loses its structural integrity. Waxy potatoes are not prey to this.

One reason seems to be that the particular make-up of starches in mealy potatoes will start to degrade at much lower temperatures then the starches in their waxy brethren – nearly 12C lower. This destroys some of the connections holding the cells together, and it puts pressure on them early in the cooking process, so they will start to split and break off much sooner than the cells of waxy potatoes. (For more detail, see this Guy Crosby article)

These are interesting things to consider when you're eyeing potatoes for a particular dish. But they might be relevant in other, very different scenarios – just consider this delightful paper, Potatoes for Human Life Support in Space. For long-term sojourns into space, being able to grow one's own food will be key, and for decades now there have been experiments looking at how potatoes would fare in growth chambers under various conditions. The most-tested breeds include both waxy and mealy varieties, so this is not a conundrum future space cooks will be able to avoid.

But there may be compensations. Chips fried in gravity approximating what you’d find on the surface of Jupiter are said be perfectly crisp.

Back here on Earth, there have been other developments. The Chinese government has announced that potatoes will become a Chinese staple food, up there with rice and wheat. Potatoes currently work as a kind of vegetable in China, rather than a starch – in one toothsome dish, they're minced into slivers, soaked in vinegar, and fried with chilis, while in another they are braised with soy sauce and anise, and both can be eaten over rice.

But potatoes becoming a staple doesn't necessarily mean that they'll be more obvious on the table than they are already. Rather than a baked russet taking the place of a bowl of a rice, it's likely that flour made from potatoes will be used in noodles, buns, and other food products, notes Fan Bai on For those uses, the decision of which variety to use will be out of the consumer's sight – and out of mind.

Source: BBC Future

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