To make the perfect french fries, you need the right potatoes -- not just the variety of potato such as Russets -- but ones that are neither too wet or too dry. A quick dip in a salt water solution will help you separate the best french fry potatoes from the rest.
The test demonstrated in the video above comes from ChefSteps, who explain that potatoes that are too wet will have a soggy texture when fried, while potatoes that are too dry will end up hard and crunchy. So:
A simple method that sorts potatoes based on their specific gravity -- an estimate of their solids content -- can be used to find potatoes that are ideal for frying. Prepare two tubs of brine: the first should be nine per cent salt relative to the weight of the water (8.25% salinity) and the other twelve per cent salt relative to the weight of the water (10.7% salinity). Plunge each potato into the nine per cent brine first; potatoes that aren't too wet should sink in this brine. Then drop these potatoes into the twelve per cent brine; potatoes that aren't too dry should float in this brine.
Go ahead and cut up and fry those potatoes that pass both tests.
Finding Perfect French Fry Potatoes [ChefSteps]