in this, S is the total oil uptake in a single fry (g) and a is the oil
taken up by a single inactive pore (g). It was found from preliminary
experiments that there is already some oil uptake before the maximum
evaporation rate is reached, so an additional term for initial
oil content should be added. The amount of fat/oil present in raw
potatoes was found to be negligible, being less than 10% of the initial
oil uptake.