Raw celery can be frozen, but the process changes the texture of the vegetable. Rather than being crunchy and having a lot of bite, defrosted celery is mushy and easy to puree or mash.
The change in texture is caused by the rupture of the celery's cell walls as the water within them expands during the freezing process. The flavor of celery is retained during freezing, however, making frozen celery ideal for use in soup bases, sauces and other recipes that require a smooth finished texture. Celery can also be frozen in larger pieces to be used as a flavor enhancer; the large chunks can be removed from the finished product once the flavor has cooked out.