Potatoes

Photo by JESHOOTS.COM on Unsplash

Potatoes are related to tomato, chilli and tobacco — members of the nightshade family, and at least potato and tomato were once considered poisonous. Which potatoes sort of are — they contain toxic alkaloids (solanine; chaconine). At normal levels these alkaloids contribute flavour, at higher levels they are bitter and dangerous. The occasional small, green tomato-like fruits of the potato are poisonous.

Greening indicates high alkaloid levels in and below the skin. Such potatoes should be peeled thickly or discarded. Potatoes should not be purchased in this condition. Greening is accelerated by light.

They are best stored between 7–10C. Much higher and they sprout and decay. Colder, and they begin to convert their starches into sugars, giving a brown core.

Potatoes contain starch, sugar, fibre, minerals and pectin, and no fat. They are wet, 70–80% water (not much less than milk at 87%). They are a good source of vitamin C (and were once used to treat scurvy). A potato contains about the same calories as an apple. All the ‘goodness’ is not in the skin — even though potato skin contains some additional nutrients, the amount of skin on a potato is so small compared to its interior that this additional ‘goodness’ is insignificant.

What is in a potato that matters when cooking? Mostly it’s the starch and pectin.

Potato contains two forms of starch — amylose and amylopectin. Starch is a poly-saccharide, that is it’s a long chain of sugar molecules (glucose). In amylose, these chains are linear and about 1,000 sugar molecules long. In amylopectin, the chains are clumped and branching, and contain about 10 times the number of molecules as amylose.

The linear nature of amylose means it can nestle up against itself and bind, and it is primarily a gelation agent. The branched structure of amylopectin can’t do this, but its complex structure interferes with water flow, making it a thickening agent.

These starches are locked together in starch granules, which adhere to the inner walls of the potato cells. The potato cell-walls themselves are made up of fibre and strengthened by pectin, and pectin binds the potato cells to each other.

The starch granules in the cells have a roughly alternating concentric structure of amylose and amylopectin (cf. onion). The two forms of starch bind strongly to each other, allowing no room for water, giving the granules a sort of dry concentric crystalline structure. The potato contains a lot of water, but it’s not found in the granules.

When heated, the starches in the granules become agitated and separate and are now free to bind with water in the cell (and with each other). They swell. Significantly. Ultimately they fill the cell itself. This is good for eating, because the granules are now a form of gel but are still contained within the potato cell.

With continued cooking, they can swell so much they break the cell wall and escape. What’s more they break up themselves releasing their starches. This is bad. We now have wallpaper glue. It’s a fine line…

A property of the gelled granules (before they burst) is that if you stop the cooking and allow the potato to cool, the amylose, amylopectin and water re-arrange themselves and form a stable structure. It is actually more stable than the original dry granule (the process is called retrogradation). So, if heating is now resumed, it is possible to heat the potato to even higher temperatures without the risk of the granules continuing to hydrate, swell and burst.

Mashing

Floury potatoes are relatively high in starch and low in water, compared to waxy varieties that are low in starch and high in water. Floury tend towards a fluffy result, whereas waxy can be creamier and smoother, but needs more mashing and so risks becoming gluey.

As heat is applied, the potato starch granules start to separate and bond with cellular water and hydrate (and swell). This process continues as the temperature rises, and by about 70C the starch occupies most of the cell — the cell itself is now container of a thick gel.

This is where we would like to stop, as further heating can swell the starch enough for it to break out of the cell. Unfortunately, we cannot stop now — at this temperature the pectin binding potato cells together has not broken down enough to allow us to separate the cells (by mashing). Pectin doesn’t begin to break down until ~85–90C. So that’s a good time to stop and mash, but it is a compromise between hydrating the starch, and breaking down the pectin, and it’s not ideal.

It’s a dilemma, but one that was solved by the instant mashed potato industry a long time ago, and that was famously brought to the attention of cooks and chefs only relatively recently (1998) by Jeffrey Steingarten. If they had not solved this, they would have been the ‘instant wallpaper glue’ industry instead.

The solution is to use retrogradation. Heat the potato to the upper limit for starch hydration (70C) and then cool it down. As the potato cools, the swelled starch granules reorganise themselves into a new structure that is stable, resists rupture and resists further hydration. The second step is to now simmer the potatoes at ~90C to break down the pectin and enable the potato cells to separate. The stabilized starch granules will not rupture or swell during this second cooking step.

Prepare the potatoes in ~2cm slices. This is a compromise because cut potatoes can lose more nutrients into the cooking water, but thin slices are necessary for the potato to be heated to 70C uniformly throughout.

Mash the potatoes with a ricer. A potato masher (or fork) is not too bad, but it is necessary to repeatedly crush already mashed potato in order to get at the last lumps, so is more damaging. With a ricer the potato is crushed just once.

