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Most professional cookbooks tell me to fry french fries two times. First at a lower temperature of about 150-170°C and then at a higher temperature about 180-190°C.
Reference: http://en.wikibooks.org/wiki/Cookbook:French_Fries#Variations
Related: What varieties of potato would be good for chips / french fries?
What are the advantages of frying the potatoes two times?
The lower (first) temperature actually cooks the potato so that it is tender, the hotter temperature (second) is what gives the crispy golden coating.
If you just did the lower temperature, your fries might be too soft. If you just did the hotter temperature they would be too too tough.
Well, that is part of what is going on. Frying at the lower temperature does get it cooked through without over-browning. However, if you kept cooking, you could get it brown without crisping. So, actually it has a lot to do with the changes to the starch molecules in the potatoes. The lower temperature frying brings the starch and water to the exterior of the fry, forming a coating, that is then crisped nicely on the outside while leaving the interior softer when cooked at the higher temp.
This blog actually explains it very nicely if you want more detail: http://aht.seriouseats.com/archives/2010/01/the-burger-lab-why-double-fry-french-fries.html
I know I'm a bit late here, but I do have an alternative to frying twice. If you don't want to fry twice, you can also boil the french fries in water that has one or two tablespoons of vinegar or lemon juice for about 15 minutes. You have to allow the fries to cool down after the boiling, at least until the steam is no longer present, before frying.
This is because the purpose of the first time cooking is to stimulate gelatinization and water loss. The fries are able to crisp due to the absence of water in the outer layer of the fries. The acid from the vinegar protects the starch from reacting with the boiling water. If you leave out the vinegar/lemon juice, the end result will not be the same.
Personally, I like this approach because it's less messy than frying twice and you can leave the pot unattended.
According to McGee, the ~10min low-temperature (250-325F) fry is the stage that the foundations of a thicker crust is formed; During this period, starch leaks from the swelling starch granules near the surface, forming a thick reinforcing "glue." If only high heat is used, this crust-thickening stage is heavily shortened because the outer granules aren't allowed to pick up water and leak starch--not enough moisture loss also contributes to crust-softening. You can progress through the two stages without pulling out your fries. Raw potatoes stored long in temperatures colder than 43F have some starch converted to sugar, making browning often too quick (it appears old potatoes are more likely to have more sugar); apparently storing your potatoes at room temperature for a few weeks will "recondition" your overly-sugary potatoes back to normal. Potato type matters too: the "mealy"-type russet gives you the dry/fluffy interior. Starch should be rinsed off the raw fries.
