The pith is the white, stringy tissue that you'll find attached to the outside of the fruit. There's also a large, white ...

Avoid white spots, which might indicate damage to the fruit during shipment. Smoother skin without large pores generally suggests a juicier orange that has a thinner pith, aka more fruit inside.

The white pith just under the skins of citrus fruit is high in pectin. Any citrus fruit can be used, but choose ones that have a thick layer of white pith for good results with this recipe.