The "bitter orange" has a three part leaf, unlike the leaf we are used to seeing on citrus trees, and the fruit is only about 1 1/2 inches in diameter. If this is the case with your trees, what you have is Poncirus trifoliata, hardy to zone 6 and used as a root stock on citrus for dwarfing and resistance to root rot. It is used in Chinese medicine and to make a very bitter marmalade. I have made the marmalade and it is improved by adding orange juice (at half the volume of marmalade but the experiments continue). IMO the best feature of this fruit is its fragrance. A small bowl can perfume a room for 1-2 weeks. There are a number of postings on Poncirus trifoliata on this forum.

If the leaf is not a three part leaf and the fruit size is about that of a sweet orange, then you are likely to have a sour orange (the Seville orange is a variety of this) which is sometimes called a bitter orange. These are used to make the traditional marmalade (which is less bitter) and the rind is used as a flavoring. One variety is used in Earl Gray Tea. Also 18th century recipes were written for these sour oranges, not the sweet oranges that we use because the sweet oranges either had not yet been introduced to Europe or were still exceedingly rare.

I hope this helps you to find a way to make use of these trees.

Cath