[recommended pre-read: Potato Science]
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:
1. Crisp, but not tough exterior
2. Fluffy, but not hollow interior
3. Light golden colour
4. Stay crisp on cooling
5. Can be pre-prepared, frozen, and cooked at the last minute in one step while still satisfying requirements 1 to 4.
Any one of these is tricky – but all five at the same time? And reproducibly and reliably too.
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.
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).
Induce retrogradation in a boiling step. 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, asap.
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 yet sure. There are some combinations I have not tried (e.g. combining pectinase with steaming, and in which order?), and some that I‘ve tried but not described (e.g. soaking in lime to create a firm surface) because this post is already too long.
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 asap (remember that they are already brined). Reserve in warm oven (120C).
Consider combining with the classic combo – battered fish.
Acknowledgement: I have consulted multiple sources for this post, but my best-bet is based on Kamozawa and Talbot.