Thick-cut chips have a common problem: golden outside, hard and starchy within. The cause is physical rather than technical.
How does heat travel into a chip?
Heat passes from the oil into the product during frying.
That transfer starts at the surface.
The surface reaches 100 degrees immediately.
Water begins evaporating and a layer of vapour forms at the surface.
Heat then travels inward by conduction.
Potato is not a good conductor of heat.
Heat moves slowly toward the centre.
The centre of a seven-millimetre chip warms in two minutes; in a fifteen-millimetre chip that rises to six.
By then the surface has long browned and begun to darken.
When does the starch gelatinise?
The interior cooking means the starch gelatinising.
Potato starch begins gelatinising between 58 and 66 degrees.
The centre has to exceed 70 degrees for full gelatinisation.
Where that temperature is not reached the starch granules stay intact.
They leave a gritty, starchy sensation on the palate.
That is the condition described as raw.
Once the centre reaches 70 degrees the structure softens and a fluffy texture forms.
Reaching that temperature in a thick cut before the surface burns is not possible; the solution separates the two operations.
| Cut | Time to reach the centre | Single fry | Recommended method |
|---|---|---|---|
| 7x7 mm | 2-2.5 minutes | Sufficient | Single fry |
| 9x9 mm | 3.5-4 minutes | Marginal | Single or double |
| 10x10 mm | 4.5-5 minutes | Insufficient | Double fry |
| 12x12 mm | 6-7 minutes | Insufficient | Double fry |
| Wedges | 7-9 minutes | Insufficient | Blanch and fry |
How does double frying rescue a thick cut?
That is the most widespread and most effective solution.
Two stages apply.
The first stage runs at a low temperature: 150 to 160 degrees.
The time is four to six minutes.
The surface does not brown at that temperature but the interior cooks.
The starch gelatinises and the texture softens.
The chips come out of that stage pale and soft.
The second stage runs at a high temperature: 180 to 190 degrees.
The time is two to three minutes.
Only the surface gets worked in that stage; browning and crispness form in that short time while the interior is already cooked.
What happens between the two stages?
That intermediate step is the most critical point in double frying.
The chips come out of the oil after the first fry.
They rest at least ten minutes.
Moisture at the surface evaporates during that time.
The surface dries and browns better in the second fry.
Without that rest the second fry starts on a wet surface and the crispness stays weak.
A long rest is possible too; the chips can wait an hour or even overnight in a fridge.
That gives the best result and is applied as preparation in businesses; the first fry happens in the morning and only the second is needed during service.
When is blanching necessary?
Double frying does not suffice in very thick cuts.
An additional step is needed for wedges and cuts above twelve millimetres.
The potatoes get boiled in water first.
The water comes to the boil and the potatoes stay five to eight minutes.
The centre cooks completely in that time.
They then get drained and dried.
Drying matters; wet potatoes make the oil spit and delay browning.
Blanched potatoes get cooled before frying; hot potatoes fall apart in the oil and lose their shape.
How is the oil temperature maintained?
That is an additional difficulty with thick cuts.
Thick chips carry more mass and drop the oil temperature further.
A kilogram of thick chips can drop oil at 180 degrees to 150.
That drop delays browning and increases oil absorption.
The solution fries small quantities.
A third of the basket's capacity suits.
A minute or two passes between batches for the oil to recover.
Using a thermometer secures that control on its own and removes the greatest uncertainty in working by eye.
Is the situation different with frozen products?
It is, and that is an important detail.
Commercial frozen chips get blanched in production and are generally fried once.
The first stage has therefore been completed at the factory.
A single fry suffices for those products.
But the time should be set by the cut.
Four to five minutes for a thick frozen cut.
They should not be thawed; a thawed product falls apart in the oil and the interior turns watery.
The instructions on the pack are a starting point, but domestic fryers work differently from commercial ones; the time should be observed and noted on the first batch.
How does it apply in an oven?
The same problem appears in an oven.
Thick chips can stay raw inside there too.
The solution is similar: baking in two stages.
The first stage runs covered with foil: fifteen minutes at 180 degrees.
Steam gets trapped and the interior cooks.
The second stage runs with the foil open: ten minutes at 220 degrees.
The surface browns and crisps.
Alternatively the potatoes get five minutes in a microwave before going into the oven; that markedly shortens the time and offers a practical solution in a domestic kitchen.
How much does the potato variety matter?
Variety decides as much as the cut.
Varieties high in starch suit frying better.
Their interior takes on a fluffy texture once cooked.
Varieties high in water and low in starch turn watery when fried.
Those suit boiling and salads better.
The word suitable for frying on the packaging indicates that distinction.
The sugar content of the potato matters too; a potato high in sugar darkens rapidly when fried and turns bitter, which is why potatoes held long in cold storage do not suit frying.
