What causes blanch time to go up when making French fries with low solids potatoes? What is the mechanism at work here?

Low solids means the potato has more moisture than a high solids or starch potato such as the Russet Burbank variety. So, what a potato processor does when they have to use a lower solids spud is dial in the right amount of cooking time and temperature as best as possible compensate for the removal of the excess water without the potato getting over cooked.  We recommend with the Norkotah variety, which typically may have less solids on average, you reduce the blanching cooking temperature, but also cook it longer. You'll have to experiment with the times and temperatures, but start by blanching 15-25 degrees cooler and for another 1-2 minutes. The final product should still be light colored and not dark, but bend slightly to the touch so it is thoroughly cooked.

With new crop potatoes, they may contain excess sugar and starches, so be sure to cut and rinse in water until the water runs clear. Hopefully this helps.  Thanks for using Idaho potatoes at your operation.