For years, I made key lime pie from a newspaper-clipped recipe for "Naples Style Key Lime Pie," using cream cheese and ordinary limes. Today, I thought I was moving forward by using real key limes, and the recipe from the America's Test Kitchen Family Baking Book (not the quick shortcuts version). This recipe uses grated lime zest, fresh squeezed lime juice, egg yolks, and sweetened condensed milk for the filling. It is baked instead of cooked on the stovetop. I used my Microplane fine grater on the limes.
My pie filling ended up bitter. My hypothesis is that the zest layer on the limes was thin, and I grated pith along with the zest. Any ideas on what went wrong?