A frozen etrog

Hi,

An etrog which has been frozen may still be valid, if one knew that the insides had not been damaged, and would not display damage before the end of Yom Tov. Of course, one would need to experiment in order to determine this.

(Rav Moshe Feinstein, Igrot Moshe Orach Chaim 1:185)

המצפה לישועה,
Mordechai