Readers tip: Zucchini with Balsamic

Last week one of my readers left this tip on my Grilled Zuccini post,

We LOVE grilled zucchini and squash. I marinate it for 15 minutes in a balsamic vinegar and olive oil dressing. Then grill them on the foreman for a fantastic dish!!

So I thought it was worth a try…

I am not sure if I did something wrong, but the skins turned really bitter? They looked and smelled yummy, Has anyone else has this problem?