It is one of life’s little ironies that although the potato is essentially fat-free, it always tastes better with a bit of butter (or other fat) added. In mash, it actually has a purpose. It gets in the way of any free starch binding to itself and forming glue — it acts to reduce gumminess. This is why a mash made with waxy potatoes, which need more mashing or greater force, is likely to require more butter. It is most easily incorporated in cold pieces. Finally, the consistency of the mash is adjusted with dairy.

Chips

The chip is probably the (western) world’s most successful fast food. So it is frustrating that a good home-cooked chip is a challenge (for me at least). To me, a chip has to satisfy all of these requirements: Crisp, but not tough exterior; fluffy, but not hollow interior; light golden colour; stay crisp on cooling; ideally, can be pre-prepared, frozen, and cooked at the last minute in one step.

So, lets start with the potato. Frying means drying, a crust won’t form in the presence of water. So, a potato that starts out with low water content is good for the crust.

Floury potatoes are lower in water content than waxy, so that’s a start. Checking the water content of a potato can be done by a floatation test. Dissolve 90g of salt in 1 litre of water. Potatoes put into this should sink. Dissolve an additional 30g salt in the same solution. The potatoes should float. In my experience, most floury potatoes more-or-less satisfy this criterion, but it’s interesting to check.

Traditionally we would now do the following: cut, wash, dry, deep fry at low temperature, drain, deep fry at high temperature, drain, salt, serve.

That’s fine. Here are some explanations and further considerations:

Cutting the chips damages cells on the surface, releasing starches and sugars. The sugars will caramelize and darken the chips on frying, and the starch can be released into the oil, degrading it. Hence the washing step. After washing, the chips can be soaked overnight in a 2% salt solution (or to taste). This will evenly brine them and add a pleasant saltiness that pervades the chip.

The first slow fry is to cook the chip, and to give time for starch in the surface cells to escape and paste together into a thicker and more robust layer. Pasting, from starch release, is the exact opposite of what we aim for in mashed potatoes. The pasted layer is firmed during the second higher-temperature fry, to form a crisp semi-impenetrable layer on the surface of the chip that retards fat penetration on cooling (see later).

There’s no need for a temperature higher than ~130C, because the chip itself will not get much hotter than ~100C until all the water in it has been turned to steam. The oil temperature will probably drop back to ~130C anyway, given the limitations in domestic pot sizes. I prefer to fry in a wok, because oil is much less likely to boil over in something that gets wider towards the top. Fry in lard or tallow. Vegetable oils (which are not in fact made from vegetables but rather seeds) are not recommended on health grounds.

The first frying step can be replaced by steaming in a pressure cooker. Under full pressure, the temperature can reach 120C, and by steaming the potato its nutrients are not lost to water.

Remember that the potato cell walls contain pectin and cells are bound to each other by pectin. So, a pectin-dissolving enzyme (Pectinase-SPL) can be used to create a surface paste. Pectinase weakens the cell walls on the surface of the chip, allowing starch release. Soaking in a 0.4% solution for 1 hour is sufficient. With this method, the first cooking step would still be a fry. As an aside, pre-brining the chips also weakens the pectin (the sodium in the salt interferes with calcium/magnesium mediated bonds in the pectin).

Another option is to induce retrogradation. This is to stop the interior of the chip becoming hollow. The down side is that it’s more difficult to get a surface pasting.

Chips mostly become oily on cooling, when oil remaining on the chip surface penetrates the chip through steam vents that were created during frying. Less oil enters the chip during frying because the oil is held off the surface of the chip by the escaping steam (the bubbling you hear). Because surface pasting strengthens the surface layer, the escape of steam occurs over more numerous but smaller vents during frying, which because of their size and oil’s viscosity, limits reabsorption of oil on cooling.

Drying aggressively on paper towels after frying is the best way to further reduce oil penetration. Oil will not drain off the chips under gravity because of oil’s high surface tension. So, draining racks, baskets or racks on the side of woks are only a first step, and chips should not be left in these devices. Surface oil needs to be wicked off with paper towels.

Oil reabsorption on cooling is important because oil and water (in the chip) combine to create a greasy effect. If this is minimized, the chip will remain crunchy for longer.

Confused? Me too. I’ve described the main factors that I think are relevant. But what the ideal combination and order is — I’m not sure.

So for now, this is my best-bet, balancing simplicity with effect:

Floury potatoes; skip the floatation test; cut 15mm thick; brine overnight; steam in pressure cooker for 5 minutes; let rest in pressure cooker as temperature (and pressure) normalize; remove, pat dry and place in one layer on rack; allow to cool to room temperature then place rack in fridge until they are dry-tacky, a few hours (the surface paste step); vacuum pack and freeze.

This can be done in bulk when you feel like it, and frozen in serving portions.

Chip cooking is then one-step as needed. Take a bag of chips from the freezer and fry them at ~200C in small batches. Dry on paper towel. Salt lightly (remember that they are already brined). Reserve in warm oven (120C).

Acknowledgement: I have consulted multiple sources for this post, but my best-bet is based on Kamozawa and Talbot.

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